Production and nature of highly luminescent spark-processed porous oxides of silicon and other elements


Material Information

Production and nature of highly luminescent spark-processed porous oxides of silicon and other elements
Physical Description:
ix, 438 leaves : ill. ; 29 cm.
Stora, Michael Emery, 1970-
Publication Date:


Subjects / Keywords:
Materials Science and Engineering thesis, Ph.D   ( lcsh )
Dissertations, Academic -- Materials Science and Engineering -- UF   ( lcsh )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )


Thesis (Ph.D.)--University of Florida, 2000.
Includes bibliographical references (leaves 429-437).
General Note:
General Note:
Statement of Responsibility:
by Michael Emery Stora.

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University of Florida
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All applicable rights reserved by the source institution and holding location.
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aleph - 025873311
oclc - 47103113
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Table of Contents
    Title Page
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    Table of Contents
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    Chapter 1. Introduction
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    Chapter 2. Conventions and organization
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    Chapter 3. Background and literature review
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    Chapter 4. Materials, methods, and equipment
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    Chapter 5. Results: Pro0cessing parameters of spark-processed silicon
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    Chapter 6. Results: Microscopy and chemical spectroscopies of spark-processed silicon
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    Chapter 7. Results: Other spark-processed elements
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    Chapter 8. Results: Other luminescent spectroscopies
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    Chapter 9. Results: Miscellaneous
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    Chapter 10. Further discussion
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    Chapter 11. Summary and conclusions
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    Chapter 12. Future work
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    Appendix A. Electroluminescence and photoconductivity
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    Appendix B. Calibration of the PL system
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    Appendix C. Computer programs
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    Biographical sketch
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Full Text







Copyright 2000


Michael Emery Stora

Dedicated to my parents, Michelle Monique and Emery Lester Stora


First and foremost, I would like to express my appreciation and heartfelt

thanks to Dr. Hummel who has served as my research advisor, graduate advisor,

supervisor, supervisory committee chairman, and mentor. His enthusiasm, support,

interest, and humor have been invaluable to me. I also wish to thank Dr. Ludwig for

his friendship, creativity, and technical contributions.

I thank Drs. Holloway and DeHoff for both serving on my supervisory

committee and for their influence as professors. It was a pleasure to learn from both

of them as their classes were among the most interesting and productive I have taken.

I graciously thank Drs. Sigmund and Srivastava for their service on my

supervisory committee.

I am grateful to my current and former coworkers including Nigel Shepherd,

William "Grif" Wise, Jeliazko Polihronov, David Burton, Damien "Cross" Reardon,

Jon Hack, Dr. Sung-Sik Chang. and Dr. David Malone for both their technical

contributions and valued friendship. I also wish to thank Dr. Philip Rack. Dr.

Jonathan Gorrell, Jay Lewis, and Loren Reith, for their friendship and constant

willingness to help.

Dr. Holloway's secretary, Ludie, deserves special recognition for being the de

facto "secretary" for all of us graduate students.

I am grateful to the staff and graduate students of the University of

Rochester's Center for Photo-Induced Charge Transfer, particularly Steve Atherton

and Chris Collison, for the equipment-time and training they so graciously provided,

Dr. Robert Walko at Sandia National Laboratories and Dr. Sey-Shing Sun at Planar

Systems for their help with constructing electroluminescent devices, and Dr. Regina

Miiller-Mach at Agilent Technology for help with photoluminescence excitation

measurements. I also wish to thank Drs. Gerd MUiller and B. C. Blasse for their

interest and helpful advice and Dr. Herbert Ruefer at Wacker-Chemitronic for

supplying the large quantities of silicon wafers used in this study.

My thanks go to all my friends in the UF Kodenkan Jiu-Jitsu Club and UF

Judo Club who are too numerous to list. I owe a great deal to Professors Alex

Limbaugh, Donald Cox, and Bill Beach, and Senseis Margaret Limbaugh, Kevin

Book (deceased), and John Nelson whose lessons of hard work, discipline, and

dedication were as important as any academic lesson.

Thanks to my college roommates and long-time friends Dr. Dmitry Golovko

and "soon-to-be Dr." David Harrison for the friendship and encouragement they have

given me over the years.

Last, but certainly not least, I wish to thank my parents, to whom this study is

dedicated, and my fiancee Heather Rose for the undying love and support they have

given me.



ACKNOWLEDGEMENTS ............................ ................. iv

A BSTRA CT .............................. ......................... viii


1 INTRODUCTION ............................................... 1

2 CONVENTIONS AND ORGANIZATION .......................... 5

U nits of M measure .................... ............................ 5
Definitions and Rules ........................................... 10
Organization of Experimental Results .............................. 13


Review of Spark-Processed Silicon Literature ........................ 14
Other Literature on Spark-Processing and Sparks in General............ 57
Review of Quantum Size-Effect Literature ......................... 61
Review of Defects in Silica and Silicon Oxynitrides ................... 71
Review of an Important Transient Defect in Silica .................... 75
General Review of Luminescence Processes ........................ 93

4 MATERIALS, METHODS, AND EQUIPMENT .................... 107

SILICO N .......................................... ......... 119

Electrical, Temporal, and Physical Processing Parameters .............. 119
Chemical Processing Parameters ................................. 180

OF SPARK-PROCESSED SILICON ............................ 209

Scanning Electron Microscopy (SEM) ............................. 209
Fourier Transform Infrared Spectroscopy (FTIR).................... 215
Secondary-Ion Mass Spectroscopy (SIMS)........................ 220


Preliminary Exploration ........................................ 235
Additional Work with Selected Elements .......................... 258
Chemical Processing Parameters ................................. 265


Variable Excitation Power ...................................... 300
High-Temperature Photoluminescent Spectroscopy ...................306
Photoluminescent Excitation Spectroscopy (PLE).................... 326
Time-Resolved Photoluminescent Spectroscopy (TRPL).............. 336

9 RESULTS: MISCELLANEOUS ................................ 367

Useful Results ................................................ 367
Interesting Results ............................................. 371
Uninteresting Results .......................................... 372

10 FURTHER DISCUSSION ...................................... 374

Final Word on Quantum Dots .................................... 374
Integrating Results from Multiple Chapters ........................ 376

11 SUMMARY AND CONCLUSIONS ............................. 389

12 FUTURE WORK ............................................. 393



B CALIBRATION OF THE PL SYSTEM .......................... 402

C COMPUTER PROGRAMS ..................................... 416

REFERENCES ..................................................... 429

BIOGRAPHICAL SKETCH .......................................... 438

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy



Michael Emery Stora

December, 2000

Chairman: Professor Rolf E. Hummel
Major Department: Materials Science and Engineering

Spark-processed silicon (sp-Si) is a novel nano-porous material with unique

optical and magnetic properties. Sp-Si exhibits two broad, intense, room-temperature

photoluminescence (PL) bands, which are located near 370 nm and 530 nm when

excited by 325 nm UV light. These are known as the UV/blue band and the green

band, respectively. In addition, a number of other elements have been show to

exhibit significant PL when spark-processed.

Several PL spectroscopic methods were used to characterize sp-Si and other

spark-processed materials. These included continuous-wave PL (CWPL), time-

resolved PL (TRPL), PL excitation spectroscopy (PLE), and temperature-dependent

PL measurements. In addition scanning electron microscopy (SEM), Fourier

transform infrared spectroscopy (FTIR), and secondary-ion mass spectroscopy

(SIMS) were utilized.

The influences of the chemical, physical, electrical, and temporal processing

parameters on the PL of sp-Si and other spark-processed elements were extensively

explored and characterized. Both PL bands of sp-Si as well as similar bands in sp-

Ge, sp-Al, and sp-Ta were found to be strongly dependent on the presence of nitrogen

in the processing ambient. The PL of sp-Si and several other spark-processed

materials was found to exhibit extremely fast sub-10 ps lifetimes. Longer-lived

components attributed to trapping and evidence of competition between the two PL

bands of sp-Si indicate charge transfer between separate absorption and emission


A model for the PL mechanisms of sp-Si is proposed. This model explains

the observed properties of both PL bands of sp-Si and may be extended to other

spark-processed materials. Absorption of UV light is attributed to a stable version of

a normally metastable defect found in silica, and many other materials, known as the

self-trapped exciton (STE). The rapid melting and solidification during spark-

processing is believed to "quench-in" this defect. PL emission is the result of charge

transfer of carriers from STEs to nitrogen-related deep defect states within the silica

band-gap. The UV/blue band is attributed to defects containing nitrogen and the

green band is attributed to more complicated defects containing two or more nitrogen

atoms, similar in nature to the N-N nearest-neighbor center in GaP.


Silicon has dominated the semiconductor industry for more than half and

century. While other materials are often used for high power switching and

rectification, the role of silicon in the microelectronics industry is of paramount

importance. The vast majority of integrated circuit technology is based on silicon

[Pea88]. Silicon is economical (costing about a tenth as much as gallium arsenide, its

nearest competitor) and easy to manufacture. The technology to dope silicon with a

number of donor and acceptor elements is mature and well behaved. The most

significant advantage of silicon is that it has a stable, adherent, and easily grown

oxide that is epitaxial and self-passivating (especially for the (100) crystal

orientation) [May90]. This oxide may be selectively patterned and etched by

hydroflouric (HF) acid, which crystalline silicon resists. Finally, silicon is non-toxic

and silicon production and processing has a minimal impact on the environment.

This is not true of gallium arsenide. These technical advantages, combined with the

maturity of, and investment in, silicon-based technology make it likely that silicon

will remain the dominant semiconductor material for the foreseeable future.

Despite its advantages, silicon has major limitations. It is both a low band-

gap and indirect band-gap material. It is limited to very inefficient optical emission

in the near-infrared (IR) region of the electromagnetic spectrum. It is poorly suited to

both electro-optical communication and display technology. For these reasons,

gallium arsenide and other III-V (or II-VI) semiconductors and their alloys are

typically used in applications where light emission is necessary, while other non-

semiconductor technologies dominate the display market. Photonics (the integration

of photo-optics and electronics) promises to help overcome basic speed and

bandwidth limitations of electronic communications, not only over long distances, as

common today, but on the inter-chip and intra-chip levels as well. This will only

become economically and technically feasible when a phosphor materials technology

is developed that is completely compatible with silicon-based microelectronics. For

these reasons, the development of silicon-based phosphors has generated much

excitement for a number of years.

In order to be successful, such a material must have a higher effective band-

gap and a much higher radiative efficiency than silicon. The second issue can be

addressed by doping silicon with isoelectronic centers or activators, but this actually

decreases the energy of emission. For a time in the nineties, it appeared that porous

silicon (PS), offered the solution to both issues. PS was discovered by Uhlir lUhl56]

and its efficient room temperature photoluminescence (PL) was reported by Canham

[Can901. While the mechanisms for the luminescence of PS are still controversial,

practical photonic devices have never materialized and the excitement has largely


A new material, known as spark-processed silicon (sp-Si) was discovered by

Hummel and Chang in 1992 IHum92|. It is a highly luminescent porous oxide

produced by an electrical discharge between an anode and a silicon cathode in air or a

mixture of nitrogen and oxygen. While sp-Si and PS have some superficial

similarities, they exhibit different emission wavelengths, emission time constants,

stability under annealing and ultraviolet (UV) exposure, microstructure. and other

important properties [Lud96a, Cha94]. Sp-Si and PS are very different materials with

unique properties which likely result from unique mechanisms.

Despite its efficient PL, useful electroluminescence (EL) from sp-Si has

proven elusive. While strong PL is no guarantee of useful EL (it is not even a

prerequisite), a more detailed understanding of the nature and mechanisms of the PL

of sp-Si may allow the design of more intelligent approaches to generate EL in sp-Si

or a future material derived from sp-Si.

The author has undertaken this study to better understand the nature of sp-Si

and its luminescent properties. This endeavor consists of two main thrusts:

correlating the properties of sp-Si to the conditions under which it was produced and

attempting to glean an understanding of the mechanisms responsible for the

luminescence of sp-Si. The former involves the development of new precision

production techniques to accurately quantify and verify the previously reported

properties of sp-Si as well as to conduct new experiments by taking advantage of the

new degrees of freedom offered by these techniques. The latter involves attempting

to identify the species and mechanisms responsible for the PL of sp-Si by conducting

various experiments to find evidence that may suggest, support, or exclude proposed


In an attempt to further understand the properties of sp-Si. the author

conducted spark-processing on a number of other elements besides silicon. It was

discovered that strong, stable, room temperature PL is not unique to silicon. In fact,

every elemental semiconductor and semi-metal (with the exception of selenium,

which is too flammable to spark-process), as well as a number of metals, exhibit PL

after spark-processing. In addition, the PL of several of these spark-processed

materials was found to be dependent on the presence of nitrogen in the processing

ambient, indicating that a common mechanism may be involved. Thus. spark-


processing of materials other than silicon evolved from a limited exploration to

become an integral part of this study.

The two goals of this endeavor have been met. The processing parameters of

sp-Si and their influence on its properties have been extensively explored. Not only

are the absorption and emission mechanisms of sp-Si well characterized, but the

author is also able to present a comprehensive theory for the luminescence of several

spark-processed materials.


Units of Measure

Physical Units of Measure

The author uses units of measurement pragmatically. Preference is given to

the International System of Units (SI), or "Metric System," when there is no obvious

alternative. SI-derived units such as the cgs (centimeter-grams-seconds) System are

used when these are more appropriate to the scale of the measurement than SI units.

As all the experiments reported in this study were performed in the United States, the

American version of the Imperial, or "English" system of units cannot be ignored. In

most cases, equipment and supplies were manufactured to US specifications. It is

difficult for the reader to have such measurements arbitrarily converted to SI units

only for the sake of conformity, as some authors do. Because SI and SI-derived units

are the standard for scientific literature, whenever an Imperial unit is used, a Metric

equivalent will follow in parentheses. If this is a scientifically important

measurement, significant digits will be stated and preserved in the conversion. If this

is a nominal measurement, than the metric conversion will be nominal as well. An

example of this is the size of semiconductor wafers, which are usually manufactured

in even-inch diameters. For example: "the 4" (10 cm) silicon wafer was . While

the wafer may very well have been manufactured to very high tolerances (4.00", for

example), 4" is a standard size and terminology used in industry. The metric

conversion is equally nominal (10 cm, not 10.16 cm).

Units of pressure deserve some special consideration. Most American

literature uses pounds per square inch (psi) for pressure above atmosphere and torr

(mtorr, Rtorr) for vacuum. The torr is a SI-derived unit defined as the pressure

necessary to support a 1 mm column of mercury against gravity. However, torr is not

a very common unit in international literature. It is also not particularly practical

(there are 760 torr in an atmosphere (atm)). The SI unit of pressure is the Pascal (Pa),

but this is not particularly practical for vacuum, and not very common in the

literature. This study will use a cgs unit, the bar (mbar. [tbar) for vacuum as well as

pressure above atmosphere, which is very common in international literature. The

[ibar is a dyne cm-2. It is quite convenient and intuitive, as 1 bar is very close to 1

atmosphere (0.9869 atm--within about 1%) and 1 mbar is very close to 3/4 of a torr

(0.7453 torr--within less than 1%). Atmosphere, or atm, in this study always refers to

the SI standard atmosphere and not the technical atmosphere. The technical

atmosphere is defined as one kilogram-force (the weight of one kilogram mass on

earth) per square centimeter. It is essentially equal to the pressure exerted per 10

meter depth of fresh water. While it is a very convenient engineering unit, and

deserves much more acceptance in the opinion of the author, it is relatively unknown

and will not be used in order to avoid misunderstandings.

Unless otherwise stated, absolute vacuum serves as the common reference

point for all pressure measurements in this study, both vacuum and above

atmosphere. For example, 1.5 atmospheres means 0.5 atmospheres above

atmospheric pressure.

Optical Units of Measure

Units of optical intensity are quite simple but are the cause of much confusion.

The author uses the following rules.

If no units are given for an "intensity", the number represents uncorrected

instrumental units (typically counts for a photomultiplier or charge-plate detector).

Quantum units represent the number of light quanta (photons) emitted at a

particular wavelength per unit time. These differ from instrumental counts in that

they take into account a wavelength-dependent correction for the sensitivity of the


Radiometric units are units of optical power (watts, for example) at a

particular wavelength. These are related to quantum units by the wavelength

dependence of the energy of a photon (the amount of energy carried by a photon is

inversely proportional to its wavelength). This is rigorously defined as the radiant

flux (intensity integrated over a sphere), but the author will use the term intensity as it

is so often used in the literature. Many sources indicate that the use of watts for

radiant flux is to be deprecated, but this has not stopped their almost universal use.

The alternative is the clumsy ergs per second. There is often a reality gap between

the people who propose standards and those who must use them.

Photopic units are obtained from radiometric units by taking into account the

wavelength-dependent sensitivity of the vision of a "typical" (and gender-less) human

being. The term photometric is more commonly encountered in the literature, but

could refer to either the photopic vision of the retinal cones (which dominates at light

levels above I millilambert (mL)), or the scotopic vision of the retinal rods (which

dominates at low light levels and is more sensitive in the blue). The author prefers

the term photopic to prevent this ambiguity. These units indicate the apparent

brightness of a light source at a given wavelength. Photopic units are derived by

multiplying radiometric units by a correction curve known as "The International

Standard Eye", or more formally, The Relative Luminosity Function of the

Commission Internationale de L'Eclairage (CIE) [Ber76, Sze811. As the sensitivity of

the eye varies with viewing angle, this curve was defined for light entering the eye in

a 2 acceptance angle (other curves exist for larger angles). This curve peaks at 555

nm, which is close to the peak wavelength of sunlight, and resembles a slightly

asymmetrical Lorentzian curve with a greater half-width in the long-wavelength

direction, and a slight shoulder near 450 nm. It has finite values from 380 nm to 770

nm and is defined as zero outside of this range. The proper term for these units is

luminous flux, which is measured in lumens (1m). It is neither an Imperial nor an SI

unit, but was defined as the apparent brightness of 1/680"h of a watt of light at 555 nm.

Luminous flux density can be measured in footcandles (fc), lux (Ix), or phot (ph) in

the Imperial. SI, and cgs systems, respectively. Flux can also be measured per unit

solid angle and per unit solid angle per unit area, with and without factors of 7t, giving

rise to candelas (cd), stilbs (sb), nits (nt), footlamberts (fL), lamberts (L), and

apostilbs (asb). There is no equivalent set of radiometric units for these as people just

use "watts per..." If all this gets confusing, the reader should just keep in mind that

these units were contrived from psychometric measurements made on small groups of

volunteers in the early years of the 20' Century. There is really no underlying

physical meaning behind these units for the reader to understand.

It is common to see statements in the literature referring to intensity (power or

its photopic equivalent) at a particular wavelength. The author has essentially done

the same thing in the proceeding paragraphs. This is rather sloppy, because, except in

the case of a theoretically monochromatic light source, power is actually proportional

to the area under the curve of the intensity vs. photon energy plot. This is usually

referred to as the "integrated intensity" in the literature. In addition, most authors use

the area under the curve of the intensity vs. wavelength plot, which is not really the

same thing. Such imprecisions are common to every field of scientific study and are

generally not problematic if the reader is cognizant of them.

When conducting photoluminescence (PL) measurements, it is generally not

possible or even desirable to know exactly how the measured quantities correspond to

true quantum or radiometric units as long as it is known that they are related by an

unknown linear proportionality constant. This is often due to uncertainty over the

spatial distribution of emitted light as well as arbitrary sensitivity adjustments made

to the instruments (such as the adjustment of optical slits, detector alignment, and

geometry). In such cases, the units of measure are referred to as arbitrary units. If

intensity is measured at different wavelengths, it makes sense to refer to arbitrary

quantum units or arbitrary radiometric units, etc. However, if intensity is measured at

only one wavelength while some other parameter is varied, there is no difference

between these wavelength-dependent arbitrary units (one arbitrary constant is as good

as another). In such cases, only the term "arbitrary units" will be used.

As a final note on optical measurements, it should be pointed out that the peak

location of an optical band may be reported in nanometers (nm) or electron-volts

(eV). These are inversely proportional to each other with a proportionality constant

of 1240 nmeV.

Definitions and Rules

Stages of Apparatus Development

In the experimental chapters of this study, the reader will encounter the terms

Stage I, Stage II, and Stage III. These refer to the degree of sophistication of the

equipment and instrumentation as it evolved during the course of the experimental

work and will be defined in detail in Chapter 4. In summary: Stage I involved spark-

processing with a Tesla coil, DC power supply, or pulsed high voltage supply from a

video monitor. Stage II involved the use of a sophisticated controllable spark-current

supply that was relatively insensitive to changes in experimental parameters. In

addition, the spark current could be crudely measured. Stage III involved the ability

to precisely measure and control the spark current to completely separate it from

other processing variables. In addition, Stage III involved the implementation of

means to prevent nonlinearities in the PL system resulting from chromatic aberrations

and scattering/diffraction at small slit openings. This finally allowed good calibration

curves to be made for the PL system which can be applied to Stage III measurements

with great confidence and to Stage I/II measurements with a fair degree of


Rules for Figures

When multiple PL spectra or peak intensities are presented in one figure, it is

to be assumed that these were measured at identical conditions or that the author has

corrected for varying conditions so that the intensities may be compared with

meaning. When PL spectra are in different figures, it should be assumed that their

intensities are not directly comparable unless otherwise stated. The term

"normalized", when applied to a PL spectrum or other data, means that the largest

value of the dependant variable was set to 1 and that all other data points were scaled

accordingly. The term "self normalized", when applied to multiple spectra or other

data sets, means that each one was individually normalized to 1 with different scaling

factors. In the case of PL spectra, this allows peak shape, width, and location to be

compared independent of peak intensity.

Except in the background and literature review chapter (Chapter 3), when the

intensities, peak intensities, or peak wavelengths of more than one PL band of a

sample or samples are shown in a figure, the shortest peak-wavelength data are

denoted by squares and the longer peak-wavelength data are denoted by circles.

When applicable, the shortest peak-wavelength data points are connected by a solid

line and the longer peak-wavelength data points are connected by a dashed line.

When only one dependant variable is plotted, squares and solid lines are used. In the

case of the UV/blue and green bands (to be defined) of spark-processed silicon (sp-

Si), the UV/blue is represented by squares and solid lines and the green is represented

by circles and dashed lines.

Multiple PL spectra are often presented in figures by two-dimensional (2-D)

projections of three-dimensional (3-D) parameter spaces. In general, the X-axis

represents the processing variable in question, the Y-axis represents wavelength, and

the vertical Z-axis represents intensity. The orientation of the Z-axis is never changed

(increases vertically). However, in order that the reader obtain a clear view of the

data, the azimuthal angle and rotation about the Z-axis vary from figure to figure. In

many cases, this results in the unfortunate but necessary reversal of the direction of

increasing numerical values of one or both of the X and Y-axes. This is better than

having one set of data obscure another and is not a problem if the reader looks closely

at the figure. The projections are made with no perspective (infinite horizon, or

"cubic perspective") and the "back planes" are drawn to assist the eye of the reader.

These 3-D figures may represent two or more samples or measurements per

approximate value of a processing parameter. In these cases, the author is forced to

resort to the use of color so that the reader can clearly differentiate between them.

Highly contrasting solid colors are used. For the case of PL spectra, the spectrum

with the highest peak intensity (of the shortest wavelength band if more than one) is

assigned the color blue followed by red, purple, and green in that order. Even for the

microfilm-reader, who will see shades of gray, this arrangement is superior to the use

of black and white.

In addition, color is used in any graph where two or more dependent variable

data sets overlap in a figure or are otherwise hard to follow with the eye. In the case

of the PL peak intensities of the UV/blue and green bands of sp-Si plotted as a

function of a processing variable, the colors used are blue and green, respectively.

There are three possible standards for the scale and range of a particular

parameter's axis in a figure. The first is a fixed range for all figures, the second is to

scale the axis to the distribution of the data in each figure, and the third is to adjust

the range and scale of the axis to best show the trend of the data that the author

wishes to demonstrate (a smaller range to amplify a small trend, or a larger range to

hide scatter when there is no trend). The author feels that the third approach is

misleading. In the case of the location of the peak intensity of PL bands, the first

approach is impractical, leaving only the second method. Accordingly, in such

figures, the Y-axis is scaled to the distribution of the data.

Organization of Experimental Results

Due to the large number of experimental parameters varied in this study and

the sheer quantity of data to be analyzed, there are several experimental chapters,

each of which may contain its own introduction/motivations, experimental, results,

and discussion sections. It would be unwieldy to attempt this with one chapter for

each where the discussion would be too far removed from most of the corresponding

results (particularly figures). The experimental chapters are arranged in a logical

order and the discussion sections will be cumulative (each may refer to previous

chapters). This is important because the results build on each other and, in many

cases, experiments were designed based on previous results. Following these

experimental chapters is a further discussion chapter (Chapter 10) covering all the

previous chapters as well as the summary and conclusions chapter (Chapter 11) and a

future work chapter (Chapter 12).

Because this is a scientific study with the primary goal of understanding the

nature of the luminescence of spark-processed materials, the author chose to use

separation of variables (scientific method) rather than the Taguchi method. Instead of

a uniform-density parameter space, the author endeavored to find a set of optimal or

near-optimal values to serve as the origin of the parameter space and to fill it with

data points along one axis (parameter) at a time. If it is ever desirable to produce

spark-processed materials for commercial uses, the empty areas of the parameter

space, which represent simultaneous large deviations from the standard values of

multiple variables, may have to be explored.


Review of Spark-Processed Silicon Literature

Hummel and Chang first observed photoluminescence from spark-processed

silicon (sp-Si) in 1992 [Hum92J. The vast majority of literature on sp-Si and spark-

processing in general has been published by Hummel and associated researchers and

students, a group that includes the author. Work by other groups has concentrated on

areas not directly related to this dissertation. These include work on the

electroluminescence of sp-Si IYua95], doping of sp-Si with hydrocarbons [Rut94],

and doping of sp-Si with rare earth elements (Stj971.

Due to the relative novelty of the field and the fact that most of the work on

sp-Si has been done locally, the traditional concept of a "literature review" chapter is

of limited applicability. This section will cover the majority of the work done by

others while reserving most of that done by the author for later chapters. However,

both were conducted concurrently and neither category was done in an intellectual

vacuum. While most of the content to be presented herein is unpublished, work by

others has strongly influenced the author and the author has played a part in the

research to be presented in this section. The following sections will cover work by

other research groups on sp-Si, work in closely related fields, and other work

applicable to the understanding of this presentation.

The majority of work published by Hummel, Ludwig, et al., has been

periodically assembled in large review articles I Hum01. Lud96a. Lud971 and this

background draws heavily on these three publications. The literature will be

presented in the as published state. Much of this information is confirmed by the

author's research, but in some cases it must be modified or even contradicted. Some

of these instances will be alluded to in this background section. Such comments and

criticisms will be indented, single-spaced, and in a bold font to clearly differentiate

them from the review.

Introduction to Spark-Processed Silicon

Spark-processing substantially modifies the properties of materials. Spark-

processed silicon has been shown to exhibit intense room-temperature

photoluminescence (PL), electroluminescence (EL), and cathodoluminescence (CL)

in the visible part of the electromagnetic spectrum as well as in the near UV and near

IR. Sp-Si also exhibits some novel magnetic properties.

When excited with 325 nm light from a He-Cd laser, sp-Si exhibits either or

both of two intense PL peaks. These have been located at approximately 385 nm and

525 nm in the published literature and are known as the UV/blue peak and the green

peak, respectively. Several processing parameters influence the absolute and relative

intensities of these two bands and will be discussed.

Measurements by the author combined with detailed
calibrations of the PL system place these peaks at about 372 nm
and 531 nm when plotted in radiometric (power) units and 373 nm
and 542 nm when plotted in quantum units.

Much has been published about the major differences between the production,

properties, luminescence, and mechanisms of sp-Si and another material known as

porous silicon (PS), which is produced by a wet chemical or electrochemical etch

JCan90J. Electropolishing of Si predates the discovery of luminescence by decades

[Uhl56, Tur58]. There is little in common between these materials, and PS will not

be discussed to any significant degree in this study. Ludwig has dealt with these

distinctions in great detail [Lud96al. It should be pointed out however, that when

excited by 325 nm light, sp-Si exhibits similar photoluminescent intensities (at

significantly different wavelengths) to PS samples purported to have quantum

efficiencies of approximately 5%. The conclusion has been drawn that sp-Si has

similar quantum efficiencies [Hum95a, Hum95b].

Theoretical models to explain above band-gap luminescence in modified

semiconductors with nanoscale features traditionally fall into three main categories:

(1) a classical quantum confinement model [Can90, Bru94, Cal94], (2) a surface-

modified quantum model [Koc93, Cam86], and (3) models based on molecular

species and localized defects (many of which will be discussed in a later section).

The first category of theories predicts a widening of the optical band-gap of

semiconductors due to physical confinement of carriers in quantum wells, wires, or

dots (one, two, or three-dimensional confinement). These models usually use a

classical particle in a box approach IOha90O or an effective mass approximation

approach IBru86, Kay881. In addition, materials with an indirect optical band-gap

(like silicon) can assume a pseudo-direct behavior due to Heisenberg-related

smearing of the momentum of confined carriers ISan921.

The second category also involves broadening of the optical gap of

nanocrystalline semiconductors, but rather than a direct recombination mechanism

across this gap, the enlarged gap hosts optically active tail states associated with the

surface interface of the quantum dot and the surrounding material. The high surface

to volume ratio of nanoscale particles makes such a mechanism plausible.

The third category assumes either the presence of luminescing molecular

species present in the matrix (often complexes of Si, 0, and H known as siloxenes)

[Fuc93, Tam94] or luminescence from highly localized defects in the oxide itself

[Tsy94. Pro93J.

Production of Spark-Processed Silicon

The method of producing sp-Si samples in the majority of published work has

evolved from the use of a Tesla coil or DC high voltage supply to the use of the

pulsed high voltage output of the flyback coil from a cathode ray tube (CRT). The

spark intensity varies from one CRT to another. The open circuit voltage is

approximately 15 kV. The repetition rate of the high voltage pulses is equal to the

horizontal scan rate of the CRT. This is 16 kHz for a television (525 columns X 30

frames per second) and is about 19 kHz for a VGA monitor. The pulses are applied

between an anode (often a sharp tungsten tip) and the silicon to be processed, which

forms the cathode, see Figure 3-1. They have typically been separated by a gap on

the order of 1 mm. The pulse width created when driving a spark gap with a VGA

monitor has been quoted as being 10 ns or 20 ns |Lud96a, HumOl I. Based on the

assumption that the power supply charges the capacitance of the spark gap (estimated

to be 30 pF) to the full potential of 15 kV after which it discharges across the gap in

10 ns. Ludwig calculated a value of 45 amps for the instantaneous current of each


This analysis is inherently flawed and will be addressed in
Chapter 5.

Spark-processing is done in air or a mixture of pure gases. As it requires a

discharge, it cannot be done in a vacuum. In air or mixtures of nitrogen and oxygen it

tungsten tip


Figure 3-1: Schematic representation of the preparation of spark-processed materials.
High-voltage DC pulses cause a discharge between an anode tip and the substrate
(cathode). Adapted from Lud96b with permission.



results in the build-up of a circular whitish-gray porous oxide layer on the surface of

the silicon. When the power source is a monitor, the growth rate is about 3-5

im/min. and the central region is often surrounded by a light brown ring that is

substantially thinner than the central region (this is not always true of samples

produced by the author, but depends on the processing conditions).

Very little surface modification occurs when the sample is the anode and the

tip the cathode. It is believed that gas molecules are positively ionized and

accelerated toward the cathode transferring kinetic energy to the substrate surface.

When a high-voltage DC potential is applied between two tungsten electrodes in air,

the cathode glows red hot and the anode remains relatively cool. The cathode is

consumed at a much greater rate.

This rapid transfer of kinetic energy melts and/or evaporates the silicon and

ejects a plume of material, which reacts with the ambient gases and recondenses on

the substrate, see Figure 3-2. The deposited material is constantly bombarded by

more ions and more material deposits on top of it. The result is a nanoporous matrix

composed largely of silicon-rich silica with a quantity of embedded silicon


The luminescence has been reported to vary with a number of processing

parameters. The area and thickness of the porous region increases with processing

time. A linear relationship between PL intensity and processing time has been

reported. The resulting PL appears to be unaffected by the doping type, doping level,

or crystalline orientation of the wafer during processing [Cha941.

W-tip ____
I (anode)

/ vaporized Si

0 enriched
SSi vapor

Si substrate

Si rich vapor

low dimensional
Si structures

a SiOx

Figure 3-2: Schematic representation of the formation of silicon nanoparticles within a
silicon-rich silica matrix during spark-processing. Adapted from Lud96b with


Morphology of Spark-Processed Silicon

Figure 3-3a shows a plan view scanning electron microscopy (SEM)

micrograph of sp-Si processed with a Tesla coil power source. Figures 3-3c and 3-4

show cross-sections of the substrate and porous region. The porous region consists of

deposited clusters surrounding deep "worm-holes". These holes appear to be

channels through which the spark reaches the more conductive substrate. In this

example, they are about 15 ptm in diameter and 200 [tm in length. Ludwig believed

that these channels represent the volume eroded by a single pulse and has calculated

an evaporation rate of 8 g/s and a current density of 107 A/cm2 [Lud97].

The author believes that these channels result from the
gradual deposition of material around the discharge sites. As the
thickness builds up, the spark is drawn to the least resistive paths
to the substrate and repeatedly strikes areas with less coverage.
This causes growth near these areas, limiting their size. As a
result, long narrow channels form through the porous region. The
author neither believes that the pore volume represents the
amount of material removed by a single discharge, nor that the
pore diameter indicates the diameter of the plasma channel, which
is likely to be highly forked at the substrate end.

Figure 3-3b shows a high resolution transmission electron microscopy (TEM)

micrograph of a sp-Si sample produced using a Tesla coil. Silicon particles with

diameters in the nanometer range can be seen (four are outlined in ink). The

crystalline orientation appears to be random and the particles are surrounded by an

amorphous matrix. Also in the figure are electron diffraction patterns of the silicon-

rich region in the upper right of the figure and of the amorphous region in the lower

left. These confirm that the silicon is crystalline but randomly oriented and the

Figure 3-3: Micrographs of sp-Si:
(a) SEM plan view. Adapted from Hum93 with permission.
(b) TEM. Some nanocrystals have been outlined. Satellite pictures show electron
diffraction patterns taken at locations indicated on the micrograph. CR =
crystalline and AM = amorphous. Adapted from Hum93 with permission.
(c) SEM cross-section at a fresh fracture. Adapted from Hum93 with permission.

Figure 3-3- continued

Figure 3-3--continued

Figure 3-4: Cross-sectional SEM micrograph of sp-Si taken at a fresh fracture.
Adapted from Lud96a with permission.

matrix is amorphous. The X-ray energy dispersion spectrum of the amorphous region

indicates that it is largely silica (SiO2) [Cha941.

The presence of silicon nanoparticles has been confirmed by the observation

of their expected effect on the Raman spectra (Stokes shift) of sp-Si vs. bulk silicon

[Cha941. Campbell and Fauchet describe how the Raman peak shift and line width

depend on particle size and shape (Cam86]. This is caused by phonon confinement.

In addition, relaxation of momentum conservation of optical phonons in nanoparticles

may cause splitting, resulting in a double Raman peak ITsu92, Wu_95]. Figure 3-5a

shows the Raman peak for bulk silicon at 520.9cm'. Figure 3-5b shows the

corresponding peak of a green luminescing sample that is shifted toward lower

energies and broadened. Figure 3-5c shows the shifted, broadened, and split peak for

a UV/blue luminescing sample. In this case, both single peak and double peak best-

fit Gaussian deconvolutions are superimposed on the data. Both samples were

produced by monitor sparking. The degree of shift and broadening is consistent with

spherical particles of 3-5 nm in diameter ICam86, Sui921.

While nanoparticles are present, there seems to be no correlation between the

size of the nanoparticles and the color or intensity of the PL. A more detailed study

found no significant difference between the Raman shift and broadening between

UV/blue and green sp-Si samples (Rup97]. Furthermore, the less luminescent edge or

"halo" regions of monitor-sparked samples appear to have a smaller particle size than

the center of the porous region, which is highly luminescent. The sizes of the

particles detected in this study, regardless of region, ranged from 8 to 15 nm. These

are much too large to be the cause of either PL band. Finally, the observed Raman

peak shifts and broadenings were not consistent with one another. This is very strong

evidence that quantum size effects are not responsible for the PL of sp-Si.

1000 1 1 1-1-1

800 -

600 -

400- ~



'^ 600- \
".': .. .* sy .^

1000 ------------------------ "--



400 ".,

200 (.). -s

460 480 500 520 540
Raman Shift [1/cm]

Figure 3-5: Room temperature Raman shifts (Xee = 488 nm, 400 W cm 2):
(a) crystalline Si 520.9 cm peak. 4.0 cm 'FWHM
(b) green luminescing sp-Si 513.8 cm 'peak, 6.5 cm FWHM
(c) UV/blue luminescing sp-Si 510.9 cm 'peak. 24.0 cm-' FWHM
Single Lorentzian fit shown for all samples (solid line), double Lorentzian fit shown for
(c) (dotted line). Adapted from Hum95a with permission.

Composition of Spark-Processed Silicon

X-ray photoelectron spectroscopy (XPS) has been performed on monitor

sparked sp-Si samples. The core binding energies of the Si 2p electrons have been

found to be 103.4 eV [Kur97]. This is very close to that of silicon in the +4 valence

state as measured in amorphous SiO2 (103.3 eV). This implies that the majority

component of the sp-Si surface is near-stoichiometric silica.

Small-spot XPS has been conducted on the cross-section of a freshly fractured

sample that was processed in air, and reveals that the composition varies with depth

[Lud96b]. The resulting depth profile is shown in Figure 3-6. The concentration of

Si02 is essentially 100% at the surface and falls monotonically as the depth increases

until the substrate is reached. The spark-processed layer in question was 250 Im in

depth. The concentration of crystalline silicon increases monotonically from 0% at

the surface to 100% at the interface with the substrate. The depth profile of two

minor constituents differed. The first represents silicon whose signal is charge

shifted. This must be due to isolated islands or clusters of silicon not connected to the

substrate and surrounded by an insulating matrix. These first appear at a depth of

about 50 [tm, and constitute about 13 at% (atomic percent) of the sample from about

100 tm to 200 Rm. The charge shifted signal finally disappears at the interface with

the substrate. Finally, nitrogen in concentrations as high as 6 at% is found at similar

depths. This signal is also exhausted at the interface with the substrate. Within the

detection limit of the instrument (approximately 1 at%), the surface appears to be

pure SiO,. Depth resolved X-ray emission spectroscopy results are consistent with

only silica to a depth of at least 200 nm (10 keV beam) ILud96bI.




50 100 150 200


Depth [pm]

Figure 3-6: Composition of sp-Si as a function of depth as derived from small-spot
XPS measurements. Adapted from Lud96b with permission.



Transmission and diffuse reflection Fourier transform infrared spectroscopy

(FTIR) spectra were taken for a monitor-sparked sample in the "fresh" state (Figure 3-

7a), after annealing at 600 C in air for 20 h (Figure 3-7b), after an additional 24 h

anneal at 800 C (Figure 3-7c), and for another sample sparked in a stream of flowing

nitrogen (Figure 3-7d). The first three spectra show strong vibrational modes

associated with Si and 0. These are labeled on the figure. Structures in the 3600

cm-1 to 3750 cm' range assigned to vibrational frequencies of OH groups including

the aSi-O-H group (silanol) are present and are shaded in the figure. It is clearly seen

that the strength of these modes decreases with annealing. OH groups are known to

desorb at temperatures above 450 C. A number of hydrogen related vibrational

modes that are seen in porous silicon produced by a wet etching process are not seen

in sp-Si. These include the scissors and wagging modes of the Si-H bond at 910 cm'

and 630-670 cm ', respectively, as well as a Si-O-H mode at 870 cm-'. A strong N2

related vibrational mode is seen in all four spectra indicating that nitrogen is

incorporated in sp-Si processed in either air or nitrogen.

Optical Properties

Room temperature photoluminescence

Sp-Si prepared in air using a monitor as a power supply exhibits two primary

photoluminescence (PL) bands under 325 nm excitation. A peak maximum near 385

nm is known as the UV/blue band and a peak near 525 nm is known as the green

band. These are shown in Figure 3-8a and 3-8b. A tail extending into the red can be

seen in samples exhibiting only the UV/blue peak. This peak can be resolved into a

discrete peak near 650 nm when sp-Si is excited by the 488 nm or 514.5 nm line of an

^ .. z (C) 6 I. ,: .-"
C -
Cn -

I. -.. ::.I-.

i (a) : (a)
i I / i/ I,

4000 3500 3000 1500 1000 500

Wavenumber [cm"1]

Figure 3-7: FTIR spectra of:
(a) freshly sparked sp-Si produced in air
(b) same sample annealed for 20 h at 800 C in air
(c) same sample after an additional 24 h anneal at 800 C in air
(d) sp-Si produced in UHP nitrogen (99.999% pure)
Adapted from Hum95a with permission.


20 -(a)
20 -



325 nm
300 mW/cm2 2-
300 K
300 400 500 600 700
Wavelength [nm]

Figure 3-8: Room-temperature PL spectra of: (325 nm 0.3 W cm2 excitation)
(a) UV/blue luminescing sp-Si
(b) green luminescing sp-Si
(c) red PL of PS shown for comparison
Adapted from Lud97 with permission.

Argon ion laser as well as when PL is measured at high temperatures. Hummel has

assigned this luminescence to a specific radiative transition rather than to an extended

distribution of states [HumO1I. The peak intensities of the UV/blue and green peaks

are similar. When excited by about 5 W of 488 nm laser light, the red peak has

similar intensities to the other peaks when excited by about 20 mW of 325 nm laser

light. This indicates a room temperature quantum efficiency ratio of about 1:250. It

is possible that the tail also exists in green samples, but it cannot be individually


The relative proportions of the UV/blue and green peaks can be changed by

varying several processing parameters. A more intense spark produces more green

and less blue. A large heat-sink under the sample produces more blue. Blowing a

stream of air on the sample during processing almost guarantees a blue emitting

sample. Heating the substrate during processing enhances the green at the expense of

the blue. It is likely that temperature is the common factor in all these observations.

While a sample can exhibit a superposition of the UV/blue and green peaks, there is

never a peak in between or a shift in the peak locations beyond that expected by the

mathematical addition of the two curves.

The PL peak of an anodically etched porous silicon sample purported to have

5% external quantum efficiency is shown in Figure 3-8c. This was measured under

identical conditions as the sp-Si peaks. The intensity of PS is comparable to both

UV/blue and green sp-Si leading to the published assertion that the quantum

efficiency of sp-Si is in the neighborhood of 5%.

The methods reported in this dissertation allow the creation
of UV/blue sp-Si several times brighter than samples of this
vintage. For the green PL, they can be over an order of magnitude
brighter. However, when the author attempted to make a direct

measurement of the quantum efficiency of a typical green sample,
the result was only about 1/3%. The brightest green samples are
about 5 times more intense than a typical green sample, which
indicates a quantum efficiency of approximately 1 -2%. This is
still quite high, and due to the methodology of the measurements,
is probably a conservative or "worst case" value. This issue is
addressed in Chapter 9.

Both major PL peaks vary linearly in intensity with excitation powers varying

from 0.01 mWcm2 to 10 mWcm2. The spectral shape is also preserved.

These PL bands of sp-Si cannot be accounted for by currently known PL

bands in stoichiometric and non-stoichiometric silica and other glasses. In Figure 3-

9, the primary peaks of sp-Si are represented by curves (a) and (b). Also shown on

this graph are a number of silica-related peaks. These samples were measured under

identical conditions and include: (c) X-ray damaged borosilicate glass exposed to 40

keV X-rays, (d) 99.999% pure silica powder, (e) Silicon-rich silica created by Si ion-

implantation (25 keV, 2 X 101 cm2), and finally, (f) industrial grade fused silica.

Within the high bandgap of silica (9 eV for crystalline silica, about 8 eV for dense

amorphous thermal silica) there are reported luminescent bands at 185 nm, 264 nm,

288 nm, 400 nm, 563 nm, and 650 nm |Fri85, Pio9O, Sta871. These have been

assigned to defect states as they have been found to increase in PL intensity under X-

ray, gamma, or neutron irradiation of silica. These bands have been successfully

excited by below-gap photon energies. 165 nm, 240 nm. 258 nm, and 620 nm

absorption bands have been found in silica. Photon energies over 5.0 eV (248 nm)

are reportedly necessary to activate luminescence at 264 nm, and 288 nm. These are

associated with triplet-to-singlet and singlet-to-singlet transitions of electrons in Si-Si

bonds across an oxygen vacancy |Toh89b, Jon76]. The 460 nm luminescence is seen

in Si-rich silica containing a low concentration of OH groups. The 650 nm emission

350 (a) 325 nm
3- ( 300 K
/ \ (b)

M \O
300 / \
l \ /--
I \ / \
I /

e\() 1000

IL 10 0
150 /

100 /2 ) \
50 / / \ \
100 / \- \ V -

O i ^ i < i i ^ i

350 400 450 500 550 600 650 700
Wavelength [nm]

Figure 3-9: PL spectra of various samples measured under identical conditions:
(a) UV/blue luminescing sp-Si
(b) green luminescing sp-Si
(c) X-ray damaged borosilicate glass
(d) SiO2 powder (99.999% pure)
(e) Si ion-implanted fused SiO2
(f) fused SiO,
Numbers indicate multiplication factors. Adapted from Hum95a with permission.

band has been attributed to a non-bridging oxygen hole center (NBOHC) [Sta87,

Gri91] or an oxygen interstitial [Lud95a, Sku84, Pro94, Shi94]. It is associated with

the 258 nm and 620 nm absorption and is unique among defects in silica due to its

small relaxation (most others have Stokes shifts of 2 eV or more). This band is

rapidly degraded under UV excitation [Hum95b].

It appears that the existing literature on silica cannot explain the main PL

peaks of sp-Si. Also, He-Cd laser radiation (325 nm) is well below the peak

absorption energy associated with all of the known emission bands except the one at

650 nm. Finally, under 325 nm excitation, the PL of monitor produced sp-Si is at least

three or four orders of magnitude more intense than that from these various defects in

silica and glasses.

Thermal dependence of the PL of sp-Si

The PL spectra of monitor-sparked sp-Si samples were measured at a variety

of temperatures. Samples with UV/blue or green luminescence lose intensity above

room temperature and gain intensity below room temperature, see Figure 3-10. The

peak location and shape of the UV/blue luminescence is unchanged, while the green

peak is blue-shifted and broadened by increasing temperature. As in the case of

lower energy excitation, the red tail of UV/blue sp-Si can be resolved as a separate

peak at high temperature. This band appears to be less susceptible to thermal

quenching than the main part of the spectrum. The optical band-gaps of Si. Si02, and

silicon quantum dots all decrease (red shift) with increasing temperature |Blu74,

Thu751. This is not observed in either primary peak. The blue shift of the green peak

with increasing temperatures resembles the behavior of some silicon oxynitrides and

400 500 600 400 500 600

Wavelength [nm]

Figure 3-10: Temperature dependence of the PL spectra of:
(a) UV/blue luminescing sp-Si
(b) green luminescing sp-Si
Measurement temperatures are indicated adjacent to curves. Adapted from Lud96b with

non-stoichiometric silicon nitride excited by below band-gap energies [Aus60,


The integrated PL intensity is plotted as a function of inverse temperature in

Figure 3-1 la. At temperatures as low as 25 K, the PL intensity of sp-Si is 10 to 20

times more intense than at room temperature. Above room temperature, the PL is

thermally quenched with activation energies of 156 meV for the UV/blue peak and

241 meV for the green peak. These values indicate a high degree of localization. The

scale of this localization can be estimated by assuming that these activation energies

correspond to excitonic binding energies. Using a simple electrostatic potential

model (AE = (47tEn0) 'e2d1) and an average of the two values, a carrier separation

distance, d, can be estimated to be 0.6 nm in Si (c=12) and 1.9 nm in SiO2 (E=3.9).

The lattice constant of Si is 0.543 nm by comparison. There are a number of ways to

trap and localize defects besides classical excitons, but this exercise demonstrates a

very high degree of localization, nevertheless.

Below room temperature, the activation energies vary with temperature. Such

effects are generally attributed to the thermal emptying of exponential band-tails or

other localized states that are inhomogeneously broadened. Such a phenomenon has

been seen in band-tail to band-tail luminescence in a-Si:H ICol801. In such cases, the

intensity varies with temperature according to the equation I(T) = 1 / (1 + ceTr'),

where c is a constant. T, is the characteristic temperature, which is a measure of the

disorder in the system. It is related to the degree of inhomogeneous broadening and is

proportional to the "width" of the density of band-tail states. If Io / I(T) 1 is plotted

logarithmically vs. temperature then a straight line should result according to the

theory. To can be determined from the slope of the line. As seen in Figure 3-11 b, the

thermal data for sp-Si fits this model very closely. In its simplest form, this model

Temperature [K]
200 100 50 33

0 10 20 30 40

1000 /Temperature [1/K]

T. 81 K 101

r ^? \ 100 6
^^ blue 10

T =96 K

(b) 10-1

0 100 200 300 400 500

Temperature [K]

Figure 3-11: Temperature dependence of sp-Si PL:
(a) Temperature dependence of the integrated PL intensity of UV/blue and green
luminescing sp-Si. Activation energies of thermal quenching are shown.
(b) Plot of II/I(T)| 1 vs. temperature for the same data. Characteristic
temperatures are shown (see text).
Adapted from Aug96 with permission.


C 100


also predicts a redshift with increasing temperature as the band-tail states closest to

the mobility edges are thermalized first. At higher temperatures, the remaining

occupied band-tail states lie further in the gap and have less potential energy.

However, real systems can be more complex. An inhomogeneously broadened state

might not lie near a mobility edge and might depopulate by a different mechanism. In

the case of SiNx, a red shift is seen at small values of X, a blue shift at high X, and no

shift at all at some intermediate X value. Thus, neither the lack of a shift in the case

of the UV/blue peak, nor the blue shift of the green peak can be taken as a significant

contradiction of the theory. The excellent fit of the integrated intensities indicates

that either exponential band-tails or exponentially broadened interband states are

involved in both species of luminescence. Measured values of the characteristic

temperature of the PL of sp-Si have varied from 70 K to 110 K. This indicates a

higher degree of disorder and band broadening than in PS where To is in the range of

40 K to 70 K [Ros93, Mus93]. It should also be noted that in PS, PL intensity is

actually decreased at very low temperatures and has a maximum between 100 K and

200 K [Zhe92, Per92]. Thus, luminescence in PS is thermally activated. In sp-Si,

thermal quenching predominates at all temperatures measured.

Figure 3-1 la is actually highly misleading. By putting straight lines and the

activation energies on this Arrhenius-plot, Augustin strongly implies that these

energies were derived from a slope in this I(T) vs. I/T plot. Correspondence with

Ludwig verified that these energies are in fact derived from the slope of an 10 / I(T) -

I vs. 1/T plot (not shown). This treatment is a consequence of the following theory

for the change in PL intensity during thermal quenching: I(T) = 1 [(Tr') / (T'- + Tn')],

where xTr and T, are the time constants of the radiative process and non-radiative

processes. In processes that are not thermally activated, T, usually is relatively

insensitive to temperature and t, is strongly temperature dependent. Some

manipulation of the previous equation yields: (Tn' / T,-') = I, / I(T) 1. The ratio on

the left can be approximated by an exponential giving: exp(-Ea/KbT) I / I(T) 1,

where Ea is the activation energy for the non-radiative process. This model is much

simpler than the treatment of Collins and is often not sufficient for amorphous or

disordered materials [Col801. However, at sufficiently high temperatures, this

thermally activated quenching usually predominates and Ea approaches a constant


Stability of sp-Si

Sp-Si is highly stable against degradation by UV light exposure, thermal

annealing, and HF acid etching. Figure 3-12 shows how the PL intensity of sp-Si

compares to that of PS with continued 325 nm UV excitation. Curves (a) and (b)

represent typical sp-Si samples. Curve (c) represents a sp-Si sample annealed in 900

C air for three hours. It is even more stable than freshly produced sp-Si. Curve (d)

represents a sp-Si sample produced in UHP nitrogen. Curves (e) and (f) represent

two typical PS samples. All curves are self-normalized to an intensity of I at zero

time. The difference between sp-Si and PS is stunning, especially after the

logarithmic intensity axis is taken into account.

Figure 3-13a shows how the PL intensity of sp-Si is influenced by annealing

in ultrapure nitrogen. For each data point, the same sample was introduced to the

furnace at the indicated temperature and held for 30 minutes. It was then removed,

allowed to cool to room temperature, and the PL spectrum was measured. Each

successive data point from low to high temperature indicates several accumulative

annealings at the indicated temperatures. The green peak is relatively stable until 600


100 (c)
\ ~(a) \
V \ (b)


: 10-1

.j por-Si
C' 10-2

: ~~~(f) ^ ^ ^ ^
325 nm, 300 mW/cm2 por-Si
room temperature
10- -----
0 5 10 15 20

Time [min]

Figure 3-12: Degradation of PL as a function of time of exposure to 325 nm 0.3 W cm2
laser light:
(a) UV/blue luminescing sp-Si
(b) green luminescing sp-Si
(c) UV/blue luminescing sp-Si annealed in air for 3 h at 900 C
(d) sp-Si produced in UHP nitrogen (99.999% pure)
(e) and (f) porous Si samples for comparison
Adapted from Hum 95b with permission.

400 800 1200 0

Annealing Temperature rC]




480 .0

~A n0

400 800 1200

Annealing Temperature [C]

Figure 3-13: Annealing characteristics of sp-Si (consecutive anneals of the same
sample in UHP nitrogen (99.999% pure) for 30 min. at each temperature):
(a) PL peak intensities of UV/blue and green PL bands
(b) PL peak positions of UV/blue and green PL bands
All data was taken at room temperature with 325 nm excitation. Adapted from Hum95a
with permission.









green band

blue band
-* *eep* ~e ** *-


C and begins to increase with annealing, tripling in intensity by 1,100 C. The

UV/blue peak remains relatively stable until 900 C when it begins to decrease

somewhat, loosing less than half its intensity by 1,100 C. The apparent peak

locations of both bands are independent of annealing temperatures as seen in Figure

3-13b (410 nm and 520 nm are instrumental locations of the peaks before correction).

The apparent increase in PL around 400 C has been attributed to quenching of non-

radiative centers from "naked" silanol groups. This is seen in the 460 nm defect

luminescence in silica, see Figure 3-14. The silica sample was boiled in pure

deionized water for 5 hours to saturate dangling silicon bonds with -OH groups.

From room temperature to about 110 C, the surface of silica is believed to be

covered with chemisorbed -OH groups which are in turn covered by physisorbed

H20. This combination does not provide good surface passivation. The H20 desorbs

between 200-250 C leaving an -OH terminated surface that provides excellent

passivation. Between about 450-600 C, the -OH groups desorb, and the passivation

is lost. Thus, similar peaks in the annealing curves are seen between 400 C and 450

C for the UV/blue and green peaks of sp-Si as well as the much less intense defect-

related luminescence in silica. The double line in the figure indicates the temperature

above which there should be no chemisorbed -OH groups. It is believed that silanol

groups enhance the luminescence of sp-Si within a narrow temperature range, but are

not directly responsible for either luminescent band. It should be noted that no

difference was seen for sp-Si after boiling in water vs. a standard air-sparked sp-Si

sample, indicating that dangling bonds are already passivated in sp-Si prepared in

damp Florida air.

The PL from sp-Si has been found to be remarkably stable against HF etching.

HF etches SiO, quickly, indicating that the excess silicon and nitrogen in the bulk

0 200 400 600

Annealing Temperature [oC]

Figure 3-14: PL peak intensities of 460 nm defect luminescence in silica after
consecutive anneals of the same sample in UHP nitrogen (99.999% pure) for 30 min. at
each temperature. Adapted from Hum95c with permission.

slow etching substantially [Cha94]. Because of this, it has been concluded that the

PL of sp-Si is a bulk, and not surface, phenomenon.

PL lifetimes of sp-Si

Nanosecond PL decay was originally reported for the PL band of sp-Si. More

recent measurements by the author have revealed much faster picosecond

luminescence using equipment with a much faster time resolution. This will be

discussed in detail in Chapter 8.

However, the original published conclusions about the speed of these

processes in sp-Si are still true (perhaps more so) and will be presented in this review.

The fast response suggests that recombination is based on geminate carriers

recombining at highly localized luminescent centers.

The ps decay of sp-Si is much faster than red-luminescing PS [Ook921 and

defects in a-SiO, [Toh89cj, which are both in the 1 10 .us range. It is also three

orders of magnitude faster than the 10 ns decay observed in blue-luminescing highly

oxidized PS IKoc93, Kov94]. The 1.9, 2.2, 2.7, and 3.1 eV transitions in silica have

reported time constants on the order of 10 us, 100 ns, 10 ms, and 100 1us, respectively

[Sta87, Nis92, Sku78, Sku84, Sku92]. All of these mechanisms are excluded as

likely candidates to explain the UV/blue and green PL of sp-Si.

Effects of processing parameters on luminescence

The intensity and color of the PL emission from sp-Si depend on several

experimental parameters including the pressure of the gas ambient, the total

processing time, and the substrate temperature. Two of the most important

parameters are the spark current and the chemical composition of the gas ambient.

Figure 3-15 shows how the UV/blue and green PL intensity change with gas

20 40 60 80

Oxygen Content [Vol %]



Figure 3-15: Peak intensities of UV/blue and green PL bands as a function of the
composition of the gas ambient during processing. X-axis shows the percentage of UHP
oxygen. The balance is UHP nitrogen. Adapted from Hum98 with permission.






composition between pure N2 and pure 02. Both gases must be present during

processing for either of these dominant bands to exist in a sample [Hum98]. Samples

sparked in pure 0O exhibit only an orange/red peak at 1.9 eV (650 nm) which is 4-6

orders of magnitude smaller in intensity than the PL from a typical sp-Si sample.

This peak has been attributed to nonbridging oxygen hole center (NBOHC) defects in

silica. The samples represented by this figure were spark-processed with a monitor as

a power source, which is essentially a constant-voltage supply. Oxygen is more

readily ionized than nitrogen. It is clear to the casual observer that as the nitrogen

concentration is increased the spark intensity rapidly decreases. Thus, without the

luxury of a constant current source, this experiment cannot differentiate between two

of the most important parameters (gas composition and current). With such a

constant-voltage supply, the intensity of both peaks is maximized at N2:02 ratios near

1:1. While nitrogen has been shown to passivate non-radiative centers in Si [Luc95],

it appears to play a more direct role in the luminescence of sp-Si.

The PL intensity of sp-Si increases with processing time, see Figure 3-16

[Aug96]. This can be explained by the continuous production of luminescing centers.

This increase is not accompanied by any change in the peak location. If the

luminescence were due to quantum dots of continuously decreasing size, one would

expect both a blue shift and nonlinear intensity increase. According to one model

[Hyb941 quantum dot optical transition probability (and thus, the efficiency) varies

with diameter by 1/d6.

As previously mentioned, heating the substrate is known to increase the

intensity of the green peak at the expense of the UV/blue peak. Figure 3-17 shows

the intensity of the green peak as a function of substrate temperature. Without a

constant-current supply, increasing substrate temperature can increase the current as














Time [min]

Figure 3-16: Intensity of the UV/blue PL peak of sp-Si as a function of processing
duration. Adapted from Aug96 with permission.

- 0-

-^ -



80 100















Substrate Temperature [C]

Figure 3-17: Intensity of the green PL peak of sp-Si as a function of the substrate
temperature during processing. Adapted from Aug96 with permission.

well because the intrinsic carrier concentration of the silicon increases. However,

because of the large relative increase of the green peak compared to the UV/blue

peak, there is definitely evidence of a thermal component to this effect.

If the spark-processing is done in flowing air, the intensity of the UJV/blue

peak is observed to increase with increasing air velocity. Figure 3-18 shows the

increase in PL intensity as a function of the air pressure applied to a nozzle blowing

on the sample during spark-processing. Such "spray cooling" is known to be a highly

efficient method of cooling a small area, and this effect is probably thermal in nature.

It is not due to additional oxygen from the flowing air. X-ray emission measurements

indicate that sp-Si processed in flowing air is less oxidized than that processed in

stagnant air, a somewhat counterintuitive observation [Kur97I.

As briefly mentioned, variations in the substrate including the type and

concentration of dopants (n-type and p-type) do not effect the peak locations of the

PL bands in sp-Si. However, a substantial difference in current can result.

Tungsten has proven to have excellent properties for use as an anode material

for spark processing, and it is often used for this reason. This led to the argument that

the UV/blue luminescence of sp-Si originates from tungsten contamination IVep971.

The luminescence of sp-Si sparked using a variety of anode materials exhibited

identical peak locations and similar PL intensities [Hum98I. The anode materials

included W. Ti, Mo, V, Ni, and, most importantly, Si. Some electrodes oxidized more

easily than others, and some exhibited an adherent insulating oxide which "choked

off" the spark. Differences in current can explain the observed differences in

intensity. These experiments conclusively rule out the role of contamination from the

anode as a mechanism for the UV/blue and green peaks of sp-Si.













10 20 30 40 50

Air Pressure [Ibs/]

Figure 3-18: Peak intensity of the UV/blue band of sp-Si as a function of the air
pressure used for spray cooling of the area being processed. Adapted from Aug96 with

Cathodoluminescence of sp-Si

Sp-Si is known to exhibit two cathodoluminesence (CL) peaks near 480 nm

and 650 nm [Lud95al. This luminescence is commonly observed in silica and glasses

and is easily explained. It appears to have no relation to the PL of sp-Si. It is beyond

the scope of this study to go into the CL in detail, but it should be pointed out that the

penetration depth of electrons (10 KeV was used for the CL work) is only a few [tm,

while silica is nearly transparent to 325 nm UV light. CL is therefore believed to be a

surface (or near-surface) phenomenon, while PL is a bulk phenomenon.

Electroluminescence of sp-Si

Some work has been done by the author on electroluminescence of sp-Si and

will be covered in Appendix A. In brief, thin film AC EL devices were found to be

very inefficient and produce a faint greenish luminescence. No meaningful spectra

could be taken of such a dim source. Roughly half (apparently the younger half) of

people asked to observe it could see it in a darkroom. Thick film (high frequency)

AC devices based on sp-Si in an oil dielectric worked significantly better (intense

enough to measure) but were short lived. The EL spectrum in this case is very similar

to the CL spectrum. It should also be noted that a much more intense and stable

broad red/IR luminescence has recently been observed by Nigel Shepherd. a member

of the same research group as the author, in a DC device based on metal contacts over

an extremely thin spark-processed layer (sparked for only seconds). Previously,

Yuan and Haneman observed visible PL in a similar system [Yua95]. However, as

the author was not involved in this work and the EL peaks appear totally unrelated to

the PL, these results will not be discussed here.

Doping sp-Si with rare-earth elements

Sp-Si has been doped with highly luminescent Ce, Eu, and Tb salts [HumOl].

This is also beyond the scope of this study except to note that the PL of these narrow

line emitting elements becomes very broad when they are dispersed in a sp-Si matrix.

The emission of Eu in sp-Si:Eu has a full-width at half-maximum of 200 nm. This

demonstrates that the disorder in sp-Si is great enough to inhomogeneously broaden a

very specific state into a wide PL band. Thus, a distribution of states is not needed to

explain the broad PL bands in undoped sp-Si.

Spark-processing of materials other than silicon

Spark-processing leads to visible PL in a large variety of materials including

Ge, Sb, Bi, Sn, Te, As, and GaAs ILud94, Lud95b]. With the exception of GaAs, this

was the work of the author and will be covered in the experimental section.

However, certain aspects of sp-Ge and sp-Zn have also been studied by others,

particularly Chang [ChaOOa, ChaOOb I.

Three deconvoluted PL peaks are reported in air-sparked sp-Ge near 420 nm

(blue), 520 nm (green), and 620 nm IChaOOa/Figure 3-19]. The author consistently

observes the first two peaks in sp-Ge but never the third. Chang sparked primarily in

air, while the majority of the author's work was in mixtures of UHP N2 and UHP 02,

which may explain the difference. The overall PL intensity increases at low

temperatures (as low as 21 K) and the increase is greater for the blue peak than the

green peak. Furthermore, the blue band of sp-Ge also maintains its peak location at

all temperatures (as does the yellow 620 nm band). This thermal behavior is very

similar to that of sp-Si.

1.2 -

S0.8 -


**b // l

II I ,
^~~~~ #- /^^

400 450 500 550 600 650 700

Wavelength (nm)

Figure 3-19: PL spectrum of sp-Ge (solid line) and Gaussian deconvolution (dashed
lines) of same into three distinct peaks. Adapted from ChaOOb with permission.

Several spark-processed metals exhibit visible PL and will be discussed in this

study. With the exception of recent conference proceedings [StoOO], only sp-Zn has

been covered in the literature [ChaOOb]. Unlike sp-Si and other sp-materials

including some metals, the PL of sp-Zn can be readily explained. A UV/blue 380 nm

PL peak is attributed to a direct band-edge transition of ZnO [Sze81], while a broad

green 540 nm PL peak is attributed to any of many well-documented non-

stoichiometric defects in ZnO [Van96, Pro95, Xu_96, Byl78, Ort97. Liu92].

Laser-processing of silicon

Pulsed laser ablation of silicon in an atmosphere containing nitrogen and

oxygen (Ip-Si) results in PL similar to, but substantially weaker than, the UV/blue

peak that results from spark-processing [HumOO]. Lp-Si exhibits no green peak

analogous to that of sp-Si.

Magnetic Properties of Sp-Si

Sp-Si exhibits unique magnetic properties that can best be described as being

strongly paramagnetic with a narrow, but clearly discernable. ferromagnetic

hysteresis loop IHac971. It is likely that spark-processing produces such a high

concentration of paramagnetic centers that their unpaired electrons are close enough

to couple, giving rise to a ferromagnetic exchange energy. Electron Paramagnetic

Resonance (EPR) studies indicate the presence of approximately 3.8 x 1020 unpaired

spins/cm3 associated with the E' center in silica. The E' center is one of the major

sources of unpaired spins in silica. The remanent magnetization (a measure of the

ferromagnetic behavior), the total magnetization (a measure of the paramagnetic

behavior) and the strength of the EPR signal (a measure of unpaired spin density)

correlate to one another and all decrease rapidly with annealing above 600 C. This

magnetic behavior appears to be independent of the PL mechanism, which is not

strongly effected by annealing in this temperature range.

The PL intensity of the UV/blue peak increases slightly when the PL is

measured in a magnetic field as high as 2 x 104 Oe. When the field is reduced, a

permanent reduction in intensity of around 6% is observed. This trend continues

when the field is reversed, see Figure 3-20 IHumOOl. This phenomenon is not easily

explained, but it is interesting to note that it does not occur to the green peak of sp-Si.

This is further evidence that the mechanisms for the two peaks are different.

No peak splitting is observed in the PL bands of sp-Si at high magnetic fields,

but it is unlikely that this effect could be observed even if present given the broad

nature of these bands.

Finally, it should be noted that once a sample is exposed to a magnetic field

over 104 Oe, it no longer displays a ferromagnetic hysteresis loop.

Other Literature on Spark-Processing and Sparks in General

Three papers on spark-processing and spark erosion not published by

researchers at the University of Florida are of interest and are discussed in this

section. St. John performed spark processing on silicon covered by erbium nitrate

salts IStj97]. The erbium was incorporated into the sp-Si structure and found to be

coordinated to oxygen and exhibit broadened PL. These results are consistent with

those discussed in the previous section.

Hsu reported the formation of titanium-rich titanium carbide nanoparticles

produced by spark erosion of titanium electrodes in an organic solvent [Hsu95I. The

reaction between Ti and C is thought to occur in the vapor phase. A bimodal size

....... I I 1 .1 1 1 1 1 1 1 ....... a.. -I T ...... M ......... Il...... 111 l11



0.0 0.5 1.0 1.5 2.0 x104

Magnetic field strength (Oe)

Figure 3-20: Evolution of the peak intensity of the UV/blue PL band of sp-Si with
exposure to a varying magnetic field. Adapted from HumOO with permission.

-2.0 -1.5 -1.0 -0.5

..t........., ......... .........l.... 1.....t.........t......... ......... ..... ..........

" - ... .. .... .... .... I

distribution of particles was found. 5-50 nm particles were attributed to the rapid

condensation of TiC and Ti vapor while larger 5-20 rtm particles were attributed to

solidification of remelted TiC:Ti.

Zhang reported on a novel method of performing spark erosion by electrical

discharge machining (EDM) with a constant voltage supply [Zha97]. An ultrasonic

repetition rate of sparks was achieved by vibrating the cutting electrode at ultrasonic

frequency. While this paper belongs to a very different field than spark-processing, a

thorough explanation of the processes that occur in a spark discharge is discussed in


If a potential below the breakdown voltage is applied between two electrodes

in air, a tiny current will flow. The conductivity is provided by gas molecules that are

ionized by various forms of ionizing radiation including UV light and cosmic rays.

This current is stable if the potential is unchanged and is sometimes referred to as the

"hidden current".

If the potential is increased further, a halo-discharge occurs. Here the current

is large enough to cause some additional ionization. However, even at this stage,

there is not enough current to create a specific ionization channel and the halo

expands in a diffuse manner.

If the potential is increased further still, the halo-discharge becomes a self-

continuous discharge. The production rate of charged particles overcomes the de-

ionization rate and the number of ionized particles increases dramatically with time.

This rapid increase in current is known as the spark discharge phase. The entire spark

discharge phase lasts about 107 to 105 seconds. During this short period, a large

amount of energy is released resulting in highly localized heating. This creates a

tremendous overpressure. Typical temperatures and pressures are on the order of

10,000 K and hundreds of atmospheres. A shock wave travels away from the spark

column through the surrounding gas.

When the number of ionized particles again reaches equilibrium, the final

stage is reached. This is known as the arc discharge phase and is stable as long as the

potential is maintained. In this phase, many ionizations and recombinations occur in

equilibrium and the current and total number of ions are constant. The arc can last a

long time and results in significant melting and/or evaporation of the cathode surface

(the heavy ions striking the cathode carry much more momentum than the electrons

striking the anode). When the potential is reduced below the point necessary to

maintain an arc, the hot column of gas rapidly cools resulting in a sudden reduction of

pressure to near vacuum. This, in turn, collapses forming another shock wave.

If a series of spark and arc discharges occur repeatedly, a large amount of

melted and vaporized material can be produced. The free material is drawn into the

gap by the vacuum associated with the end of the arc discharge and blown outward by

the shock wave associated with the spark discharge. At the same time, ions are being

produced and the cathode is being melted/evaporated (mostly in the arc phase). This

repetition results in a continuous mixing and remixing of molecules from the gas

phase and the cathode in an environment with an abundance of available thermal and

electrical energy to drive chemical reactions.

Understanding the nature of sparks and how they develop and progress in time

provides the necessary background needed to interpret the microstructure of sp-Si and

the effects of changing the spark-processing parameters on the PL of sp-Si. These

processing parameter variations will be presented in detail in Chapter 5 and newer

SEM micrographs of sp-Si will be presented in Chapter 6.

Review of Quantum Size-Effect Literature

While a very strong case against the involvement of quantum confinement or

"quantum dots" in the PL of sp-Si has been made in the literature, the topic deserves a

closer second look. if only for the reason that nanoscale silicon structures are present

in the material.

Radiative Lifetimes in Quantum Systems

The radiative lifetimes of sp-Si will be discussed in Chapter 8. It is difficult to

compare the lifetime of sp-Si to published lifetimes of quantum structures, because

there is substantial disagreement among the published lifetimes for both experimental

and theoretical treatments of quantum systems.

Beginning with the experimental papers: Bimberg observed lifetimes of

"dozens of picoseconds" in GaAs/AIGaAs and InAs/GaAs quantum dots |Bim97]. It

should be pointed out that these materials already exhibit efficient direct PL in the

bulk without quantum effects and the PL was only in the range of 1.1 to 1.3 eV even

for dot diameters as small as 0.5 to 2.5 nm.

Linnros prepared Si nanoparticles in a Si02 matrix by ion-implantation of Si

into silica (Lin971. The dosage was varied by changing the thickness of an

amorphous silicon (a-Si) implantation mask above the silica layer. Phase segregation

of the excess Si was achieved with a 1 hour 1100 C anneal under N,. The resulting

PL wavelength varied with the implantation dose (longer wavelength for more Si) and

was believed to correlate with particle size. In all cases, the PL decay took the form

of a stretched exponential similar to that reported for PS. The primary exponential

time constants decreased exponentially with increasing PL energy from 20 to 50 as

for 1.45 eV PL to 5 to 15 is for 1.9 eV PL. The extended tails were attributed to

interactions between adjacent quantum dots. The PL was believed to be excitonic

even at room temperature.

Okamoto observed low temperature PL in 2D Si/SiO2 single quantum wells at

low temperature (2 K) [Oka97]. A small size-dependent PL peak was seen

superimposed on a much larger size-independent peak which was located at 1.6 eV

and had a decay time of 1.1 ms. The size-dependent portion had the following

locations and decay times: 1.8 eV and 0.3 ms for a 0.6 nm well, 1.7 eV and 0.6 ms for

a 1 nm well, and 1.5 eV (no decay time reported) for a 1.3 nm well. Stokes shifts for

resonant excitation were from 0.1 eV to just under 0.4 eV, which is consistent with

theoretically expected exciton binding energies in quantum dots. The majority of the

luminescence was attributed to defects at the Si/SiO, interface while the small size-

dependent portion was attributed to a quantum size effect.

Min et al. prepared Ge quantum dots in SiO2 by ion-implantation of a super-

saturated concentration of Ge IMin96]. No correlation was seen between nanocrystal

size and the peak energy and lifetimes of the PL. In addition, the PL shows only

weak temperature dependence, which is inconsistent with the published theory for Ge

quantum dots. The PL was identical to that seen in silica damaged from Xe

implantation with no quantum dots present. The luminescence was attributed to

radiative defect centers in the SiO, matrix.

In a theoretical paper, Takagahara and Takeda used first-principle calculations

to describe the development of pseudo-direct behavior in quantum dots of indirect-

gap materials such as Si and Ge [Tak921. The interesting claim is made that the

resulting band folding in a nanocrystal is similar to the local action of an isoelectronic

center such as the nitrogen impurity in GaP:N. The calculated lifetimes were found

to vary from the ns range for 1 nm dots to the ms range for 3 nm dots of either Si or

Ge. Effective gaps were calculated to be 1.5 eV for 3 nm particles for both Si and

Ge. To achieve 3.0 eV gaps, sizes as small as 1.3 nm in Si and 1.7 nm in Ge were


The large disagreement in time constants for processes attributed to quantum

confinement makes it difficult to argue that a process is not due to such effects based

on a measured time constant. However, there seems to be no experimental evidence

of time constants in the 1-10 ps range for known, well-characterized, quantum

structures. This is important because, as will be shown in Chapter 8, the primary time

constants of sp-Si are in this sub-10 ps range.

Particle Size vs. Energy Gap

Several of the papers in the preceding section also discuss the correlation

between PL peak wavelengths and particle size or the lack of such agreement. It is

evident, even in theoretical models, that very small particles would be needed to

account for the UV/blue peak in sp-Si. Takagahara's calculation that a 1.3 nm Si

quantum dot would have an absorption band-gap of 3.0 eV is in close agreement with

the simpler effective mass approximation model. If one considers the high Stokes

shift observed in quantum systems (hundreds of meV) and the fact that the UV/blue

emission of sp-Si (with 325 nm excitation) is at 3.3 eV (not 3.0 eV), then a gap on the

order of 4 eV would be necessary for a quantum dot model of this PL band. This

would correspond to a particle size significantly smaller than 1 nm. In addition, there

seems to be a wavelength vs. particle size discrepancy between theoretical papers and

experimental reports in the literature. The wavelengths often do not correlate well to

particle size, or when they do, they are often longer for a given particle dimension

than predicted by theory.

It is interesting that even in a very well characterized system, such as that

studied by Okamoto, where some luminescence that correlated with size was found, it

appeared to contribute only slightly to the total luminescence intensity. Thus,

assigning the PL bands of sp-Si to non-size effect related systems is not necessarily a

challenge to the validity of quantum theories. Such phenomena may simply be

masked by more prominent ones.

Papers by Gotza and Wu and their coworkers provide more examples of well-

defined quantum structures that fail to produce observable luminescence. Gotza et al.

prepared arrays of free-standing isolated 2-3 nm diameter silicon wires using reactive

ion etching of silicon-on-insulator wafers followed by a self-limiting oxidation

[Got981. The buried insulator layer not only isolated the wires electrically from the

substrate, but served as an effective etch stop. These wires were then passivated by a

layer of a-SiN,:H followed by an anneal in a reducing atmosphere (forming gas).

Under 325 nm excitation, a broad luminescence could be seen with a peak

wavelength which depended on the value of x in the SiN,:H capping layer. In

addition, PL bands were observed at 400 nm, 480nm, 560 nm, and 650 nm. All of

these emission bands were attributed to various defect states in SiN, and SiO2. Wu et

al. observed PL spectra from nanocrystalline Si:H that has no correlation with particle

size, even for mean particle sizes as low as 2.2 nm IWu_981. The PL was attributed

to the radiative recombination of carriers at interface defects in the nanocrystalline


Excitonic Nature of Quantum Dots

Several of the papers already discussed report or predict that various

nanoparticle systems show excitonic behavior, even at room temperature. According

to Yip, quantum systems should exhibit strong Stark-effect red-shifting and intensity

quenching due to an applied electric field:

In response to an electric field, the electron and hole
eigenstates follow the contours of the linear energy gradient in
opposing directions so that a large red-shift necessarily induces a
strong polarization of the eigenstates with the corresponding reduction
in the oscillator strength of the absorption resonance. [Yip98, 8021

The same should follow for the emission resonance as well, which differs only

by the Stokes shift corresponding to the exciton binding energy. While Yip worked

with InAsP/InP and InAsP/InGaP quantum wells, this quenching may be expected to

be even more prominent in indirect-gap materials that rely on relaxation of the

momentum selection rules for luminescence. By reintroducing a spatial dependence

to the eigenstates that increases the expectation value of the hole/electron separation

distance, the external field decreases the localization of the system. In addition to the

decrease in resonance that occurs in direct-gap quantum systems, there should be an

additional decrease in the oscillator strength from the loss of Heisenberg-related

momentum smearing.

There is a strange disagreement about the quantum confined Stark-effect in

the literature. In bulk (non-quantum) semiconductors which show excitonic behavior

(often at low temperatures), the Stark-effect results in a blue-shift because the binding

energy of the exciton subtracts from the gap energy. When the exciton is destroyed

by the electric field, the emission energy increases by an amount equal to the binding

energy. Haung describes the same phenomenon in quantum confined systems

[Hau891. While this blue shift contradicts the red-shift described by Yip et al., Haung

confirms that highly confined quantum systems are excitonic at room temperature and

that moderate electric fields cause large decreases in the oscillator strength of such

systems. While the author finds the argument of Yip more compelling, it may not be

necessary to resolve the red/blue-shift dispute for purposes of this study.

If a distribution of quantum dot sizes were responsible for either broad PL

band in sp-Si, it may be impossible to observe any Stark red or blue-shift because its

magnitude may be insignificant compared to the total width of the band. However,

intensity quenching would apply to each nanoparticle in isolation and should be seen.

A total lack of any Stark-effect quenching in the UV/blue band sp-Si will be reported

in Chapter 9.

Limitations of the Various "Size-Effect" Models

An ideal theory would be capable of predicting the whole range of

semiconductor properties from a single atom to an infinite bulk crystal. This is

clearly not available. Different approaches are used for different scales or ranges.

Bloch functions do a good job of predicting the band structure of infinite crystals.

other models can account for surface relaxation in finite crystals, etc. The quantum

confinement approach, especially in its simplest form (the particle in a box) seeks to

extrapolate the behavior of small particles from their bulk properties. At the other

extreme end of the spectrum, molecular models seek to explain the behavior of

molecules and small clusters by building them up from individual atoms and atomic

bonds. More sophisticated "quantum" models blur the line as to exactly what

category they belong to. Of course, the natural system is not changing, only the

model. The ultimate test of any model is how well it works at predicting the behavior

of real systems. A number of papers, considered under the broad umbrella of

quantum confinement literature, blur these lines between the molecular approach and

the quantum size effect approach.

Khurgin reports that real Si/SiO2 quantum dot systems do not exhibit

luminescence that corresponds well to the ideal model where the blue shift is

inversely proportional to the square of the particle diameter [Khu96I. After

acknowledging that this lack of correlation has led to models based on oxidation and

surface states. Khurgin suggests that the discrepancy may be accounted for by

considering that a majority of the luminescence may come from a small portion of the

size distribution of particles. A model similar to the effective mass approximation,

but that takes into account all three lattice directions, demonstrates that the oscillator

strength (OS) of luminescence depends only on the smallest dimension of a

nanoparticle, not on the average dimension. Furthermore, the OS is proportional to

d6, where d is the smallest dimension. With such a nonlinear relationship, the mean

particle size measured by Raman or other means is less important than the size of the

smallest population of particles, which will contribute most of the luminescence.

While the author agrees with this in principle, the d6' relationship is questionable as

nowhere in the derivation did Khurgin account for the finite barrier of a Si/SiO,

quantum dot. A more rigorous treatment by Hybersten finds that the OS of a

quantum dot varies with d 6 only for the idealized case of an infinite barrier and with

as little as d3 for finite barriers IHyb94|. Finally, Khurgin et al. achieved a close fit

to empirical results by assuming a 2 nm size distribution about the mean. These

results fit data for systems exhibiting luminescence between 1.7 and 2.7 eV. Despite

the good fit between this modified theory and the observations, Khurgin has an

excellent grasp of the limitations of the quantum confinement model:

It is important to note that this statement is not contradictory to
the alternative model: since size of the NC's [nanocrystals] is of the
same order of magnitude as the size of the exciton in the silicon oxide
or siloxene, and/or the spatial extent of the surface state, the
probability of electron-hole recombination determined by the volume
in which they are confined, should be roughly the same for the exciton,
surface state, or quantum confinement. In this respect, making the
distinction between the quantum confinement, surface state, and defect
makes very little sense on the nanometer scale. IKhu96, 1243]

Quantum Dots as Large Molecules

Three papers that blur traditional lines are discussed in this subsection.

In the first paper, Delley uses a density functional approach to calculate the

(absorption) band-gaps of small silicon structures ranging from a single silicon atom

to a 3 nm (706 Si atom) nanoparticle as well as an extrapolation all the way up to bulk

silicon fDel931. The claim is made that dangling bonds would completely mask the

effective band gap, so all dangling bonds were considered to be hydrogen terminated.

After correcting for self-energy considerations using a Green's function dynamically

screened interaction approximation, the calculated band-gaps at both extreme ends of

the size scale are very close to the known values (within 0.1 eV for bulk Si and within

0.3 eV for pentasilane). Through a symmetry consideration, the model also predicts

the pseudo-direct gap behavior that occurs in small particles. This may be about as

close as one can get to an ideal model for Si particles of all scales, at least in these

respects. The model predicts that the band-gap varies with N"3 where N is the

number of Si atoms in the cluster (this is essentially d' for spherical clusters) while

the OS is nearly constant for clusters of 1-30 atoms and decreases exponentially with

the number of atoms above 30. A band-gap of over 6 eV is calculated for a single

silicon atom and a band-gap of 2.5 eV is calculated for a 3 nm (706 atom)

nanoparticle. The latter is claimed to correspond to the smallest observed particles at

the time of submission and to be in agreement with the observed absorption energy


In the second paper, Ebihara reports on the PL and EL of polydihexylsilane

(PDHS) [Ebi971. While no models are presented, if the preceding papers are

accurate, such a small molecule is the logical extension of the concept of a silicon

quantum dot to smaller sizes and higher energies. PL emission is reported to be at

3.35 eV (370 nm) at 77 K and 3.23 eV (384 nm) at 4.2 K. The quantum efficiency is

less than 0.1 % at 300 K and it increases to almost 0.7% at temperatures near 4.2 K.

The trend is fairly linear. At room temperature, this peak is close in energy to the

UV/blue peak of sp-Si but, as will be seen in Chapter 8, the thermal quenching of sp-

Si is nonlinear. Also, Ludwig observed no thermal shift in the UV/blue peak. but the

author did observe a blue shift with increasing temperature.

In the third and final paper, Filonov presents a fascinating report on the

calculated electronic properties of Si,4 clusters with varying degrees of oxygen

involvement 1Fi981. The Si,4 cluster is highly symmetric and corresponds to a

silicon nanoparticle about 0.8 nm in diameter. Filonov starts with a fourteen atom

lattice "fragment" with a vertical direction corresponding to the bulk Si <111> axis

and atoms at their standard diamond lattice sites. There are 16 Si-Si bonds and 24

dangling bonds that are hydrogen terminated. This initial molecule is, thus. Si14H24.

The effect of adding oxygen to the structure is determined by building additional

prototype molecules one additional oxygen atom at a time. Two possibilities were

studied: replacing terminating hydrogens by -OH groups and inserting -0- bridges

between Si-Si bonds. The second scheme is limited to the addition of 16 oxygens.

one into each of the 16 Si-Si bonds, and the decision was made to limit the number of

oxygen atoms incorporated by both schemes to 16. To this end, 21 prototype

molecules were constructed: Si14H24, Si4H2,(OH), with x between 1 and 16, and

Si14H240, with x between 1 and 16 (the following values of x were skipped in both

cases: 2, 4,7, 9, 11, and 13). The latter structure essentially approaches a silica, not

silicon, quantum dot. For each x value, the lattice relaxation of the structure is

estimated using an MM2 routine and the lowest energy locations for the oxygen

atoms are selected. The candidate structures are then further optimized using

molecular orbital theory. The specific theory used was MO LCAO theory using the

Modified Neglect of Diatomic Overlap-Parametric Method 3 (MNDO-PM3) in the

restricted Hartree-Fock approximation. Only valence electrons were considered as

individual bodies. Core electrons were modeled as part of the nucleus. This method

allowed both the ground and excited states to be modeled, neatly giving absorption,

emission, and Stokes shift energies for every structure. The simplest Si 14H24 structure

has an absorption band-gap of about 4.4 eV and an emission energy of 3.7 eV, giving

fairly close agreement with more common quantum confinement models.

Hydrogen replacement by -OH groups at the surface resulted in little change

in the emission energy of the cluster but there was a continuous decrease in the

absorption energy with increasing -OH incorporation. This also resulted in a

decreasing Stokes shift with more -OH that was attributed to a decrease in physical

relaxation of the excited state due to mutual repulsion among the electronegative -OH

groups at the surface. The addition of bridging -0- in the bulk of the dot gave

significantly different results. The Stokes shift was essentially constant for values of

x between 0 and 10. which is attributed to the lack of influence of the oxygen on the

surface that undergoes relaxation. The bulk is already too tightly bonded to suffer

significant relocation of atoms. The conclusion is made that the Stokes shift in a

quantum dot is essentially a function of surface termination. As x approaches 10, the

absorption and emission energies decrease in a linear fashion. With x between 14 and

16, the energy levels are radically different (they suddenly increase about 2 eV) and

the Stokes shift is tiny. It is actually negative when x is 16, indicating that the ground

state is unstable. When x is 12. the energies and Stokes shift are intermediate

between the two forms. It is in this range that the molecule looses its silicon

character and takes on properties similar to silica. The overall ground state energies

of the oxygen-incorporated structures are lower than those of the -OH terminated

ones and are more likely to occur in nature.

The excitation and emission wavelengths and constant Stokes shift of the

bridging oxygen-incorporated structure are remarkably similar to the behavior of the

UV/blue peak of sp-Si when excited by various wavelengths. However, this is only

true over a limited range of excitation energies so it is probably a coincidence. This

will be addressed in detail in Chapter 10.

Review of Defects in Silica and Silicon Oxynitrides

Before concluding that known defects in silica and silicon oxynitrides are not

responsible for the luminescence of sp-Si, one should consider some broader

luminescence literature beyond that which was reviewed in publications by Hummel,

Ludwig, et al. This review will concern itself with the phenomenological

characteristics of such luminescence only, instead of attempting to review the theory

of the luminescence from defects that do not apply to sp-Si. Absorption bands at 165

nm, 240 nm, 258 nm, and 620 nm have already been discussed as well as emission

bands at 185 nm. 264 nm, 288 nm. 400 nm. 563 nm. and 650 nm. All of these have

been ruled out as candidates for the UV/blue (375 nm) or green (525 nm) emission

bands of sp-Si. Some additional emission bands, as well as additional evidence

regarding these already presented, are discussed.

Low-Energy Emission Bands

States with emission energies of 2 eV or less are often reported for silica.

Three common examples are 1.7 eV (729 nm), 1.9 eV (653 nm), and 2.0 eV (620

nm). Kenyon observed all three emission peaks in silicon-rich silica [Ken96]. The

1.7 eV peak was attributed to oxygen deficient centers (ODCs), while there was no

attribution for the 2.0 eV peak. Interestingly, the 1.9 eV center was attributed to

quantum structures because it is red-shifted and looses intensity with annealing.

However, this band may be the same as the previously discussed 650 nm band.

Tohmon also observed 1.9 eV luminescence and claimed that it is a well-known

oxygen vacancy related defect IToh89a]. Shimizu-Iwayama observed the 2.0 eV and

1.7 eV bands but not the 1.9 eV band in ion-implanted SiO2:Si [Shi941. The 2.0 eV

center was destroyed by annealing at 600 C and its intensity followed that of the

electron spin resonance (ESR) signal of the E' center (believed to be a Si dangling

bond at an oxygen vacancy) in SiO.. The 1.7 eV emission was attributed to carrier

recombination at defects on the surface of Si nanoparticles. Clearly, there is

disagreement among the authors regarding the source of sub 2.0 eV luminescence in

silica. However, as the primary peaks of sp-Si are above this energy range, they can

be ruled out nevertheless.

Involvement of Other Elements

Several authors have reported on emission related to impurities in silica or

luminescence from silicon oxynitrides. Augustine observed 2.2 eV emission (564

nm) under 2.54 eV (488 nm) excitation from SiONy:H plasma-enhanced chemical

vapor deposition (PECVD) films made with a N20 source [Aug951. This

luminescence increased 10 times after a 20 minute rapid thermal anneal (RTA) and

was attributed to bandtail-to-bandtail emission of the oxynitride. Stathis observed the

ESR signal of a photoinduced defect in silica that he attributed to nitrogen

involvement [Sta84]. Two models were proposed: -Si-O-N-O-Si- and

,Si-O-N-Si.. Finally, Poumellec reports 7 absorption bands and associated

emission bands in SiO,:Ge [Pou97]. Clearly, the incorporation of other elements into

silica can be significant. In sp-Si. likely elements for incorporation are nitrogen,

tungsten (already ruled out as contributing to luminescence), and carbon (from

organic contamination). The luminescence of sp-Si produced in mixtures of nitrogen

and carbon dioxide are reported in Chapter 5. Carbon is associated with a third peak

in sp-Si located near. but distinguishable from, the green peak. This peak is not seen

in samples not deliberately processed in carboniferous gases, even when a measurable

amount of carbon contamination is present. This leaves only nitrogen as a viable

candidate for the UV/blue and green emission peaks.

Higher-Energy Emission Bands

Gee and Imai both observed similar defects in silica irradiated by much

different means JGee79, Ima88]. In neutron irradiated silica. Gee observed the 4.3

eV (288 nm) emission band previously reported in this chapter and found it to be

associated with a 7.6 eV excitation peak attributed to an ODC. Imai observed this

same emission and excitation peak in silica exposed to ArF excimer laser radiation.

An excitation band for the same emission was also found at 5.0 eV. All three peaks

were reduced in intensity after annealing in oxygen. Tohmon placed these absorption

peaks at 7.6 eV and 5.2 eV and claimed three associated emission peaks at 4.3 eV, 2.7

eV (459 nm), and 1.9 eV (653 nm) [Toh89]. Imai attributed the 7.6 eV absorption to

a relaxed oxygen vacancy and the 5.0 absorption to an unrelaxed vacancy. In an

unrelaxed oxygen vacancy, the Si atoms are near the same positions they would

occupy without a missing oxygen. The unbound electrons on the Si atoms have a

pseudo-bonding interaction with each other over such a long distance (-Si *Si-).

The potential of this system is rather high when the separation is large. If the Si

atoms are displaced somewhat, the system can reach a lower potential energy

configuration known as the relaxed vacancy. In the literature, the unrelaxed vacancy

is sometimes represented by aSi/ \Sia to distinguish it from the relaxed vacancy:

*Si-Si-. As these states do not reproduce the observed excitation spectrum of sp-Si

(to be presented in Chapter 8), these defects can be ruled out as well. However, a

related defect will be discussed in the next section.

Several authors report absorption bands in silica near 5 eV. Tohmon observed

a 5.25 eV absorption associated with 3.1 eV (400 nm) emission that he attributed to

2-coordinated silicon with two excess electrons. IToh89] While this emission peak is

near that of the UV/blue band of sp-Si when excited by 325 nm (3.8 eV) light, it will

be shown that this is not the case when sp-Si is excited by energies closer to 5 eV.

Kohketsu also observed similar peaks at 5.17 eV and 5.06 eV [Koh89]. The former is

associated with emission at 4.3 eV (288 nm) and 3.1 eV (400 nm), and the latter is

associated with emission at 4.2 eV (295 nm) and 3.0 eV (413 nm). The decay of the

3.1 eV band had a time constant of 80 ats, which will be shown to be completely

incompatible with the UV/blue emission from sp-Si. Kohketsu attributed the former

to excess Si in SiO, and the latter to small Si clusters containing at least 4 Si-Si

bonds. Bagratashvili reported absorption bands at 248 nm (5.0 eV) and 242 nm (5.12

eV) that he associated with emission peaks at 280 nm (4.4 eV) and 455 nm (2.7 eV)

for the former, and 296 nm (4.2 eV) and 396 nm (3.1 eV) for the latter. The former

was attributed to simple ODCs while the latter was attributed to ODCs associated

with germanium impurities. The 455 nm emission was reported to increase in

intensity as the temperature was increased from 290 K to 514 K. This is in contrast to

the peaks of sp-Si, which are thermally quenched, not activated. Finally, Griscom

reported these absorption peaks at 5.0 eV and 5.15 eV lGri91]. Clearly, there is much

disagreement in the literature on these similar peaks. None of which appear to fit the

behavior sp-Si. Griscom also discusses a 3.8 eV absorption band that may be linked

to peroxy linkages or interstitial Cl2 molecules.

Review of an Important Transient Defect in Silica

The observed properties and theory of one particular defect should be

discussed in detail, because, as will be shown, its absorption curve matches the

excitation spectrum of the green PL peak of sp-Si. This transient defect found in both

crystalline and amorphous silica is believed to be the self-trapped exciton (STE).

An excellent review of the properties and theory of the STE has been written

by Itoh Iito891. While the band-gap of silica in either the amorphous or crystalline

form is a matter of some debate, SiO2 of both varieties appears to have a direct

transition at 10.4 eV. Even the question of how to define a band-gap in an amorphous

material is controversial. The author prefers to call the separation between the

mobility edges the "gap" and consider local tail states to be within the gap. Often

quoted numbers such as 8 eV for amorphous and 9 eV for crystalline silica are for

indirect transitions and may include a significant portion of the tail states in the

former case. If one measures the optical absorption at many temperatures and

rigorously defines the temperature-independent convergence of the Urbach tails to

define the indirect gap, the numbers become 9.1 eV for crystalline and 8.7 eV for

amorphous silica.

Electromagnetic radiation above 10.4 eV, as well as ionizing radiation and

electron bombardment, have long been known to introduce transient defects in both

amorphous and crystalline silica. In amorphous (but not crystalline) silica, this

transient defect is credited with being a precursor of many of the defects discussed in

the previous chapter. Under such excitation, a 2.8 eV emission is observed. Its decay

is exponential in crystalline silica and both non-exponential and blue-shifted with

time in amorphous silica (this is characteristic of inhomogeneously broadened donor-

acceptor pair emission). Figure 3-21 shows the transient optical absorption reported

by Itoh for this defect. A 5.2 eV absorption peak with a 4.2 eV satellite peak are

observed. It is remarkably similar for both amorphous and crystalline silica

indicating that this state likely arises from the reorganization of bonds in the SiO2

backbone common to silica in all its forms. This transient state has a triplet ODMR

(Optically Detected Magnetic Resonance) signal shift an order of magnitude stronger

than that observed for a recombining free electron and hole. The most important

evidence comes from the observation that this transient state has a large volume

change on the order of one SiO. molecular volume per defect.

-I- 1


I /


I ""I I I
3.0 40 SO 6.0

Figure 3-21: Transient optical absorption spectra of the defect believed to be the self-
trapped exciton in crystalline and amorphous silica. Adapted from Ito89.


The Model for the Self-Trapped Exciton

The popular model to explain these observed properties is that of a self-

trapped exciton in silica, usually abbreviated STE. This is actually a double exciton

consisting of an oxygen vacancy with two extra electrons and an adjacent peroxy

radical with two holes. Both the defect as a whole, as well as each site individually,

have paired spins and are diamagnetic. The structure of this defect is shown in Figure

3-22. Note that in a Si02 crystal, the peroxy group must invade the adjacent

interstitial site for room and this configuration makes the STE very similar to the

close Frenkel pair defect in crystalline silica (and it can be considered a donor-

acceptor pair in amorphous silica as well). Most authors simply call it the Frenkel

defect, but Itoh is careful in pointing out that it is an isomer of the idealized Frenkel

defect because the peroxy group occupies both the standard oxygen site and part of

the adjacent vacancy site, giving a smaller vacancy-interstitial separation than in the

classical Frenkel pair. The classical interstitially-bonded Frenkel defect for silica

would have the extra oxygen simply inserted into the silica linkage in a non-resonant

manner instead of as a closely bonded peroxy radical. Thus, it is more rigorous to

represent the STE as mSi- Si-O,2-Si. than as mSi -Si-O-O+-Si-. This

distinction is not trivial, for reasons that will be addressed shortly.

The large Stokes shift between 10.4 eV excitation and 2.8 eV luminescence is

attributed to the lattice relaxation necessary to break one Si-O bond and create the

subsequent peroxy radical.

In another publication, Itoh makes several additional important points

Iltoh88j. Evidence is building that the STE is common in insulating solids with

strong electron-phonon coupling. Since SiO, is a "lone-pair" semiconductor with the






Figure 3-22: Proposed structure of the self-trapped exciton in silica consisting of an
oxygen vacancy with two electrons and an adjacent peroxy radical with two holes.
Adapted from Ito89.

valence band strongly associated with electrons at oxygen sites and the conduction

band strongly associated with electrons at silicon sites, such coupling is unavoidable

in silica. Strong experimental evidence of the STE exists for Y205 and A1203 as well.

It would not be unreasonable to expect the STE to exist in all oxides.

Upon closer examination, Itoh found that the STE can decay through a

completely intrinsic process and give the previously reported 2.8 eV luminescence as

well as decay by charge transfer to other nearby defects through an extrinsic process

resulting in 2.5 eV luminescence. The 5.2 eV transient absorption peak and the 4.2

eV satellite peak are not from separate absorption centers but are a superposition of

two transitions of the same center. In an optical bleaching experiment, the fractional

change in the optical density across the entire transient absorption spectrum of the

STE was found to be completely independent of probing energies between 3.0 and

5.4 eV.

Due to the degree of polarization of the 2.8 eV intrinsic luminescence of the

STE, Itoh was able to calculate that the STE is created by the breaking of one Si-O

long bond followed by the oxygen entering a peroxy arrangement with the only other

oxygen which is long-bonded to that same silicon atom. This is important because

Robertson has challenged the model of the STE on the basis that 10.4 eV is not

sufficient to break "three Si-O bonds" [Rob85]. It seems that Robertson is adhering

to the "ball and stick" model of the peroxy ion being two oxygen atoms with a normal

bond between them. It appears much more reasonable to the author that one Si-O

long bond is broken and the free oxygen atom resonantly rearranges its electrons with

the neighboring oxygen without ever breaking the bonds that the second oxygen atom

shares with its silicon neighbors.

It is believed that the 5.2 eV transition results from one of the excess electrons

at the vacancy site being promoted from a bonding to an anti-bonding orbital while

the 4.2 eV transition results from one of the same electrons being physically

transported to a higher orbital of the peroxy radical site, see Figure 3-23. The STE is

metastable, which is probably due to the tetrahedral pseudo-bonding that occurs

across the vacancy (the ground state of the STE has the two excess electrons in

bonding orbitals of the vacancy and two holes in the anti-bonding orbitals of the

peroxy radical). The STE decays with a time constant roughly on the order of

milliseconds if left alone and nearly instantaneously if excited by either transition.

The whole structure becomes unstable if one of the excess electrons cannot "see" the

other across the vacancy or if the peroxy radical has even one electron in an anti-

bonding orbital. Consequently, a single-exciton STE is never seen because there is

no energetic advantage to its formation. The STE relaxes nearly all of the 10.4 eV

used in its formation and is only about 2.8 eV in potential above the ground state.

There is a significant metastable energy barrier between the STE and the ground state

that can be overcome by energies in the range of 3.0 to 4.5 eV. However, the driving

force to return to the ground state is not that large and this may explain why the STE

readily transfers the offending electron to another defect in the extrinsic process,

probably by propagating it along the silica backbone and possibly even accepting a

lower energy replacement in the same manner. To occur with any likelihood, this

charge transfer process would have to be very fast since the optical bleaching itself is

known to be very fast.

The lifetime of the STE is about I ms |Hay84| in crystalline silica and about

10 4ts [Tani881 in amorphous silica. The decay of the former is in the form of a

stretched exponential or power law while the latter is exponential. Non-exponential

5.2 eV absorption AV
4.2 eV absorption

2.8 eV luminescence

Si-Si 0-0

Figure 3-23: Ground state of the self-trapped exciton (STE) in silica with two electrons
in bonding orbitals of the Si *Si pseudo-bond and two holes in the anti-bonding orbitals
of the peroxy radical along with proposed optical transitions:
2.8 eV emission from the intrinsic decay process of the STE to normal silica
5.2 eV absorption promoting a vacancy electron to an anti-bonding orbital
4.2 eV absorption transferring a vacancy electron to a higher orbital of the peroxy
Adapted from Ito88.

decay is usually observed in donor-acceptor pair luminescence when there is a

distribution of pair separation distances. This also explains the observed blue shift

with time seen in the PL of the a-SiO2 STE. While the favorable bonding

arrangement of the doubly charged STE (filled bonding orbitals in a pseudo-bond

across the vacancy in a configuration some authors refer to as a "relaxed vacancy"

and filled bonding orbitals at the peroxide with empty anti-bonding orbitals at both

sites) makes this state nearly stable, the driving force for the destruction of the STE

certainly comes from the very large strain it creates in the material. While one would

expect this strain to be more severe in the crystalline material, the faster time constant

of the amorphous case may be explained by the lower diffusivity of the oxygen in

crystalline silica. Once the oxygen occupies an interstitial site, it must displace other

lattice atoms to return to the vacancy site.

There are important points in three more papers, which involved Itoh, but with

either Tanimura or Tanaka as primary authors. These are discussed in chronological


Tanimura et al. studied the formation of the STE when silica is exposed to

electron bombardment and found the formation process was nonlinear with electron

dose |Tani83 I. This supports the idea that the singly ionized STE is unstable and that

a two-step ionization process is necessary for the formation of the metastable STE.

The current must be high enough that there is a significant likelihood of the same
"mer" on the silica backbone being doubly ionized within the transient lifetime of the

unstable singly ionized STE. In addition, the volume change of the STE was

carefully measured and found to be at least one molecular volume and essentially the

same for both crystalline and amorphous silica.

Tanaka et al. verified the volume change reported by Tanimura et al. and also

discussed the likelihood of a STE explanation for transient absorption bands observed

in As2Se3 [Tan851.

Finally, Tanimura et al. performed extensive experimental work on electron

irradiated amorphous silica JTani88]. The main transient absorption peak was found

to be 5.3 eV for amorphous silica rather than the 5.2 eV reported for crystalline silica,

while the satellite band remained at 4.2 eV in both materials, see Figure 3-24. The

transient absorption of high purity silica as well as silica with high concentrations of -

OH and Cl is identical indicating that this defect involves the backbone structure of

SiO2. A Gaussian deconvolution of the spectrum indicates the two absorption peaks

are 5.30 eV with a full-width half-maximum (FWHM) of 0.78 eV and 4.20 eV with a

FWHMof 1.16eV.

Polished silica plates were placed on a 78 K cold finger and irradiated with a 2

MeV 20 ns electron pulse from a Febetron 707 accelerator creating electron-hole

pairs in concentrations up to 2 x 1018 cm-3. The time evolution of the PL intensity at

450 nm and the optical absorption at 250 nm were monitored following the electron

pulse. Finally, a 308 nm laser pulse from a XeCI excimer laser was used to bleach

the transient STE center. Figure 3-25 shows the resulting PL and transmission curves

vs. time. t, represents the electron pulse and t, represents the laser pulse. The width

of the laser pulse and the data sampling rate were not stated. The creation of centers

at te and the destruction of centers at t, are too fast for the instrument to clearly

resolve. The destruction or bleaching of the center verifies that its excited state is

unstable. It should be noted that these plates were only 1 cm thick, yet the material

experienced a 70% reduction in the transmission of 250 nm UV light. The absorption


So ) a-SiO2
(&A ) a-SiO2:OH
0 I a-SiO2: CI

0.4 -


3 5 6
Photon energy (eV)

Figure 3-24: Transient optical absorption spectra of the self-trapped exciton in
amorphous silica showing no difference among high-purity silica. -OH-rich silica, and
Cl-rich silica. Peaks at:
5.30 eV with a FWHM of 0.78 eV
4.20 eV with a FWHM of 1.16eV
Adapted from Tani88.





-S1 I.-

Figure 3-25: Decay of the self-trapped exciton (STE) in amorphous silica:

(a) PL intensity of decay emission (monitored at 450 nm)

(b) optical transmission of sample (monitored at 250 nm)

te denotes the 2 MeV electron pulse that creates the STEs while t, denotes the 308 nm

laser pulse that bleaches the STEs. Adapted from Tani88.



m L


B -

I m m It E

at the 5.3 eV (234 nm) peak would be about 20% stronger than it is at 250 nm.

Clearly, the STE is a strong absorber of UV light.

Additional Evidence of the Nature of the Self-Trapped Exciton

A number of other sources offer some additional clues to the nature of the

STE. Hayes performed OMDR measurements on crystalline quartz [Hay84]. In

quartz, as well as amorphous silica, each oxygen atom is bound to two silicon atoms

by one long bond (1.612 A) and one short-bond (1.607 A). Each silicon atom has two

oxygen atoms connected by long bonds and two by short bonds. Hayes observed that

long bonds are weaker and preferably broken. This is consistent with the structure

presented by Itoh. A physical explanation of the difference in lifetimes for the STE in

amorphous and crystalline silica has already been presented. However, Hayes

attributes the 1 ms lifetime of the STE in its unexcited state in c-SiO, to his

observation that the transition from the STE to the standard silica bonding

configuration is spin-forbidden. If this explanation for the time constant is true (as

opposed to the limited diffusivity argument previously proposed), one must make the

argument that the lOOx faster time constant in the amorphous case comes from some

relaxation of the spin selection rules that is not seen in the crystalline case.

Silin discusses the transient defect in silica which he also attributes to the STE

and describes how it can give rise to more traditional defects like the various E'

centers, NBOHCs, and neutral peroxy bridges in amorphous silica ISi1801. Careful

reading of this paper indicates that Silin is actually discussing the singly-ionized

version of the STE because of a specific reference to "an electron in an anti-bonding

orbital" which makes the exciton unstable. This may be the critical difference

between the mechanism that leads to defect formation and that which leads to the

metastable doubly-ionized STE.

The Physics of Self-Trapping in Polar Solids

Some explanation of what exactly "self-trapping" means and why it occurs

should be made. Mott and Davis are a good source for this but can get quite involved

for the casual reader IMot77]. In general, self-trapping occurs when an electron,

hole, or exciton interacts with the lattice in such a way that it becomes distorted and

the combined system has a lower energy than the free carrier. It is called self-

trapping because the defect did not exist prior to being induced by the carrier; rather,

the carrier created the defect to minimize the total energy of the system. Such a state

is essentially a molecular polaron. The hindered-rotor problem and the Jahn-Teller

effect from solid-state physics are analogues of self-trapping. Self-trapping occurs

only when the lattice distortion is large enough to create a global minimum in the

energy vs. coordination coordinate plot, see Figure 3-26 for an example. A local

minimum is not sufficient for self-trapping at finite temperature as such metastable

states are very shallow and free carriers can find their equilibrium position within a

few lattice vibrations. Such thermalization is a femtosecond process (Bla94j. Strong

interactions between the lattice and carriers occur in silica because it is a "lone-pair"

insulator with a valence band made up of oxygen 2p-orbitals. This, combined with a

reasonably large band-gap, guarantees that nearly every outer-shell electron in the

solid is at an oxygen site. This strong polar nature of the lattice provides the needed

carrier-phonon coupling. Self-trapping has been reported in many oxides including

MgO, CaO. SrO, ZnO, BeO, and Ai20,.



(a) q

"a sdiii)

w^ W Uidi ___
0 ^<^ ^~~


(c) W (ii)

Figure 3-26: Theoretical self-trapping mechanism for a hole:
(i) denotes the elastic energy, (ii) denotes the electronic energy, and (iii) denotes the total
(a) no polaron is formed
(b) a polaron is formed with trapping energy W
(c) the case of trapping by a pre-existing defect.
Adapted from Mot77.

The Polar Nature of Oxides and "Lone-Pair" Insulators

The domination of the valence band by oxygen in an oxide is explained in

simple terms by Fowler [Fow86J. Taking SiO, as an example, each silicon atom

contributes 4 valence electrons while each oxygen atom has 6 electrons in its outer

shell. There are also two oxygen atoms for every silicon atom. This means that there

are 12 valence electrons contributed by oxygen for every 4 contributed by silicon.

Even if the bonding were covalent, the oxygen would contribute 75% of the total

valence electrons. However, the Si-O bonding is highly ionic with the electrons

favoring the oxygen. For this reason the valence band is almost totally dominated by

electrons at oxygen sites.

In this same paper. Fowler also discusses the nature of the bonding across the

oxygen vacancy in silica and supports the idea of a pseudo-bond bridging the


Even with the structural constraints imposed by the crystalline
environment the silicons will approach to =2.5 A 12.35 A for
amorphous apart.... The suggestion that the 0 vacancy is better
thought of as a Si-Si bond seems to be valid. IFow86, 1741

In addition. Fowler discusses the STE in some detail. The paper is supportive

of the position of Itoh, Tanimura. and Tanaka but does mention that some controversy

exists on the subject. For example, based on the same evidence for the relaxation of

the oxygen vacancy, Fowler points out that one would expect the oxygen vacancy

associated with the STE to lead to a contraction and not an expansion as observed.

The discussion closes with:

Although no calculations have yet been reported which bear on
the stability of this self-trapped exciton, it is reasonable to assume that
the system has found the lowest energy atomic arrangement consistent

with its being an excited electronic state; in a configuration-coordinate
picture it is at the minimum of the excited-state curve. Apparently
there are no low-energy crossovers to the ground-state configuration
coordinate since radiative de-excitation occurs. [Fow86, 179]

Summary of the Properties of the STE

Mott has stated unambiguously that the coordination coordinate is a stress-

related concept |Mot75]. Taking this into consideration, we can conclude that the

STE is chemically and electronically stable, but is not favored elastically. This elastic

constraint is relatively small in magnitude, as the chemical and electronic stability of

the STE relaxes all but 2.8 eV of the 10.4 eV potential of the classical exciton.

The singly-ionized STE and the electronically excited STE are unstable. The

excited STE decays either by returning to its own ground state by a fast charge

transfer process or by the destruction of the STE itself (driven by the elastic strain).

This charge transfer process has been observed and must be at least as fast as the

destruction process to be observed at all.

The STE is formed spontaneously from hole-electron pairs in silica and can be

formed by any process that provides the requisite 10.4 eV of energy needed to create

the pair. The lifetime of the STE in amorphous silica is on the order of 10 ps.

A Thought Experiment Involving the STE

Rather than leave the reader in the dark for several chapters as to why

the STE is important, the author believes that a small amount of argument

foreshadowing the eventual conclusions of this study is appropriate at this

point in the discussion. Consider the following thought experiment: What