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Controlled Surface Topography of Transparent Conducting Oxide for Enhanced Light Extraction in Light Emitting Devices

Permanent Link: http://ufdc.ufl.edu/UFE0042112/00001

Material Information

Title: Controlled Surface Topography of Transparent Conducting Oxide for Enhanced Light Extraction in Light Emitting Devices
Physical Description: 1 online resource (158 p.)
Language: english
Creator: Gupta, Sushant
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: chemical, light, micro, transparent, zinc
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Solid-state lighting (SSL) is an emerging technology with potential to replace the conventional lighting. Energy efficiencies exceeding that of ?uorescent and high-intensity-discharge (HID) technologies have been demonstrated for SSL. Equipped with high efficiencies SSL technology has potential to impact energy consumption as artificial lighting alone amounts to more than $230 billion annually or > 8.9% of total energy consumption. Internal quantum efficiencies nearly 100% have been reported for both inorganic and organic LEDs. However, the external quantum efficiencies are still very low. The major impediment for high external quantum efficiencies is the photon extraction from various layers in the device. The focus of this work is to enhance the light extraction efficiency in light emitting devices. To realize this goal, surface topography of transparent conducting oxide layer has been controlled on two scales: micro-level texturing and nano-level roughness. The introduction of micro-scale texture at TCO/active layer interface enhances the light output by extracting the TCO and active guided modes. By controlling the nano-scale roughness the detrimental effects of surface roughness, large particle inclusions and pinholes are eliminated. Hence, the first goal of this research is to control the surface roughness of the TCO thin film to reduce the effect of such anomalies on the properties of the subsequent active layer and the performance of the LED device. In order to achieve this, a chemical mechanical polishing (CMP) process was developed for controlling the surface roughness of zinc oxide (ZnO). ZnO was used as the TCO material as it has been extensively researched as a replacement material for commercially used indium tin oxide. CMP process parameters along with slurry chemistry were systematically varied to develop a CMP process for ZnO. Based on the findings, a CMP mechanism for ZnO polishing has been proposed. Effects of slurry additives such as surfactants and salts were also studied to further improve the surface quality and CMP performance. Highly smooth zinc oxide surface (with RMS roughness between 3-6 angstroms) was obtained using developed CMP process for zinc oxide thin films. The second part of this work is focused on the enhancement of extraction efficiency by controlling the micro-topography. This involves the study of periodic micro-scale textures (microlens, micropyramid and cylinder) for light extraction applied at different interfaces. Ray-tracing simulations were performed to understand the ray dynamics of these features. Two device constructions were simulated with texture at: (A) glass/air interface and (B) glass/TCO interface (corrugated device). The impact of feature parameters (diameter and h/d ratio or contact angle) on luminous intensity and angular distribution of extracted photons was studied. It was found that the introduction of these textures results in increase in the escape probability of the photon due to change in their directionality after multiple reflections. The enhancement in light extraction is obtained from the extraction of higher angle photons in most cases. Based on the ray-tracing simulations, microlens, micropyramid and microcylinder arrays were fabricated on the glass substrate. Photoluminescence measurements were performed using evaporated Alq3 (as photoluminescent material) and Al (as back reflector) layers over the substrates with controlled topography of ZnO TCO layer in corrugated device configuration. The textured TCO device showed more than 2X improvement as compared to control device with no structure which matches closely with the simulation results. This work demonstrated that by controlling the topography of the TCO layer more than 1.5 fold increase in extraction efficiency can be achieved. This enhancement can be further increased by texturing the glass/air interface to reduce substrate mode.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sushant Gupta.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Singh, Rajiv K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042112:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042112/00001

Material Information

Title: Controlled Surface Topography of Transparent Conducting Oxide for Enhanced Light Extraction in Light Emitting Devices
Physical Description: 1 online resource (158 p.)
Language: english
Creator: Gupta, Sushant
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: chemical, light, micro, transparent, zinc
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Solid-state lighting (SSL) is an emerging technology with potential to replace the conventional lighting. Energy efficiencies exceeding that of ?uorescent and high-intensity-discharge (HID) technologies have been demonstrated for SSL. Equipped with high efficiencies SSL technology has potential to impact energy consumption as artificial lighting alone amounts to more than $230 billion annually or > 8.9% of total energy consumption. Internal quantum efficiencies nearly 100% have been reported for both inorganic and organic LEDs. However, the external quantum efficiencies are still very low. The major impediment for high external quantum efficiencies is the photon extraction from various layers in the device. The focus of this work is to enhance the light extraction efficiency in light emitting devices. To realize this goal, surface topography of transparent conducting oxide layer has been controlled on two scales: micro-level texturing and nano-level roughness. The introduction of micro-scale texture at TCO/active layer interface enhances the light output by extracting the TCO and active guided modes. By controlling the nano-scale roughness the detrimental effects of surface roughness, large particle inclusions and pinholes are eliminated. Hence, the first goal of this research is to control the surface roughness of the TCO thin film to reduce the effect of such anomalies on the properties of the subsequent active layer and the performance of the LED device. In order to achieve this, a chemical mechanical polishing (CMP) process was developed for controlling the surface roughness of zinc oxide (ZnO). ZnO was used as the TCO material as it has been extensively researched as a replacement material for commercially used indium tin oxide. CMP process parameters along with slurry chemistry were systematically varied to develop a CMP process for ZnO. Based on the findings, a CMP mechanism for ZnO polishing has been proposed. Effects of slurry additives such as surfactants and salts were also studied to further improve the surface quality and CMP performance. Highly smooth zinc oxide surface (with RMS roughness between 3-6 angstroms) was obtained using developed CMP process for zinc oxide thin films. The second part of this work is focused on the enhancement of extraction efficiency by controlling the micro-topography. This involves the study of periodic micro-scale textures (microlens, micropyramid and cylinder) for light extraction applied at different interfaces. Ray-tracing simulations were performed to understand the ray dynamics of these features. Two device constructions were simulated with texture at: (A) glass/air interface and (B) glass/TCO interface (corrugated device). The impact of feature parameters (diameter and h/d ratio or contact angle) on luminous intensity and angular distribution of extracted photons was studied. It was found that the introduction of these textures results in increase in the escape probability of the photon due to change in their directionality after multiple reflections. The enhancement in light extraction is obtained from the extraction of higher angle photons in most cases. Based on the ray-tracing simulations, microlens, micropyramid and microcylinder arrays were fabricated on the glass substrate. Photoluminescence measurements were performed using evaporated Alq3 (as photoluminescent material) and Al (as back reflector) layers over the substrates with controlled topography of ZnO TCO layer in corrugated device configuration. The textured TCO device showed more than 2X improvement as compared to control device with no structure which matches closely with the simulation results. This work demonstrated that by controlling the topography of the TCO layer more than 1.5 fold increase in extraction efficiency can be achieved. This enhancement can be further increased by texturing the glass/air interface to reduce substrate mode.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sushant Gupta.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Singh, Rajiv K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042112:00001


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1 CONTROLLED SURFACE TOPOGRAPHY OF T RANSPARENT CONDUCTING OXIDE FOR ENHANCED LIGHT EXTRACTION IN LIGHT EMITTING DEVICES By SUSHANT GUPTA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIA L FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Sushant Gupta

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3 To my parents, family, friends and a special someone

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4 ACKNOWLEDGMENTS I would like to express my heartiest gratitude to Prof. Rajiv K. Singh for his support and guidance with out which none of this work would have been possible His encouragements and suggestions made the research more interesting and challenging. His patience and faith made me successfully complete this endeavor even after changing 3 projects. The dedication to his work and sleepless nights he spent for submission of proposals, motivated me to go that extra mile. He has been a great advisor and a mentor not just for my research at UF but also on a persona l level. I would like to acknowledge my supervisory committee: Prof. S. J. Pearton Prof. D Norton Prof. Timothy Anderson, Prof. V. Craciun and Prof. Henry Hess for serving on my supervisory committee, for their time and suggestions. I would like to than k, Tyler Parent, my manager at Maxim Integrated products for his belief and confidence in me during my short stay as an intern. His candid advices helped me shape my career and currently guiding me to choose the suitable career path. His faith in my abilit ies instilled confidence in me and gave me opportunity to lead and represent the CMP team for development of a next generation device. My sincere gratitude for my mentor Dr. Kamal Mishra, for his guidance and training which helped me work on various tools independently and demonstrate my skills. None of my success at Maxim would been possible without the support of Jack Huang Kiyoko, Sudhir Chopra, Viral Shah, Ching Tang, and other team members and technicians who made it so easy to transition in to Maxim culture. Special thanks to Anuranjan, who was the reason for getting this great internship experience. No words can be enough to express my gratitude for him and Dhriti (his wife) for providing me with place to stay early on in California and welcoming me in their family. Tvisha (Chiya), y ou might not be able to

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5 read this now but your little mischief and lovely ways of making me smile made me forget the world and made the life look like a fairy tale. I am trul y thankful to you three for making me feel like a part of your family, without you California stay would have been totally different and very difficult. I am thankful to all my past and present Karthik Ramani, Dr. Seung Young Son, Dr. Feng Chi, Dr. Sejin Kim, Dr. Ta ekon Kim, Dr. Jaesoek Lee and Myoung Hwan Oh, Jungbae Lee, Kannan Balasundar and Yu Liu Specials thanks to NRF staff: Bill Lewis, David Hays, Al Ogden and Dr. Brent Gila for their guidanc e and help in troubleshooting my experiments. Also, I am grateful to Dr. Mark Davidson, Jason Rowland and Chuck Rowland for their help with device fabrication trials I would like to thank Dr. Franky So, Dr. Do Young Kim and Wooram Youn for their guidance and help with PL device fabrication and measurements. I am grateful to all the people I met, all the roommates (I changed too many apartments) who became my friends and made the stay at Gainesville a wonderful learning experience. For making Gainesville fun and exciting, large number of my friends played a role and I would like to thank, Aniruddh, Vibhava, Abhishek, Richa, Karam, Arul, Parnitha Bhabhi, Anu, Ashutosh, Abu, Isha, Mamta, Ashwini and Preeti. Special thanks to Purushottam (PK) for being sort of an advisor and a mentor after Dr. Singh. I benefitted tremendously f rom h is guidance and fruitful discussions on my research work. I can not say enough to express my gratitude and appreciation for Neetu who has been my best friend and family away from family. Her relentless support and encouragement pushed me to go on and overcome hurdles in my research and in a

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6 way in life She has always been there for me and her presence made me feel that there is always someone on whom I can always rely upon. She makes it so easy to share even the most embarrassing moments with her. A nother person who made a big impression on me is Toral. We have been friends only for a year now but with her time losses its count. She has been a source of support, encouragement and inspiration to carry on during the last few arduous semesters at UF. A lthough outwardly she might be very different person but I know from inside she is caring and loving. I cannot ever forget our exercising and biking endeavors, HPNP discussions and of course her delicious cooking. I am thankful to Toral for coming in to my li fe and showing me a different per spective to see life. This acknowledgment would not be complete without thanking my parents and principles I can not imagine myself w here I am right now. I am indebted by their endless love and innumerable sacrifices which made me a successful person today. I am thankful to my sister Surbhi for her love, patience and friendship. I express my gratitude towards: bade and badi chachi, chot i chachi, Rupali, Pragya, Sonu bhaiya, choti boa, Ashu and all others those I have missed for their love and support. I dedicate this dissertation to Sangeeta didi and a special someone for their abysmal love, support, encouragement, inspiration and sacri fices without which I would have settled for

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7 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TAB LES ................................ ................................ ................................ .......... 10 LIST OF FIGURES ................................ ................................ ................................ ........ 11 LIST OF ABBREVIATIONS ................................ ................................ ........................... 15 ABSTRACT ................................ ................................ ................................ ................... 16 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 19 Transparent Conducting Oxides ................................ ................................ ............. 19 Importance of TC Os ................................ ................................ ......................... 19 History of TCO ................................ ................................ ................................ .. 20 Zinc Oxide: A Promising TCO ................................ ................................ ................. 21 Propertie s of ZnO ................................ ................................ ............................. 22 ZnO Applications ................................ ................................ .............................. 23 TCO Roughness ................................ ................................ ................................ ..... 23 Light Extractio n ................................ ................................ ................................ ....... 25 Basic Concepts ................................ ................................ ................................ 26 Light Extraction Efficiency ................................ ................................ ................ 27 Improving Light Out coupling ................................ ................................ ............ 28 Device shaping ................................ ................................ .......................... 28 Photon recycling ................................ ................................ ........................ 28 Phot on scattering ................................ ................................ ....................... 29 Summary ................................ ................................ ................................ ................ 30 2 LITERATURE REVIEW ................................ ................................ .......................... 36 TCO Surface Rou ghness ................................ ................................ ........................ 36 Light Extraction ................................ ................................ ................................ ....... 38 Device Shaping ................................ ................................ ................................ 39 Interface Modifi cation Techniques ................................ ................................ .... 41 Surface roughening ................................ ................................ .................... 41 Photonic crystals ................................ ................................ ........................ 43 Patterned substrates ................................ ................................ .................. 45 External Out couplers ................................ ................................ ....................... 47 Other Techniques ................................ ................................ ............................. 51 3 OUTLINE OF RESEARCH ................................ ................................ ..................... 62

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8 4 CHEMICAL MECHANICAL POLISHING OF ZINC OXIDE ................................ ..... 64 Introduction ................................ ................................ ................................ ............. 64 Experimental Procedure ................................ ................................ ......................... 66 Results and Discussion ................................ ................................ ........................... 67 Effect of Applied Down pressure and Platen Speed ................................ ......... 68 Effect of Particle Size ................................ ................................ ....................... 69 Effect of Slurry pH ................................ ................................ ............................ 70 Effect of P article Concentration ................................ ................................ ........ 71 Zinc Oxide CMP Mechanism ................................ ................................ ............ 71 Conclusion ................................ ................................ ................................ .............. 75 5 EFFECT OF SLURRY ADDITIVES: SURFACTANT AND SALTS .......................... 83 Introduction ................................ ................................ ................................ ............. 83 Experimental Procedure ................................ ................................ ......................... 84 Results & Discussion ................................ ................................ .............................. 85 Effect of Surfactant ................................ ................................ ........................... 85 Effect of Salt ................................ ................................ ................................ ..... 87 Conclusion ................................ ................................ ................................ .............. 88 6 RAY TRACING SIMULATIONS ................................ ................................ .............. 94 Introduction ................................ ................................ ................................ ............. 94 Device Simulation Parameters ................................ ................................ ................ 96 Results and Discussion ................................ ................................ ........................... 98 Device A: Texture at Substrate/Air Interfa ce ................................ .................... 98 Pyramid texture ................................ ................................ .......................... 98 Microlens texture ................................ ................................ ........................ 99 Cylindrical textur e ................................ ................................ .................... 100 Device B: Texture at Substrate/TCO Interface ................................ ............... 101 Conclusion ................................ ................................ ................................ ............ 104 7 EXPERIMENTAL PROCEDURES FOR FABRICATION OF TEXTURES ............ 115 Experimental Procedure ................................ ................................ ....................... 115 Substrate Preparation ................................ ................................ .................... 115 Structure Fabrication ................................ ................................ ...................... 115 Cone/pyramid texture ................................ ................................ ............... 115 Microlens texture ................................ ................................ ...................... 117 Cylindrical texture ................................ ................................ .................... 117 Results and Discussion ................................ ................................ ......................... 118 Cone/Pyramid Stru cture ................................ ................................ ................. 118 Microlens Structure ................................ ................................ ........................ 119 Cylindrical Structure ................................ ................................ ....................... 120 Conc lusion ................................ ................................ ................................ ............ 121

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9 8 MICRO TEXTURED TRANSPARENT CONDUCTING OXIDE FOR ENHANCED LIGHT EXTRACTION EFFICIENCY ................................ ................ 128 Introduction ................................ ................................ ................................ ........... 128 Experimental Procedure ................................ ................................ ....................... 129 Substrate Preparation ................................ ................................ .................... 129 TCO Fabrication ................................ ................................ ............................. 130 Device Fabrication ................................ ................................ .......................... 131 Ray tracing Simulations Parameters ................................ .............................. 132 Resu lts and Discussion ................................ ................................ ......................... 133 Conclusion ................................ ................................ ................................ ............ 136 9 CONCLUSIONS ................................ ................................ ................................ ... 142 Chemical M echanical Polishing of ZnO ................................ ................................ 143 Light Extraction ................................ ................................ ................................ ..... 144 Corrugated Device ................................ ................................ ................................ 145 LIST OF REFERENCES ................................ ................................ ............................. 147 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 158

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10 LIST OF TABLES Table page 1 1 Host and associated dopants in typical TCO systems ................................ ....... 34 1 2 Properties of ZnO ................................ ................................ ............................... 35 4 1 Fit parameters for the modified Preston e quation ................................ ............... 82 4 2 Dissolution rates and change in RMS roughness. ................................ .............. 82 7 1 Maximum microlens height from square base pyramid of height H .................. 127 7 2 RIE etch recipe for silica etching ................................ ................................ ...... 127

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11 LIST OF FIGURES Figure page 1 1 A pplication s of transparent conducting oxides. ................................ .................. 31 1 2 Materials space for semimetals, good metals, transparent conductors and semiconductors based on n correlations. ................................ ....................... 32 1 3 Schematic depicting Snell's law and total internal reflection. .............................. 33 1 4 Illustration of escape cone defined by the critical angle. ................................ ..... 33 1 5 Illustration depicting waveguiding of rays with emission angle greater than critical angle. ................................ ................................ ................................ ....... 33 2 1 Dark spot formation in LED active layer ................................ ............................ 53 2 2 Evolution ) LEDs. ........... 53 2 3 (a) Optical image of AlGaInP/GaP truncate d inverted pyramid LED and (b) is schematic of a TIP LED device. ................................ ................................ .......... 54 2 4 SEM micrographs of an N face GaN surface etched by a KOH based PEC method. ................................ ................................ ................................ .............. 54 2 5 SEM image of Oblique angle deposited GRIN ITO layer with refractive index profile. ................................ ................................ ................................ ................. 55 2 6 (a) Schematic depicting fabrication of PDMS nanowires and (b) is the scannin g electron micrograph of a fabricate ... 55 2 7 (a) Top lit view, (b) schematic cross section view of the PXLED and (c) radiation patterns of the LEDs. ................................ ................................ ........... 56 2 8 (a) Schematic of a PC OLED device with SiO 2 /SiN x PC layer with intensity profiles. ................................ ................................ ................................ ............... 56 2 9 (a) Schematic drawing of GaN based device on s tripe PSS [104] and (b) cross sectional SEM of LEPS GaN grown on PSS. ................................ ........... 57 2 10 Cross section SEM micrographs of GaN grown on V gr ooved sapphire with a ................................ ........................... 57 2 11 (a) Top view SEM image of CWE PSS and (b) is a schematic diagram of device on CWE PSS. ................................ ................................ ......................... 58

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12 2 12 (a) Schematic diagram and (b) cross section TEM micrograph showing lateral epitaxial overgrowth of GaN layer on 6H SiC substrate with SiO 2 mask ................................ ................................ ................................ .................. 58 2 13 S EM of a PDMS microlens array. ................................ ................................ ....... 59 2 14 Schematic of InGaN QWs LEDs structure utilizing SiO 2 /PS microspheres microlens array. ................................ ................................ ................................ .. 59 2 15 SEM image of (a) 1 D triangular and (b) 2 D taper like gra tings on the sapphire backplane ................................ ................................ ........................... 6 0 2 16 Cross sectional SEM image of bottom of 6H SiC substrate with moth eye structure fabricate d using metal nanomask ................................ ....................... 60 2 17 Schematic diagram of the OLED with the embedded low index grid (LIG) in the organic layers. ................................ ................................ .............................. 61 4 1 AFM micrograph of the annealed zinc oxide film. ................................ ............... 76 4 2 X ray diffraction patterns of the as deposited and annealed ZnO films. ............. 76 4 3 Remo (red li ne represents the linear fit) ................................ ................................ ..................... 77 4 4 inset) ( ) with respect to linear velocity. Red line represents the linear fit. ................................ 77 4 5 Graph representing ). ................................ .............. 78 4 6 Removal rates versus pH of the slurry. Inset represents the average RMS roughness. ................................ ................................ ................................ .......... 78 4 7 AFM micrographs of zinc oxide films po lished with (a) pH 3.1, (b ) pH 7.0 and (c) pH 10.6 slurry ................................ ................................ ................................ 79 4 8 Removal rate variation with weight percent solid loading o f the polishing slurry solution ................................ ................................ ................................ .... 79 4 9 Plot shows zeta potential wi ). ................................ ................................ ........... 80 4 10 Optical transmission curves for the glass, unpolished and the samples polished with acidic and basic pH slurries. ................................ ......................... 80 4 11 (a) X ray reflectivity curves and (b) schematic showing fitting results. ................ 81

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13 5 1 /min) variation with SDS concentration in slurry whereas RMS roughness is depicted by circles ( ). ....... 89 5 2 Plot illustrating ) variation with respect to the concentration of TX 100 surfactant in the slurry. .......................... 89 5 3 ) with respect to the varying concentration of KCl salt in the slurry solution. ................................ ................... 90 5 4 AFM micrograph of the annealed zinc oxide film (scale bar is in ). .................. 90 5 5 AFM micrographs for z inc oxide films polished with SDS slurry with SDS concentration of (a) 5 mM, (b) 10 mM, (c) 16 mM and (d) 30 mM. ..................... 91 5 6 AFM micrographs for zinc oxide films polished with Triton X 100 slurry w ith TX 100 concentration of (a) 0.322 g/l, (b) 0.64 g/l and (c) 1.28 g/l. .................... 92 5 7 AFM micrographs for zinc oxide films polished with (a) 0.1M, (b) 0.2 M and (c) 0.3 M KCl salt added to the poli shing slurry. ................................ ................. 92 5 8 Optical transmission curves for samples polished using slurries with 0.1 M KCl, 5 mM SDS and 0.322 g/l TX 100. ................................ ............................... 93 6 1 Schematic depicting different device structures studied using ray tracing simulations. ................................ ................................ ................................ ....... 106 6 2 Enhancement in out coupled rays from a pyramid array textured substrate (Device A) rep resented by (a) a n d (b) whereas (c) represents the maximum simulated fractional powers obtained for the different base diameters. ............ 107 6 3 Angular distribution of the light out coupled from dev ice A with pyramid texture for active layer refractive index (a) n active = 1.7 and (b) n active = 2.5. ...... 108 6 4 (a) and (b) are e nhancement in light out coupling for a microlens array with devic e A configuration. The maximum simulated fractional powers at different base diameters are depicted by (c). ................................ ................................ 109 6 5 Angular distribution of the total light out coupled from device A with micro lens texture for (a) n active = 1.7 and (b) n active = 2.5. ................................ .................. 110 6 6 (a) and (b) depict the light out coupled for a cylinder type texture at the substrate/air interface while (c) represents the m aximum simulated fractional powers for different diameters. ................................ ................................ ......... 111 6 7 Angular spread of the light out coupled from device A with cylindrical texture for active layer refractive index of (a) 1.7 and (b) 2.5. ................................ ...... 112

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14 6 8 Light extraction enhancement s from a microlens texture at the substrate/TCO interface (Device B) are depicted in (a) and (b) while (c) represents the maximum simulated fra ctional powers for different diameters. ......................... 113 6 9 Far field angular spread of the light out coupled from device B at different microlens n active of (a) 1.7 and (b) 2.5. ..... 114 6 10 Far field angular distribution of photons out coupled from the TCO layer at different microlens heights for n active ................ 114 7 1 Schematic illustrating the mask designs. ................................ .......................... 123 7 2 Process flows for fabrication of (a) cone /pyrami d and microlens and (b) cylindrical arrays. ................................ ................................ .............................. 123 7 3 ................................ ................................ ............................ 124 7 4 ................................ ................................ ....................... 124 7 5 Schematic of microlens height derived from pyramid of heig ht H. .................... 125 7 6 AFM micrographs and line profile of microlens structure after CMP of the .. 125 7 7 ................................ ................................ ............................ 126 7 8 SEM images of fabricated cylind rical structure of different magnifications, (a) 2700 X, at 45 tilt and (b) 6000 X (top view). ........ 126 8 2 SEM micrograph illustrating the polished Al:ZnO on the microlens pattern of ................................ ........... 138 8 3 Normalized photoluminescence intensity of Alq 3 with Al back reflector for (a) plain microlens patterned glass, (b) as deposited Al:ZnO and (c) polished Al:ZnO on microlens textured substrate. ................................ .......................... 139 8 4 Enhancement in light out coupling in the corrugated device with (a) n active = 1.7 and (b) n active = 2.5. The simulated fractional power s for different diameter of the microlens at h/d = 0.5 are represented by (c). ................................ ........ 140 8 5 Simulated angular distribution of extracted mode for device with (a) n active = 1.7 and (b) n active = 2.5. ................................ ................................ ..................... 141

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1 5 LIST OF ABBREVIATION S AFM Atomic force microscopy AZO Aluminum doped zinc oxide CMP Chemical mechanical polishing ELOG Epitaxially laterally overgrown GaN GaN Gallium nitride IEP Isoelectric point ITO Tin doped i ndi um oxide LED Light emitting diode/device LEO L ateral epitaxial overgrowth MQW Multi quantum well OLED Organic light emitting diode/device PC Photonic crystals PL Photoluminescence PSS Patterned sapphire substrate RMS Root mean square SAS S econdary alkyl su lfate SDS S odium dodecyl sulfate SEM Scanning electron microscopy TC Transparent conductors TCO Transparent conducting oxide TDD Threading dislocation TFEL Thin film electroluminescent device TX 100 Triton X 100 ZnO Zinc oxide

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16 Abstract of Dissertation Pre sented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CONTROLLED SURFACE TOPOGRAPHY OF T RANSPARENT CONDUCTING OXIDE FOR ENHANCED LIGHT EXTRACTION IN LIG HT EMITTING DEVICES By Sushant Gupta August 2010 Chair: Rajiv K. Singh Major: Materials Science and Engineering Solid state lighting (SSL) is an emerging technology with potential to replace the conventional lighting. Energy efficiencies exceeding that of high intensity discharge (HID) technologies have been demonstrated for SSL Equipped with high efficiencies SSL technology has potential to impact energy consumption as artificial lighting alone amounts to more than $230 billion annually or > 8.9% of total energy consumption I nternal quantum efficiencies nearly 100% have been reported for both inorganic and organic LEDs. However, the external quantum efficiencies are still very low. The major impediment for high external quantum efficiencies is the photon extraction from various layers in the device. Th e focus of this work is to enhance the light extraction efficiency in light emitting devices To realize this goal, surface topography of transparent conducting oxide layer has been controlled on two scales: micro level texturing and nano level roughness The introduction of micro scale texture at TCO/active layer interface enhances the light output by extracting the TCO and active guided modes. By controlling the nano scale roughness the detrim ental effects of surface roughness, large particle inclusions and

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17 pinholes are eliminated Hence, the first goal of this research is to control the surface roughness of the TCO thin film to reduce the effect of such anomalies on the properties of the subse quent active layer and the performance of the LED device. In order to achieve this, a chemical mechanical polishing (CMP) process was developed for controlling the surface roughness of zinc oxide (ZnO). ZnO was used as the TCO material as it has been exten sively researched as a replacement material for com mercially used indium tin oxide CMP process parameters along with slurry chemistry were systematically varied to develop a CMP process for ZnO. Based on the findings, a CMP mechanism for ZnO polishing ha s been proposed. Effect s of slurry additives such as surfactants and salts w ere also studied to further improve the surface quality and CMP performance. Highly smooth zinc oxide surface (with RMS roughness between 3 6 ) was obtained using developed CMP pr ocess for zinc oxide thin films. The second p art of this work is focused on the enhancement of extraction efficiency by controlling the micro topography. This involves the study of periodic micro scale textures (microlens, micropyramid and cylinder ) for li ght extraction applied at different interfaces Ray tracing simulations were performed to understand the ray dynamics of these features. Two device constructions were simulated with texture at: (A) glass/air interface and (B) glass/TCO interface (corrugate d device) The impact of feature parameters (diameter and h/d ratio or contact angle) on luminous intensity and angular distribution of extracted photons was studied. It was found that the introduction of these textures results in increase in the escape pr obability of the photon due to change in their d i r ectionality after multiple reflections. The enhancement in light extraction is obtained from the extraction of higher angle photons in most cases. Based

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18 on the ray tracing simulations, microlens, micropyram id and microcylinder arrays were fabricated on the glass substrate. Photoluminescence measurements were performed using evaporated Alq 3 (as photoluminescent material) and Al (as back reflector) layers over the substrate s with controlled topography of ZnO T CO layer in corrugated device configuration The textured TCO device showed more than 2 X improvement as compared to control device with no structure which matches closely with the simulation results This work demonstrated that by controlling the topograph y of the TCO layer more than 1.5 fold enhancement in extraction efficiency can be achieved This enhancement can be further increased by textur ing the glass/air interface to reduce substrate mode.

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19 CHAPTER 1 INTRODUCTION Transparent Conducting Oxides Tran sparent conductors are the m aterials displaying the remarkable combination of high electrical conductivity and optical transparency These materials already f orm the basis of many important technological applications Transparent conducting oxides represen t an important class of materials which comprises of doped metal oxides having both optical transparency and electric conductivity. The rapid development of transparent conducting oxides ( TCOs ) has lead to a tremendous advancement and commercialization of active and passive electronic and optoelectronic devices ranging from transparent heating elements for windows to transparent thin film circuitry. Some of the most common optoelectronic applications are: fl at panel displays [1, 2] organic [3] as well as inorganic [4] light emitting diodes (LEDs) photovoltaics [5, 6] and arc hitectural window applications [7] low e windows [8, 9] electro chromic devices [10] and anti static coatings [2, 3, 6, 9, 11 13] as shown in Figure 1 1 Importance of TCOs In solid state physics, the free electron model is a simple model describing the behavior of valence electrons in a crystal structure of a metallic solid [14] The electron gas model of solids applies equally to the electrons in the conduction band of both metals and semiconductors. As per the Bolt zmann formulation, the electrical conductivity of such a material is directly proportional to the density of free carriers ( n ) and thei r mobility ( ) The mobility is in turn directly proportional to the free carrier resistivity relaxation time (the time between resistive scattering events), and is inversely proportional to the carrier effective mass Hence, it has been demonstrated that the

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20 transpar ent conducting oxides exhibit intermediate characteristics between the highly conducting and reflecting elemental metals and the low carrier density, high mobility prototypical semiconducting materials such as Si as shown in Figure 1 2 [15] Transparent conducting oxides belon g to an important class of materials characterized by relatively high values of both the electron density and the electron mobility sufficient for them to exhibit substantial electrical conductivity as transparent solids especially in form of thin films wh ich dominate the bulk of application areas of such materials. Hence, TCOs are conjugate property materials in which conductivity is strongly coupled to the lossy part of the refractive index. As explained materials like metals, that are highly conductive d o not normally transmit visible light, while highly transparent media like oxide glasses are electrically insulating in nature. Partial transparency and good conductivity can be obtained in very thin film s of metals such as aluminum or silver but have limi ted scope [16, 17] The fundamental properties governing the structure/property relationships driving these properties of conductivity and transparency need to be understood in order to achieve these properties. The only way to obtain transparency along with good conductivity is to intentionally create electron degeneracy in a wide band gap (> 3 eV) oxide by controllably introducing non stoichiometry or by using doping. This is achieved in oxides of various transitio n metals such as tin, indium, zinc, gallium, cadmium and their multi component alloys [2, 11] History of TCO TCOs have been around for a long time, the first report of a TCO film occurred material occurred in 1907 when a sputtered thin film of cad mium metal underwent incomplete thermal oxidation after heating in air [18] In this cas e, not only the electrical conductivity of the thin film changed with time, also the cadmium oxide was

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21 representative of a free carrier like conductor in which the oxygen deficiency donate free carriers to the defect (trap) energy levels near the conductio n band of CdO. Since this early discovery, other reports followed around 50 years ago in 1951 and 1952 [19, 20] establishing the coexisting of optical transparency and electrical conductivity. Similar reports followed suite for Indium oxide [21] Since the se early reports, the technological interest in transparent conductors has grown tremendously. Newfound applications of these films for heated windows and to diminish heat losses from sodium lamps fueled the development of TCOs which led to concentrated re search effort leading to development of doped films of SnO 2 and In 2 O 3 :Sn (indium tin oxide or ITO) with excellent electrical and optical properties [22, 23] Zinc Oxide: A Promising TCO In last decade, tin doped indium oxide (ITO) has become commercially used material for TCO application. However, due to limited nature of world indium reserves and ubiquitous use of solid s tate devices in displays, lighting and solar cells; a shortage of indium may occur in near future. In fact, a factor of ten increase in indium pricing has been observed in recent years [5, 11] Extensive research ha s been expanded to find an alternative for ITO. For practical use as a transparent electrode, a TCO material should exhibit a resistivity of the order of 10 cm or less and an average transmittance above 80% in the visible range [2, 12 ] A list of researched oxides and associated dopants with their qualitative resistivity and toxicity are listed in Table 1 1 In addition to the low resistivity on the order of 10 4 requir ement fabrication at a temperature below 200 C along with the feasibility of obtaining low thicknesses of approximately 15 to 100 nm are required for most of the applications of TCO materials Therefore, it is difficult to use practically cadmium oxide

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22 b ased and titanium oxide based TCO materials because of the toxicity of cadmium and the required high temperature heat treatment for titania oxide based TCOs [11] Particularly, in titanium oxide and titanium oxide based TCO thin films, a deposition and heat treatment at a high temperature above about 300 C and/or an epitaxial growth on a single crystal substrate are necessary in order to obtain a low resistivity [24 26] The impurity doped ZnO, In 2 O 3 and SnO 2 films have been prepared on glass substrates u sing various deposition methods. However, the obtained minimum resistivities of impurity doped SnO 2 and In 2 O 3 films have essentially remained unchanged for more than the past twenty years whereas those of impurity doped ZnO films are still decreasing [12] As a result doped z inc oxide is identified as the only promising and practical alternative for ITO because of inexpensive and non toxic nature of zinc oxide [12, 27 31] T he electrical and optical properties of ZnO comparable to the ITO have been reported in past [32 35] However, electrical resistivity was unstable in intrinsic ZnO at higher temperature s due to annihilation of oxyge n vacancies responsible for conductivity in intrinsic oxide [35] Since then numerous reports on multi component TCs based on ZnO [36 38] or on intentionally doped ZnO [25, 39, 40] have been published. The propertie s of ZnO that makes it suitable for TCO application are delineated in next section. Properties of ZnO Among the vastly r esearched material space, zinc oxide has caught lot of attention because of its large band gap (3.37 eV) large exciton binding energy o n the order of 60 meV and high electron affinity (4.35 eV) [3, 41 43] ZnO normally grows in the hexagonal (wurtzite) crystal structure with a = 3.25 and c = 5.12 ZnO exhibits a strong room temperature, near b and edge UV ph otoluminescence peak at ~3.2 eV [40]

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23 The well overlapped wave function of the conduction band of ZnO results in small electron effective mass between 0.23 0.35m e where m e is the mass of a free electron hence high electron hall mobility of 200 c m 2 /V s [40, 44] Basic material properties of ZnO have been tabulated in Table 1 2 Although pure zinc oxide is an insulator but slightly reduced form of zinc oxide h as oxygen vacancies which give rise to free electron associated with metal ion. The intrinsic defect levels lie approximately 0.01 0.05 eV below the conduction band [45] In addition, intentional impurities such as gallium, aluminum, indium, boron, fluorine etc have been added to obtain stable conductivity [3, 12] ZnO Applications In addition to ZnO used as a TCO material, it is extensively researched fo r various other applications. In recent years, z inc oxide has become an important material in the field of optoelectronics for UV and blue light emitting devices and lasers [40, 45 47] surface acoustic wave devices [48, 49] piezoelectric transducers [50] and sensors [51, 52] Zinc oxide e xhibits a large band gap (3.37 eV) and a high exciton binding energy (~60 meV) [42] making it suitable for UV and blue light emitting devices and lasers. The lack of a native substrate for GaN growth has led to a search for suitable choices of substrate Wurtzite type single crystal zinc oxide substrates have been demonstrated as a good template for the epitaxial growth of GaN thin films. Apart from ZnO single crystals, zinc oxide thin films have been applied as a buffer layer for growth of GaN on sapphire and SiC substrates [48] TCO Roughness Zinc oxide single crystal has been used as a substrate for epitaxial growth of GaN thin films [53 55] Zinc oxide thin films have been investigated as a buffer layer for

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24 growth of GaN on sapphire and SiC substrates [48, 56 58] In addition, doped ZnO thin films are being extensively re searched for their use as transparent conducting electrodes [40] However, both commercially available zinc oxide single crystals and as grown zinc oxide thin films have a rather high surface roughness, large particle inclusions and pinholes which hampers the growth of subsequent high quality layers [59 62] Such defects influence the growth of subsequent layers when zinc oxide thin films are used for buffer layer applications or influence the interface between zinc oxid e layer and the active layer in light emitting diodes [47, 52] In organic light emitting diodes, these anomalies lead to color degradation and localized electrical shorts undermining the performance and lifetime o f these devices. Further, in thin film silicon solar cells, high roughness of the underlying ZnO layer leads to poor growth of the subsequent silicon layers. In microcrystalline Si (c Si:H) solar cells, surface roughness, orientation and grain size of the ZnO film affects the grain size and quality of the c Si:H layer [62 64] In addition to this, increased interfacial area between rough transparent conducting oxide (TCO) and the p layer due to TCO roughness correl ates qualitatively to the observed decrease in open circuit voltage ( V OC ) and fill factor for the device. Vanecek et al., have reported that the absorption between 500 1100 nm wavelength increases with increase in RMS roughness at ZnO/back reflector inte rface [65] To address these issues due to ZnO roughness, we developed a chemical mechanical polishing process for smoothening of the zinc oxide surface. There are very few reports published on the zinc oxide polishing. Lucca et al. first reporte d the polishing of ZnO surface. In their work, the photoluminescence (PL) response of mechanically polished and chemo mechanically polished ZnO single crystal

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25 surface was compared [66] They showed that PL response of chemo mechanically polished ZnO single crystal (either O face or Zn face) was two orders of magnitude higher than the mechani cally polished ZnO, due to better surface roughness and reduced sub surface damage. However, detailed study of the chemo mechanical polishing process was not carried out. Recently, Lee et al. published a report on the effect of various chemical mechanical polishing (CMP) process parameters on morphology & optical properties of the sputtered zinc oxide thin films [59] However, the report does not provide a comprehensive study of the process parameters such as slurry pH, polishing pressure and particle size and concentration. Some of the other reported techniques for controlling surface roughness of the z inc oxide thin films involve irradiation by gas cluster ion beam for smoothening of the surface topography [61] ; controlling gro wth rate, thickness and doping concentration in chemical vapor deposition grown ZnO to tune the roughness [67] and bi layer structure of ZnO involving deposition of second smooth ZnO layer using atomic layer deposition [68] However, using ion beam/ electron cyclotron resonance etching to sputter material disrupts the crystal structure of the underlying zinc oxide film [61, 69] A CMP process however provides a good control over the surface roughness with achievable roughnesses down to angstrom level without disrupting the crystal structure of the ZnO f ilm. Hence, this work focuses on development and understanding of a CMP process for polycrystalline ZnO thin film. Light E xtraction The luminous intensity of any light emitting device is dependent on two factors: internal quantum efficiency (IQE) and light coupling The IQE is dependent on the active material quality and properties the charge transport and recombination in the device

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26 and device fabrication Tremendous progress has been made in improving the properties and the quality of active semiconducto r materials. Internal quantum efficiency as high as 100% in both LED and OLED devices has been reported [70 72] Despite of the high IQE the overall efficiency of the device is largely limited due to poor light ou t coupling/ extraction efficiency. The primary reason for reduced light extraction is the large disparity between the refractive indices of active layer and surrounding media. This difference in refractive indices causes total internal reflection of majorit y of the generated photons, as explained below. Basic Concepts different optical densities, it changes directionality. The bending is given by, (1 1) where, 1 is the angle of incidence, 2 is the angle of refraction, n 1 and n 2 are the refractive indices of medium 1 and 2, respectively (see Figure 1 3 ). When light is traveling from denser medium to lighter medium at a ngle greater than what is known as critical angle of incidence ( C ), the sine value of angle of refraction is greater than unity. refracted light. All rays having angl e of incidence greater than C are reflected back in the medium or suffer total internal reflection. The critical angle, C is given by, (1 2) When C is viewed in three dimensions, it results in a cone as shown in Figure 1 4 ofte n termed as escape cone. The emitted photons that have angle of incidence inside

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27 the escape cone are extracted out of the surface whereas the rest are reflected back in to the layer. In parallelepiped geometry since, the angle of incidence remain s same, th e reflected fraction of emitted photons are wave guided inside the layer until they are finally absorbed ( Figure 1 5 ) Light Extraction Efficiency A simple expression can be derived for the maximum emission effici ency per escape cone ( coupling ) f or the isotropic emission from the active layer: (1 3) where n a and n s are refractive indices of ambient and semiconductor. Of ten in OLEDs, the semiconductor lay er stack is reflective or cathode is reflective. This provides the photons another chance to be out coupled after reflection from the back side towards the substrate/air interface. Thus reflection opens another escape cone for photons to escape and coupli ng ca n be calculated as [73] (1 4) For the typical LED or OLED dev bis (1 naphthyl) diphenyl biphenyl diamine (NPB) with index of refraction 2.4 and 1.7, the critical angle is c ~ 24 and 36 and the extraction efficiency would be as low as 4% and 10% for a simple semi infinite sol id geometry without back reflector Thus 95 to 90% of generated light is being trapped inside the layer.

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28 Improving Light Out coupling Numerous methods of improving c ou pling have been r eported, including the use of device shaping [74 76] photon recycling [70, 71] and photon scattering [73, 77 87] techniques. These techniques have been discussed briefly in this chapter and a detailed a ccount has been provided in chapter 2. Device s haping Chip shaping for improving light extraction efficiency has been known since with 0.1 watt power output as compared to previous best of few milliwatts for a GaAs based device [76] Since then several other chip shapes such as a truncated cone shape [75, 88] Weierstrass sphere [88] truncated ellipsoid [88] paraboloid [88] truncated inverted pyramid [74] truncated hexagonal pyramid [89] rhombohedral [90] triangular [90] and slanting wall [91] have been proposed and fabricated. Device shaping presents numerous limitations in terms of processability and cost and has limited applications Transforming the device shape using pack aging has been a successful and a commercially used technique. O ptical grade epoxy is used for fabricating these hemispherical shaped packaging H owever, use of millimeter size packaging is not practical for various large scale, integrated devices such LED displays. In addition this method presents index matching issues limiting its applicability. Photon r ecycling Another way to change the directionality of the reflected photon is photon re absorption and re emission [92] The photon recycling is dependent on IQE of the device. Using photon recycling, an IQE of ~100% and remarkable external quantum efficiency of 72% have demonstrated [71] Because IQE is dependent on the quality of

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29 the semiconductor material, the l ight out coupling hence the external quantum efficiency (EQE) is sensitive to the parasitic losses High EQE using photon recycling demands high quality of material to reduce the parasitic loss mechanisms or makes it susceptible to slight decrease in IQE. Practically, IQE is usually less than 100% and dec reases with aging. Particularly in OLED s, active layer is considered usually absorptive (non radiative absorption) and hence very thin films (< 150 nm) are often used to minimize absorption losses. Therefor e, this technique is not widely applied for light extraction from electroluminescent devices. Photon s cattering Different methods have been adopted to scatter photons at different interfaces to change the directionality of the photon s thereby increasing t he probability of p hoton s to out couple. Some of the interface modification techniques include simple surface roughening techniques involving: natural lith ography, self assembled microbeads, photoelectrochemical (PEC) etching, graded refractive index layer s, anodic aluminum oxide template [79, 92 96] ; photonic crystals [97 103] ; patterned substrates [104 108] metallic nanowire array [84] and eve n incorporation of low index grid between ITO/organic interface [83] and 2D SiO 2 nanorods between ITO/glass interface [109] However, the se methods are often accompanied by changes in the radiation pattern, exhibit an undesirable angle dependent emission spectrum, or employ costly or complex processing methods [85] Application of external out couplers provide significant enhancement in light extraction without invoking such undesirable att ributes. External out couplers such as hemispherical dome [76] pyramids [81, 110] cylinders and lenses [73, 85, 111 113] have as been considered for improving the light extraction.

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30 Summary In this chapter a brief introduction to transparent condu ctors or transparent conducting oxides, their history and various TCO materials was presented. ITO, a commercially used TCO has Indium as its main constituent which is an expensive material due to low availability of Indium. Zinc oxide on other hand is ine xpensive and the combination properties of Z n O makes it a suitable candidate for replacement. Basic properties of ZnO were presented in this chapter followed by other applications of ZnO. It was elucidated in this chapter that the surface roughness of ZnO whether used for TCO application or other applications can be detrimental to device performance and reliability. Other then issue of TCO roughness, optoelectronic devices such as LED, OLED or TFEL devices suffer from poor light out coupling as low as < 20 % despite of nearly 100% internal efficiencies reported for both LEDs and OLEDs. Some of the basic concepts of light trapping, methods and techniques for extract wave guided photons were discussed in this chapter. A detailed literature review on TCO roughn ess issue and adopted techniques and light extraction methods and techniques developed so far are described in chapter 2.

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31 Figure 1 1 Applications of transparent conducting oxides.

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32 Figure 1 2 M ateri als space for semimetals, good metals, transparent conductors and semiconductors based on n correlations (electron carrier density and electron mobility, respectively). Data shown are for room temperature measuremen ts. Constant conductivity contours are shown as straight lines ( [15] Reproduced by permission of The Royal Society of Chemistry )

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33 Figure 1 3 Schematic depict i ng Snell's law and total internal reflection when light travels from materials with different refractive index (n 1 > n 2 ). Figure 1 4 Illustr ation of escape cone defined by the critical angle when light enters from optically denser medium (n 1 ) to lighter medium (n 2 ). Figure 1 5 Illustration depicting waveguiding of rays with emission angle greater than critical angl e.

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34 Table 1 1 Host and associated dopants in typical TCO systems (adapted from ref [11] ) Binary Dopant Resistivity Toxicity ZnO Al, Ga, B, In, Y, Sc, V, Si, Ge, Ti, Zr, Hf, F CdO In, Sn In 2 O 3 Sn, Ge, Mo, T i, Zr, Hf, Nb, Ta, W, Ta x Ga 2 O 3 Sn SnO 2 Sb, As, Nb, Ta TiO 2 Nb, Ta Ternary MgIn 2 O 4 GaInO 3 (Ga, In) 2 O 3 Sn, Ge CdSb 2 O 6 Y SrTiO 3 Nb, La Ternary Multi component Resistivity Toxicity Z n 2 In 2 O 5 Zn 3 In 2 O 6 ZnO In 2 O 3 system In 4 Sn 3 O 12 In 2 O 3 SnO 2 system CdIn 2 O 4 CdO In 2 O 3 system Cd 2 SnO 4 CdSnO 3 CdO SnO 2 system Zn 2 SnO 4 ZnSnO 3 ZnO SnO 2 system ZnO In 2 O 3 SnO 2 system CdO In 2 O 3 SnO 2 system ZnO CdO In 2 O 3 SnO 2 system :

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35 Table 1 2 Properties of ZnO Property Value Lattice parameters, 300 K a 0 3.2495 c 0 5.2069 a 0 /c 0 1.602 Density 5.606 g/cm 3 Crystal structure Wurtzite Me lting point 1975C Thermal conductivity 0.6,1 1.2 Refractive index 2.008,2.029 Energy gap 3.4 eV, direct Exciton binding energy 60 meV

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36 CHAPTER 2 LITERATURE REVIEW TCO S urface R oughness Many techniques have been adopted to synthesize ZnO thin films m ainly, techniques such as sputtering [29, 50, 114] pulsed laser deposition [40, 47] low pressure chemical vapor deposition [62, 115, 116] or spray pyrolysis [117 119] Irrespective of the deposition method deposited thin films have high surface roughness, large particle inclusions and pinholes [60 62, 12 0 122] Such defects influence the growth of subsequent layers when TCO thin films are used in various optoelectronic applications. Liu et al. studied the formation of dark spots due to surface roughness and spikes in the sputtered ITO layer in an OLED de vice with ITO/PEDOT:PSS/polyfluorene/Al stack [123] The spikes in the ITO film result in local melting of the polymer and migration of indium according to electric fi eld to eventually depositing on cathode surface. Indium has higher work function and continued indium migration lead to increased heat generation and growth of dark spot in size. They compared the failure performance of smoothened ITO surface by etching us ing saturated KOH/isopropanol solution. The device fabricated on as deposited rough ITO shows formation of dark spot within 15 min of operation at 4 V in nitrogen glove box with 50% reduction in light output whereas chemically etched relatively smooth ITO device gave very stable performance with no dark spot formation after 10 hours of operation under sam e operating conditions. Figure 2 1 represents the SEM image of the Al cathode surface, illustrating dark spot formation. Jung et al. reported mechanical po organic light emitting diodes (OLEDs) fabricated on the polished ITO surface showed 10 times better electroluminescence and

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37 current injection than the OLEDs based on as received ITO surface [121] The n ucleation of diphenyl N,N bis(3 methyl phenyl) 1,1 biphenyl 4,4 diamine ( TPD ) on ITO was found to be dependent on the feature s on the ITO surface They attributed the increased EL and current injection to reduced surface and change in surf ace O in the ITO layer. Choi et al. also showed improved optical and I V characteristics of polished ITO layer using chemical mechanical polishing process and the MEH PPV/polished ITO structure [124, 125] These rep orts reveal that surface roughness of TCO can adversely affect the device performance and lifetime. Besides application of ZnO as a TCO material, it is also known to be an effective substrate for GaN growth [53 55] or buffer layer for GaN growth on Si, sapphire and SiC substrates [48, 56 58, 122] The ZnO surface morphology affects the GaN film growth on Si substrates as studied by Xue and group [122] It was reported that increase in annealing temperature of ZnO buffer layer leads to reduction in film defects resulting in smoother film up to 900C, beyond which dissociation of ZnO in to Zn and O 2 occur s and film quality deteriorates The growth of GaN on rougher ZnO results in early coalescence of GaN islands hence, increased defect density in so deposited GaN film evident from their SEM, AFM images and observed increased PL intensity for GaN film grown on annealed ZnO film at 900C. In addition, ZnO films also find application in optoelectronic integrated circuits as optical waveguides where surface undulations close to dimension of wavelength lead to scattering and propagation losses [61, 120, 126] These reports elucidate the need for polishing of zinc oxide whether ZnO is used as TCO in organic LED or as substrate or buffer layer in GaN based devices.

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38 Several techniques have been reported to for reducing the s urface roughness, large particle inclusions and pin holes in ZnO films or ZnO single crystals. Some of the reported techniques for controlling surface roughness of the zinc oxide thin films involve irradiation by gas cluster ion beam for smoothening of the surface topography followed by etching of damaged layer in 0.1% HCl solution [61] controlling growth rate, thickness and dopin g concentration in chemical vapor deposition grown ZnO to tune the roughness [67 ] bi layer structure of ZnO involving deposition of second smooth ZnO layer using atomic layer deposition [68] and annealing of ZnO film at different temperatures [122] However, using i on beam/ electron cyclotron resonance etching to sputter material, disrupts the crystal structure of the underlying zinc oxide film [61, 69] Furthermore, these techniques do not provide a good control over the surf ace roughness. There are very few reports published on the zinc oxide polishing. To our knowledge, Lucca et al. first reported the polishing of ZnO surface. In their work, the photoluminescence (PL) response of mechanically polished and chemo mechanically polished ZnO single crystal surface was compared [66] They showed that PL response of chemo mechanically polished ZnO single crystal (either O face or Zn face) was two orders of magnitude higher than the mechanically polished ZnO, due to better surface roughness and reduced sub surface damage. However, detailed study of the chemo mechanical polishing process was not carried out. Recently, Lee et al. published a paper on the effect of various chemical mechanical polishing (CMP) process parameters on morphology and optical properties of the sputtered zinc oxide films [59] Light Extraction It is well known that the large difference in refractive index between semiconductor and air limits th e escape of generated photons due to the emitting angle of large fraction

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39 of these photons are larger than the critical angle ( c ) defined by eq. 1 2 which get trapped or confined by total internally reflection. For isotropic photon emission from an is given by eq. 1 3. For the typical bis (1 naphthyl) diphenyl biphenyl diamine (NPB) with index of refraction 2.4 and 1.7, the critical angle is c ~ 24 and 36 and the extraction efficiency would be as low as 4% and 10% for a simple s emi infinite solid geometry. Clearly, light extraction efficiencies are very low and to achieve extraction efficiencies of 70% or greater for a cost effective general purpose lighting source, simple parallelepiped geometries will not work [127] Much success has been achieved in improving light extraction through tailoring the overall shape of the chip, surface texturing of the devices and other photon manipulation techniques to enhance the light extraction. Figure 2 2 illustrates the evolution of various light extractions methodologies in inorganic LEDs. Some of different photon extraction tech niques adopted for inorganic and organic LEDs have been illustrated in this chapter. Device Shaping Chip shaping for improving light extraction efficiency has been known since device with 0.1 watt power output as compared to previous best of few milliwatts for a GaAs based device [76] The authors attributed the increase in output primarily to the reduction internal reflection due to hemispherical design of the device. Franklin et al. published a report on funnel shaped chip in a short time gap from Ca rr group [75] They

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40 proposed that theoretically using a truncated cone shape of cone angle 45 a device efficiency of 18% can be achieved. Although shaping methods used were not very efficient and practical. Other shapes such as Weierstrass sphere, truncated ellipsoid, truncated cone and paraboloid have also been studied [88] In 1999, Krames and group report ed a practical method for shaping of die by saw dicing using beveled saw blade [74] The truncated inverted pyramid (TIP) structure reduces the mean free path of the generated photon within the device reducing internal losses (figure 2 3 ). Further, using TIP geometry with 35 vertex angle and multiwell structure they demonstrated a record peak efficiency of exceeding 100 lm/W at 100 mA dc for orange e mitting (610 nm) device and 68 lm/W for 598 nm emitting device. Thompson et al. reported the truncated hexagonal pyramid shaped LED based on ZnO and GaN wafer bonding for the purpose of improving LED efficiency by fabricating highly transpa rent and shaped p type electrode [89, 128] This ZnO wafer bonded LED provide twofold enhancement, (1) a light extraction efficiency advantage over conventional LEDs because of the reduction of internal light reflec tions and (2) reduction in internal loss mechanism because of low absorptio n p type electrode and reduction of mean photon path due to truncated pyramid geometry. The external quantum efficiency (EQE) was 2.2 times higher at 20 mA of current injection over conventional device. There have been other geometries and shaping strategi es such as rhombohedral [90] triangular [90] chip and slanting walls [91] which have shown enhancement in light extraction over regular parallelepiped chip structure. However, shaping is costly and complex procedure for light extraction and other photon randomization or redirection techniques ha ve been demonstrated in literature which are equally or more efficient and are easily realized experimentally.

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41 Interface Modification Techniques Surface r oughening Chaotic photon trajectories assures the maximum statisti cally feasible output coupling from the LED through the escape cone [92] A ngular randomization by photon recycling is not practical since it is vulne rable to the slightest deterioration in material or back reflector quality. A more practical approach is interface modification by texturing of surface. Schnitzer et al. took advantage of random surface texturing using natural lithography to cause cha otic photon trajectories [92] Using self assembled 200 nm polystyrene beads as etch mask and etching the AlGaAs surface with Cl 2 assis ted Xe + ion beam, a 30% external efficiency was demonstrated. Huh et al. micro roughened the semiconductor surface in GaN based LED [79] The average roughness obtained using Pt metal cluster mask and wet etching of GaN surface was 5 6 nm. The I V curve of LED device showed reduced series resistance attributed to higher contact area between GaN and light transmitting Pt l ayer. The power efficiency of micro roughened device was 62% higher than conventional LED owing to lowered forward voltage and enhanced light extraction. Fujii et al used hexagonal cone shaped roughness etched using photoelectrochemical (PEC) etching of N face n GaN in KOH and in presence of Xe light source [96] Figure 2 4 represents the SEM micrograph of GaN surface after a 10 min etch process creatin g ~500 nm high hexagonal cones. The roughened N face n GaN resulted in improved output power by a factor of 2.3 as compared to non textured device. Graded refractive index (GRIN) layers have also been applied at air/semiconductor interface to extraction li ght due to their anti reflective properties [129] In a GRIN layer, the refractive index is gradually varied between the refractive index of active semiconductor and refractive index o f air. Kim et al. deposited

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42 ITO GRIN layer using oblique angle deposition technique on Ga N surface in GaInN LED (Figure 2 5 ) [77, 130] A 24.3% enhancement in light output was observed for GRIN ITO antireflective la yer as compared to device with dense ITO layer. The mechanism for improved light extraction is attributed to the reduction in Fresnel reflection at ITO/air interface. Tsai et al. grew naturally textured GaN device using a growth interruption step and a sur face treatment using biscyclope ntadienyl magnesium (CP 2 Mg) simultaneously to form a plurality of nuclei sites on the surface of a p type cladding layer [131] A rough surface having truncated pyramids was f ormed after further growth of p type contact layer. A light output power enhancement of 66% was observed at 20 mA drive current. Lee et al. used roughened GaN layer with ITO/Al 2 O 3 /Al stack as back reflector to extract 1.6 fold higher light as compared to n on roughened LED device [93] They used adhesive layer bonding technique followed by wet etching to c reate triangle like roughness. Recently, Cheng et al. created chaotic mesh like surface roughening to improve light extraction [95] Using porous anodic alu minum oxide (AAO) template, they created PDMS meshed surface which resulted in 46% enhancement in light out couplin g from the OLED device (Figure 2 6 ). Kang et al. nanotextured GaN surface using etching and ITO nanospheres [94] They roughened the p GaN and n GaN surface by an interesting wet etching technique to create ITO nanosphere by taking advantage of faster etch rates at ITO grain bou ndaries and subsequent short duration dry etch of GaN. As compared to non textured LED, p GaN, n GaN and both p GaN and n GaN roughened device showed 16, 12 and 33 % higher light output, respectively without detrimental effect of dry etch process on GaN el ectrical properties.

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43 Photonic c rystals There has been remarkable progress in photonic crystal recently. The photonic crystals (PC) are periodic variations in the refractive index of a dielectric material in one or more dimensions Such variation of refract ive index at wavelength of light scale give rise to forbidden photon energy bands as well as dramatic changes to the photon density of states [127] Similar to electronic band gap in case of electrons, the photon energy band allows manipulation of the photons in fundamentally new ways, including enhancing the radiative recombination rate It can be utilized to inhibit the emission of the guided modes or redirect trapped light into radiated modes. In LEDs, photonic crystals can promote increase in the radiance by increasing the directionality of the extracted light [132] For example, Fan and group for the first time elucidated b y simulating 2 dimensional GaAs PC slab ( n 1.55 m) that the extraction efficiencies of ~80% can achieved over broader range of frequencies unlike 1 dimensional resonant microcavities [86, 87] They analyzed two structures one with the dielect ric slab with triangular lattice of infinite length air holes and other one with dielectric pillars of infinite and finite length. The periodicity of these structures opened up the gap and set the upper frequency limit. The modes between the gap are scatte ring out t hat would have otherwise been trapped leading to average enhancement up to ~70% and 80% in case of air holes and dielectric pillar structure, respectively Several reports have been published ever since on use of PCs to extract light from various inorganic LED, OLED and thin film electroluminescent devices (TFEL) [97, 99, 100, 102, 103, 133 136] Han Youl et al. described both the simulation and experimental results on effect of air hole depth on light extr action [101] Their findings suggest that the light extraction decreases as the depth of air holes decreases

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44 indicating that it is desirable to transfer the PC pattern through the slab. The gain in light extraction as large as 8 fold and 13 fold was observed for photonic crystal slab with SiO 2 and air cladding, respectively. Also, square lattice showed better light extraction as compared to triangular lattice due to greater density of leaky modes. Wierer et al. demon strated triangular lattice PC on n GaN layer in a InGaN based MQW LED structure [132] A schematic of PXLED is depicted in Figure 2 7 with optical image of lit device and angular distribution curves. Four different photonic crystal lattices were investigated with lattice constants and ho le diameters as 270, 295, 315, and 340 nm, and 200, 220, 235, and 250 nm, respectively. The holes of 100 nm depth with 75 sidewall angle were created using e beam lithography and dry etching. The photonic crystal LED ( PXLEDs ) displayed heavily modified fa r field radiation patterns with increased radiance up to 1.5 times The increased out coupling was correlated to Bragg scattering of the guided modes out the top of the LED in the report Lee et al. applied the PC in an OLED device by creating 200 nm SiO 2 pillars on the glass substrate (Figure 2 8 ) [136] The optimized PC pattern with 200 nm SiO 2 rods embedded in Si N x and lattice constant of 600 nm exhibited a simulated light extraction efficiency of 80% which translated to experimentally observed 50% enhancement in light extraction over viewing angle of 9040. The integrated light effi ciency with triangular lattice PC Y 2 O 3 :Eu 3+ thin film device was found to be approximately 4.0 and 4.7 times higher f or the nanorods and airholes as reported by Oh et al [135] In case of a square lattice they integrated PL intensit y 3.0 and 2.8 times that of non PC TFEL device. Th ey attributed the observed enhanc ement in extraction ef fi ciency to the scattering of the forward emission excited directly by inward UV and the scattering of re excited forward emission

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45 by backscattered UV (from mercury excitation source) through the leaky and/or Bragg scattering produced by the 2D periodic array. Patterned s ubstrates Tadatomo et al. first time reported the use of patterned sapphire substrates for growth of InGaN/GaN multi quantum well (MQW) UV LEDs (see F igure 2 9 ) [137] They fabricated parallel grooves with dimensions of ridges, gr direction which resulted in a 24% external quantum efficiency at emission peak of 382 nm and 20 mA operating current. The results were approximately 5 folds improvemen t over the conventional LED which they attributed to the reduction in defect density to 1.5x10 8 cm 2 Chang et al. claimed to have 35% increase in the electroluminescent (EL) intensity without the shift in EL emission peak with stripes patterned along direction [138] Additionally, better reliability of InGaN/GaN MQW LED on patterned s apphire substrate ( PSS ) was found with maximum output at higher power and lower decay in EL intensity after 72 hours burn in test. Hsu and group also reported similar results with 25% increase in output power correlating to reduced threading dislocation i nduced non radiative recombination [ 139] All these reports indicate that the enhancement of optical output power is attributed to the effective suppression of leakage current using the patterned sapphire substrates owing to reduced dislocation defects. Lee et al. argued that besides the el imination of threading dislocations due to the lateral growth of GaN on top of stripe patterned sapphire substrate, enhancement in light extraction could be due to extraction of guided mode because of stripe pattern [104] They observed an improvement of light output efficiency of their PSS LED to be about 20% which is higher than their simulation

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46 results (13%). Improvement in efficiency of their device over conventional LED was mainly attributed to the pattern on sapphire substrate whereas the additional improvement in light output efficiency as compared to their simulation results was suggested to be related to the elimination of threading dislocations with better epitaxial quality. Yamada et al. used dual approach in order to enhance the external quantum efficiency [105] One is to use the PSS for effective scattering of the emission light from the active layer. Second is to use R order to reduce the absorption of the emission light by the electrode, they patterned the electrode in mesh shape. Using there approach, an output power of 22 mW with 35.5% external efficiency and 18. 8 mW with 34.9% external efficiency at operating current of 20 mA at room temperature was observed for InGaN based n UV LED of emission wavelength at 400 nm and 426 nm, respectively. Wang et al. developed a Cantibridge technique to reduce the threading dis location (TDD) in the GaN film [140 142] The technique involves creation of V shaped grooves on sapphire substrate by wet etching using H 2 SO 4 acid. Presence of V grooves on sapphire surface resulted in selective gr owth of GaN only on the (0001) mesas and no GaN growth occurs on the sidewalls of the V grooves as shown in F igure 2 10 Recently, Lee et al. reported using the Cantibridge technique to fabricate GaN based light emitting diodes on ch emical wet etched PSS (CWE PSS) with V shaped pits on top surface ( F igure 2 1 1 ) [108] The reduction in defect density to 3.62 x 10 8 cm 2 using CWE PSS substrate resulted in 12.5% enhancement in the internal quantum efficiency. They showed using theoretical calculation that the V shaped pits are stronger diffuser, increasing the probability of trapped photons to escape and achieving a 20% enhancement in

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47 estimated light extraction efficiency. Additionally, V shaped gr ooved PSS demonstrated additional 7.8% enhancement over planar sapphire due to superior guided light extraction efficiency. A combined 63.5% light extraction efficiency was observed as compared to 49.1% for the GaN device on flat sapphire without V shaped pits. GaN grown on patterned SiO 2 (epitaxially laterally overgrown GaN, ELOG) has also been developed for reducing TDD (Figure 2 1 2 ) [106, 143 145] Mukai et al. grew InGaN LED on ELOG and sapphire substrate. The d evice on ELOG substrate showed significantly low leakage current [145] However, the light output results illustrated no change in output power for due to presence of SiO 2 mask layer However, Lee et al recently used inverted hexagonal pyramid dielectric mask (IHPDM) to grown GaN device on sapphire [106] Compared to conventional lateral epitaxial overgrowth (LEO), their technique provided 3 fold merit Firstly, because large size etch pits generally originate from the nanopipes ( open core screw dislocations ), they selectively eliminate most defective region. Secondly, t he regrowth thickness of GaN layer can be controlled less than 5 due to small d iameter of these etch pits. Lastly, embedded inverted pyramid structures acts as strong light scattering centers. The ray tracing simulation for device with IHPDM showed 56% increase in light extraction and increase in output by a factor of 1.41 was experi mentally observed. Recently, Cho et al. reported 80% increase in light output power of InGaN LED at 20 mA using pyramidal SiO 2 mask over non LEO LED and 30% over standard square slab type LEO LED [1 07] External Out c ouplers External out couplers are any structures created on emitting side of a light emitting device for increasing the external mode. Several structures have been considered for

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48 improving the light extraction as external out couplers s uch as hemispherical dome [76] pyramids [81, 110] cylinders and lenses [73, 85, 111 113] S pherical shapes have been known for improving external efficiency as they subtend a large solid angle of emitted rays resulting in extraction of light prev io usly trapped in the substrate [73] Madigan et al. demonstrated the effect of spherical subs trate features on external coupling efficiency and the far field emission pattern. The light extraction increased by factor of 3 by attaching macro sized spherical lens using index matching gel on the backside of the OLED. However, use of millimeter sized lens is not practical for many applications without index matching issues. Mller et al. suggested an alternative, simpler technique to fabricate ordered array of micro sized lens without the need for alignment with respect to LED device. A 10 um diameter microlens square array of poly dimethyl siloxane (PDMS) was fabricated using mold transfer method which was attached to emission side of glass substrate (Figure 2 1 3 ). Using this scheme, 1.5 fold increase in light output over unlensed device was observed. Choi et al. integrated the microlens on micro LED substrate [111] Microlenses were fabricated using thermal reflow method, aligned to the emitters forming the array. Light output measured using Si detector at 5 mm distance from micro LED was 23% higher than same for an unlensed device. The observed enhancement was attributed to the concentrating effect of microlens. Peng et al. studied the effect of lens diameter and height on light extraction capability of the microlens array [112, 146] The simulation showe d that hemispherical lens results in maximum out coupling efficiency. With optimized microlens array, efficiency as high as 85% can be achieved whereas the experimental observed efficiency was 70% for LED device with optical epoxy microlens array. This dis crepancy was attributed to the

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49 absorption loss in microlens layer media and deviation of fabricated microlenses from spherical profile. Using similar approach but with slight modification Nakamura et al., fabricated pyramidal microlens array and studied th e effect of using high refractive index substrate in conjunction with microlens array [81] Interestingly, the high refractive index substrate did not show any enhancement in light extraction whereas pyramidal microlens structure resulted in similar extraction efficiencies as report by Peng et al. Ee et al. fabricated SiO 2 /polystyrene microlens films utilizing low cost rapid connective deposition on InGaN based device [113] An improvement in integrated luminescence of 270% was observed using SiO 2 /PS microlens arrays A sche matic of InGaN based device with SiO 2 /PS microlens array is depicted in Figure 2 1 4 Chen et al. fabricated the macro pyramidal array light enhancing layer on an OLED. The pyramidal array with demonstrated a gain factor in luminescence intensity of 2.03. Greiner analyzed the microlens and micropyramid array as the external out couplers in an OLED device [147] The simulation results elucidated that the reflectance of OLED stack or back reflector is of paramount importance since the probab ility of a photon to escape in first pace through various structures is only a fraction of total escape probability. In addition, the aspect ratio of microlens or micropyramid structure has strong influence on the out coupling efficiency as escape probabil ity of higher angle photons reduces with aspect ratio. Compact packing of these structures results in better out coupling efficiency because any flat region increases the probability of TIR. Greiner findings also illustrated that the exact geometric nature of the structure influence angular distribution much strongly as compared to the out coupling efficiency. Using these findings he was able to

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50 demonstrate 80% extraction efficiency from substrate for microlens, micropyramid and scattering particles with al uminum back contact OLED device. Chang et al. studied textured encapsulating top surface (cone, hemisphere, prism, inverted hemisphere and inverted cone) on LED [80] The simulation results showed prism type structure as most efficient structure, demonstrating a 35% enhancement in light extraction efficiency with type structure concurred well with simulation results. Bao et al. reported increase in light extraction by 18.5 and 31.9% for imprinted 1D and 2D taper type grating on polymer encapsulation top surface on back side of sapphire substrate in GaN LED (Figure 2 15 ) [148] Moth eye structure etched on InGaN based LED has been demonstrated by Kasugai et al. for light extraction enhancement a s shown in Figure 2 1 6 [149, 150] GaN based blue LED was fabricated on 6H SiC substrate with moth eye structure on the bottom side using self assembled Au nanomask and RIE of SiC. Periodic cones of mean height 400 600 nm and periodicity 160 nm resulted in output power 3.8 times the convention LED at 20 mA drive current. Theoretical calculations using rigorous coupled wave analysis (RCWA) along with 3 periodic moth eye structure ~80% transmittance can be achieved for broad range of incidence angle. The calculated light extraction efficiencies for periodic moth eye structure and pseudo periodic moth eye structure were 4.27 and 4.16 times higher than conventional LED. Hon g et al. fabricated similar pseudo moth eye structures using UV lithography and ICP etching on p GaN layer with 200 300 nm diameter and 80 150 nm height [151] They reported an enhancement in PL intensity by a gain factor 5 7 due to scattering of photons by these moth eye structures. Recently, Kim et al. demonstrated

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51 27% enhancemen t in light out coupling using deeply etched mesa holes (DEMH) at reasonable forward voltage [78] However, a n increase in reverse currents in DEMH LED was observed attributed to an increased surface leakage through mesa holes. Other Techniques Numerous other techni ques for light extraction have been also been reported in literature. Few of them have been discussed here to cover the breadth. M etallic nanostructures have been shown to have much higher scattering cross section than the transparent structures due to the localized electromagnetic enhancement [152] Hsu et al. fabricated gold nanowire array to increase light out coupling in an OLED device [84] A 450 nm pe riod array doubled the extraction efficiency for an Alq 3 device. Using metallic nanowire array they demonstrated not only increased light extraction but demonstrated that a nanowire grid can also function as narrow band color filter by changing the periodi city of the nanowires. To improve light extraction from ZnS:Mn thin film phosphors, Do et al studied the effects of introducing various two dimensional (2D) SiO 2 nanorod arrays between ITO/Glass interface [109] They obtained a ~6.4 fold enhancement in the CL extraction efficiency at the interface between ZnS and air Recently, Sun et al. demonstrated increased light extraction efficiency by incorporating a low index grid between ITO/Organic layer interface (Figure 2 1 7 ) [83] Low index grid only out couples waveguided mode not the glass mode for which they used simple microlens array. An out coupling of ~2.3X highe r over conventional OLED was observed which can be further improved to ~3.5 fold using even lower index grid (n LIG <1.45). Although a signi fi cant increase in ext was observed for the several reported methods such as device shaping, photonic crystals, patt erned substrates, surface roughening and so on but the se methods are often accompanied by changes in the

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52 radiation pattern, exhibit an undesirable angle dependent emission spectrum, or employ costly or complex processing methods [85] Application of external out couplers provide significant enhancement in light extraction without i nvoking such undesirable attributes. In this work, effect of feature size and aspect ratio of different pyramid, lens and cylindrical textures have been studied using ray tracing simulations. The simulation results were substantiated by experimentally fabr icating devices with these structures.

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53 Figure 2 1 Dark spot formation in LED active layer (adapted with permission from ref. [123] ) Figure 2 2 Evolution of extraction efficie ) LEDs showing extraction efficiency as high as 80%. For the red triangles, the acronyms are: AS = Absorbing Substrate; DBR = Distributed Bragg Reflector; RS = Reflective Substrate. For the blue squares, the acronyms are: CC = Conventional Chip; FC = Flip Chip; PS = Patterned Sapphire; VTF = Vertical Thin Film; TFFC = Thin Film Flip Chip; ITO = Indium Tin Oxide (reproduced with permission from ref. [127] )

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54 Figure 2 3 (a) Optical image of AlGaInP/GaP truncated inverted pyramid LED and (b) is schematic of a TIP LED device (reproduced with permission from ref. [74] ) Figur e 2 4 SEM micrographs of an N face GaN surface etched by a KOH based PEC method where (a) is after 2 min etch and (b) after 10 min etch ( reproduced with permission from ref. [96] ) (a) (b)

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55 Figure 2 5 SEM image of Oblique angle deposited GRIN ITO layer with refractive index profile ( reproduced with permission from ref. [77] ) Figure 2 6 (a) Schematic depicting fabrication of PDMS nanowires and (b) is the The inset shows the cross section of the surface ( reproduced with permission from ref. [95] ) (a) (b)

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56 Figure 2 7 (a) Top lit view, (b) schematic cross section view of the PXLED and (c) radiation patterns of the LEDs at = 0 and 90 (bottom plot). The upper left insets indicate the direction of the measurements with respect to the lattice ( reproduced with permission from ref. [132] ) Figure 2 8 (a) Schematic of a PC OLED device with SiO 2 /SiN x PC layer. Far field in tensity profiles are observed for (b) the conventional OLED and (c) the PC OLED while (d) and (e) are Intensity profiles along the horizontal line (A B) and diagonal line (C D), respectively. (Dotted lines: conventional OLED, solid lines: PC OLED) ( reproduced with permission from ref. [153] ) (a) (b) (a) (b) (c) (d) (e)

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57 Figure 2 9 (a) S chematic drawing of GaN based device on stri pe PSS [104] and (b) cross sectional SEM of LEPS GaN grown on PSS aligned to direction ( reproduced with permission from ref. [137] ) Fig ure 2 10 Cross section SEM m icrographs of GaN grown on V grooved sapphire with f t er 150 min growth time ( reproduced with permission from ref. [142] ) (a)

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58 Figure 2 1 1 (a ) Top view SEM image of CWE PSS and (b) is a schematic diagram of device on CWE PSS ( reproduced from ref. [108] ) Figure 2 1 2 (a) Schematic diagram showing lateral epitaxial overg rowth of GaN layer on 6H SiC substrate with SiO 2 mask and (b) c ross section TEM micrograph of a laterally overgrown GaN layer on a SiO 2 mask ( reproduced with permission from ref. [144] ) (a) (b) (a) (b)

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59 Figure 2 1 3 SEM of a PDMS microlens array The inset is detailed side view of the lenses illustrating accurate replication of the mold shape ( reproduced with permission from ref. [85] ) Figure 2 1 4 Schematic of InGaN QWs LEDs structure utilizing SiO 2 /PS microspheres microlens array ( reproduced with permission from ref. [113] )

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60 Figure 2 1 5 SEM image of (a) 1 D triangular and (b) 2 D taper like gratings on the sapphire backplane. Each inset displays the corresponding image of profile (reproduced from ref. [148] ) Figure 2 1 6 Cross sectional SEM image of bottom of 6H SiC substrate with moth eye structure fabricated using metal nanomask Simulation results of transmittances for (a) convention al structure (b) pseudo periodic moth eye structure (dotted line) and (c) periodic moth eye structure (dashed line) ( reproduced with permission from ref. [149] )

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61 Figure 2 1 7 Schem atic diagram of the OLED with the embedded low index grid (LIG) in the organic layers ( reproduced with permission from ref. [83] )

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62 CHAPTER 3 OUTLINE OF RESEARCH Th is work focuses on enhancement in light extraction efficiency in light emitting devices by controlling the surface topography of TCO thin films. Zinc oxide is a promising material which has been extensively researched as a replacement material for commercially used indium tin oxide TCO material. This work has been primarily focused on ZnO as TCO material. T he first objective of this researc h is to control the surface roughness, large particle inclusions and pin holes on the zinc oxide thin film to reduce the effect of such anomalies on the properties of the subsequent active layer and the performance of the LED device. Second objective is en hancement of light extraction efficiency by creating controlled periodic micro topography. As part of the first goal, a chemical mechanical polishing process for controlling the surface roughness of ZnO was developed. A thorough study was conducted to deve lop an understand ing of the zinc oxide CMP process due to limited literature o n the polishing of zinc oxide material For enhancement in light extraction, s urface textures consisting of hexagonal arrays of cones/pyramids, microlens and cylinders were fabri cated These structures are known to improve light extraction but thorough understanding of ray dynamics needs to be developed. The ray tracing simulations were carried out to elucidate the ray dynamics of these structures with respect to the effect of fea ture size and aspect ratio when applied to different interfaces in a LED device structure. These two strategies were finally combined in a thin film photo luminescent device involving Alq 3 as the photoluminescent material, to demonstrate the efficacy of the se textures in improving light extraction

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63 In this dissertation, zinc oxide chemical mechanical polishing results have been presented in chapter 4 In this chapter, a detailed study of various CMP variables such as pressure, platen speed, pH, slurry partic le size and concentration was carried out. Based on the results obtained, a zinc oxide CMP mechanism was proposed. The effect of slurry additives such as surfactants and salt are illustrated in chapter 5 As part of second objective, t he ray tracing simula tions were performed on hexagonal arrays of pyramids, microlenses and cylinders with different base diameters and aspect ratios. The simulation results for these micro textures when applied to different interfaces in glass/ZnO/ active layer stack with back reflector are presented in chapter 6 The chapter 7 describes the experimental procedure for fabrication of the hexagonal arrays of cones/pyramids, microlenses and cylinders. The strategies of introducing controlled micro textures and polishing of zinc ox ide surface to reduce surface roughness and surface defects were combined and tested using PL measurements The experimental results of P L measurements with polished Al:ZnO TCO layer an d micro texture at the glass/TCO interface are described in chapter 8 Chapter 8 also details the ray tracing simulation results to support the observed ex perimental findings. Chapter 9 summarizes the conclusions of zinc oxide polishing results with both ray tracing simulation and experimental results for light extraction usi ng various textures at different interfaces.

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64 CHAPTER 4 CHEMICAL MECHANICAL POLISHING OF ZINC OX IDE Introduction In recent years, z inc oxide has become an important material in the field of optoelectronics for UV and blue light emitting devices and lase rs [40, 45 47] piezoelectric transducers [50] and sensors [51, 52] due to i ts large band gap (3.37 eV) and a high exciton binding energy (~60 meV) [42] Zinc oxide single crystal has been used as a substrate for epitaxial growth of GaN thin films [53 55] Zinc oxide thin films have been investigated as a buffer layer for growth of GaN on sapphire and SiC substrates [48, 56 58] In addition, doped ZnO thin films are being extensively researched for their use as tran sparent conducting electrodes [40] However, both commercially available zinc oxide single crystals and as grown zinc oxide thin films have a rather high surface roughness, which hampers the growth of subsequent high quality layers. Zinc oxide thin films are l argely fabricated using techniques such as sputtering [29, 50, 114] pulsed laser deposition [40, 47] low pressure chemical vapor deposition [62, 115, 116] or spray pyrolysis [117 119] Irrespective of the deposition method they have high surface roughness, large particle inclusions and pinholes [59 62] Such defects influence the growth of subsequent layers when zinc oxide thin films are used for buffer layer applications or influence the interface between zinc oxide layer and the active layer in light emitting diodes [47, 52] In addition, in organic light emitting diodes, these anomalies lead to color degradation and localized electrical shorts hence, undermining the performance and lifetime of these devices. Further, in thin film silicon solar cells, high roughne ss of the underlying ZnO layer leads to poor growth of the subsequent silicon layers. In microcrystalline Si (c Si:H) solar cells, surface roughness,

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65 orientation and grain size of the ZnO film affects the grain size and quality of the c Si:H layer [62 64] In addition to this, increased interfacial area between rough transparent conducting oxide (TCO) and the p layer due to TCO roughness correlates qualitatively to the observed decrease in open circuit voltage ( V OC ) and fill factor for the device. Vanecek et al., have reported that the absorption between 500 1100 nm wavelength increases with increase in RMS roughness at ZnO/back reflector interface [65] These observations have motivated investigation of zinc oxide polishing to achieve a smooth surface. Planarization of zinc oxide provides the opportunity for formation of controlled surface texture and eliminates the detrimental unwanted/uncontrolled large particle inclusions/sharp features on zinc oxide s urface. There are very few reports published on the zinc oxide polishing. To our knowledge, Lucca et al. first reported the polishing of ZnO surface. In their work, the photoluminescence (PL) response of mechanically polished and chemo mechanically polish ed ZnO single crystal surface was compared [66] They showed that PL response of chemo mechanically polished ZnO single crystal (either O face or Zn face) was two orders of magnitude higher than the mechanically polished ZnO, due to better surface roughness and reduced sub surface damage. However, detailed study of the chemo mechanical polis hing process was not carried out. Recently, Lee et al. published a paper on the effect of various chemical mechanical polishing (CMP) process parameters on morphology & optical properties of the sputtered zinc oxide thin films [59] However, the report does not provide a comprehensive study of the process parameters such as slurry pH, polishing pressure and particle size and concentration. Some of the other reported techniques for controlling surface roughness of the zinc

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66 oxide thin films involve irradiation by gas cluster ion beam for smoothening of the surface topography [61] ; controlling growth rate, thickness and doping concentration in chemical vapor deposition grown ZnO to tune the roughness [67] and bi layer structure of ZnO involving deposition of second smooth ZnO layer using atomic layer depositio n [68] However, using ion beam/ electron cyclotron resonance etching to sputter material, disrupts the crystal structure of the underlying zinc oxide film [61, 69] Furthermore, these techniques do not provide a good contr ol over the surface roughness. In this work, a thorough study of the chemical mechanical polishing process for zinc oxide thin film polishing was performed in order to delineate the zinc oxide polishing process and to achieve low surface roughness ZnO thin films. We investigated the effect of various CMP variables such as pH, applied pressure, platen speed, particle size and concentration on the polishing of zinc oxide films. The CMP performance was evaluated by measuring the removal rates and surface rough ness obtained from thickness measurements and atomic force microscopy (AFM) scans followed by optical and x ray reflectivity (XRR) measurements. Experimental Procedure A Struers Tegra Force 5 table top polisher and Cabot Microelectronics D100 polishing p ads were used for all the CMP experiments. The polisher variables such as revolutions per minute (rpm) and pressure were varied from 60 120 rpm and 2.5 5 psi, respectively, whereas slurry parameters namely, slurry pH, abrasive particle size and slurry soli d loading were varied systematically to understand the effect of each variable on the polishing performance. C olloidal silica particles ( Levosil S.p.A. Italy) with different particle sizes (35 135 nm) were used throughout the study. Post CMP cleaning wa s performed using pH 10.6 (adjusted using KOH) deionized (DI) water on a

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67 buffing pad for 1.5 minutes followed by ultrasonication in acetone, methanol and DI water. For the study, silicon p type substrates (1x1 inch 2 ) were sputter coated with 25020 nm thic k ZnO film. The ZnO films were deposited using RF sputtering at 250 W power, 5 mTorr working pressure with target to substrate distance of 80 mm and no external substrate heating. All depositions were carried out at 1.5% oxygen mixed with argon. After depo sition, the samples were annealed at 150C for 1 hour under ambient atmosphere. The unpolished films were characterized by x ray diffraction (XRD) and atomic force microscopy (AFM, Veeco Dimension 3100). The x ray diffraction was performed on the as depos ited and annealed samples on a Philips APD 3720 machine. The surface morphology, before and after CMP was investigated using AFM performed in contact mode using a silicon nitride tip. Film thicknesses were measured using spectroscopic ellipsometry ( Woollam EC110 Ellipsometer ) for calculating removal rates. X ray reflectivity (XRR) measurements were performed on a Panalytical MRD X'Pert System whereas optical transmission spectra of the ZnO films deposited on corning glass were measured in the visible region using an Ocean Optics HR4000 high resolution spectrometer Results and Discussion Figure 4 1 shows the AFM micrograph for the sputter deposited film after annealing at 150C for 1 hour. The annealed zinc oxide films consist of grains varying from 500 to 3000 in size with an average RMS roughness of 266 Figure 4 2 shows the x ray diffraction patterns for the as deposited and annealed samples. The XRD pattern of as deposited and annealed ZnO film shows dominant (002) c axis orientation. The increased (002) peak intensity and reduced full width half maxima

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68 (FWHM) of the annealed ZnO film are due to improvement in the film quality and crystallinity. Annealing in ambient atmosphere resulted in a shift in (002) ZnO peak from 33.87 to 34.31. The average crystallite size deduced from FWHM using Scherrer equation [154, 155] increased from 154 to 192 Effect of A pplied D own p ressure and P laten S peed For pressure variation, samples were polished at 60 rpm, at a constant slurry flow rate of 60 ml/min and at a basic pH (pH ~ 10.6). Down pressures above 4 .5 psi (32.1 kPa) were not considered in this study since high pressure polishing is not desirable due to higher defectivity and low yield. Figure 4 3 represents the variation of removal rate with respect to the applied pressure during polishing. With incr easing pressure, contact area between the surface and slurry abrasive s increases resulting in the observed increase in material removal. The average RMS roughness of ZnO films was reduced to as low as 5 after polishing. Similarly, for studying the vari ation of removal rate with platen speed, down pressure was fixed at 2.5 psi with a constant slurry flow rate of 60 ml/min and a polishing time of 2 min. The variation of removal rate with linear velocity shows a typical CMP behavior, it increases with the increase in platen speed (see figure 4 4). Preston equation is often used to fit the removal rate variation with pressure and platen speed. According to Preston equation: ( 4 1) where RR is the removal rate, P is the pressure, V is the linear velocity and K is the represented by the red line in figure 4 3 and 4 4. However, careful analysis of fitting parameters illustrate that the Preston equati on is inconsistent as the removal rate

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69 showed non zero intercepts zero intercepts, slightly modified form of Preston equation as suggested by Luo et al [156] was used (eq. 4 2). ( 4 2) where, P th is a constant representing a threshold pressure and R c is a constant removal due to purely chemical reaction. The constants K P th and R c were calculated from the figure 4 3 and 4 4 using least square method and are enumerated in Table 4 1 The calculated values of Preston constant, K from figure 4 3 and 4 4 were compared and are within 5% error, suggesting the consistency of the equation with the experimental data. Effect of P article S ize The variation of removal rate and RMS roughness (inset) with respect to varying particle size is represented in Figure 4 5. The change in removal rate with varying particle size is within CMP variations. However, as expected the samples p olished with smaller particle slurry show better roughness which tends to saturate for particle size above 55 nm. This is related to the indentation depth given by the relation: ( 4 3) where, is the particle size, P app is the appl ied load, E is the effective elastic modulus of the two surfaces involved. As seen from the equation ( 4 3), smaller particles due to shallow indentation will lead to lesser material removal and hence better surface roughness whereas large particles will ca use large indentations resulting in higher surface roughness or defects.

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70 Effect of S lurry pH To investigate the effect of slurry pH on the material removal, pH of the slurry solution was varied from basic to acidic pH. The zinc oxide removal rates increase dramatically with decrease in the slurry pH from ~10.6 to ~3 as represented in Figure 4 6. A maximum removal rate of ~ 670 /min was achieved at a pH of ~3. The 10 fold increase in the removal rates in acidic conditions (pH ~3) as compared to the basic pH (~10.6) illustrate that the removal rates in the ZnO polishing process are strongly dependent on pH of the slurry. The inset in figure 4 6 represents the RMS roughness values with respect to the slurry pH. The RMS roughness trend demonstrates an increase i n roughness with increasing pH H owever, after careful examination it was found that samples polished at higher pH were under polished due to lower polishing rates at those pH values. W hen the samples were polished for 2 minutes with pH 10.6 slurry, the su rface roughness w as comparable to that at low pH. The AFM images of zinc oxide films polished with different pH slurries are depicted in figure 4 7. For ease of comparison, the Z scale of the images was adjusted. After polishing, the zinc oxide surfaces a re highly smooth and exhibit RMS roughness of 6 as compared to the initial roughness of 266 The RMS roughness values of <2 were observed for the ZnO crystals polished under same conditions (results not shown), indicating the role of grain size in t he final roughness after polishing of the polycrystalline ZnO thin films. Interestingly, the surface roughness values obtained with acidic as well as basic pH slurry (over polished) were very close to each other (< 1 variation in RMS roughness) in spite of an order of magnitude difference in the polishing

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71 rates. To explain such nature of zinc oxide CMP process, the zinc oxide polishing mechanism has been discussed in detail in the later section. Effect of P article C oncentration The solid content of the sl urry was varied from 2 to 10 wt% at slurry pH ~3. The variation of removal rate with respect to solid loading is represented by the plot in figure 8 with the average RMS roughness graphed in the inset. The removal rate increases with increasing concentrati on however, the RMS roughness plateaus above 5 wt% slurry loading. Zinc O xide CMP M echanism A typical CMP process can be broken down into two components: (1) the formation of chemically modified surface layer and (2) mechanical removal of the surface laye r [157 159] In case of metal oxides, the formation of chemically modified surface layer can be classified into two sub processes: (1) hydration of oxide [160, 161] and (2) bonding of abrasive particles with the material [157, 158] The hydrolysis of oxide is largely affected by pH of the solution and the useful pH range depends upon the dissolution of oxide and hydroxide formed [160, 162] For zinc oxide, it is stable between a pH of 6 to 12, whereas dissolution of zinc oxide is relatively high in acidic and high alkaline pH (>12) range In alkaline pH, zinc oxide forms zinc hydroxide, which has an inhibiting effect on the dissolution of zinc oxide. Depending upon the pH of the solution (in alkaline range), zinc hydroxide further transforms into zincate or bizincate ions via eq. ( 4 4) and ( 4 5). These zincate ions also passiva te the zinc oxide surface due to their low solubility in aqueous solution at pH 10 [163] ( 4 4 )

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72 ( 4 5) ( 4 6) On the other hand, dissolution of ZnO in acidic pH conditions is given by the eq. ( 4 6). In acidic pH, zinc oxide dissolution is faster due to direct reaction with hydrogen ions to give zin cic ions. To delineate such chemical interactions of zinc oxide with the slurry, the polished samples were kept in different pH slurry solutions for 5 minutes. The polished samples with RMS roughness < 6 were used for this study to highlight the change i n RMS roughness due to dissolution. The dissolution rates and change in the RMS roughness with respect to slurry pH are tabulated in Table 4 2 The dissolution rate of 37 /min was observed for pH 3 slurry solution whereas the solubility of zinc oxide was found to be negligible for other pH values tested. The change in RMS roughness was 17 for pH 3 in contrast to negligible change for slurries with pH 7 11 in 5 minutes. This confirms the response of zinc oxide under various pH conditions of slurry solutio n in accordance to the above discussion. Besides dissolution, pH of the slurry also affects the mechanics of the material removal in the CMP process [161] An oxide surface can become charged by absorbing either H + or OH ions depending on the pH of the solution. Such electrostatic charg ed sites can affect the area of contact between the abrasive particles and the metal oxide surface hence, influencing the material removal during CMP process. The number of such charged sites and surface charge can be measured using streaming potential/zet a potential measurements. The zeta potential s of the annealed zinc oxide film and colloidal silica particles (the slurry abrasive used in this study) were measured to understand the development of surface charge and interaction of the two surfaces

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73 under va rious pH ranges. The zeta potential of zinc oxide film and colloidal silica particles with respect to the pH of media solution are plotted in figure 4 9. Isoelectric point (IEP) of silica particles occur at pH 2.75 whereas it occurs at pH 8.5 for the zinc oxide film. Interestingly, zinc oxide surface and silica particles are oppositely charged between the pH region 2.75 8.5, implying increased interaction between the zinc oxide surface and the silica abrasives in this range during polishing. Based on the zinc oxide dissolution and zeta potential measurements, CMP of the zinc oxide can be divided in to three pH regions: (1) pH greater than ZnO IEP, (2) region between the IEP of zinc oxide and silica and (3) pH less than silica IEP. For pH greater than zinc oxide IEP (pH > 8.5), the zinc oxide and silica particles surface have same charge which reduces the interaction between the two surfaces. The zinc oxide dissolution is limited by the passivation of zinc oxide by hydroxide/zincate ions, resulting in lower removal rates. Similarly, when solution pH < 2.75 (the IEP of silica), the two surfaces have same charge. However, ZnO dissolution is high in such high acidic conditions leading to higher removal rates and poor surface roughness due to dominant dissolution process. In contrast, the region between pH 3 8, zinc oxide and silica surfaces are oppositely charged leading to increased particle surface interactions. Additionally, chemical dissolution increases as pH of the solution decreases. The combined effect of chemistry and mechanical aspect of the CMP process synergistically mitigate the effect of dissolution on surface roughness with beneficial increase in removal rate s Figure 4 10 represents the optical transmission curves for plain glass substrate, unpoli shed zinc oxide deposited substrate and zinc oxide substrates polished with

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74 acidic (pH ~3) and basic pH (pH~10.6) slurries. The curves have been normalized with respect to plain glass. A transmittance of 88%, 91% and 94% at the wavelength of 632.8 nm was f ound for the as deposited, acidic pH slurry and basic pH slurry polished samples, respectively. The polished samples show better transmittance which can be attributed to the reduction in surface roughness and exclusion of peaks and large particles. Additio nally, the substrate polished using the basi c pH slurry shows higher acidic pH slurry. The sample polished with basic pH slurry shows higher transmission in the region above ~520 nm whereas it shows lower transmiss ion below that as compared to both unpolished and acidic pH slurry polished samples. This behavior can be related partially to the shifting of curves due to thickness variation between the films and chemical interaction of the zinc oxide surface with the p olishing slurry as discussed above. The x ray reflectivity measurements were performed on the unpolished, acidic pH slurry and basic pH slurry polished samples. Figure 4 11(a) depicts the measured intensity versus angle whereas the schematic depicting the simulation results obtained by fitting the XRR curves is shown in figure 4 11(b). As seen from the figure 4 11(a), both acidic and basic pH slurry polished ZnO films show similar curves whereas as deposited film shows some dissimilarity. To model the XRR d ata, we have assumed a surface layer composed of ZnO, adsorbed carbon and water. According to the fit parameters, a surface roughness of 5 6 was observed for polished ZnO films (with both acidic & basic slurries) whereas 38 roughness was observed for t he unpolished ZnO film. These results concur well with the RMS roughness calculated from the AFM

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75 micrographs. The observed dissimilarity between polished and unpolished samples can be attributed to the surface roughness difference and the presence of thick surface layer for the unpolished ZnO surface. Conclusion H ighly smooth zinc oxide surface with RMS roughness less than 6 was achieved Removal rates as high as 670 /min were achieved at slurry pH of ~3. Polished zinc oxide films showed higher transmiss as compared to the unpolished surface. When various process variables were varied typical CMP behavior was observed. Modified Preston equation was applied to explain the removal rate variation with pressure and platen vel ocity. The modified Preston equation was found to be consistent with the experimental results and accounts for the non zero intercepts. Low polishing rates were observed at higher pH (> 7) whereas at low pH (~3) an order of magnitude increase in the remova l rates was observed without any detrimental effects on the surface roughness. The zinc oxide CMP process at different slurry pH values was explained by the pH dependence of the zinc oxide dissolution in aqueous solution and particle surface interactions. X ray reflectivity simulation results concurred well with the RMS roughness values for the ZnO films deduced from the AFM images.

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76 Figure 4 1 AFM micrograph of the annealed zinc oxide film (scale bar is in ). Figure 4 2 X ray diffraction patterns of the as deposited and annealed ZnO films. RMS=3 1

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77 Figure 4 3 Removal (red line represents the linear fit). Inset plots RMS roughness ( ) versus pressure. Figure 4 4 Re inset ) ( ) with respect to linear velocity. Red line represents the linear fit.

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78 Figure 4 5 Graph representing depicts the RMS surface roughness given by ( ). Figure 4 6 R emoval rates versus pH of the slurry Inset represents the average RMS roughness

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79 Figure 4 7 AFM micrographs of zinc oxide films polished with (a) pH 3.1, (b) pH 7.0 and (c) pH 10.6 slurry. Scale bar is in Figure 4 8 Removal rate variation with weight percent solid loading of the polishing slurry solution. Measured RMS roughness is shown in the inset. (b) RMS=6 (c) RMS=5 (a) RMS=6

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80 Figure 4 9 Plot shows zeta potential wi and the zinc oxide thin film ( ). Figure 4 10 Optical transmission curves for the glass, unpolished and the samples polished with acidic and basic pH slurries.

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81 Figure 4 11 (a) X ray reflectivity curves and (b) schematic showing fitting results (values indicate film thickness and roughness, respectively).

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82 Table 4 1 Fit parameters for the modified Preston equation Plot R c (/min) P th K(x10 4 Pa 1 ) RR versus pressure 9 1.235x10 4 2.69 RR versus velocity 2.78 Table 4 2 Dissolution rates and change in roughness for the samples soaked in different pH slurries pH Dissolution Rate (/min) () 10.5 <1 ~1 9 <1 ~1 7 ~2 3 3 37 17

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83 CHAPTER 5 EFFECT OF SLURRY ADD ITIVES: SURFACTANT A ND SALTS Introduction Surfactants are surface active agents that can modify the surface properties of liquids or solids. In the field of integrated circuit fabrication, adsorption of surfactants at the solid liquid interface has been mainly applied to the stabilization of particulate dispersions in chemical mechanical planarization [164, 165] and wet cleaning [166 168] Surfactants can selectively ad sorb at any interface Spontaneous adsorption of surfactants on surface can cause charge reversal Pr esence of surfactants is also known to result in lubrication between the two interacting surface s [165] This concept has been applied to CMP, not only for particulate stabilization but for the maximization of the topographical selectivity to increase planarization efficiency [169] Surfactant layers can also adsorb on a metal or a dielectric surface and act as inhibi ting layers against unfavorable chemical etchin g or corrosion during CMP [164, 170] T he addition of salt into the solution increases the i onic strength of the solution The increase in ionic strength of the slurry solution results in screen ing of the charges o n various surfaces, lowering the repulsion between polishing pad, particles or wafer surface depending upon the pH of the slurry [171] T he charge screening enables a closer contact between the pad and the wafer surface ; and between the particles and the wafer surface This phenomenon is known to increas e the frictional forces between the pad and the wafer surface [161, 171] Additionally, it has been suggested that some transient agglomerates ma y for m during polishing in the presence of salt which can further increase the frictional forces between different interacting surfaces in the CMP process [161, 171, 172]

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84 In this chapter, the effect of slurry addit ives such as surfactants and salt has been studied to increase the polishing efficiency of zinc oxide The concentration s of surfactants (anionic and non ionic) and potassium chloride (KCl) salt were varied to understand the effect of these additives on ch emical mechanical polishing (CMP) of zinc oxide films. The CMP performance was evaluated by measuring the removal rates and surface roughness obtained from thickness measurements and AFM scans. Finally, optical measurements were performed to evaluate the o ptical properties of polished zinc oxide surface. Experimental Procedure CMP experiments were carried out on a Struers Tegra Force 5 table top polisher with a Cabot microelectronics D100 polishing pad. The effect of concentration of surfactants namely, sod ium dodecyl sulfate (SDS) and Triton X 100 (TX 100) ; and potassium chloride (KCl) salt on the surface roughness of zinc oxide was studied. The concentrations of additives were varied systematically to understand the dependence of polishing performance on t he concentration. The polishing was carried out at 2.5 psi pressure with platen speed of 60 rpm under acidic pH (~3) conditions. Colloidal silica (Silbond Corp.) with mean particle size of 75 nm was used for all the experiments. Silicon p type substrates ( 1x1 inch 2 ) were sputter deposited with 25020 nm thick ZnO film. The ZnO oxide films were deposited using RF sputtering at 250 W power, 5 mTorr ambient pressure with target to substrate distance of 80 mm and no external substrate heating. All films were de posited at 1.5% oxygen mixed with Argon and were subsequently annealed at 150C for 1 hour in ambient atmosphere. The CMP performance was evaluated by measuring the RMS roughness and removal rate. The surface morphology, before and after CMP was investiga ted using

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85 atomic force microscope (AFM, Veeco Dimension 3100). Initial and final thicknesses were measured using spectroscopic ellipsometer ( Woollam EC110 Ellipsometer ). The optical transmission spectrum was measured on samples deposited on glass using an Ocean optics spectrophotometer (HR4000). Results & Discussion It is known that surfactants in a polsihing slurry can effect CMP process in two ways, (1) modifying particle particle and (2) particle surface interactions [165] By tuning the concentration of surfactant, optimal CMP performance in terms of removal ra tes and surface roughness can be achieved. In addition, ionic strength of the slurry solution also affects the mechanical aspect of CMP process by charge screening between various surfaces. To elucidate the effect of slurry additives such as surfactants an d salts, ionic and non ionic surfactants and KCl salt concentrations were varied systematically. The CMP variables such as pressure and platen speed, were fixed at 2.5 psi, 60 rpm whereas slurry pH, particle size and concentration were ~3, 75 nm and 5 wt % respectively as per optimized process parameters described in chapter 4 Effect of Surfactant Sodium dodecyl sulfate (SDS) surfactant was used for the polishing experiments. Figure 5 1 represents the variation of removal rate and RMS roughness (inset) wit h SDS concentration. A reduction of greater than 18 % in removal rates w as observed as compared to the reference slurry. Additionally, the zinc oxide removal rate decreases with increasing concentration of the SDS surfactant whereas RMS roughness shows sl ight increase. This can be attributed to the surfactant zinc oxide surface interactions. At slurry pH ~3, the ZnO surface is positively charged and silica particles are negatively charged according the zeta potential curves ( chapter 4, F igure 4 9 ). SDS bei ng an

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86 anionic surfactant attaches to the positively charged zinc oxide surface t hereby reducing the charged sites on the surface. This charge screening effect contributes to the reduced interaction between the zinc oxide surface and the silica particles a s compared to the case with no surfactant. The reduction in mechanical removal can be correlated to the observed increase in RMS roughness as chemical dissolutio n of zinc oxide slightly dominates The concentration of Triton X 100 surfactant in the slurry was varied to study the effect of non ionic surfactant on the polishing of zinc oxide. Figure 5 2 represents the variation of removal rate with respect to the concentration (in g/l) of TX 100. The removal rates with triton X 100 surfactant were found to b e lower than the rates observed for slurry without any surfactant. Addition of non ionic surfactant is known to have lubricating effect between the surfaces. Also, non ionic surfactant forms a surfactant layer on substrate, silica particle and polishing pa d surface which leads to the charge screening effect causing observed decrease in removal rates as compared to the slurry without the TX 100 surfactant. However, removal rate shows slight increase with increasing concentration of TX 100 while RMS roughness illustrates a decreasing trend with respect to increasing surfactant concentration. This corresponds to the fact that as surfactant concentration increases the screening (reducing electrostatic repulsion) between silica particles and CMP pad (negatively c harged in pH range 4 9 [161] ) th ereby the number of particles between pad and zinc oxide substrate increases. Such increase in the number of particles between pad and substrate leads to higher friction due to larger contact area hence higher observed polishing rates. However, due

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87 to the distribution of down force (reduction of force per particle) surface roughness decreases with increasing concentration of surfactant [161] Effect of Salt Salts increase the removal rates in a typical CMP process by increasing the ionic strength of slurry solution resulting in screenin g of charge. Such charge screening leads to the reduction in repulsive forces between various surfaces as depicted by the increase in removal rate with salt concentration in F igure 5 3. The removal rate s for concentrations greater than 0.2 M w ere observed to be even more than that for reference slurry (without salt, 670 /min). The increase in the surface roughness of the polished zinc oxide surface with the addition of salt can also be related to the increas ed frictional forces. Additionally, it is suggest ed that some transient agglomerates may form during polishing in presence of salts in the slurry due to local variations in the particle concentration. Such increased mechanical removal and formation of transient agglomerates cause the observed increase in roughness as depicted by F igure 5 3 (inset). AFM micrograph for the sputter deposited film after annealing at 150 C for 1 hour is represented by F igure 5 4. The AFM image elucidates that the zinc oxide films consist of grains varying from 500 to 3000 in size with an average RMS roughness of 266 The AFM micrographs of zinc oxide films polished with different concentration s of SDS, TX 100 and KCl in the slurry are represented in F igure 5 5, 5 6 and 5 7, respectively. The zinc oxide surfaces after pol ishing were highly smooth and exhibit RMS roughness between 3 6 as compared to the initial roughness of 266 RMS roughness between 3 6 after polishing irrespective of pH or slurry additives

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88 indicates that the minimum roughness is limited by the grai n structure of the polycrystalline zinc oxide films Figure 5 8 is the optical transmission plots for plain glass substrate, as deposited zinc oxide on glass substrate and zinc oxide on glass substrates polished with different slurry compositions. The cur ves have been normalized with respect to the plain glass substrate. The average transmittance varies between 92 93 % for the as deposited zinc oxide film and polished zinc oxide with slurry consisting of 0.1 M KCl, 5 mM SDS and 0.322 g/l TX 100. The observe d difference in the spectrum of various films polished with different additives is due to variation in the final thickness es of these films. Conclusion Effect of anionic and non ionic surfactant s on the zinc oxide CMP was studied. Anionic surfactant ( SDS in this case ) was found to lower the polishing rates and increase the RMS roughness with increas ing concentration. Addition of 5 mM SDS was found to give the best surface finish with RMS roughness ~3 However, r everse trends were obtained with Triton X 1 00 (non ionic) surfactant due to reduced repulsive forces between slurry particles and CMP pad The removal rates increased to as high as 900 /min with addition of KCl salt at the cost of surface roughness. The RMS roughness of polycrystalline film of ZnO was found to limit at or greater than 3 for any slurry com positi on. This behavior is related to the film morphology dependence (grain size) of CMP process for zinc oxide Optical transmission of zinc oxide films deposited on glass substrate exhibited an average transmission between 92 93% for various slurry compositions.

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89 Figure 5 1 /min) variation with SDS concentration in slurry whereas RMS roughness is depicted by circles ( ). Figure 5 2 Plot illustrating ) variation with r espect to the concentration of TX 100 surfactant in the slurry.

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90 Figure 5 3 Removal ) with respect to the varying concentration of KCl salt in the slurry solution. Figu re 5 4 AFM micrograph of the annealed zinc oxide film (scale bar is in ). RMS=31

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91 Figure 5 5 AFM micrographs for zinc oxide films polished with SDS slurry with SDS concentration of (a) 5 mM, (b) 10 mM, (c) 16 mM and (d) 30 mM (scale bar is in )

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92 Figure 5 6 AFM micrographs for zinc oxide films polished with Triton X 100 slurry with TX 100 concentration of (a) 0.322 g/l, (b) 0.64 g/l and (c) 1.28 g/l (scale bar is in ) Figure 5 7 AFM micrographs for zinc oxide films polished with (a) 0.1M, (b) 0.2 M and (c) 0.3 M KCl salt added to the polishing slurry (scale bar is in )

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93 Figure 5 8 Optical transmission curves for samples polished using slurries with 0.1 M KCl, 5 mM SDS and 0.322 g/l TX 100.

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94 CHAPTER 6 RAY TRACING SIMULA TIONS Introduction Light extraction efficiency is primar y reason for low overall efficiency of light emitting devices. Although a signi fi cant increase in ext was observed for several reported methods such as device shaping, photonic crystals, patterned substrates, surface roughening etc. but the se methods are often accompanied by changes in the radiation pattern, exhibit an undesirable angle dependent emi ssion spectrum, or employ costly or complex processing methods [85] Applicat ion of external out couplers provide significant enhancement in light extraction without invoking such undesirable attributes. Several structures as external out couplers have been considered for improving the light extraction such as hemispherical dome [76] pyramids [81, 110] cylinders a nd lenses [73, 85, 111 113] S pherical shapes have been known for improving external efficiency as they subtend a large solid angle of emitted rays resulting in extraction of light prev iously trapped in the substra te [73] Madigan et al. demonstrated the effect of spherical features on external coupling e fficiency and the far field emission pattern. The light extraction increased by factor of 3 by attaching macro sized spherical lens using index matching gel on the backside of the OLED. However, use of millimeter sized lens is not practical for many applic ations due to alignment and index matching issues. Mller et al. suggested an alternative, simpler technique to fabricate ordered array of micro sized lens without the need for alignment with respect to LED device. A 10 m diameter microlens square array of poly dimethyl siloxane (PDMS) was fabricated using mold transfer method which was attached to em ission side of the glass substrate T his

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95 architecture resulted in 1.5 fold increase in the light output over unlensed devi ce. Nakamura et al., fabricated pyramidal microlens array and studied the effect of using high refractive index substrate in conjunction with microlens array [81] Interestingly, t he high refractive index substrate did not show any enhancement in light extraction whereas pyramidal microlens structure resulted in extraction efficiency enhancement by a factor of 1.5. Greiner analyzed the microlens and micropyramid array as the externa l out couplers in an OLED device [147] The simulation res ults elucidated that the reflectance of OLED stack or back reflector is of paramount importance since the probability of a photon to escape in first pace through various structures is only a fraction of total escape probability. In addition, the aspect rat io of microlens or micropyramid structure has strong influence on the out coupling efficiency as escape probability of higher angle photons reduces with aspect ratio. Compact packing of these structures results in better out coupling efficiency because any flat region increases the probability of TIR. Greiner findings also illustrated that the exact geometric nature of the structure influence angular distribution much strongly as compared to the out coupling efficiency. Using these findings he was able to d emonstrate 80% extraction efficiency from substrate for microlens, micropyramid and scattering particles with aluminum back contact OLED device. In this chapter, Monte Carlo ray tracing simulations in 3D have been performed to understand the behavior of py ramid, microlens and cylinder arrays as external out couplers. A comparative study on effect of feature size and aspect ratio of pyramid, lens and cylindrical features on output intensity and angular distribution of out coupled photons ha s been conducted u sing ray tracing simulations. For the first time, c orrugated

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96 device structures with micro n scale textures applied to substrate/TCO interface were analyzed to simulate the controlled micro scale topography on TCO surface The enhancement in light output and angular spread of out coupled intensity from corrugated structures were studied for microlens structure with different feature size s and h/d ratio s Commercially available LightTools (v6.3.0) ray tracing simulation software from Optical Research Associate s was used for all the simulations. The details of simulation parameters and obtained results are discussed below. Device S imulation P aram e ters Textured substrates with pyramid, microlens and cylinder arrays were simulated. Figure 6 1 is the schematic of t he device simulated with the glass as substrate, a TCO layer and a light emitting layer. The light emit ting layer of and refractive index, n active = 2.5, 1.7 over ZnO layer as the TCO with thickness, n ZnO = 1.9 and optical density (O.D.) = 0.09691 was placed on a glass substrate (t = 0.5 mm, n = 1.46). The two refractive indices of active layer were considered for simulation to understand the effect of refractive index of the active layer on the ray dynamics The n active of 1.7 represents typical refractive index of organic materials whereas n active = 2.5 is representative of inorganic mat erials. The device size was 1x1 mm 2 with 100% reflecting sides. This structure represents the simplified structure of a n OLED/TFEL type of device. The values of the material refractive index and thicknesses may change in actual device but the ray dynamics will hold true for classical ray optics regime. T wo kinds of structures were simulated (A) texture on the top side of glass substrate which results in enhancement in extracted mode and (B) texture on the bottom side (TCO and active layer side) of glass sub strate for extraction of TCO/active layer mode (Figure 6 1). For device structure A, pyramid, microlens and cylinder arrays with different base

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97 diameters and aspect ratios were simulated whereas for device structure B only microlens was simulated. The cyl inder arrays and pyramid structure were not considered for the simulation in device structure B configuration due to deposition of conformal film on straight wall structures is difficult and pyramid structure presents sharp edges which can serve as site fo r probable shorts. The film thickness of ac tive layer was changed to 0.15 m for device B simulations to match closely with the device fabricated with Tris(8 hydroxyquinolinato)aluminium ( Alq 3 ) organic active layer with n active = 1.7. For comparison, the n active value equal to 2.5 was also sim ulated with same thickness of 0.15 m The rear surface of active layer was made reflective to replicate the metal back contact present in a typical OLED device. The absorption of 20% was deliberately incorporated in the properties of reflecting rear surface to account for absorption losses in the active layer and metal back contact. The emission of the active layer was assumed to be isotropic with 550 nm used as the emission wavelength for all simulations. The ray tracing simulations were performed using 100,000 rays for measuring out coupli ng using large plane detectors near front and rear side of the device. The angular distribution of the out coupled rays was measured using a far field detector centered to the device with 1 million rays used for the simulation. Both planar and far field de tector measured total fractional power of the out coupled rays. The aspect ratio of pyramid structures was varied by changing the contact angle from 0 85 whereas for microlens and cylinders, height to diameter (h/d) ratio was varied from 0 to 0.5 ( 0.5 bei ng hemisphere) and 0 to 1 respectively. The base diameters (1, 3, 6, 10, 14 and Device A a l l the structures with individual texture elements arranged in hexagonal pattern with 3 m spacing The side of the pyramid

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98 base was calcu lated as considering an inscribed square in the circular base. For Results and Discussion Device A: Texture at S ubstrate/ A ir I nterface Pyramid t exture Figure 6 2 (a) & (b) represents the light out coupled from a pyramid array for device configuration A normalized to a planar device for the active layer with refractive index of 1.7 and 2.5 respectively It can be seen from the figures that irrespective of the refractiv e index of active layer the enhancement in light out coupling increases with contact angle for a given base diameter Extraction of substrate mode is higher for the device with larger base diameter. Interestingly, the enhancement shows slight decrease for structures having contact angle greater than 55. This loss in light extraction at higher contact angles is because of re entry of the extracted rays from a cone into the adjacent cone. Such re entrant rays finally get absorbed in the different layers of the device leading to loss in luminance intensity Figure 6 2(c) plots the maximum obtained enhancement with respect to the diameter of pyramids for active layer with different refractive indices Lower refractive index active layer demonstrates higher enh ancement due to absence of active layer mode as the refractive index of active layer is less than the TCO layer Figure 6 3 (a) and (b) represent the angular distribution curves for different diameter structures at most efficient contact angle for n active = 1.7 and 2.5, respectively The angular distribution curves illustrate a deviation from Lambertian distribution profile of a planar device. A superposition of peak for angles greater than critical angle (~43 )

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99 can be seen from the curves. The observed in crease in intensity at higher angle is a result of extraction of emitted photons by the pyramidal structures at angles greater than critical angle. The intensity in forward direction within escape cone is expected to be reduce d due to introduction of pyram id structure. However, it remains same or demonstrates slight increase than the planar device as the reflected photons incident in forward direction on the surface are recovered after multiple reflections. Microlens t exture Microlens array texture demonstr ates light extraction as high as ~ 1 87 X for n active = 1.7 and ~ 1 7 4 X for n active = 2.5 The light out coupling efficiency increases both with h/d ratio and base diameter of the lens as depicted in F igure 6 4 (a) and (b). A s observed in case of pyramid, the trend with respect to aspect ratio and diameter remain unaffected by the refractive index of the active layer. The lower refractive index active layer demonstrates higher enhancement values (see F igure 6 4 (c)) because of n active being lower than n ZnO whi ch results in e limination of active layer mode A strong dependence of light out clearly evident from the figure. The hemispherical lens (h/d = 0.5) demonstrates maximum efficiency in light extraction for a given diameter. A microlens structure reduces the extraction of photons emitted at normal incidence due to curvature of the feature. However, the reduction in the light extraction of normal incident photons is more than compensate d by the enhancement from the extraction of higher angle photons. In addition, even the se reflected photons are directed back towards substrate by the mirrored back side with changed directionality due to non parallelepiped geometry of the substrate The m ultiple reflections and changed directionality upon reflection improves the probability of such photons to be extracted. Figure 6 5 represents the

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100 angular distribution of extracted mode photons for the most efficient h/d ratio structure s for each diameter The angular spread of photons elucidates the extraction of higher angle photons although the enhancement is observed for both forward and at higher angles The observed improvement of the extracted light intensity is related to the simultaneous increase i n photon extraction due to back reflector and extraction of higher angle photons coupled with the focusing effect of lens structure. Cylindrical t exture The simulation of cylindrical structures divulge s a different behavior as compared to the pyramid and microlens structures. The reason for such disparity can be explained by the difference in geometry of these structures. The cylindrical structures are straight wall features with flat top unlike inclined or spherical profile o f the pyramid and microlens respectively with no flat region. In case of cylinders, the introduction of vertical wall leads to improved light out coupling however, efficacy of the cylindrical structure is limited to low values of h/d ratio (< ~0.4) as evident from Figure 6 6. The fla ttening observed for enhancement curves for the cylindrical features at low h/d can be explained by the losses due to re entry of extracted photons from one cylinder into to no significant increase in light o ut coupling as compared to one This behavior is a result of steep increase in the area of top surface of cylinder as compared to surface area of the side walls which undermines the enhancement in light extraction. The simulation results show enhancements as high as ~1. 68 and 1. 57 folds for n active = 1.7 and 2.5 respectively for base diameter of 14 m. The angular spread of extracted photons is depicted in Figure 6 7. The light extraction in direction normal to the surface show no significant improvement,

PAGE 101

101 corroborating the reason of reduced efficacy of cylindrical structure with increase in diameter of the features. The formation of butterfly wing like features between 45 90 illustrates the redirection of higher angle photons into escape cone However, the increase is less drastic and the intensity distribution is less spread as observed for pyramid and microlens arrays. The observed low improvement in light extraction in cylindrical structure as compared to the pyramid and microlens can be due to parallelepiped geometry of the features which limit s the ability of texture to randomize the directional ity of reflected ray s Hence, the probability of photon extraction is not significantly enhanced even after multiple reflections from back side reflector. Device B: Texture at S ubstrate/TCO I nterface Only 4% and 10% of generated photons are extracted final ly from the device with n active = 2.5 and 1.7, respectively. Simple calculations show that t he a ctive layer mode account for trapping of ~ 82% of generated photons whereas TCO layer mode accounts for 8% of the 18% entering TCO layer for n active = 2.5, n TCO = 1.9 and n substrate = 1.46 system Similarly, in case of n active = 1.7, since active layer mode is absent the combined TCO/active layer mode amounts for trapping of roughly 76 % of generated photons in TCO layer without taking into account the high absor ption losses in organic layer These calculations illustrate a clear need to extract the TCO and active layer modes which account for significant amount of generated photons. Few reports have been published on attempts to reduc e TCO/active layer modes beca use of the increased probability of electrical shorts due to intentionally introduced roughness at active layer interface [82 84, 109, 123] This restricts the feature dimensions much less than film thicknesses. In this section, a n attempt has been made to understand the ray dynamics when microlens textures with dimensions much greater than the emission

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102 wavelength are applied on the substrate at active layer side. In order to understand the ray dynamics in such syste m, simulations were carried out with varying diameter and h/d ratios of microlens texture applied on the active layer side. Due to the spherical profile of a lens structure, the shape is ideal from fabrication view point and has less probability of possible shorts due to sharp features or edges. Further, the dimensions considered for these structures are in micron range, making microlens structu res locally flat compared to the scale of active layer thicknesses. It should be noted here that as the film thickn esses (TCO and active layer) are much less than the height of microlens features a corrugated geometry of the device is obtained assuming conformal nature of the deposited films. The simulation results on the enhancement in light extraction are presented by Figure 6 8 (a) and (b) for the microlens texture at substrate/TCO interface (device B configuration) whereas (c) presents the maximum fractional powers obtained for different diameters and the two refractive indices of active layer considered here In o rder to understand the effect of feature dimensions alone on the light extraction efficiency, t he simulations were carried out with fixed number of rays (100,000 rays) eliminating the effect of increased surface area. The enhancement demonstrates a steep i ncrease with increasing h/d ratio with maximum enhancement of 1.45 and 1.55 for n active = 1.7 and 2.5, respectively. A significant portion of generated photons are waveguided within active layer due to high refractive index of the layer as compared to TCO ( n ZnO = 1.9). This active layer mode is absent in case of n active = 1.7, as the refractive index is less than n ZnO The application of texture on the active layer side distorts the parallelepiped geometry of active layer leading to extraction of the activ e

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103 layer mode. Additional photons extracted results in higher observed relative enhancement in higher refractive index material as compared to lower index material. However, the absolute extracted power of the photons in n active = 2.5 are still less as comp ared to the n active = 1.7 (see Figure 6 8 (c)). Additionally although smaller diameter structure shows better enhancement but the difference in enhancement values is very sma ll This small difference in enhancement can be attributed to the fact that as fe ature size increases the radius of curvature of the feature reduces resulting in reduc ed efficacy of the texture. Figure 6 9 depicts the angular spread of photons for n active = 1.7 and 2.5 with 6 m microlens base diameter at different microlens heights The angular spread illustrates Lambertian spread as the measured output is extracted mode from glass substrate without any texturing. However, intensity increases with height (or h/d ratio) of the microlens feature as observed from the enhancement curves for a given diameter. The increase in intensity in all direction is attributed to the reduction in TCO/active layer modes and extraction of higher angle photons leading to higher fraction of generat ed photons entering substrate Simulation results also illustrate a reduc tion in enhancement after h/d of 0.3 for n active = 1.7 and 0.4 for n active = 1.7 as observed from both enhancement and angular distribution plots The reason for this decrease can be understood from the far field angular distribution of photons represented by Figure 6 10 In the figure, substrate has been immersed in a medium of refractive index same as that of substrate. This leads to measure ment of only the photon out coupled from th e TCO layer. With special emphasis on the forward direction in the figure it can be inferred that the enhancement is coming from increased extraction in forward direction. However, as

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104 h/d (or height) reaches close to hemispherical profile the distribution becomes well spread and more off normal photons are extracted as compared to forward direction photons. The increase in forward for h/d less than certain value and off forward for h/d greater than that v alue can be explained by considering the fact that a microlens in the device B configuration is also acting like a concave mirror. Because there is no texture at glass/air interface the out coupling of photons from this interface is still restricted by the escape cone ( c ~ 43) a nd has Lambertian distribution Hence, the extracted angular spread is a superposition of Lambertian distribution of glass/air interface over the non Lambertian spread of photons entering the substrate thereby preventing the higher angle photons to escape reducing overall efficiency for high aspect ratio structures Conclusion Light extraction efficienc ies of various external light out couplers such as pyramid, microlens and cylinder arrays w ere analyzed. The use of such external light out coupler s in conjunction with back side mirror results in enhancement of light due to extraction of higher angle photons and change in directionality of the reflected photons outside the escape cone. In case of pyramids, the intensity of extracted light increased with contact angle less than ~55 after which the structure demonstrates slight decrease due to re entry of the extracted photons back into device. The ray simulations elucidated similar behavior of the microlens array s Microlens arrays demonstrate higher light extraction efficiency and circular symmetry in angular distribution of extracted photons. In contrast, efficacy of the cylinder arrays is limited to the lower h/d ratio s and the light extraction efficiency becomes independent of increase in feature diameter In conclusion, microlens and pyramid arrays were found to be

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105 more efficient structures as compared to cylindrical array s The trends remain independent of refractive index of the active layer as expected because these features only af fect the substrate mode Ray dynamics slightly change s when the microlens structure is applied at the substrate/TCO interface in the device B architecture. Application of microlens texture at this interface changes the geometry of the TCO and active layer to a corrugated geometry. The deviation from parallelepiped geometry leads to increased extraction of TCO/active mode in case with n active = 1.7 and higher extraction of both TCO and active layer mode i n case of n active = 2.5. The enhancement increases ste eply up to 0.3 and 0.4 h/d ratio for n active = 1.7 and n active = 2.5, respectively. There after decrease in extraction efficiency was observed in simulated results. The increase in extraction of normal angle photons from TCO layer up to 0.3 and 0.4 h/d rat io for n active = 1.7 and n active = 2.5, respectively was found to be res ponsible for highest enhancement at these h/d ratios when fraction of photons exiting TCO layer were measured alone However, at higher h/d ratios the extraction of higher angle photon s increased but due to Lambertian profile of extracted light at glass/air interface the combined effect of the TCO extracted and glass extracted mode profiles resulted in the observed decrease in the overall enhancement.

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106 Figure 6 1 Schematic depicting different device structures s tudi ed using ray tracing simulations

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107 Figure 6 2 Enhancement in out coupled rays from a pyramid array textured substrate (Device A) for (a) n active = 1.7 and (b) n active = 2.5 whereas ( c) represents the maximum simulated fractional power s obtained for the different base diameters of pyramid array s and refractive indices of active layer

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108 Figure 6 3 Angular distribution of the light out coupled from device A with pyramid texture for active layer refractive index (a) n active = 1.7 and (b) n active = 2.5

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109 Fi gure 6 4 Enhancement in light out coupling for a microlens array with device A configuration represented for (a) n active = 1.7, (b) n active = 2.5. The maximum simulated fractional power s at different base diameters for the two refractive ind ices of active layer are depicted by (c).

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110 Figure 6 5 Angular distribution of the total light out coupled from device A with microlens texture for (a) n active = 1.7 and (b) n active = 2.5

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111 Figure 6 6 (a ) and (b) depict the l ight out coupled normalized to a planar device for a cylinder type texture at the substrate/air interface (Device A) for n active = 1.7 and 2.5, respectively wh ile (c) represents the maximum simulated fractional power s for different diameters and active layer refractive ind ices

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112 Figure 6 7 Angular spread of the light out coupled from device A with cylindri cal texture for active layer refractive index of (a) 1.7 and (b) 2.5

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113 Figure 6 8 L ight extraction enhancement from a microlens texture at the substrate/TCO interface (Devi ce B ) normalized to the planar device is represented by (a) for n active = 1.7 and (b) for n active = 2.5 while (c) represents the maximum simulated fractional power s for different diameters and refractive ind ices of active layer

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114 Figure 6 9 Far field a ngular spread of the light out coupled from device B at different microlens heights for and n active of (a) 1.7 and (b) 2.5 Figure 6 10 Far field angular d istribution of photons out coupled from the TCO layer at different microlens heights for n active = 1.7

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115 CHAPTER 7 EXPERIMENTAL PROCEDU RES FOR FABRICATION OF TEXTURES Experimental Procedure Substrate Preparation D evices were fabrica ted on 1x1 inch 2 glass substrates (Corning 2497). As received glass substrates were cleaned in acetone, methanol, deionized water in a sonicator For wet etching not only the etch selectivity between etch mask material and the etched material but the int erface between the etch mask and the etched material is also important. To remove organic residue p iranha clean with 2:1 sulfuric acid and hydrogen peroxide was carried out immediately before deposition of etch mask layer Samples were kept in p iranha sol ution for 20 30 min and were rinsed thoroughly with DI water and blow n dried with nitrogen The substrates were placed in nitrogen filled oven at 120C for 15 min to remove the adsorbed water from the surface. Structure Fabrication Three different types of features (pyramid/cones, cylinder and microlens) were fabricated on the substrate with base diameters of 6, 10 and 14 m. Figure 7 1 shows the mask design along with the dimensions and lattice spacing Figure 7 2 is the p rocess flow schematic for different features fabricated. The description of p rocess flows has been divided in to three sub sections based on structure type as described below Cone/ p yramid t exture For any etching process the etch selectivity between etch mask material and the etched material is an important parameter to be consider ed when selecting a material for etch mask Amorphous silicon (a Si) demonstrates infinite etch selectivity with

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116 respect to glass in buffered oxide etchant (BOE) commonly used for glass etching. For this reason a Si was chosen as a etch mask for creating cone/pyramid structures. The cleaned substrates were deposited with amorphous silicon after 30 s Ar plasma pre clean in the Kurt J Lesker CMS 18 Multi Source RF sputter tool. A ~3 000 thick a Si film was deposited at room temperature at the power setting of 200 W and 5 mTorr chamber pressure using argon as the sputter gas. Silicon target was pre sputtered for 5 min to remove any oxide or impurity layer on the target surface. The substrate s with sputtered a Si films were annealed at 400C for 6.5 hours in n itrogen ambient at 100 mTorr pressure The annealed substrate s w ere primed with HMDS vapors in a YES oven. Immediately after priming, the substrates were patterned using standard UV photolithography technique. A ~ 1.2 m thick S1813 photoresist was sp u n on the substrate s followed by exposing to 365 nm Hg i line UV radiation using a Karl Suss MA6 Aligner. The photoresist was soft baked at 105C for 30 minutes prior to UV exposure and was post baked at 120C for about 30 minutes. Standard AZ300 MIF developing solution was used for 1 min to develop the pattern. Patterned photoresist (PR) coated substrate s w ere descummed in a reactive ion etcher (RIE/ICP, Unaxis SLR) under oxygen plasma for 20 s. The RIE conditions used for descumming process were 100/600 W RF/IC P power with 20 sccm O 2 flow at chamber pressure of 5 mTorr. To transfer pattern from PR to the a Si layer standard SF 6 chemistry was used in a STS deep reactive ion etcher (DRIE) with 800 W RF power and 12 W DC platen power Cone/pyramid structures on gl ass substrates were etched using a 6:1 buffered oxide etchant (BOE) solution structure 6:1 BOE solution was diluted by adding 15 vol % 30 44% hydrochloric acid

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117 End point for BOE etch was determined by first visual sign of Si mask curling, when substrate was retracted from the BOE solution. To ensure complete removal of a Si mask, DRIE step was repeated before final clean with acetone, methanol and DI water. Microlens t exture Cone/pyramid structures fabricated according to the method described above were used to create micr olens profile using chemical mechanical fabrication (CM F ) technique. Substrate with cone structures fabricated as above were polished using standard silica slurry on a soft politex pad. Details of the CMP process are described in ref [173] Post polish cleaning was carried out by buffing with DI water followed by acetone, methanol and DI water rinse in an ultrasonicator. Cylindrical t exture Dry etching process was used to attain high aspect ratio vertical wall cylindrical f eatures. Glass substrates were coated with low frequency PECVD silica deposited at 60 W (RF) and 300C by flowing 7.2 sccm SiH 4 mixed with 1279 sccm N 2 O and 352 sccm N 2 For etching PECVD silica, chromium (Cr) metal was used as the etch mask. The S1813 PR was spin coated at 5000 rpm for a nominal thickness of ~1.2 m To pattern Cr mask layer, lift off process was used by image reversing the positive S1813 photoresist after soft bake and exposing the PR layer to UV radiation in presence of a photomask For image reversal, PR was exposed to ammonia (NH 3 ) gas in a YES vacuum oven at 90C temperature. The NH 3 reacts with acid in the exposed resist rendering it insoluble in the developer. The proceeding flood exposure causes acid to form in the previously unexposed areas allowing them to be removed in development step, l eaving behind negative image of the first exposure. A 3500 of Cr metal was

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118 deposited using DC sputter after the lithography process. Metal deposited substrates were sonicated in acetone to remove PR for Cr lift off. Dry etching of PECVD silica was perf ormed in an Unaxis reactive ion etcher with inductively coupled plasma module using CHF 3 recipe. A 600 W ICP power with 30 W RF power was applied with 25 sccm of CHF 3 and 3 sccm of O 2 gas under 5mTorr pressure. An etch selectivity of 33:1 was obtained betw een PECVD silica and Cr metal mask for the given optimized recipe. After etching of the cylindrical structures of desired heights controlled by changing the etch time the Cr metal mask was removed using Transgene chromium etchant. Substrates were finally cleaned with acetone, methanol and DI water and blow dried using N 2 gas. The topography of the fabricated structures was characterized using atomic force microscope (AFM, Veeco Dimension 3100) and field emission scanning electron microscope (JEOL SEM 6335 F). Results and Discussion Cone/Pyramid Structure Various masking strategies have been published on micropatterning of glass. Metal masks such as Cr/Au [174 176] or Cr/Cu [177, 178] have been reported. LPCVD poly Si [177] PECVD a Si [179] or even bulk Si [180] are also commonly used etch mask s Amorphous Si is resistant to hydrofluoric (HF) acid and offers an infinite etch selectivity with glass (SiO 2 ) etching in HF [178, 17 9] However, the a Si mask/ glass interface and internal stress es in a Si layer are two very important parameters that determine the performance of the silicon masking layer. Poor adhesion between the two surfaces and internal stress in masking layer can l ead to premature peeling of mask in the etchant solution. To create a good interface between silicon mask and glass, after

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119 standard acetone, methanol and DI water rinse in an ultrasonicator, the glass substrate s w ere further stripped of any organic residue using p iranha solution (a mixture of 2:1 sulfuric acid and hydrogen peroxide) for 20 30 min and rinsed in DI water thoroughly. Substrates were subjected to 400C for 6.5 hours to reduce the internal stress in the deposited silicon layer on glass. Figure 7 3 represents the SEM micrographs at 45 tilt angle for cone/pyramid structures fabricated SEM images reveal that due to isotropic nature of wet etching process the cones/pyramids have concave slant walls. The low m agnification SEM images (not shown here) illustrate highly smooth etched glass surface with very low density of wet etching related defects such as notching, pinholes etc. These observations in conjunction with observed high aspect ratio demonstrate the go od interface between Si mask and glass and optimum internal stress in a Si layer required to withstand long etch times. AFM was performed to measure the height of fabricated structures. Figure 7 4 (a) and (b) are the A FM topographs of cones/pyramid structures A s maximum h/d ratio achievable is 1:2 due to isotropic nature of wet etching process. The obtained h/d ratio in this work was ~1:2.6 The glass etch rates of ~ 0.6 /min were observed hi obtained The addition of HCl in the BOE solution dissolves the insoluble oxides of alkali metals present in glass acting as micro masks. This helps in accel erating the glass etching and obtaining smooth surface after the etch process. Microlens Structure Microlens structures were created by polishing the cone/pyramid structures. The details of the process are describe in ref [173] Because the microlens feature is created

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120 from a cone/pyramid structure, it limits the maximum achievable aspect ratio according to the Figure 7 5 The maximum height of microlens is calculated according to equa tion 7 1 and is tabulated in Table 7 1 ( 7 1) where, h represents the height of the segment (microlens) of a sphere, H is the height of square base pyramid assuming the pyramid has vertex angle of 45 and square base with side x ( x = r r is radius of pattern). The AFM and SEM micrographs of fabricated structures are given in F igure 7 6 and Figure 7 7 Figure 7 6 also shows a line profile along the center of lens feature extracted from AFM dat a The rounding of surface after polishing can be obse rved from the curves. It should be noted that the polishing time to reach the maximum achievable lens height varies with the base diameter of pyramid structure The variability of smoothening rates was observed to be high which is attributed to the variation in local pressure with contact area of the feature (higher pressures for smaller features) For this reason a good control over polishing is difficult. In addition to polishing variability, the conca ve geometry of fabricated cone/pyramid structure also leads to deviation of obtained microlens h/d ratios (approximately 0.21, 0.17 and 0.21 for 6, 10, and 14 ) from the calculated height values. SEM images in Figure 7 7 are obtained at 45 tilt angle. T he images illustrate rounding of the pyramid tip confirming the AFM results. Cylindrical Structure Cylindrical structures were created using dry etch process as creation of vertical wall features necessitates anisotropic etching. The Corning 2947 glass sli de used in this study is a soda lime glass which has various metal oxides other than silica such as

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121 Na 2 O, MgO, CaO and Al 2 O 3 These metals do not form gaseous products in a dry etch process which makes the dry etching of glass to be physical in nature The etch selectivity between masking layer and the glass is severely impeded by the physical nature of glass etching. To avoid such complications, 10 thick PECVD silica layer was deposited on the glass substrate. Chromium metal used as etch mask for the dr y etching of PECVD silica presented an etch selectivity (oxide:Cr) of ~33:1 using CHF 3 plasma chemistry at 30/600 RF/ICP powers. The etch selectivities and etch rates for silica obtained by variation of RIE process variables have been tabulated in Table 7 2 Figure 7 8 represents the SEM images of fabricated cylinder array of 14 base diameter. The Figure 7 8 (a) reveals the straight wall structure of the cylinder with height of cylinder approximately 5.38 m. The top view of the cylindrical pillar is presented in Figure 7 8 (b). The figure illustrates a very close match between the mask and fabricated pillar diameter (see annotation ). However, the side walls are highly rough which is due to striations in the photoresist layer and formation of fluorinated amorphous carbon residue a side pro duct of CHF 3 etch Conclusion Cone/pyramid arrays with aspect ratio as high as 1:2.6 were fabricated on the Corning 2947 glass using sputtered a Si mask and 6:1 buffered HF solution. The etch rates of 0.6 /min and 4.5 /min were observed for the Corning 2947 soda lime glass when etched with 6:1 BOE and diluted BOE with15 vol % 30 44% HCl respectively Pre clean of glass substrate was found to be of utmost importance in the wet etching process for better adhesion between glass and a Si mask. The release of internal stress in th e masking layer after optimum post annealing reduces pre mature failing of the etch mask. Smoothening process to create spherical profile from pyramid

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122 structures was tested for high er aspect ratio structures using CM F process As expected features with smaller diameters present poor process control as compared features with larger diameters. Microlens arrays with aspect ratio as high as 1:6 were fabricated using the CM F process. Cyli ndrical structures with aspect ratio up to 1.7:1 were fabricated using CHF 3 dry etch recipe and Cr metal mask.

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123 Figure 7 1 Schematic illustrating the mask design s Figure 7 2 Process flow s for fabrication of (a) cone/pyramid and microlens and (b) cy lind rical arrays

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124 Figure 7 3 SE M micrographs of cone/pyramid texture of (a) 6 m, (b) 10 m and (c) 14 m base diameter (at tilt angle of 45 ) Figure 7 4 2D /3D (a) (b) (c)

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125 Figure 7 5 Schematic of microlens height derived from pyramid of height H. Figure 7 6 AFM micrographs and line profile of microlens structure after CMP of the

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126 Figure 7 7 SEM micrographs of microlens features with base diam eter (a) 6 (b) 10 and (c) 14 (at 45 tilt ) Figure 7 8 SEM images of fabricated cylind rical different magnifications (a) 2700 X at 45 tilt and (b) 6000 X (top view) (a) (b) (a) (b) (c) (a)

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127 Table 7 1 Maximum microlens height from square base pyramid of height H h 6 2.12 4.24 1.00 10 3.53 7.07 1.67 14 4.95 9.9 2.34 Table 7 2 R IE etch recipe for silica etching RF1 (W) ICP Power (W) Gas mixture Gas flow (sccm) Chamber ( mTorr) Oxide removal (nm/min) Cr removal (nm/min) Selectivity (Oxide:Cr) 50 600 SF 6 /Ar 20/5 5 115.5 7.6 15 50 600 CHF 3 /O 2 25/3 5 139 4. 8 29 50 600 CHF 3 /O 2 25/10 5 109.5 15. 7 7 30 600 CHF 3 /O 2 25/3 5 112 3.4 3 3

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128 CHAPTER 8 MICRO TEXTURED TRANSPARENT CO NDUCTING OXIDE FOR ENHANCED LIGHT EXTRACTION EFFICIENC Y Introduction Only 4% and 10% of the generated photons are extracted finally from the device with n active = 2.5 (inorganic active layer materials) and 1.7 (organic active layer materials), respectivel y. Simple calculations show that the active layer mode account s for trapping of ~ 82% of generated photons for n active = 2.5 and n TCO = 1.9 system whereas TCO layer mode again accounts for 8% of the 18% photons entered TCO layer in same system with n substr ate = 1.46. I n case of OLED s the refractive index of the active layer is roughly 1.7 which is less than or close to that of TCO layer. This eliminates the guided mode in the active layer. The T CO or combined TCO /active layer mode still amounts for trappin g of roughly 76 % generated photons in TCO layer. These calculations illustrate a clear need to extract the TCO and active layer modes which account for significant amount of generated photons. Few reports have been published on attempts of redu cing TCO/act ive layer modes because of the increased probability of electrical shorts due to intentionally introduced roughness at active layer interface [82 84, 109, 123] In order to avoid shorts due to light extraction struc tures, the feature dimensions are restricted to much less than film thicknesses. At these scales ( wavelength or sub wavelength range ) the enhancement in light extraction is often accompanied by undesirable change in the radiation pattern, exhibit angle de pendent emission, or employ costly or complex processing methods [83 85] In this chapter an attempt has been made to understand the ray dynamics when microlens textures with dimensions much greater than the emissi on wavelength are applied on the substrate at active layer side. In order to understand the ray dynamics in

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129 such system, ray tracing simulations ha ve been carried out with varying diameter and h/d ratios of the microlens texture applied on the active layer side. Due to spherical profile of a lens structure, the shape is ideal from fabrication viewpoint and has less probability of possible shorts due to sharp features or edges. Further, the dimensions considered for these structures are in micron range, maki ng microlens structures locally flat compared to the scale of active layer thicknesses In addition, results on chemical mechanical polishing of TCO layer in order to reduce the nano scale roughness have been presented in this chapter Alq 3 laye r with alum inum back metal w ere evaporate d on the fabricated microlens textured substrates to experimentally evaluate the efficacy of microlens structure and corroborate the simulation results Experimental Procedure Substrate Preparation D evices were fabricated on 1 x1 inch 2 glass substrates (Corning 2497). As received glass substrates were cleaned in acetone methanol, deionized water in a sonicator and 30 min in piranha solution followed by deionized water rinse and nitrogen blow drying For creating texture s micr o pyramid pattern s w ere fabricated on the substrate s using standard lithography with S1813 photoresist to define pattern on the Si etch mask The cleaned substrates were first deposited with amorphous silicon after 30 s Ar plasma pre clean in the Kurt J Le sker CMS 18 Multi Source RF sputter tool. A ~3 000 thick a Si film was deposited at room temperature at the power setting of 200 W and 5 mTorr chamber pressure using argon as the sputter gas. Silicon target was pre sputtered for 5 min to remove any oxide or impurity layer on the target surface. The substrates with a Si were annealed at 400C for 6.5 hours The annealed substrate s w ere primed with HMDS vapors in a YES oven. Immediately after priming, the

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130 substrates were patterned with ~ 1.2 m thic k S1813 photoresist spun on the substrate The photoresist was expos ed to 365 nm Hg i line UV radiation using a Karl Suss MA6 Aligner to create the pattern The photoresist was soft baked at 1 12 C for 2 minutes prior to UV exposure and was post bake d at 12 5 C for 2 minutes on CEE hotplate Standard AZ300 MIF developing solution was used for 1 min to develop the pattern. Patterned photoresist (PR) coated substrate s were descummed in a reactive ion etcher (RIE/ICP, Unaxis SLR) under oxygen plasma for 2 0 s. The RIE conditions used for descumming process were 100/600 W RF/ICP power with 20 sccm O 2 flow at chamber pressure of 5 mTorr. To transfer pattern from PR to the a Si layer SF 6 chemistry was used in a STS deep reactive ion etcher (DRIE) with 800 W R F power and 12 W DC platen power P yramid structures on glass substrates were made by etching glass with a 6:1 buffered oxide etchant (BOE) solution or 15% HCl/BOE solution The patterned substrate with micro pyramidal array w ere polished using standard si lica slurry on a soft politex pad. Details of the CMP process are described elsewhere [173] Post polish cleaning was carried out by buffing with DI water followed by acetone, methanol and DI water rinse in an ultrasonicat or Immediately before deposition of transparent conducting oxide layer of Al doped zinc oxide, a 30 minute p iranha clean with 2:1 sulfuric acid and hydrogen peroxide was carried out followed by rinse with DI water and nitrogen blow drying The Al doped zi nc oxide was deposited on the cleaned substrate using a RF sputter tool, details of which are described in next section. TCO Fabrication Al doped ZnO (AZO) films were deposited using Kurt J Lesker CMS 18 Multi Source RF sputter tool For the study, all sub strate s were coated with 1500 thick film of Al:ZnO on the patterned side of the substrate The RF power used was 1 5 0 W

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131 whereas working pressure of 10 mTorr with target to substrate distance 60 mm and substrate temperature of 255 C was found to be optimum with reasonably good conductivity of the AZO film with high optical transmission The Al doped ZnO depositions were carried out in pure argon atmosphere The as deposited AZO films were polished using silica slurry with 0.5 wt% secondary alkyl sulfate (SA S) surfactant at pH ~3 The CMP down pressure was kept constant at ~3.5 psi with linear velocity reduced to ~ 4. 8 m/min and slurry flow rate fixed to 70 ml/min. Post CMP cleaning was performed using pH 10.6 (adjusted using KOH) deionized (DI) water on a buf fing pad for 1.5 minutes followed by ultrasonication in acetone, methanol and DI water. Resistivity of the deposited AZO thin films w as characterized using Pro4 four probe resistivity system ( Lucas Signatone Corp. ) with Keithley 2601 source meter O ptical transmission was measured using Ocean optics HR4000 spectrophotometer. S urface morphology of the deposited films was investigated by Fie ld Emission Scanning Electron Microscope (FESEM, JEOL JSM6330F) Device Fabrication A 1500 thick t ris(8 hydroxyquinoli nato)aluminium (Alq 3 ) layer was evaporated on to the substrate s on the patterned side after sonicating samples in acetone and isopropanol (IPA) for 15 minutes each To form the backside metal reflector 600 aluminum metal was deposited on top of Alq 3 lay er. The Alq 3 and Al layers were deposited through a shadow mask to define the active layer area over patterned portion of the substrate. Three different sets of device s were fabricated viz ., Alq 3 and Al layer deposited on (a) patterned glass, (b) patterned glass with AZO layer and (c) patterned glass with polished AZO layer with un patterned glass used as a reference with similar layer stack

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132 Photoluminescence measurements were carried out in Perkin Elmer LS 55 fluorescence spectrometer using a front surfac e solid sample measurement assembly. The measurements were made in reflective mode due to geometry of the sample assembly and presence of Al back reflector. Ray tracing Simulations Parameters Textured substrates with microlens arrays were simulated for bas e diameter 6, 10 and 14 m for h/d ratio varied between 0.1 0.5 The device architect ure simulated was kept close to the experimentally fabricated device with 0.15 m thick active light emitting layer (Alq 3 ) having refractive index, n active = 1.7 over the TCO layer. The ZnO as the TCO layer with thickness, t = 0.2 m n ZnO = 1.9 and optical density (O.D.) = 0.09691 was placed on a glass substrate (t ~ 8.4 m n = 1.46) to simulate the complete stack Due to the software limitations instead of having array of texture elements arranged in hexagonal pattern, the simulations were simplified by considering only one texture element with square field area equal to that of the hexagonal unit cell area in actual texture All vertical sides of each layer were made 10 0% reflecting whereas bottom surface of Alq 3 layer was made 80% reflecting to account for absorption losses in the Alq 3 layer and metal back contact. The emission of the active layer was assumed to be isotropic with 550 nm emission wavelength for all the s imulations. Since, the software strictly operates in classical ray regime; the simulation results are independent of the wavelength of emission light. The ray tracing simulations were performed using 100,000 rays for measuring the out coupl ed photons using large plane detectors near front and rear side of the device. The angular distribution of the out coupled rays was measured using a far field detector centered to the device with 1 million rays used for the

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133 simulation. Both planar and far field detector m easured total fractional power of the out coupled rays. Results and Discussion The electrical and optical properties of deposited Al doped zinc oxide films were characterized on a plain glass substrate without pattern s Four probe measurements were made t o characterize the electrical resistivity of the deposited films. The as deposited films had average sheet resist ance of 5 k Figure 8 1 is the optical transmission plot for plain glass reference and as deposited AZO film. The film shows transmission as high as ~93% for a film thickness of ~1450 as compared to the plain glass substrate. Figure 8 2 depicts SEM micrographs of the fabricated microlens after polishing of Al:ZnO film The SEM images illustrate that only microlens tops are polished whereas on rest of the area zinc oxide grains are still discernable, illustrating no polishing. Due to close packing of the featur es and high aspect ratio the field area is relatively inaccessible to the pad asperities to cause any polishing. Further, the feature height, diameter and pad mechanical properties affect the ability to polish the surface with closely packed features. Thi s is evident from the extent of polished area along the slope of the microlens, 6 m has more polished area followed by 10 and 14 m base diameter microlens. The SEM micrographs also elucidate the conformal nature of deposited A ZO films. It should be noted here that because the film thicknesses (TCO and active layer) are much less than the height, the diameter and periodicity of the texture. The deposited subsequent films are conformal which results in corrugated geometry of the device. Photoluminescence me asurements were performed to investigate the enhancement in light out coupling efficiency of the microlens texture. Figure 8 3 plots

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134 relative PL intensity with respect to the un patterned reference for 6, 10 and 14 m diameter microlens texture on (a) patterned glass substrate, (b) patterned glass substrate with AZO film and (c) patterned glass substrate with polished AZO film. The obtained broa d PL peak centered at 520 nm matches closely with the literature reported peak position for Alq 3 emission [181] The UV excitation wavelength used was 325 nm. Figure 8 3 (a) elucidates the enhancement in PL output o btained using the structures of different diameters as compared to the reference non patterned glass substrate. The results demonstrate increase in enhancement with increasing diameter of the microlens with maximum enhancement of ~ 4 fold for 14 m microlens texture. Figure 8 3 (b) and (c) are relative PL intensities of the substrates with AZO and polished AZO The relative PL intensit ies demonstrate a maximum enhancement of 2.1 and 2.6 X for as deposited Al:ZnO and polished Al:ZnO samples, respect ively. The PL spectra for substrate with Al doped zinc oxide layer show broadening of Alq 3 peak and additional shoulder peaks between 450 460 and 480 490 nm wavelength. The broadening and appearance of the peak shoulders are related to the weak deep level PL emission peaks of AZO film which occur in the range of 510 600 nm [182, 183] Further, it should be noted here that the measured PL intensity is the cumulative of enhancement due to micro texture and the increase d surface area measured at an angle of 30 with respect to surface normal of the substrate. Ray tracing simulations were performed to understand the origin of improvement in the light extraction due to corrugated device structure. Figure 8 4 (a) plots the simulated enhancement in light extraction efficiency using microlens arrays of different diameters. The plotted curves take in to account the fraction al increase in the surface

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135 area relative to planar device to obtain the total combined enhancement. For co mparison, simulation results for n active = 2.5 have also been presented here (see Figure 8 4 (b)). Figure 8 4 (c) depicts the fractional power output for n active = 1.7 and 2.5 at h/d ratio of 0.5. The ray tracing simulation results presented in chapter 6 s how that the smaller diameter is slightly efficient and a maxim a is observed in enhancement versus h/d plot (see Figure 6 8). However, when the increase in surface area is taken in to account, the increase in number of generated photons compensates for the decrease in the extraction efficiency of structure with larger diameter and greater h/d ratio. The simulations results for high refractive index active layer ( n active = 2.5) demonstrate more enhancement as more light is trapped in the active layer with hi gh refractive index which is extracted from the active and TCO layer by the microlens texture. The far field angular spreads of the extracted photons ( depicted by Figure 8 5 ) reiterate the similar trend observed with total out coupled light when they are m ultiplied by the fractional increase in area. The angular distribution curves show increase in received power at all angles proportionally. However, there is preferential enhancement in forward direction as it was obse rved in chapter 6 (Figure 6 10). T he i ntroduction of microlens features at glass/TCO interface actually leads to preferential enhancement in forward direction of the extracted photons from the TCO layer. Th is enhancement leads to observed improvement in out coupled photons. The resultant angul ar spread of the finally extracted photons from substrate is superposition of the non Lambertian profile of glass/TCO and Lambertian spread of non textured glass/air interface. The angular distributions of extracted photons from simulated stack with n activ e = 2.5 demonstrate similar results but the spread is expected ly compact as compared to that for n active = 1.7.

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136 The reason for this behavior is high refractive index of the active layer which opens another guided mode in active layer with escape cone angle ~ 50 at TCO/active layer interface. Based on the height calculated from AFM scans on the textured surface (see Figure 7 5), the fabricated features have h/d ratio approximately 0.21, 0.17 and 0.21 for 6, 10 and 14 m, respectively. The enhancement values obtained from the ray tracing simulations for h/d ratio of 0.2 and measured at detection angle of 30 are close to the observed experimental result s The slight deviation from the simulation results can originate from the approximation of absorption withi n the Alq 3 and the reflectivity of the Al layer in the simulation model or from interference effect between broad closely occurring emission peaks of Alq 3 and AZO around 520 nm and the thickness variation in the film thicknesses which can alter the measure d absolute intensity. In addition, the simulation software used for this study simulates ray tracing only in classical ray optics regime. However, the film thicknesses of TCO, active layer and metal contact are much smaller than emission wavelength which i nvokes addition al q uantum mechanical phenomena. It has been shown in literature that changes in refractive index at sub wavelength scales demonstrate micro cavity effects which are known to cause spatial and spectral changes in the emitted light [184] Conclusion The microlens structures with h/d ratio ~ 0.2 were fabricated with 1500 thick Al doped zinc oxide thin film. The SEM micrographs elucidate that the polishing of AZO film was limited to tip of the features due to compactness and relatively high aspect ratio of the features. The enhancement in light extraction efficiency was experimentally determined by measuring the PL intensity of evaporated Alq 3 layer on top of the AZO

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137 layer. The experimentally observed extraction efficiency from the PL results show maximum enhancement as high as 2.6 folds for the polished Al doped zinc oxide film. The experimental values are close to the simulated ray tracing results. The res ults presented here illustrate that the introduction of structures at glass/TCO interface leads to more extraction of photons from active layer and TCO layer. The cumulative effect of improvement in extraction efficiency and increased surface area results in the observed enhancement of more than 1.5X. The angular spread obtained from simulation follows the Lambertian profile of a rectilinear geom etry providing further room for enhancement of the extraction efficiency by placing textures on the glass/air int erface

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138 Figure 8 1 Optical transmission of plain glass and Al doped ZnO film. Figure 8 2 SEM micrograph illustrating the polished Al:ZnO on the microlens patt ern of (a) (b) (c) (a)

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139 Figure 8 3 Normalized photoluminescence intensity of Alq 3 with Al back reflector for (a) plain microlens patterned glass, (b) as deposited Al:ZnO and (c) polished Al:ZnO on microlens textured substrate.

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140 Figure 8 4 Enhancement in light out coupling in the corrugated device with (a) n activ e = 1.7 and (b) n active = 2.5. The simulated fractional power s for different diameter of the microlens and active layer refractive indices at h/d = 0.5 are represented by (c).

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141 Figure 8 5 Simulated a ngular distribution of the extracted mode for device with (a) n active = 1.7 and (b) n active = 2.5

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142 CHAPTER 9 CONCLUSIONS The increasing energy needs of the world have led to a quest for alternative energy efficient lighting sources. This has led to treme ndous research efforts in SSL devices such as thin film EL devices, OLEDs and LEDs. One of the main issues limiting the performance of all these devices is the light extraction efficiency limit of currently used architectures. Most of these devices are now achieving close to 100% internal quantum efficiencies, however, the external quantum efficiencies are usually limited to 20% or lower. Hence, there is an urgent need to push the limit of extraction efficiency higher in order to tap all the photons generat ed in the device. Hence, this work has focused on enhancing the light extraction efficiency in light emitting devices. In order to enhance the extraction efficiency surface topography of transparent conducting oxide layer has been controlled on two scales: micro level texturing and nano level roughness. The introduction of micro scale texture at TCO/active layer interface enhances the light output by extracting the TCO and active guided modes. By controlling the nano scale roughness the detrimental effects of surface roughness, large particle inclusions and pinholes are eliminated. For controlling the surface roughness of ZnO (the TCO considered in this work), a CMP process was developed, the results and findings of which are summarized below. For enhancing the light extraction efficiency periodic micro scale textures (microlens, micropyramid and cylinder) applied at di fferent interfaces were investigated using ray tracing simulations To corroborate the ray simulation results, photoluminescence study was conducted on the fabricated substrate with periodic micro scale arrays. The

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143 findings of these ray tracing simulations and photoluminescence study confirmed the ligh t extraction enhancement with these structures as summarized below. Chemical Mechanical Polishing of ZnO As part of the first goal of this research a zinc oxide polishing process was developed in order to control the surface roughness of the TCO layer Hig hly smooth zinc oxide surface with RMS roughness es between 4 6 were obtained with developed CMP process Removal rates as high as 670 /min were achieved at slurry pH of ~3. Acidic and basic pH slurry polished zinc oxide films showed higher transmission (91% equation was applied to explain the removal rate variation with pressure and platen velocity. The modified Preston equation was found to be consistent with the experimen tal results and accounts for the non zero intercepts. Low polishing rates were observed at higher pH (> 7) whereas at low pH (~3) an order of magnitude increase in the removal rates was observed without any detrimental effects on the surface roughness. The zinc oxide CMP process at different slurry pH values was explained by the pH dependence of the zinc oxide dissolution in aqueous solution and particle surface interactions. X ray reflectivity simulation results concurred well with the RMS roughness values for the ZnO films deduced from the AFM images. Effect of anionic and non ionic surfactant on the zinc oxide CMP was also investigated Anionic surfactant, SDS in this case was found to lower the polishing rates and increase the RMS roughness with increasi ng concentration. Addition of 5 mM SDS was found to give the best surface finish with RMS roughness ~3 However, r everse trends were obtained with Triton X 100 (non ionic) surfactant due to reduced repulsive forces between slurry particles and CMP pad be cause of the screening of surface

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144 charges by the surfactant molecules adso rbed on the pad and silica particles The removal rates increased to as high as 900 /min with addition of KCl salt but at the cost of surface roughness. The lowest RMS roughness of ZnO polycrystalline films after polishing was found to limit at or greater than 3 This is related to the film morphology (grain size) dependence of the CMP process for zinc oxide and the change s in surface particle and particle pad interactions by addit ion of different slurry additives. Optical transmission of zinc oxide films deposited on glass substrate exhibited an average transmission between 92 93% for various slurry compositions. The effects of various CMP process variables and slurry chemistry and additives were delineated in this work which resulted in understanding of CMP process for zinc oxide and achieving highly smooth zinc oxide surface after chemical mechanical polishing. Light Extraction Light extraction efficienc ies of various external lig ht out couplers such as pyramid, microlens and cylinder arrays w ere analyzed. The use of such external light out couplers in conjunction with back side mirror results in enhancement of light due to extraction of higher angle photons and change in direction ality of reflected photons outside the escape cone. In case of pyramids, the intensity of extracted light increased with contact angle less than ~55 after which the structure demonstrates slight decrease due to re entry of the extracted photons back into device. The ray simulations elucidated similar behavior of microlens array. Microlens arrays demonstrate higher light extraction efficiency and circular symmetry in angular distribution of the extracted photons. In contrast, efficacy of the cylinder arrays is limited to the lower h/d ratio s and light extraction efficiency becomes independent of increase in the feature diameter after M icrolens and pyramid arrays were found to be more efficient structures as

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145 compared cylindrical array s The trends remain independent of refractive index of the active layer as expected because these features only affect the substrat e mode In addition the enhancement in light out coupling originates from the extraction of photons outside the escape cone by changing their directionality upon multiple reflections Ray dynamics slightly change when the microlens structure is applied at the substrate/TCO interface Application of microlens texture at this interface changes the geometry of TCO and active layer to a corrugated geometry. The deviation from parallelepiped geometry leads to increased extraction of TCO/active mode in case with n active = 1.7 and higher extraction of both TCO and active layer mode in case of n active = 2.5. The enhancement increases steeply up to 0.3 and 0.4 h/d ratio for n active = 1.7 and n active = 2.5, respectively. Thereafter, decrease in extraction efficiency w as observed in simulated results. The increase in extraction of normal angle photons from TCO layer up to 0.3 and 0.4 h/d ratio for n active = 1.7 and n active = 2.5, respectively was found to be responsible for highest enhancement at these h/d ratios when f raction of photons exiting TCO layer w ere measured alone. However, at higher h/d ratios the extraction of higher angle photons increased but due to Lambertian profile of extracted light at glass/air interface t he combined effect of TCO extracted and glass extracted mode profiles resulted in the observed decrease in the overall enhancement. Corrugated Device For the first time, micro scale textures applied to glass/TCO interface to form corrugated device structures were analyzed and demonstrated experimenta lly. The microlens structures with h/d ratio ~ 0.2 were fabricated with 1500 thick Al doped zinc oxide thin film. The polishing of the AZO films was limited to tip of the features due to compactness and relatively high aspect ratio of the features. The e nhancement in light

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146 extraction efficiency was experimentally determined by measuring the PL intensity of evaporated Alq 3 layer on top of the AZO layer. The experimentally observed extraction efficiency from the PL results show maximum enhancement as high a s 2.6 folds for polished Al doped zinc oxide film. The experimental values were close to the simulated ray tracing results. The results presented here illustrate that the introduction of structures at glass/TCO interface leads to more extraction of photons from active layer and TCO layer. The cumulative effect of extraction efficiency improvement and increased surface area leads to observed enhancement of more than 1.5X. The angular spread obtained from simulation follows the La mbertian profile of a rectili near geometry providing further room for enhancement of extraction efficiency by applying textures at glass/TCO interface to reduce substrate mode

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158 BIOGRAPHICAL SKETCH Sushant Gupta was born in 198 4 in Delhi India. He completed his high school in 2001 from Delhi. He graduated from the Delhi College of Engineering Delhi, India in 2005 polymer science and chemical technology. He interned at two prestigious national laboratori es in India, Bhabha Atomic Research Centre, Mumbai and National Physical Laboratory, Delhi. The research experience at these national laborator ies motivated him to further his career by pursuing graduate studies. science and engineering from University of Florida in 2008 while continuing with his Doctorate at same place. During graduate studies, he worked on several projects during his doctorate studies at UF, including synthesis of polymeric thin films for Fuel cell applications, fabrication of PTFE thin films using pulse electron deposition technique and organic LED material thin film fabrication using pulse laser deposition. In 2008, he interned at Maxim Integrated products, San Jose, CA where he work ed in chemical mechanical polishing (CMP) area His skills and in depth knowledge in material science and CMP were greatly appreciated and he was given the task to manage the project independently. H is recommendations and suggestions on the projects were w ell received. The internship opportunity for him was a great learning experience both professionally and personally. His doctoral research was in the area of light out coupling from solid state lighting devices. H e developed a CMP process for zinc oxide p olishing and fabricated textured ZnO films for improving the light extraction efficiency of light emitting devices. He hopes to join a semiconductor manufacturing company to demonstrate his acquired skills and knowledge He feels learning is a continuous process and the journey has just begun.