GALLIUM NITRIDE-BASED ELECTRONIC DEVICES
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
To my parents and my wife, for their love and support.
I would like to express my deepest thanks to Dr. Fan Ren for introducing me to
this exciting field and for his patient guidance as my research advisor. His hard work to
provide an extremely well-equipped lab and high standards for research, along with a
very professional work environment, gives each of his students a unique opportunity to
gain valuable experience.
I must also express my thanks to Dr. Stephen J. Pearton for his invaluable
knowledge on III-V material processing and devices which has been very helpful to me;
and to Dr. Fred Sharifi for letting me use his facilities and providing a lot of help.
I would also like to thank Drs. Cammy R. Abernathy, Timothy J. Anderson and
Chang-Won Park for being members of my committee and spending time on my behalf. I
especially appreciate the lab mates from our group and collaborators from Dr. Pearton's
I am forever indebted to my parents, who always encouraged me to achieve
excellence in every aspect of life and supported me throughout my educational
endeavors; and to my wife for her selfless support and never-ending understanding.
TABLE OF CONTENTS
ACKN OW LED GM EN TS ...................................................................... iii
ABSTRACT ........... ........... ........... ....... ..................vi
1. INTRODUCTION .......................... ....................... .......... 1
1.1 Gallium Nitride-Based M materials ................... ..... .......... ..... ........... 5
1.2 Gallium Nitride-Based Optoelectronic and Electronic Devices ..................11
1.2.1 Gallium Nitride-Based Optoelectronic Devices.................................11
1.2.2 Gallium Nitride-Based Electronic Devices .......................... ............13
2. GALLIUM NITRIDE-BASED DEVICE PROCESSING...........................18
2.1 Chlorine/Argon(C12/Ar) High Density Inductively Coupled Plasma Damage in
GaN Schottky Diodes ........................ ............. .......... ......... 19
2.1.1 Introduction................................... ................. 19
2.1.2 Experimental M ethods ............. ....................... ..........................20
2.1.3 Results and Discussion ............................................... .......... ..21
2.1.4 Sum m ary and Conclusion..................... ....................................... 31
2.2 Effect ofN, Inductively Coupled Plasma Treatment on n-AlGaN/GaN
OhmicContacts................................................................ ......... 31
2.2.1 Introduction.......... ...................... ... ............31
2.2.2 Experimental M ethods ............ ...................... ................... ..... 33
2.2.3 R results and D discussion .................................................................34
2.2.4 Sum m ary and Conclusion ............ ........... ................ .... ............. 37
3. GALLIUM NITRIDE AND ALUMINUM GALLIUM NITRIDE HIGH VOLTAGE
POWER RECTIFIERS ............... ............... ........................... 43
3.1 Introduction....................... ...... ................. ..... ...... ............43
3.2 Gallium Nitride Schottky Rectifiers with 3.1 kV Reverse Breakdown
V oltage ............................................ ....... ..... .. ................. 46
3.3 Aluminum Gallium Nitride Schottky Rectifiers with 4.1 kV Reverse Breakdown
V oltage ................... ........................................... ....... 58
3.3.1 Introduction............................. .................. ........ 58
3.3.2 Experimental M ethods ............ ...................... ................... ..... 59
3.3.3 Results and Discussion ............................................... ............60
3 .3 .4 Su m m ary ............ ...... ............ ... ................ ..... ............ 6 5
3.4 Temperature Dependence and Current Transport Mechanisms in AlxGajxN
Schottky Rectifiers........... ........................ ............. .......... 65
3.4.1 Introduction.................................. .......... ........ 65
3.4.2 Results and Discussion ................... ........................... ................66
3.4.3 Sum m ary............. ......... .................... .. ... .... .............71
3.5 Lateral AlxGal-xN Power Rectifiers with 9.7 kV Reverse Breakdown
Voltage ................... ..................................... ........ 74
3.5.1 Introduction................................... ......... ........ 74
3.5.2 Experim ental M ethods................. ................. ........... .............. 74
3.5.3 Results and Discussion ........................................................... 75
3.5.4 Summary and Conclusion................. ...................... ................78
3.6 Vertical and Lateral GaN Rectifiers on Free-Standing GaN
Substrate........... ................ ..................................... ... .......... 83
4. GALLIUM NITRIDE p-i-n POWER RECTIFIERS........... .. .......... 93
4.1 Comparison of GaN p-i-n and Schottky Rectifiers Performance....................93
4.1.1 Experimental Methods ............ .................................. ...............94
4.1.2 Results and Discussion ...........................................................95
4.1.3 Sum m ary and Conclusion ............ ............................. .............. 107
5. GALLIUM NITRIDE-BASED BIPOLAR DEVICES.......... ............ 112
5.1 Gallium Nitride pnp Bipolar Junction Transistors Operated to 250 C........... 112
5 .1.1 Introdu action ................................................ ................... .. ..112
5.1.2 Experim ental M ethods ................................... .......... ........... ...112
5.1.3 Results and Discussion .................................... ..................114
5.1.4 Summary and Conclusion......................................................... 118
5.2 Direct-Current Characteristics of pnp AlGaN/GaN Heterostructure Bipolar
Transistors ................... ......... .................... ....... ... .........118
5.2.1 Introduction.................. ................. ............................ 118
5.2.2 Experimental Methods ............................................ ................120
5.2.3 Results and Discussion ................................. .... ............ 121
5.2.4 Summary and Conclusion ......................................... ..126
5.3 Self-Aligned Small-Area GaN/AlGaN Heterojunction Bipolar
Transistors ........... ................... ...................... ........ 126
5.3.1 Introduction................... .................... ................... ........126
5.3.2 Experimental Methods ............... ................. ......... .............129
5.3.3 Results and Discussion .................................... ..................130
5.3.4 Summary and Conclusion ............ ......... .................. ..133
REFERENCES ................... .............................. 137
BIO GR APH ICAL SK ETCH ....................................................................... 145
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
GALLIUM NITRIDE-BASED ELECTRONIC DEVICES
Chairman: Fan Ren
Major Department: Chemical Engineering
Gallium Nitride (GaN) and related materials (especially AlGaN) have recently
attracted a lot of interest for applications in high-power electronics capable of operation
at elevated temperatures and high frequencies. The AlGaInN system offers numerous
advantages. These include wider bandgaps, good transport properties, the availability of
heterostructures (particularly AlGaN/GaN), the experience base gained by the
commercialization of GaN-based laser and light-emitting diodes; and the existence of a
high-growth-rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing
very thick layers or even quasi-substrates. These attributes have led to rapid progress in
the realization of a broad range of GaN electronic devices.
AlxGal-xN (X=0-0.25) Schottky rectifiers were fabricated in a lateral geometry
using p'-implanted guard rings and rectifying contact overlap onto an SiO2 passivation
layer. The reverse breakdown voltage (VB) increased with the spacing between Schottky
and ohmic metal contacts, reaching 9700 V for Al0.25Ga0.75N and 6350 V for GaN,
respectively, for 100 .im gap spacing. Assuming lateral depletion, these values
correspond to breakdown field strengths of <9.67x105 V-cm-1, which is roughly a factor
of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (VB)2/RoN,
where RON is the on-state resistance, was in the range 94-268 MW-cm-2 for all the
devices. Edge-terminated Schottky rectifiers were also fabricated on quasi-bulk GaN
substrates grown by HVPE. For small diameter (75 [tm) Schottky contacts, VB measured
in the vertical geometry was -700 V, with an on-state resistance (RON) of 3 mQOcm2,
producing a figure-of-merit VB2/RON of 162.8 MW-cm2.
Gallium nitride (GaN) p-i-n diodes were also fabricated. A direct comparison of
GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher
reverse breakdown voltage for the former (490 V versus 347 V for the Schottky diodes),
but lower forward turn-on voltages for the latter (-3.5 V versus -5 V for the p-i-n
diodes). The reverse current in both types of rectifiers was dominated by surface
perimeter leakage at moderate bias. Finally, all of the devices we fabricated showed
negative temperature coefficients for reverse breakdown voltage due to high defect level,
which is a clear disadvantage for elevated temperature operation.
Bipolar devices are particularly interesting for high current applications such as
microwave power amplifiers for radar, satellite and communication in the 1-5 GHz
range, powers >100 W and operating temperatures >425 C. We demonstrated pnp
Bipolar Junction Transistors (BJT) and pnp Heterojunction Bipolar Transistors (HBT) for
the first time. For power microwave applications, small area self-aligned npn
GaN/AlGaN HBTs were attempted. The devices showed very promising direct current
For the last three decades or so, the III-nitride semiconductor material system has
been viewed as highly promising for semiconductor device applications at blue and
ultraviolet (UV) wavelengths in much the same manner that its highly successful As-
based and P-based counterparts have been exploited for infrared, red and yellow
wavelengths. As members of the III-V nitrides family, A1N, GaN, InN and their alloys
are all wide band gap materials, and can crystallize in both wurtzite and zinc-blende
polytypes. Wurtizite GaN, A1N and InN have direct room temperature bandgaps of 3.4,
6.2 and 1.9 eV, respectively (Figure 1-1). In cubic form, GaN and InN have direct
bandgaps, while A1N is indirect. In view of the available wide range of direct bandgaps,
GaN alloyed with A1N and InN may span a continuous range of direct bandgap energies
throughout much of the visible spectrum well into the ultraviolet wavelengths. This
makes the nitride system attractive for optoelectronic device applications, such as light
emitting diodes (LEDs), laser diodes (LDs) and detectors which are active in the green,
blue or UV wavelengths . Although similar applications based on InGaAlP
heterostructures have been successfully demonstrated, this material system is limited to
about 550 nm. The addition of III-V nitrides to the family of device-quality
semiconductors is essential for developing full-color displays (Fig. 1-2), coherent sources
required by high density optical storage technologies, and very likely devices for signal
Lattice Constant (A)
Figure 1-1 Bandgap of hexagonal (a-phase) InN, GaN and
versus lattice constant.
A1N and their alloys
Figure 1-2 The various ternary and quaternary materials used for LEDs with
the wavelength ranges indicated.
* Direct Bandgap
o Indirect Bandgap
AlP q ZnSe
and illumination application. Particularly, the combination of GaN-based blue and green
LEDs with GaAs-based red LEDs forms the basis for large-scale full displays and white
light illumination. The solid-state white-light source generated by mixing the primary
colors in a light scrambling configuration would provide not only compactness and high
lifetime, but also would reduce power consumption by 80-90% compared to
incandescent or fluorescent light sources.
Another area gaining a lot of attention for III-V nitrides is high-temperature/high-
power electronics [2,3,4,5]. The interest stems from two intrinsic properties of this group
of semiconductors. The first is their wide bandgap nature. The wide bandgap materials,
such as GaN and SiC, are promising for high-temperature applications because they go
intrinsic at much higher temperatures than materials like Ge, Si and GaAs. It means that
GaN power devices can operate with less cooling and fewer high-cost processing steps
associated with complicated structures designed to maximize heat extraction. The second
attractive property of III-V nitrides is that they have high breakdown fields. The critical
electric field of the breakdown scales roughly with the square of the energy band gap,
and is estimated to be >4 MV/cm for GaN , as compared to 0.2 and 0.4 MV/cm for Si
and GaAs, respectively.
GaN also has excellent electron transport properties, including good mobility, and
high saturated drift velocity , thus making this material suitable for general electronics
and promising for microwave rectifiers, particularly. The material properties associated
with high temperature, high power, and high-frequency application of GaN and several
conventional semiconductors are summarized in Table 1-1. It is anticipated that GaN may
eventually prove to be superior to SiC in this area.
Table 1-1. Comparison of 300K semiconductor material properties .
Si GaAs GaN A1N 6H-SiC
Bandgap (eV) @300 oC 1.1 1.4 3.4 6.2 2.9
indirect direct direct direct indirect
Electron mobility (cm2/V s), RT 1400 8500 1000 (bulk) 135 600
Hole Mobility (cm2/V s), RT 600 400 30 14 40
Saturation velocity (cm/s), 107 1 2 2.5 1.4 2
Breakdown field (V/cm), 106 0.3 0.4 >5 4
Thermal conductivity (W/cm) 1.5 0.5 1.5 2 5
Melting temperature (K) 1690 1510 >1700 3000 >2100
CFOM* 1 8 489 458
*CFOM=X ~vsEB2/(Z PsEB2)Sl, combined Figure of Merit for high
temperature/high power/high frequency application.
The strongest feature of the III-V nitrides compared to SiC is the heterostructure
technology it can support Quantum well, modulation-doped heterointerface, and
heterojunction structure can all be made in this system, giving access to new spectral
regions for optical devices and new operation regimes for electronic devices. From this
point of view, III-V nitrides can be considered the wide-bandgap equivalent of the
AlGaAs/InGaAs system which has set the modern benchmark for microwave device
Other attractive properties of III-V nitrides include high mechanical and thermal
stability, large piezoelectric constants and the possibility of passivation by forming thin
layers of Ga203 or A1203 with band gaps of 4.3 and 9.2 eV, respectively. In addition, A1N
has received considerable attention for its insulating property , particularly as a
potential isoelectronic insulator for GaAs field effect transistors (FETs).
1.1 Gallium Nitride-Based Materials
Substantial research on III-V nitrides growth was initiated in the early 1960s.
However, they have trailed behind the easier-to-grow Si and GaAs semiconductors on the
development curve. Nearly 30 years later, Si and GaAs have been pushed to their
theoretical limits, while nitrides are just beginning to show their promise. The
technological spin-offs came late because ideal substrates could not be found and the
consequent growth of GaN thin films contained substantial concentration of defects and
had high n-type background. Even in films with relatively small background electron
concentration, p-type doping could not be achieved until recently.
One particular difficulty in the growth of GaN thin films is the unavailability of
sufficiently large (>1 cm) single crystals for use as substrate for homoepitaxial growth.
Thus up to now, heteroepitaxial growth has been a practical necessity and the choice of
substrate is critical. Possible substrate materials should have low thermal expansion and
lattice mismatch with the grown crystals. Also, they should be unaffected by the growth
chemistries (such as NH3 or H2) at high growth temperatures (in excess of 1000 C in
some cases). Under these constraints, sapphire (A1203) and SiC are the most popular
substrate materials used currently. When hexagonal GaN is grown on the (0001) basal
plane of A1203, a lattice misfit of -13% exits at the growth temperatures. A high density
of threading dislocations is observed in GaN layers. The residual strain is comparable to
the lattice misfit between 6H-SiC and GaN, and the result is comparable with dislocation
densities observed . Today, SiC substrates, though more costly, are of increasing
interest for high temperature and high-power devices like transistors due to their good
thermal conductivity and possibility of n- and p-type doping. The materials with a close
lattice match with GaN, such as LiAlO2  and LiGaQO , were also used for
epitaxial substrates. However, the grown GaN lacked the desired electronic properties
due to either the rough growth or unintentional contamination from the substrates. The
ideal candidate substrate is clearly a GaN wafer. Several research groups are
investigating the growth of the bulk GaN crystals and very thick films through various
techniques [13-15]. However, commercially available large area GaN wafers appear to be
several years away. The nitride community is, therefore, challenged with growing of
heteroepitaxial films having large MISFITs.
Many epitaxial thin-film growth processes have been developed, including
molecular beam epitaxy (MBE) [16,17], hydride vapor-phase epitaxy (HVPE) [13-
15,18], metal organic chemical vapor deposition (MOCVD) [19-24], and derivatives of
these methods. In the past few years, MOCVD [19-24] has evolved as a leading
technique for production of III-V nitride optoelectronic and microelectronic devices. One
remarkable application worth mentioning is the achievement of super-bright blue LEDs
. Characteristics of this method include the use of high purity chemical sources, a
high degree of composition control and uniformity, high growth rates, large scale
manufacturing potential and the ability to grow abrupt junctions.
Initially the growth of GaN was performed directly on sapphire and SiC
substrates, with large crystalline defects threading vertically from the substrate interface
through the newly deposited thin film. The wafer usually had rough surfaces mainly
caused by the 3D-growth mode. In 1986, Amano et al.  succeeded in remarkably
improving the GaN surface morphology as well as the electrical and optical properties by
depositing a thin low-temperature A1N buffer layer before the high-temperature growth
of GaN. The essential role of this buffer is to serve as a template for the nucleation of
growth and promote lateral growth of the GaN film due to the decrease in interfacial free
energy between the film and the substrate. Although the buffer layer has reduced the
effects of the lattice mismatch, the densities of the threading defects in these thin films
are still in the range of 109-1010 cm2, and on the order of one million times higher than in
other semiconductor systems such as Si and GaAs. These defect-laden materials, to date,
have had a surprisingly small effect on the performance of both optical and electronic
devices, but they may raise major questions as to the long-term stability of these devices.
It is unlikely that the full promise of GaN and related alloys can be realized without a
major reduction in the defect densities in the as-grown materials.
In 1994, the lateral epitaxial overgrowth (LEO) technique was used to further
improve the quality of the heteroepitaxially grown GaN, resulting in a marked reduction
in defect density . In this method, a layer of GaN grown by MOCVD is covered with
100-200 nm of amorphous SiO2 and Si3N4 with ex situ techniques. Small circular or
rectangular "windows" are then etched through to the underlying GaN. A GaN film is
then regrown under conditions such that growth occurs epitaxially only in the windows
and not on the mask. If growth continues, lateral growth over the mask eventually occurs.
Since most of the extended dislocations propagate in the growth direction through GaN,
very few threading dislocations are visible in the regrown GaN that extends laterally over
the mask. Marchand et al.  observed that the density of dislocations reaching the
surface of LEO GaN was in the 104-105 cm2 range, while the film over the window
regions still contained high levels of the threading defects. Figure 1-3 compares the cross-
section transmission electron microscopy (TEM) of a typical MOCVD growth (a) and
LEO GaN (b).
A refined approach to a nearly dislocation free GaN substrate for devices can be
used by two successive LEO steps with the mask of the second step positioned over the
opening defined by the mask of the first step, thus blocking the defects that have grown
out of the first windows. This complicated procedure offers the possibility of eliminating
the disadvantages of heteroepitaxy, and will be important until GaN substrates become
In addition to growing GaN films with low defect densities, another key
requirement for fabricating devices is the ability to precisely control the desired electrical
properties of the thin film. In general, wide bandgap semiconductors are difficult to dope
due to native defects. When the enthalpy for defect formation is lower than the band gap
energy, the probability of generating a defect increases with the bandgap, i.e., the energy
released by donor-to-acceptor transition. Particularly for GaN, MOCVD grown material
is commonly n-type, and N-vacancy was long believed to be the dominant donor. Many
attempts have been made to avoid N-vacancy formation by growing GaN at high
pressures and high temperatures [25,26]. Efficient n-type doping of GaN through
incorporation of Si during the growth proved relatively easy to achieve. High doping can
also be achieved by implanting Si or Group VI donors. Recently, Burm et al.  have
shown a shallow Si implant at high dose to produce a doping density of 4x1020 cm-3
resulted in an extremely low Ohmic contact resistance of 4x10-8 Qcm2 using Ti/Au
Figure 1-3 Cross-section TEM of typical MOCVD grown GaN using
a A1N buffer on Sapphire (left) and typical LEO GaN
(right, after ).
Since conductivity is proportional to the product of carrier concentration and Hall
mobility, another goal for GaN used in device applications is to obtain the highest Hall
mobilities possible [28,29]. As can be seen, the experimental data are roughly half of the
calculated value, possibly due to significant scattering from impurities and defects in the
The III-V nitrides are expected to be made p-type by substituting Column II
elements such as Zn, Mg Be and Ca on Ga sites to form single acceptors. However, all of
these divalent elements form deep acceptors, the shallowest being Mg with an ionization
level of 0.17 eV which is still many kTs above the valence bandedge of GaN . At this
acceptor level, one should only expect <10% of the Mg atoms to be ionized at room
temperature, which means the Mg concentration needs to be approximately two orders of
magnitude larger than the desired hole concentration. When MOCVD is used as the
growth method, it has been difficult to obtain p-type conductivity. It was later found that
hydrogen plays a crucial role in passivating the Mg acceptors, and creates a neutral
complex Mg-H that prevents the formation of holes in GaN . It was first shown by
Amano et al.  that p-type conductivity could be achieved by activating Mg-doped
GaN using low-energy electron irradiation. Nakamura then showed that the activation of
Mg can also be realized by thermal annealing at -7000C . Note that MBE grown GaN
doped with Mg may be p-type without a thermal activation process, because of the
absence of hydrogen and H-N radicals during growth. In addition, p-type doping was also
achieved by implant of Ca or Mg into GaN, followed by high-temperature annealing
(-1100 C) [34,35]. The highest hole concentration reported so far is ~1018 cm3, and the
typical hole mobility is very low, often 10 cm2/V-s or below, but allowing the realization
of p-n junctions. Achieving low-resistance Ohmic contacts to the GaN layers with poor p-
type doping concentrations has proven troublesome. Recently, Brandt et al.  found
that by compensating Be with O, a neutral dipole is formed that does not scatter the holes.
Hence a record high hole mobility of 150 cm2/V-s was obtained. This may be the ideal
contact layer for GaN based devices.
1.2 Gallium Nitride-Based Optoelectronic and Electronic Devices
1.2.1 Gallium Nitride-Based Optoelectronic Devices
The current level of the progress in the development of GaN commercially viable
devices, namely GaN based-LEDs, LDs and UV detectors, has been the direct result of
the realization of high-quality layers of GaN, AlGaN, InGaN, and relatively recent
achievement of p-type conduction in GaN. The first p-n junction LED was demonstrated
by Amano et al.  in 1989. Then, Nichia Chemical Industries announced the
commercial availability of blue LEDs with high efficiency and luminous intensities over
1 cd . Since then, high-brightness single quantum-well structure blue, green, and
yellow InGaN LEDs with luminous intensities above 10 cd [37,38] have been
commercialized. In 1996, Nakamura et al.  reported the first current-injection GaN-
based LDs with separate confinement heterostructure, and subsequently achieved
continuous-wave (CW) lasing at room temperature . Figure 1-4 shows the cross-
section of a nitride-based laser diode. The active layer is an InGaN multiquantum well
with a large number of well layers. Gallium nitride (GaN) and AlGaN were used as the
waveguide and cladding layers, respectively. The mirror facet was formed by numerous
methods, including dry etching, polishing or cleaving.
InGaN QWs active
Figure 1-4 Cross-sectional view of a typical structure of GaN-based
Surprisingly, the high-density dislocations resulting from the heteroepitaxial
growth on sapphire in these optical devices did not appear to be efficient non-radiative
centers, as they are in other III-V materials. However, the crystalline defects do affect
device reliability. Nichia used the LEO growth technique for their blue LDs and achieved
an increase in device lifetime from a few hundred hours to an estimated 10,000 h .
Another major problem limiting diode performance is high specific contact resistance of
Ohmic contact on the p-GaN side of the junction. Present lateral GaN lasers suffer
significant IR drops due to poor p-type doping and Ohmic metallization.
1.2.2 Gallium Nitride-Based Electronic Devices
The nitride material growth technology that supports the optical device efforts has
also proven to be compatible with the development of electronic devices. In the past
several years, the electronic device development has emphasized field effect transistor
(FET) structures, because this important class of devices places smaller demands on the
growth and fabrication technique compared to bipolar transistors. The rapid progress that
has been made, especially in modulation-doped FETs (MODFETs), has been sufficient to
show that GaN and related alloys will play a significant role in the future development of
high temperature, high power and high-frequency electronic devices [42-45].
GaN-based transistors have a unique combination of high current density, high
breakdown electric field, and good thermal conductivity, that enable previously
unrealizable microwave power performance for solid state transistors. For microwave
transistor performance, two figures of merit (FOMs) have been developed for comparing
the inherent semiconductor material capabilities. These FOMs are Johnson's FOM
(VsatEc)2 and the Baliga's high frequency FOM (gEc2), where Ec is the critical breakdown
field, Vsat is the electron saturation velocity and |j is the low field electron mobility. Fig.
1-5 shows these figures of merit normalized to silicon for all the potential microwave
semiconductor materials. The FOM comparison clearly shows the advantage of the GaN
material system .
Figure 1-6 shows a GaN/AlGaN heterostructure. Due to the large conduction band
discontinuity, the electrons diffusing from the large bandgap AlGaN into the smaller
bandgap GaN form a two-dimensional electron gas (2DEG) in the triangle quantum well
at the interface, which is the hallmark of MODFET. The sheet carrier density of the
2DEG was found to be further enhanced by the strong pizeoelectronic effect in GaN.
Pizeoelectronic coefficients in nitrides were measured to be about an order of magnitude
higher than in traditional Group III-V semiconductors . Theoretical simulations have
predicted a high peak electron velocity of -3 x107 cm/s  and an electron mobility of
-2000 cm2/V-s in the GaN channel at room temperature at a carrier concentration of 1017
cm-3 . Gaska et al.  found the highest measured Hall mobility at room temperature
was 2019 cm2/V-s, and increased approximately fivefold to 10,250 cm2/V-s below 10 K
for growth on 6H SiC substrate.
In 1993, Khan et al.  demonstrated the first AlGaN/GaN MODFET, with a gm
of 23 mS/mm and 2DEG mobility of 563 cm2/V-s at 300 K. They also reported the first
microwave results with ft of 11 GHz and fmax of 14 GHz . In the early stages, the
MODFETs exhibited very low transconductances and relatively poor frequency response.
This is consistent with the defect-laden nature of the early GaN and AlGaN layers. With
improvements in the materials quality, the transconductance, current capacity, and drain
breakdown voltage are all increased to the point that GaN-based MODFETs are now
strong contenders in the arena of high-power devices and amplifiers. To date, the highest
power density achieved for a 0.45x125 |tm GaN MODFET is 6.8 W/mm at 10 GHz and
associated gain of 10.65 dB. The operation temperature has been pushed to 750C by
employing a thermally stable Pt/Au gate contact .
Johnson's (series 1) and Baliga's (series 2) high frequency
figure-of-merit normalized to silicon.
+ E EC
---- -- ----I ------------------E
a .........-o...-- 0 ..... E F
Figure 1-6 Conduction band structure of a modulation-doped structure.
p 150 ESeriesl
Si GaAs InP SiC GaN
The published performances of epitaxial GaN-based MESFETs show that all the
required components for a MESFET-based technology are in place [49, 50]. That is, an
appropriate high resistivity buffer and sub state combination has been developed for
doped layer epitaxial growth, FET channels can be grown with thin n+ contact layers on
which Ohmic contact with adequate contact resistances have been achieved, gate
metallizations that can pinch off the channel and support a high drain bias have been
demonstrated, and it has shown that both mesa etch and implant isolation can be used to
define the active device area. Recently, an all implanted GaN junction FET, an Si3N4
gated GaN MISFET , and a Ga203 (Gd204) gated GaN MOSFET with reasonable
performance were also reported. These types of devices potentially have an advantage
over MESFET, especially at high temperatures due to low reverse leakage currents.
So far, few reports exist on development of GaN-based bipolar transistors [54,55].
Basically the device performance is limited by the difficulty in growth and processing
related to the buried p-type layer and the small minority carrier lifetime. It is still far from
commercialization of these devices, but their developments will follow the material
improvements in the new decade, and much impetus comes from defense applications
where ultrawide bandwidth and linearity are desired.
Group III-V nitrides offer a valuable combination of electrical, optical and
pizeoelectrical behavior, and enable the fabrication of LEDs, LDs, detectors, and
transistors. In the past, the poor quality of the materials, the lack of p-type doping, and
the absence of reliable processing procedures thwarted engineers and scientists from
fabricating these useful devices. However, the 1990s have brought significant advances
in the sophistication of growth techniques, the purity of the chemicals used for film
deposition, the controlled introduction and activation of selected impurities, and progress
in processing techniques. Most of the aforementioned obstacles have been sufficiently
overcome, and the electronic and optical devices have been demonstrated and partially
commercialized. Market projections show that GaN-based blue and green LEDs will
represent most of the estimated US$ 3 billion per year GaN-based device market by
2006. In transistors, GaN can go where no other semiconductors have gone before. The
future development in this area will definitely be fueled by the increasing demand for
high-temperature, high-power applications. From materials science to device engineering,
from laboratory research to commercial products, III-V nitride technologies have shown a
late but exciting development.
GALLIUM NITRIDE-BASED DEVICE PROCESSING
While further improvements in the III-V nitride materials quality can be expected
to enhance device operation, further device advances will also require improved
processing technology. Owing to their wide bandgap nature and chemical stability, GaN
and related materials present a host of device processing challenges, including poor p-
type doping (by implantation), difficulty in achieving reliable low-resistance p-Ohmic
contacts, high temperatures needed for implant activation, lack of efficient wet etch
process, generally low dry etch rates and low selectivity over etching masks, and dry etch
damage. These problems constitute a major obstacle to successful demonstration and
commercialization of some GaN-based devices, such as bipolar transistors and power
switches, whose performance are much more affected by the immature fabrication
techniques. To fully exploit these device applications, a number of critical advances are
necessary in the areas of implantation doping and isolation, high-temperature thermal
processing, Ohmic contact to p-type material, dry etching process, and device
passivation. The current state-of-the-art results on advanced GaN processing were have
been reviewed . In this chapter, the results from C12/Ar high density inductively
coupled plasma damage in GaN Schottky diodes and N2 inductively coupled plasma
discharge treatment on n-AlGaN/GaN Ohmic contacts will be presented.
2.1 Chlorine/Argon (C12/Ar) High Density Inductively Coupled Plasma damage in
GaN Schottky Diodes
Precise pattern transfer during fabrication of GaN-based devices requires use of
dry etching methods with relatively high ion energy in order to break the strong Ga-N
bonds (8.92 eV/atom) . Under those conditions generally some ion-induced damage
remains in the GaN after dry etching, along with the possibility of a non-stoichiometric
near-surface region due to preferential loss of atomic nitrogen in the form of N2 [57-59].
The Ga etch product in C12-based discharges is GaC13, and this is less volatile than N2
both from a pure chemical vapor presence and from a preferential sputtering viewpoint.
There has been relatively little work on understanding the effects of plasma
processes on the electrical characteristics of GaN. Exposure to pure Ar discharges was
found to produce higher reverse-bias leakage currents in p-n junction structures compared
to use of Ar/N2 discharges . Even relatively low power reactive ion etching (RIE)
conditions were found to deteriorate the quality of Schottky contacts deposited on
plasma-etched n-GaN [61,62]. The preferential loss of nitrogen from the GaN surface
does improve the specific contact resistance of n-type ohmic contacts because of the
creation of a degenerately doped surface layer , but may increase the average sheet
resistance of the GaN. Previous results have shown that exposure of GaN to H2 or N2
Inductively Coupled Plasmas (ICP) prior to deposition of Schottky contacts creates a
damaged region -500 A deep that can be essentially restored to its original characteristics
by annealing at 750 C. There are also situations where GaN device structures use a metal
contact as a self-aligned etch mask. In this case it is of interest to examine the effects of
plasma exposure on samples with existing Schottky contacts.
2.1.2 Experimental Methods
GaN Schottky devices already have the contacts in place. The degradation of
reverse breakdown voltage (VB) and Schottky barrier height (iB) was strongly dependent
on the incident ion energy and flux. Both annealing and UV ozone treatment were
employed to try to remove the plasma damage.
Planar diodes were fabricated on nominally undoped (n-1017cm3) GaN layers -3
jtm thick grown on an n (101 cm-3) GaN buffer on a c-plane A1203 substrate .
Ohmic contacts were formed with lift-off Ti/Au subsequently annealed at 600 OC,
followed by evaporation of the 250 |tm diameter Pt(250 A)/Au(1500 A) Schottky
contacts through a stencil mask.
The samples were briefly exposed (-10secs controlled by the system software) to
10C12/5Ar (total gas load 15 standard cubic centimeters per minute) ICP discharges in a
Plasma Therm 790 reactor. During the ignition stage of the discharge, the dc self-bias
takes -2 secs to reach its final value. From limited measurements we found that damage
saturates in this time frame. The gases were injected directly into the source through
electronic mass flow controllers, and the 2MHz source power was varied from 100-1000
W. The samples were placed on an rf-powered (13.56 MHz, 5-300 W), He backside-
cooled chuck. Process pressure was hold constant at 2 m Torr.
The current-voltage (I-V) characteristics of the diodes were recorded on a HP
4145A parameter analyzer. Barrier heights (iB) and ideality factors (n) accurate to +5%
were obtained from the forward I-V characteristic according to the relationship :
e( eV 2-1
J=A**T2exp( )[exp(n 1)]
Where J is the current density, A** the effective Richardson constant, T the
measurement temperature (25 C), e the electronic charge and k is Boltzmann's constant.
The reverse breakdown voltage (VB) was defined as the voltage at which the current
density was 3.06x 104 mA/cm2 (i.e. a current of 15 mA).
Some diodes were annealed at temperatures up to 800 OC for 30 secs under N2
after plasma exposure, while others were treated in UV-ozone at 25 C for periods up to
20 minutes in a Jelight 200S system, followed by rinsing in HC1 solutions. Auger
Electron Spectroscopy (AES) was performed in some cases on blanket (unmetallized)
2.1.3 Results and Discussion
Figure 2-1 shows some typical I-V characteristics from GaN diodes after
exposure to the ICP C12/Ar discharges at fixed ICP source power and varying rf power.
The latter parameter controls the average energy of ions (predominantly Ar+ and C12 in
this case) incident on the samples. There is a clear degradation in VB as this rf chuck
power is increased. Control diodes not exposed to the plasma had I-V characteristics that
were similar to curves 1 and 2, with VB of 38 V and 6B of 0.82 eV.
The dependence of VB and 6B on rf chuck power is shown in the upper part of
Figure 2-2. Both of these parameters, at least initially, decrease with increasing power.
The 6B values saturate beyond 50 W. The main effect on 6B is from damage created
around the contact periphery. This would expected to saturate once a N2-deficient region
S 0 .000 ................................................................. .
-0.015 I I
-40 -30 -20 -10 0
Figure 2-1 I-V characteristics from GaN diodes after C12/Ar plasma
exposure (300W source power, 2m Torr) with different rf
is created because much of the resultant 6B is still determined by the unexposed region
under the contact metal. Under these conditions, the dc chuck self-bias increases from -
105 V at 50 W to -275 V at 200 W. The average ion energy is roughly the sum of this
voltage plus the plasma potential which is 20-25 V in this system under these conditions.
After plasma exposure, the diode ideality factor was always >2, which is a further
indication of the degradation in electrical properties of the structures. The results are
consistent with creation of an ion damaged, non-stoichiometric GaN surface region. This
region exists in the plasma-exposed area outside the metal contacts. Note that the GaN
etch rate increases monotonically with rf chuck power (Figure 2.2, bottom), but this more
rapid removal of material is not enough to offset the greater amount of damage caused by
the higher-energy ion bombardment. We believe the GaN must be non-stoichiometric and
hence more n-type at the surface because of the sharp decreases observed in VB. In the
case of semiconductors such as GaAs where ion bombardment creates more resistive
material by introduction of deep compensating levels rather than shallow donor states, the
breakdown voltage is generally found to increase with exposure to plasmas [65-67].
The dependence of VB and 6B on ICP source power is shown in Figure 2-3 (top).
While 4B continues to decrease as the ion flux increases, VB initially degrades but shows
less of a decrease at higher source powers (Figure 2-3, bottom). This is most likely a
result of the continued decrease in the self-bias at higher source power. This also leads to
a decrease in GaN etch rate above 500 W. The results of Figure 2-2 and 2-3 show that
both ion energy and ion flux are important in determining not only the GaN etch rate, but
also the amount of residual damage in the diodes.
As mentioned previously, past measurements on ICP damaged GaN surfaces have
established the damage depth as being of order 500 A. One method for trying to remove
the damaged material between the contacts is by oxidizing it by UV/ozone (03) exposure,
followed by stripping of the oxide. Figure 2-4 shows the dependence of VB and BB on UV
ozone treatment time. In each case after the oxidation, a 1:20, HCl: H20 solution was
used for removal of the oxidized material. While there is some improvement in both
parameters up to 5 min, there is no further improvement for longer times. We assume the
oxidation distance is diffusion-controlled (i.e. dependent on 17), and from preliminary
measurements we believe that only -30 A of GaN is oxidized and removed for 5 minute
UV ozone exposure. Therefore the process would have to be repeated approximately 15-
20 times to remove the damaged region of the GaN, assuming the oxidation rate remains
50 100 150 200
rf Power (W)
Figure 2-2 rf chuck power dependence of VB and 6B in C12/Ar plasma
exposed GaN diodes (top) and of dc chuck self-bias and GaN
etch rate under the same conditions (bottom)
. . . . . . . . . . . -B . . . . . . . . . . . .
control 4. --
S I I
, I I
ICP Power (W)
Figure 2-3 ICP power dependence of VB and 6B in C12/Ar plasma exposed
GaN diodes (top) and of dc chuck self-bias and GaN etch rate
under the same conditions (bottom)
-*- etch rate
-0- dc bias
I I I I I
the same deeper into the material. Use of a stronger HC1 solution improves the VB value
compared to use of the 1:20 solution (Figure 2-5), but there is no improvement in 4B. We
emphasize that the damaged GaN is the exposed region outside the contact area. This will
lead to reductions in VB by increasing the surface conductivity and degrade 6B by incr
easing leakage current at the contact periphery.
Figure 2-6 shows the effect of anneal temperature on the recovery of VB and 4B.
There is a clear improvement in VB for anneals in the range 500-700 C, and little change
thereafter and it remains lower than the unetched control value. However, 4B changes
very little with annealing. These results are somewhat different than in the case where the
surface is exposed to the ICP discharge, annealed and then the Schottky contact is
deposited. For that sequence, essentially full recovery of the electrical characteristics was
obtained for 750 C annealing. In the present case where the contact is in place we
believe the metal begins to react with the GaN at -600 OC, accounting for the lack of
recovery of 6B at higher temperatures.
The effect of annealing time at fixed temperature (700 C) on VB and 4B is shown
in Figure 2-7. The improvement in both parameters is saturated beyond 60 secs. It would
be expected that the recovery mechanism should be most critically dependent on
temperature since most defect annealing processes involve dissociation and diffusion of
defects and their complexes. In this case, the recovery would be dependent on the square
root of annealing time and exponentially on temperature.
To establish the chemical state of the GaN surface at different stages, AES was
performed on an unmetallized sample. Figure 2-8 shows surface scans before (top) and
40 1 I I
control value (unetched)
30 O,+HCI (1 min)
I I I I
control value (unetched)
0 5 10 15 20
03 Time (min.)
Figure 2-4 UV ozone oxidation time dependence of VB (top) and 4B
(bottom) in C12/Ar plasma exposed GaN diodes. After
oxidation, the samples were rinsed in 1HCl: 20H20 for 1 min.
Figure 2-5 Dependence of VB (top) and B (bottom) on process condition in Cl2/Ar
plasma exposed GaN diodes. After UV ozone oxidation, the samples
were rinsed in 1HCl: 20H20 for 1 min or aqueous HCI (35-38%) for
control value (unetched)
control value (unetched)
. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . .
1: as etched
12 r\3 5 i\
0. %-v .1 ^_-
3: 03 (5min)+HCI (1:20, 1 min)
4: 03(5min.)+HCI(1:20, 1min.)
5: 03 descum(5min.)+HCI(1:20, 1min.)
Anneal Temperature (C)
Figure 2-6 Annealing temperature dependence of VB (top) and 4B
(bottom) in Cl2/Ar plasma exposed GaN diodes. Anneal time
was 30 sec at each temperature.
control value (unetched)
30 sec anneal
Control value (unetched)
Control value (unetched)
700 C anneal
Anneal Time (min)
Figure 2-7 Annealing time dependence (at 700 OC) of VB (top) and 4B
(bottom) in C12/Ar plasma exposed GaN diodes.
-r -r -r -r I- -r
Control value (unetched)
I I - I -
after (lower) exposure to a 500 W source power, 50 W chuck power C12/Ar discharge.
The main change is a reduction in the N2 signal in the latter sample (by -20%),
confirming the preferential loss of this element during dry etching. Subsequent annealing
at 700 C in N2 restored some of this deficiency (Figure 2-8, bottom).
2.1.4 Summary and Conclusion
The main points of our study may be summarized as follows:
ICP Cl2/Ar discharges degrade the performance of GaN Schottky diodes,
with ion energy and ion flux both playing important roles.
UV ozone oxidation of the surface and subsequent dissolution of the
oxidized region in HC1 provides some restoration of the electrical
properties of the GaN.
Annealing at 700 to 750 C also restores some of the initial reverse
breakdown voltage characteristics, but little change in 4B for Pt/Au
contacts on GaN.
The degradation mechanism appears to be creation of a conducting, non-
stoichiometric (N2-deficient) near-surface region on the GaN.
2.2 Effect of N2 Inductively Coupled Plasma Discharge Treatment on n-AlGaN/GaN
Both the dc and rf performance of AlGaN/GaN High Electron Mobility
Transistors (HEMTs) are strongly dependent on the specific contact resistance of the
source/drain contacts [68-77]. There have been four basic classes of metallization
employed for n-type ohmic contacts to GaN-based materials, namely Al [78-80], Ti or
5 N' Si
KINETIC ENERGY (eV)
Figure 2-8 AES surface scans from GaN (top) or after (center) C12/Ar plasma
exposure, and subsequent annealing at 700 OC for 60 seconds
TiN [81-86], W or other refractory metals [87-90] or multilayers such as Ti/Al/Ni/Au
[91-93] which appear to give wider process windows by reducing oxidation of the Ti [91-
93]. Modifications to the GaN by high temperature annealing  or reactive ion etching
[91,95] to produce preferential loss of nitrogen can improve n-type ohmic contact
resistance by increasing electron concentration in the near-surface region. In all cases, the
best specific contact resistitity has been achieved after annealing the metallization at 900-
950 C [91,96,97].
We have previously found that exposure of n- or p- type GaN to high density
Inductively Coupled Plasmas (ICP) degrades the rectifying properties of subsequently
deposited Schottky contacts . The degradation mechanism is loss of nitrogen, as
described above. To this point, there have been no investigation of the effect of ICP
exposure on the properties of n-type ohmic contacts, especially on HEMT structures
where the contact resistivity can be high due to presence of AlGaN donor and contact
layers. In this paper we report the results of a systematic study to understand the effect of
ion energy, ion flux and exposure time of N2 ICP discharges on the contact resistance of
Ti/Al/Pt/Au metallization on AlGaN/GaN HEMTs.
2.2.2 Experimental Methods
The AlGaN/GaN structures were grown by rf plasma activated Molecular Beam
Epitaxy on (0001) sapphire . After nitridation of the surface, at low temperature, 300
A thick A1N buffer was grown, followed by a 1 |tm undoped GaN layer grown at 750 C
under Ga-rich growth conditions. This was followed with a 30 A undoped Alo.15Gao.s5N
spacer layer, 100A Alo.15Gao.s5N donor layer (Si-doped, n=1019 cm3) and a 100 A
undoped Alo.15Gao.85N cap layer. A schematic of the structure is shown in Figure 2-9.
Typical room temperature sheet electron densities were -3.5x1012 cm-2, with Hall
mobilities of-400 cm2 V1 sec-1 .
The N2 plasma exposures were carried out in a Plasma Therm 790 reactor, in
which the ion flux is controlled by a 1500 W ICP source operating at 2 MHz, and the ion
energy is controlled by rf power (13.56MHz) applied to the sample chuck. The N2 gas
was injected into the source at a total flow rate of 15 standard cubic centimeter per
minute and process pressure was held constant at 2 m Torr. After plasma exposure, e-
beam deposited Ti(200 A)/Al(800 A)/Pt(400 A)/Au(1500 A) was patterned by lift-off and
annealed under N2 in an AG associates Heatpulse 610T system. The specific contact
resistance was obtained from Transmission Line Method (TLM) measurement using gap
spacings of 2, 4, 8, 16 and 32 |tm. In some cases the plasma exposed AlGaN/GaN
structures were examined by Atomic Force Microscopy (AFM) and Auger Electron
Spectroscopy(AES) for measurement of surface morphology and composition,
2.2.3 Results and Discussion
Figure 2-10 shows the measured contact resistances for the Ti/Al/Pt/Au
metallization on unexposed (control) samples, as a function of post-deposition annealing
temperature. We will use this data for comparison with the plasma-exposed samples.
Note that a value of 7x 103 -.cm2 was obtained for 950C annealing.
The effect of rf chuck power on contact resistance of the N2 plasma exposed
samples is shown in Figure 2-11. In this case the ICP source power was held constant at
300 W (equivalent to an ion flux of 4x 1016 cm-2.sec-1). The lowest contact resistances
were obtained for samples exposed at 40 W chuck power and subsequent annealed for
100 A Alo0.Gao.ssN cap layer
100 A Alo.1sGao.ssN donor layer, 1x1019 cm3
30 A Alo0.1Gao.ssN spacer layer
lp.m UID GaN buffer layer
300 A AIN buffer layer
Figure 2-9 Schematic of AlGaN/GaN HEMT structure.
Anneal for 30 sec
10-31 1 1 1 1 1
0 200 400 600 800 1000
Anneal Temperature (oC)
Figure 2-10 Contact resistance for Ti/Al/Pt/Au metallization as a function of
annealing temperature for AlGaN/GaN structures not exposed to
N2 discharges prior to metal deposition.
30 sec at 950 OC, producing a value of 2x104 fQcm2. This is approximately a factor of
three improvement over contacts annealed at the same temperature on control samples.
The ion energy at this condition is roughly -125 eV, the sum of the dc self-bias(lower part
of Figure 3) and plasma potential (about -25 eV under these conditions).
We fixed the rf chuck power at 40 W and examined the effect of varying the ICP
source power during the plasma exposure (Figure 2-12). For annealing at 800 or 950 OC,
there is a broad minimum in contact resistance centered at 300 W source power. We
believe that at lower powers the ion flux is too low to produce efficient preferential loss
of the nitrogen, while at higher fluxes there are large concentrations of defects created
that degrade current transport in the AlGaN. Note that while flux increases with source
power, the ion energy decreases slightly due to the higher plasma conductivity.
The improvement in contact resistance saturated with exposure time, as shown in
Figure 2-13. This result is not unexpected, since part of the surface is removed by
sputtering during plasma exposure and the creation of an N2-deficient surface region will
come to an equilibrium condition. The plasma exposure did not roughen the AlGaN
surface, as shown by the AFM scans of Figure 2-14. The root-mean-square (RMS)
roughness of the controlled sample was 1.3 nm, compared to 1.0 nm for the sample
exposed to a 300 W source power, 40 W rf chuck power N2 discharge for 30 secs. It is
likely that at high chuck power, corresponding to high ion energies, surface roughening
should be more prevalent.
To confirm that the mechanism for the contact resistance improvement was loss
of nitrogen, we performed AES measurements. Surface scans before and after N2 plasma
exposure (300 W source power, 40 W rf chuck power, 30 sec) showed that the average
composition of N in the top 100 A of the surface dropped from 10% in the control sample
to 7.1% in the plasma exposed sample (Figure 2-15). Scanning electron microscopy of
the contact metallization showed good morphology and edge definition for both the
control and plasma exposed samples.
2.2.4.Summary and Conclusion
ICP N2 discharges were used to improve contact resistances on
AlGaN/GaNHEMT structures by inducing preferential loss of nitrogen from the near-
I I I I I I
ICP Power: 300W
r ^^^^----^ o -O-
--- -As Deposited
S-0-- 800 C anneal (30 sec)
---- 950 C anneal (30 sec)
S I I I I I I
10 20 30 40 50 60
rf Power (W)
Contact resistance for Ti/Al/Pt/Au metallization as a function of rf
chuck power for AlGaN/GaN structures exposed to ICP N2
discharges prior to metal deposition, for several annealing
temperatures (top) and dc self-bias as a function of rf power (bottom).
r T T T T
ICP Power: 300W
200 400 600
ICP Power (W)
Figure 2-12 Contact resistance for Ti/Al/Pt/Au metallization as a function of
ICP power for AlGaN/GaN structures exposed to ICP N2 discharges
prior to metal deposition, for two annealing temperatures (top) and
dc self-bias as a function of ICP power (bottom).
rf Power: 40W
--- 800 C anneal (30 sec) .
--- 950 C anneal (30 sec)
rf Power: 40W
10 1 I
S Exposure Conditions: 30W rf, 400W ICP
0 0 m20 10 m0 81n
1 -E-4 -*-As Deposited
--0- 800 C anneal (30 sec)
E -5-- 950 ,C anneal (30 sec)
0 20 40 60 80 100
Exposure Time (sec)
Fig. 2-13 Contact resistance for Ti/Al/Pt/Au metallization as a function of
exposure prior to ICP N2 discharges for AlGaN/GaN structures
annealed at several different temperatures.
surface 100 A) region. The N2 plasma chemistry is a good choice for this application,
since it produces light ions (N2+, N) for bombardment of the AlGaN surface that do not
create heavy lattice disorder and associated trapping states that could degrade current
transport in the semiconductor. It also avoids the chemical effects of H2 or 02 discharges
on the AlGaN surface. Under optimized conditions, the contact resistance of Ti/Al/Pt/Au
metallization deposited on the plasma exposed samples and subsequently annealed at 950
C was lowered by a factor of 3 relative to unexposed contact samples annealed in the
same fashion. This is a simple and effective method for reducing ohmic contact resistance
on AlGaN/GaN HEMTs.
. ... ...................
...... ....... .......
:. ......... ....
::. ~. .... ..... :. .
..... ;:::::.,......-.. :..".... ... ..
i ...~~. = : ".,,,,,,.,..............
:.... ..... ..:. ...
Figure 2-14 Atomic Force Microscopy (AFM) scans of AlGaN/GaN
structure before (top) and after (bottom) exposure to an ICP N2
discharge (300 W source power, 40 W rf chuck power, 30 secs).
.. .... ..
.. _.. ... ..
: .... ",,....
.. ..... :::.. .-
............ ............. ...
....... . . .
..... .. .....
0 -I I I I I I I I
200 400 600 800 1000 1200 1400 1600 1800 2000
200 400 600 800 1000 1200 1400 1600 1800 2000
Figure 2-15 Auger Eelectron Spectroscopy (AES) surface scans of
AlGaN/GaN structure before (top) and after (bottom) exposure to
an ICP N2 discharge (300 W source power, 40 W rf chuck power,
GALLIUM NITRIDE AND ALUMINUM GALLIUM NITRIDE HIGH VOLTAGE
There is a strong interest in developing wide bandgap power devices for use in the
electric power utility industry [3, 100-102]. With the onset of deregulation in the
industry, there will be increasing numbers of transactions on the power grid in the US,
with different companies buying and selling power. The main applications are in the
primary distribution system (100-2000 kVA) and in subsidiary transmission systems
(1-50 MVA). A major problem in the current grid is commentary voltage sags, which
affect motor drives, computers and digital controls. Therefore, a system for eliminating
power sags and switching transients would dramatically improve power quality. For
example it is estimated that a 2-second outage at a large computer center can cost US$
600,000 or more, and an outage of less than one cycle, or a voltage sag of 25% for two
cycles, can cause a microprocessor to malfunction. In particular, computerized
technologies have led to strong consumer demands for less expensive electricity,
premium quality power and uninterruptible power.
The basic power electronics hierarchy would include the use of widegap devices
such as Gate Turn-Off Thyristors (GTOs), MOS-Controlled Thyristors (MCT) or
Insulated Gate Bipolar Transistors (IGBTs) combined with appropriate packaging and
thermal management techniques to make subsystems (such as switches, rectifiers or
adjustable speed devices) which then comprise a system such as Flexible AC
Transmissions (FACTS). Common power electronics systems, which are inserted
between the incoming power and the electrical load include uninterruptible power
supplies, advanced motors, adjustable speed drives and motor controls, switching power
supplies, solid-state circuit breakers and power conditioning equipment. About 50% of
the electricity in the US is consumed by motors. Motor repairs cost -US$ 5 billion each
year and could be dramatically reduced by high power electronic devices that permit
smoother switching and control. Moreover, control electronics could dramatically
improve motor efficiency. Other end uses include lighting, computers, heating and air-
Some desirable attributes of next generation, widegap power electronics include
the ability to withstand currents in excess of 5 kA and voltages in excess of 50 kV,
provide rapid switching, maintain good thermal stability while operating at temperatures
above 250 C, have small size and light-weight, and be able to function without bulky
The primary limits of Si-based power electronics are as follows:
Maximum voltage ratings <7 kV
Multiple devices must be placed in series for high-voltage systems.
Insufficient current-carrying capacity
Multiple devices must be placed in parallel for typical power grid
Conductivity in one direction only
Identical pairs of devices must be installed in anti-parallel for switchable
Inadequate thermal management
Heat damage is a primary cause of failure and expense.
High initial cost
Applications are limited to the highest-value settings.
Large and heavy components
Costs are high for installation and servicing, and equipment is unsuitable for
For these reasons, there is a strong development effort on widegap power devices,
predominantly SiC, with lesser efforts in GaN and diamond, which should have benefits
that Si-based or electromechanical power electronics cannot attain. The higher standoff
voltages should eliminate the need for series stacking of devices and the associated
packaging difficulties. In addition these widegap devices should have higher switching
frequency in pulse-width-modulated rectifiers and inverters.
The absence of Si devices capable of application to 13.8 kV distribution lines (a
common primary distribution mode) opens a major opportunity for widegap electronics.
However, cost will be an issue, with values of US$ 200-2000 per kVA necessary to have
an impact. It is virtually certain that SiC switches will become commercially available
within 3-5 years, and begin to be applied to the 13.8 kV lines. MOS Turn-Off-Thyristors
involving a SiC GTO and SiC MOSFETare a promising approach . An inverter
module can be constructed from an MOS turn-off thyristor (MTO) and a SiC power
Packaging and thermal management will be a key part of future power devices.
For current Si IGBTs, there are two basic package types the first is a standard attached
die, wire bond package utilizing soft-solder and wire-bonds as contacts, while the second
is the presspack, which employs dry-pressed contacts for both electrical and thermal
paths [104,105]. In the classical package the IGBTs and control diodes are soldered onto
ceramic substrates, such as A1N, which provide electrical insulation, and this in turn is
mounted to a heat sink (typically Cu). Thick Al wires (500 mm) are used for electrical
connections, while silicone gel fills the package . In the newer presspack style, the
IGBT and diode are clamped between Cu electrodes, buffered by materials such as
molybdenum or composites , whose purpose is to account for the thermal expansion
coefficient differences between Si and Cu. The package is again filled with gel for
electrical insulation and corrosion resistance.
3.2 Gallium Nitride Schottky Rectifiers with 3.1 kV Reverse Breakdown Voltage
The GaN materials system is attractive from the viewpoint of fabricating unipolar
power devices because of its large bandgap and relatively high electron mobility [106-
109]. An example is the use of Schottky diodes as high-voltage rectifiers in power
switching applications [106-108, 8]. These diodes will have lower blocking voltages than
p-i-n rectifiers, but have advantages in terms of switching speed and lower forward
voltage drop. Edge termination techniques such as, field rings on filed plates, bevels or
surface ion implantation are relatively well-developed for Si and SiC and maximize the
high voltage blocking capability by avoiding sharp field distributions within the device.
However, in the few GaN Schottky diode rectifiers reported to date [106, 107], there has
been little effort made on developing edge termination techniques. Proper design of the
1013 1014 1015 1016 1017
Drift Region Doping Concentration (cm )
Schottky Metal Ohmic Metal
Figure 3-1 The calculation of reverse breakdown volatge as a function of
doping concentration and standoff region thickness based on a
'- o GaN Schottky Diode
I I"" I '" "
edge termination is critical both for obtaining a high breakdown voltage and reducing the
on-state voltage drop and switching time.
Based on the punch through model, Figure 3-1 shows a plot of avalanche and
punch through breakdown of GaN Schottky diodes calculated as a function of doping
concentration and standoff layer thickness. It can be seen that 20 kV device may be
obtained with -100 |tm thick GaN layer with doping concentration <1015 cm3.
In this chapter we investigate on the effect of various edge termination techniques
on the reverse breakdown voltage, VB, of planar GaN Schottky diodes which deplete in
the lateral direction. A maximum VB of 3.1 kV at 250C was achieved with optimized edge
termination, which is a record for GaN devices. We also examined the temperature
dependence of VB in mesa diodes and found a negative temperature coefficient of this
parameter in these structures.
The GaN was grown on c-plane A1203 substrates by MOCVD using
trimethylgallium and ammonia as the precursors. To create a Schottky rectifier with high
breakdown voltage, one needs a thick, very pure GaN depletion layer. Figure 3-2 shows
SIMS profile of H and other background impurities in a 2 |tm thick, high resistivity (107
Q-cm) GaN layer grown by MOCVD. The reverse breakdown voltage of simple Schottky
rectifiers fabricated on this material was > 2 kV, a record for GaN. Notice that in this
material the hydrogen concentration is at the detection sensitivity of the SIMS apparatus.
The amount of hydrogen present in GaN after cooldown from the growth temperature
will depend on the number of sites to which it can bond, including dopants and point and
line defects. In the absence of p-type doping, it is clear that the number of these sites is <
8x107 cm-3 under our growth conditions.
^ 1E+20 -
Z GaN (counts)-> uJ
S 1E+19 Z
o5 11 I-
o 1E+02 0
0 0.5 1 1.5 2 2.5 3
Figure 3-2. SIMS profiles ofH and other background impurities in as-
grown, MOCVD Schottky rectifier structure.
For vertically-depleting devices, the structure consisted of a 1 |tm n+(3 x 108 cm3,
Si-doped) contact layer, followed by undoped (n-2.5x1016 cm-3) blocking layers which
ranged from 3 to 11 tm thick. These samples were formed into mesa diodes using ICP
etching with C12/Ar discharges (300 W source power, 40 W rf chuck power). The dc self-
bias during etching was -85 V. To remove residual dry etch damage, the samples were
annealed under N2 at 800 oC for 30 s. Ohmic contacts were formed by lift-off of e-beam
evaporated Ti/Al, annealed at 700 oC for 30 s under N2 to minimize the contact
resistance. Finally, the rectifying contacts were formed by lift-off of e-beam evaporated
Pt/Au. Contact diameters of 60-1100 |tm were examined.
For laterally depleting devices, the structure consisted of 3 |tm of resistive (107
Q/0) GaN. To form Ohmic contacts, Si+ was implanted at 5 x1014 cm-2, 50 keV into the
contact region and activated by annealing at 150 OC for 10 s under N2. The Ohmic and
rectifying contact metallization was the same as described above.
Three different edge termination techniques were investigated for the planar
1. Use of a p-guard ring formed by Mg' implantation at the edge of the Schottky
barrier metal. In these diodes the rectifying contact diameter was held constant at
124 |tm, while the distance of the edge of this contact from the edge of the Ohmic
contact was 30 |tm in all cases.
2. Use of p-floating field rings of width 5 mm to extend the depletion boundary
along the surface of the SiO2 dielectric, which reduces the electric field crowding
at the edge of this boundary. In these structures a 10 [m wide p-guard ring was
used, and one to three floating field rings employed.
3. Use of junction barrier controlled Schottky (JBS) rectifiers, i.e., a Schottky
rectifier structure with a p-n junction grid integrated into its drift region.
In all of the edge-terminated devices the Schottky barrier metal was extended over
an oxide layer at the edge to further minimize field crowding, and the guard and field
rings formed by Mg+ implantation and 1100 C annealing.
Figure 3-3 shows a schematic of the planar diodes fabricated with the p-guard
rings, while the lower portion of the figure shows the influence of guard ring width on
3p1m undoped GaN
400 A GaN buffer layer
Figure 3-3 GaN power rectifiers with p-guard ring for edge terminations.
0 .0 .........
10 20 30
Guard Ring Width ([im)
Figure 3-4 Current-Voltage characteristics of GaN power rectifiers with p-
guard ring for edge terminations (top), and effect of p-guard ring
on the reverse breakdown voltage of GaN power rectifiers
3 am undoped GaN
400 A GaN buffer layer
Figure 3-5 GaN power rectifiers with floating-field ring for edge terminations.
g 2000 1 1
0 10 20 30
Guard Ring Width (pLm)
Figure 3-6 Effect of floating field ring on the reverse breakdown voltage of
GaN power rectifiers.
Effect of Junction Barrier Control on the reverse breakdown
voltage of GaN power rectifiers.
- without junction barrier control
-*-with junction barrier control
1 1Opm guard ring + 1 float field ring
2 10pm guard ring + 2 float field rings
3 10pm guard ring + 3 float field rings
lO0mu guard ring without junction barrier control
3 pm undoped GaN
400A GaN buffer layer
Figure 3-8 GaN power rectifiers with Junction Barrier Control.
VB at 25 oC. Without any edge termination, VB is -2300 V for these diodes. The forward
turn-on voltage was in the range 15-50 V, with a best on-resistance of 0.8 Q cm2. The
figure-of-merit (VB)2/RoN was 6.8 MW/cm2. As the guard-ring width was increased, we
observed a monotonic increase in VB, reaching a value of -3100 V for 30 [m wide rings
(Figure 3-4). The figure-of-merit was 15.5 MW/cm2 under these conditions. The reverse
leakage current of the diodes was still in the nA range at voltages up to 90% of the
Figure 3-5 shows a schematic of the floating field ring structures, while Figure 3-6
shows the effect of different edge termination combinations on the resulting VB at 25 oC.
Note that the addition of the floating field rings to a guard ring structure further improves
VB, with the improvement saturating for a three-floating field ring geometry.
Figure 3-7 shows the effect of the junction barrier control on VB, together with a
schematic of the p-n junction grid in Figure 3-8. In our particular structure we found that
junction barrier control slightly degraded VB relative to devices with guard rings and
various numbers of floating field rings. We believe that with optimum design of the grid
structure we should achieve higher VB values and that the current design allows Schottky
barrier lowering since the depletion regions around each section of the grid do not
completely overlap. This is consistent with the fact that we did not observe the decrease
in forward turn-on voltage expected for JBS rectifiers relative to conventional Schottky
The results of Figures 3-3 to 3-6 are convincing evidence that proper design and
implementation of edge termination methods can significantly increase reverse
breakdown voltage in GaN diode rectifiers and will play an important role in applications
at the very highest power levels. For example, the target goals for devices, intended to be
used for transmission and distribution of electric power or in single-pulse switching in the
subsystem of hybrid-electric contact vehicles are 25 kV standoff voltage, 2 kA
conducting current and a forward voltage drop <2% of the standoff voltage. At these
power levels, it is expected that edge termination techniques will be essential for
The devices designed for vertical depletion had lower on-state voltages than the
lateral diodes, due to the fact that a highly-doped n+ contact layer can be included in the
epitaxial structure, obviating the need for implantation. However, we have not yet
perfected the ability to grow resistive GaN on top of conducting GaN and, therefore, the
depletion layers in the vertical devices typically had lightly n-type conductivity (2x 1016
to 5x10 16 cm-3). The typical on-state resistances were 6-10 m Q cm2, with reverse
breakdown voltages at 25 C of 200-550 V (depending on doping level and layer
thickness). The maximum figure-of-merit in these devices was higher than for the planar
diodes, reaching values as high as 48 MW/cm2.
In summary, GaN Schottky diodes with vertical and lateral geometries were
fabricated. A reverse breakdown voltage of 3.1 kV was achieved on a lateral device
incorporating p-type guard rings. Several types of edge termination were examined, with
floating field rings and guard rings found to increase VB. The best on-state resistance
obtained in these lateral devices was 0.8 Q-cm2. In mesa diodes incorporating n+ contact
layers, the best on-state resistance was 6 m Q-cm2, while VB values were in the range
200-550 V. These GaN rectifiers show promise for high power electronics applications.
3.3 Aluminum Gallium Nitride (AIGaN) Schottky Rectifiers with 4.1 kV Reverse
There is a strong interest in developing high current, high voltage switches in the
AlGaN materials system for applications in the transmission and distribution of electric
power and in the electrical subsystems of emerging vehicle, ship, and aircraft technology
[108, 110, 111]. It is expected that packaged switches made from AlGaN may operate at
temperatures in excess of 250 C without liquid cooling, therefore reducing system
complexity, weight, and cost. In terms of voltage requirements, there is a strong need for
power quality enhancement in the 13.8 kV class, while it is estimated that availability of
20-25 kV switches in a single unit would cause a sharp drop in the cost of power flow
control circuits. Schottky and p-i-n rectifiers are an attractive vehicle for demonstrating
the high-voltage performance of different materials systems, and blocking voltages from
3-5.9 kV have been reported in SiC devices [112-114]. The reverse leakage current in
Schottky rectifiers is generally far higher than expected from thermionic emission, most
likely due to defect states around the contact periphery . To reduce this leakage
current and prevent breakdown by surface flashover, edge termination techniques such as
guard rings, field plates, beveling, or surface ion implantation are necessary [115,116,3].
However, in the GaN rectifiers reported so far, there has been little effort in employing
edge termination methods and no investigation of the effect of increasing the band gap by
use of AlGaN.
We study on the reverse breakdown voltage (VRB) of AlGaN Schottky rectifiers
for different Al compositions (0-0.25) and on the effect of various edge termination
techniques in suppressing premature edge breakdown. A maximum VRB of 4.3 kV was
achieved for Al0.25Ga0.75N diodes, with very low reverse current densities. At low reverse
biases the rectifiers typically show currents which are proportional to the contact
perimeter, whereas at higher biases the current is proportional to contact area. The
forward current characteristics show ideality factors of 2 at low bias (Shockley-Read-
Hall recombination) and 1.5 at higher voltage (diffusion current).
3.3.2 Experimental Methods
The undoped AlxGa- xN layers were grown by atmospheric pressure
metalorganic chemical vapor deposition at 1040 OC (pure GaN) or 1100 C (AlGaN) on
(0001) oriented sapphire substrates. The precursors were trimethylgallium,
trimethylgaluminum, and ammonia, with H2 used as a carrier gas. The growth was
performed on either GaN (in the case of GaN active layers) or A1N (in the case of AlGaN
active layers) low temperature buffers with nominal thicknesses of 200 A. The active
layer thickness was -2.5 .im in all cases and the resistivity of these films was of order 107
Q-cm . To form ohmic contacts in some cases, Si + was implanted at 5x1014 cm2,
50 keV into the contact region and activated by annealing at 1150 OC for 10 s under N2.
The contacts were then formed by lift off of e-beam evaporated Ti/Al/Pt/Au annealed at
700 C for 30 s under N2. The rectifying contacts were formed by lift off of e-beam
evaporated Pt/Ti/Au (diameter 60-1100pm). A schematic of the planar diodes is shown
in Figure 3-9. The devices were tested at room temperature under a Fluorinert ambient.
On the GaN diodes, we also examined the use of three different edge termination
methods, namely p-guard rings formed by Mg + implantation at the edge of the rectifying
contact, use of p-type floating field rings of width 5 im to extend the depletion boundary
along the edge of a SiO2 passivation layer and finally, use of junction barrier controlled
Schottky rectifiers (a rectifier with integrated p-n junction grid in its drift region). In all
of these edge-terminated diodes the Schottky metal was extended over a SiO2 layer at the
edge to minimize field crowding.
3.3.3 Results and Discussion
Figure 3-10 shows current-voltage (I-V) characteristics from two different
diodes. The GaN device employed 30 [m wide p-guard rings. This was found to be the
most effective edge termination method for these structures, producing an increase in VRB
of -800 V over devices without any passivation or edge termination, i.e., breakdown
occurred at 2.3 kV in the control diodes and 3.1 kV in devices with guard rings. The use
of guard rings or floating field rings each produced improvements in VRB over the control
diodes, with increases in the range 200-800 V. By sharp contrast, junction barrier control
was unsuccessful in our structures, leading to decreases in VRB of 300-400 V. We believe
this is due to Schottky barrier lowering because of the depletion regions around each
section of the grid not completely overlapping in our initial design. The best on resistance
(RON) achieved for GaN diodes was 0.8 Q-cm2, producing a figure-of-merit (VRB)2/RoN
of 15.5 MW-cm 2. Figure 3-10 also shows an I-V characteristic from an Al0.25Ga0.75N
rectifier, without any edge termination or surface passivation. In this case VRB was 4.3
kV, which is far in excess of the values reported previously for GaN rectifiers, i.e., 350-
450 V [116,117]. The on resistance of the AlGaN diodes was higher than for pure GaN,
due to higher ohmic contract resistance. The lowest RON achieved was 3.2 Q-cm2, leading
to a figure-of-merit of-5.5 MW cm2.
Figure 3-11 shows the variation of VRB with Al percentage in the AlGaN active
2.5 gm undoped AlxGal-xN
200A A1N buffer layer
Figure 3-9 Schematic of AlGaN power rectifiers without edge termination.
-4000 -3000 -2000 -1000 0
Figure 3-10 Room temperature I- Vcharacteristics from an
layers of the rectifiers. In this case we are using the VRB values from diodes without any
edge termination or surface passivation. The calculated band gaps as a function of Al
composition are also shown, and were obtained from the relation:
E, (x)= Eg,GaN ( ) + Eg,AlN x-bx( -x) 3-1
where x is the A1N mole fraction and b is the bowing parameter with value 0.96 eV
. Note that VRB does not increase in linear fashion with band gap. In a simple
theory, VRB should increase as (Eg)15, but it has been empirically established that factors
such as impact ionization coefficients and other transport parameters need to be
considered and that consideration of Eg alone is not sufficient to explain measured VRB
behavior. The fact that VRB increases less rapidly with Eg at higher A1N mole fractions
may indicate increasing concentrations of defects that influence the critical field for
The reverse I-V characteristics of all of the rectifiers showed I ac V05 over a
broad range of voltage (50-2000 V), indicating that Shockley-Read-Hall recombination
is the dominant transport mechanism. The current density in all devices was in the range
5-10x 10-6 A cm 2 at 2 kV. At low biases (25 V) the reverse current was proportional to
the perimeter of the rectifying contact, suggesting that surface contributions are the most
important in this voltage range. For higher biases, the current was proportional to the area
of the rectifying contact. Under these conditions, the main contribution to the reverse
current is from under this contact, i.e., from the bulk of the material. It is likely that the
high defect density in heteroepitaxial GaN is a primary cause of this current. The forward
I-V characteristics showed that the current density was proportional to exp(-eV/2kT) at
lowest voltages (up to current densities of -5 x 10-4 A cm 2) and to exp(-eV/1.5kT) at
> 5000 4.0
> 4000 -3.8
r 3000 3.6
S 2000 3.4
0 10 20 30
Percentage of Al in AIxGal.xN (%)
Figure 3-11 Variation of VRB in AlxGal-xN rectifiers without edge
termination, as a function of Al concentration. The Band
gaps for the AlGaN alloys are also shown.
higher voltages (current densities in the range 103-1.5x102 A cm2). These results are
consistent with Shockley-Read-Hall recombination as the dominant mechanism at low
bias, followed by diffusion current at higher voltage. Qualitatively similar behavior has
been reported previously for SiC Schottky rectifiers.
When pushed beyond breakdown, the diodes invariably failed at the edges of the
rectifying contact, as shown in Figure 3-12. As described earlier, the use of metal field
plate contact geometries with SiO2 as the insulator and either guard rings or floating field
rings significantly increased VB. These rectifiers generally did not suffer irreversible
damage to the contact upon reaching breakdown and could be re-measured many times.
Figure 3-12 Scanning electron microscopy micrographs of AlGaN
rectifiers before (top) and after (bottom) pushing the
applied bias beyond the value for breakdown.
In summary, Schottky rectifiers on high resistivity AlxGal-x N epi layers
produced reverse breakdown voltages up to 4.3 kV for A10.25Ga0.75N diodes without edge
termination. The current transport mechanisms were investigated as a function of bias
voltage, with Shockley-Read-Hall recombination being dominant over a broad range of
conditions. Minimizing electric field crowding at the covers of the rectifying contact was
effective in increasing the breakdown voltage. The AlGaN materials system appears
promising for high voltage applications.
3.4 Temperature Dependence and Current Transport Mechanisms in AlxGal-xN
P-i-n rectifiers are expected to have larger reverse blocking voltages than
Schottky rectifiers, but inferior switching speeds and higher forward turn-on voltages.
GaN Schottky rectifiers with reverse breakdown voltage (VRB) to 3.1 kV have been
demonstrated when p+ guard rings and metal overlap onto a dielectric are employed as
edge termination techniques. Use of Al.25Ga0.75N instead of GaN produced VRB values
up to 4.3 kV.
Since this type of device is intended for elevated temperature operation, there is a
need to understand the current transport mechanisms, the origin of the reverse leakage
current and the magnitude and sign of the temperature coefficient for VRB. In this section
all of these properties are investigated. Over a broad range of voltages, the reverse
leakage current is proportional to the diameter of the rectifying contact indicating that
surface periphery leakage is the dominant contributor. The temperature coefficient for
VRB was found to be negative for both GaN and AlGaN, even in edge-terminated devices.
3.4.2 Results and Discussion
The GaN and A10.25Ga0.75N layers were found to be resistive (-107 cm). Each
was grown on c-plane Al203 substrates by metal organic chemical vapor deposition using
conventional precursors and growth temperatures of 1040 (GaN) or 1100 C
(Al0.25Ga0.75N). The layer thicknesses were 2.5-3 jm. Schematics of the completed
rectifiers are shown in Fig. 3-13. The GaN devices employed p+ guard rings formed (7
jlm wide) by Mg/P+ implantation, n source/drain region formed by Si implantation
(annealing was performed at 1150 OC for 10 s under N2) and overlap of the rectifying
contact onto a Si02 passivation layer. The AlGaN devices did not use any edge
termination techniques. The contacts on all rectifiers were formed by lift-off, with the
ohmic metallization annealed at 700 OC for 30 s under N2. The rectifying contact
diameters were 45-125 jim with a separation of 124 jim between these contacts and the
Current-voltage (I-V) characteristics from both types of rectifiers are shown in
Figure 3-14 as a function of measurement temperature. The most obvious feature of the
data is that there is a negative temperature coefficient for VRB. The only previous
information for GaN-based devices comes from GaN/AlGaN heterostructure field effect
transistors in which a value of +0.33 V-K 1 was found , and from linearly graded
GaN p+pn+ junctions, in which a value of +0.02 V K 1 was determined . In both
cases the VRB values were more than an order of magnitude lower than in the present
Figure 3-15 shows the variation of VRB with temperature. The data can be
represented by a relation of the form:
VR = VRB [1+P(T-T,)] 3-2
where = -6.0+0.4 V K 1 for both types of rectifiers. However, in Schottky and p-i-n
rectifiers we have fabricated on more conducting GaN, with VRB values in the 400-500 V
range, the values were consistently around -0.34 V K1. Therefore, in present state-of-
the-art GaN rectifiers, the temperature coefficient of VRB appears to be a function of the
magnitude of VRB. Regardless of the origin of this effect, it is clearly a disadvantage for
GaN. While SiC is reported to have a positive temperature coefficient for VRB there are
reports of rectifiers that display negative 3 values . One may speculate that
particular defects present may dominate the sign and magnitude of 3, and it will be
interesting to fabricate GaN rectifiers on bulk or quasibulk substrates with defect
densities far lower than in heteroepitaxial material.
The forward turn-on voltage VF of a Schottky rectifier can be written as
nkT J 3-3
VF= ( )+ nB + RON JF
where n is the ideality factor, k is Boltzmann's constant, T is the absolute sample
temperature, e the electronic charge, JF the forward current density (usually taken to be
100 A cm 2) at VF, A** the Richardson constant, 4B the barrier height (-1.1 eV in this
case), and RON the on-state resistance. The typical best VF values were -5 V for GaN
3 gm undoped GaN
400A GaN buffer layer
Ti/Al/Pt/Au Pt/Ti/A Ti/Al/Pt/Au
2.5 gm undoped AlxGalxN
200A AiN buffer
Figure 3-13 Schematic of GaN (top) and AlGaN (bottom) rectifiers. The
GaN devices employ several edge termination techniques.
-5000 -4000 -3000 -2000 -1000
Figure 3-14 I V characteristics as a function of temperature for GaN
(top) and AlGaN (bottom) rectifiers.
1000 GaN (edge terminated)
0-- I I
0 50 100 150 200
Figure 3-15 Temperature dependence of V RB for GaN and AlGaN rectifiers.
and -7.5 V for Al0.25Ga0.75N, with best RON values of 50 and 75 mQ cm2, respectively.
The ideality factors derived from the forward I-V characteristic were typically -2 for
both GaN and Al0.25Ga0.75N for biases up to -2/3 of VF. This is consistent with
recombination being the dominant current transport in this bias range. At high voltages, n
was typically -1.5 for both types of rectifiers, indicating that diffusion currents were
dominant. Beyond -2xVF, series resistance effects controlled the current. This behavior
is often reported for SiC junction rectifiers, while Schottky rectifiers in that materials
system show ideality factors of 1.1-1.4. In our GaN devices, the higher ideality factors
may reflect the high compensation levels in the material.
Figure 3-17 shows the reverse current (IR) at -100 V reverse bias for GaN and
AlGaN rectifiers of different contact diameter, for three different measurement
temperatures. Since IR oc contact diameter, this indicates that under these conditions the
reverse current originates from surface periphery leakage. Similar results were obtained
for the GaN rectifiers as shown in Figure 3-16. The activation energy for this periphery
leakage was -0.13 eV, which may represent the most prominent surface state giving rise
to the current. At voltages approximately 90% of the breakdown values, the reverse
current was proportional to contact area, indicating that bulk leakage is dominant under
3.4.3 Conclusion and Summary
In conclusion, the temperature dependence of VRB has been measured in high
breakdown GaN and AlGaN Schottky rectifiers. The temperature coefficient is negative,
which is a significant disadvantage for devices intended for high temperature operation,
and there are indications that it is a function of VRB. The forward current conduction
makes a transition from recombination to diffusion currents. The reverse leakage current
originates from surface components around the rectifying contact at modest voltages.
This current is thermally activated with an energy of 0.13 eV. The yield of acceptable
devices (i.e., with VRB at least 90% of the maximum found on a wafer and RON within
50% of the best values obtained) was rather small (-15%), so there is still much
Figure 3-16 Reverse current at -100 V bias for AlGaN rectifiers
measured at three different temperatures.
I I I I I
25C, 1.21x104 cm-2
i D 1.35
100C, 1.21x10 '4 cm-2
I I I I I
S150C, 1.21x104 cm -
Ilu RI D
Al 2Ga .7N Rectifier
25C, 1.21x10-4 cm-2
AIl 0Ga. N Rectifier
100C, 1.21x1O4 cm-2
I I I I I
AIl .Ga7 N Rectifier
150C, 1.21x10-4 cm'2
Figure 3-17 Reverse current at -100 V bias for AlGaN rectifiers
measured at three different temperatures.
development needed on both materials and processing.
3.5 Lateral AlxGa-_xN Power Rectifiers with 9.7 kV Reverse Breakdown Voltage
There have been advances in developing GaN and AlGaN power rectifiers which
are key components of inverter modules for power flow control circuits. Vertical
geometry GaN Schottky rectifiers fabricated on conducting materials typically show
reverse breakdown voltages (VB) 750 V whereas lateral devices on insulating GaN and
AlGaN have VB values up to 4.3 kV.
Since the predicted breakdown field strength in GaN is of order 2-3 x 106 V-cm1
[121, 110], there appears to be much room for improvement in rectifier performance and
a need to understand the origin of reverse leakage currents, breakdown mechanisms, and
the effect of contact spacing on VB. In this letter we report on the variation of VB with
Schottky-to-ohmic contact gap spacing in AlxGal xN diodes (x = 0-0.25) employing p-
guard rings and extension of the Schottky contact edge over an oxide layer for edge
termination. VB values up to 9700 kV were achieved for Al0.25Ga0.75N rectifiers, with
breakdown still occurring at the edges of the Schottky contact. The reverse leakage
current just before breakdown is dominated by bulk contributions, scaling with the area
of the rectifying contact.
3.5.2 Experimental Methods
The rectifiers were fabricated on resistive (-107 Qcm) layers of 2.5-3 itm thick
GaN or AlGaN grown on c-plane A1203 substrates at 1040-1100 C by metalorganic
chemical vapor deposition. To create n regions for ohmic contacts, Si+ ions were
implanted at 5x 1014 cm2, 50 keV, and activated by annealing at 1150 OC for 10 s under
N2. It is important to control both the heating and cooling rates to avoid cracking of the
AlGaN layer. Mg+ implantation at 5x 1014 cm2, 50 keV was used to create 30 [im
diameter p-guard rings at the edge of the Schottky barrier metal. The rectifying contact
diameter was 124 [im in most cases, while the distance of this contact from the edge of
the ohmic contact was varied from 30-100 im. The Schottky metal was extended over a
SiO2 layer deposited by plasma-enhanced chemical vapor deposition in order to minimize
field crowding. Ohmic contacts were created by lift off of e-beam evaporated Ti/Al/Pt/Au
annealed at 750 C for 30 s under N2. The Schottky contacts were formed by lift off of e-
beam evaporated Pt/Ti/Au. A schematic of the completed rectifiers is shown in Fig. 3-18.
Current-voltage (I-V) characteristics were recorded on a HP4145 parameter analyzer,
with all testing performed at room temperature under a Fluorinert ambient.
3.5.3 Results and Discussion
Figure 3-19 shows the measured VB values for GaN and Al0.25Ga0.75N rectifiers as
a function of the gap spacing between the rectifying and ohmic contacts. For gaps
between 40 and 100 rim, VB is essentially linearly dependent on the spacing, with slopes
of 6.35x105 V.cm-1 for Al0.25Ga0.75N and 4.0x105 V.cm-1 for GaN. We assume the
deviation from these values at shorter spacing is due to the fact that the p-guard ring
almost covers this region. In vertical geometry diodes VB is related to the maximum
electric field strength at breakdown EM, through the relation :
V = EW /2 34
B rM" B
2.5-3 Lim undoped AlxGal_
200A AIN buffer
Schematic of lateral geometry AlGaN rectifiers
using edge termination.
80Al Ga N
0 25 0 75
20 40 60 80 100
Gaps between Schottky and Ohmic Metals (pm)
Figure 3-19 Effect of Schottky-ohmic contact gap spacing on
VB for GaN and Al0.25Ga.75N rectifiers.
where WB is the depletion width at breakdown. In our laterally depleting devices the
surface quality will dominate the onset of breakdown, which is reflected in the lower
breakdown field observed. However, given the current state of defect densities in
epitaxial GaN, the lateral geometry seems the most promising, for the time being, for
achieving very high VB values. Quasi-substrates of GaN, produced by thick epi-growth
on mismatched substrates and subsequent removal of this template, are soon to be
commercially available. In some cases the background doping in these is as low as
7.9x1015 cm 3 which makes feasible the use of these thick (200 [lm) freestanding GaN
films for vertically depleting rectifiers.
Figure 3-20 shows some I-V characteristics from the 100 [lm gap spacing GaN
and Al0.25Ga0.75N rectifiers. The best forward turn-on voltages, VF (defined as the forward
voltage at a current density of 100 A cm 2) was -15 V for GaN and -33 V for
Al0.25Gao.75N. These are much higher than the values obtained on more conducting GaN
films, where VF is typically 5-8 V. Note, however, that the ratio VB/VF is still very high
for the resistive diodes, with values ranging from 294 to 423. The specific on-state
resistance for a rectifier is given by
Ro = (4VB2/. EM) + p -s Ws+ Rc 3-5
where s is the GaN permittivity, l the carrier mobility, S and Ws are substrate resistivity
and thickness, and Rc is the contact resistance. The best on-state resistances we achieved
were 0.15 Q-cm2 for GaN and 1 Q-cm2 for Al0.25Ga0.75N, leading to figure of merits
(VB)2/RoN of 268 MW-cm 2 and 94 MW-cm 2, respectively. At low reverse voltages
(2000 V), the magnitude of the reverse current was proportional to contact diameter. As
the diodes approached breakdown the reverse current was proportional to contact area,
suggesting bulk leakage becomes dominant.
The variation of VB with Al percentage in the AlGaN layer of the rectifiers is
shown in Figure 3-21, along with the calculated bandgaps. VB does increase with
increasing bandgap Eg, but is not proportional to (Eg)1.5 as expected from a simple
theory. The presence of bulk and surface defects will have a strong influence on VB, and
these are not well controlled at this stage of AlGaN rectifier technology.
To place our results in context, Figure 3-22, Figure 3-23 and Figure 3-24 show a
compilation of RON, reverse leakage current and forward turn-on volatges versus VB data
for state-of-the-art SiC and GaN Schottky diode rectifiers, respectively, together with
theoretical curves for Si, 6H, and 4H-SiC and hexagonal GaN. Our results for high
breakdown GaN devices show the on resistances and forward turn-on voltages are still
well above the theoretical values and more work is needed to understand current
conduction mechanisms, the role of residual native oxides on contact properties, and
impact ionization coefficients in GaN.
3.5.4 Summary and Conclusion
In conclusion, lateral geometry Al Gal-xN Schottky rectifiers employing edge
termination show reverse breakdown voltages up to 9.7 kV. These breakdown voltages
scale with contact spacing and the rectifiers appear promising for high power electronics
-10000 -8000 -6000 -4000 -2000 0
Current-voltage characteristics of GaN and Alo.25Gao.75N
Al Percentage in AIGaN (%)
Figure 3-21 Variation of VB with Al percentage in the AlGaN
layer of the rectifiers.
1 00 / AIGaN UF
SE. Diode Rectifiers GaN UF
10- GaN (UF) i
o im SiC-ABB* /
"U *GaN-Caltec //
u" 10-2 SiC-Purdue
Ql 'Si GaN-UF
E *SiC-NCSU* -
0 10-3 -- .--
- 4H-SiC SiC-RPI
10-4 .. .. .
101 102 103 104
Breakdown Voltage (V)
RON = su sub ,
VRB = breakdown voltage
[ = carrier mobility
Ec = critical field for breakdown
Psub Wsub = resistivity/thickness of substrate
Figure 3-22 On-state resistance vs VB for wide band gap Schottky
rectifiers. The theoretical performance limits of Si, SiC, and
GaN devices are shown by the solid lines.
SiC AND GaN DIODE RECTIFIERS
SiC (Siemens, '97) I
SiC (Purdue, '97)
SSiC (NCSU, '95) 0 SiC (Philips, '97)
I GaN (UF, '99) 0 SiC (Cree,'97)
lD-GaN (Caltech, '99) GaN (UF)
- SiC (RPI, '98) AIGaN (UF) |
SiC (ABB, '97)
SiC (Cincinnati, '97)
with Barrier Lowering
-I I I I I I I
800 1200 1600
JR = AT 2 exp[ -(- A )]
JR = reverse current density
A** = Richardson's constant
T = absolute temperature
B = Schottky barrier height
A4B = image-force induced barrier lowering
Reverse leakage current JR VS VB for wide band gap
GaN (UF) AlGaN (UF, 00)
GaN (UF, 00) '
SiC AND GaN DIODE RECTIFIERS
nkT J +
n = ideality factor
T = absolute temperature
A** = Richardson's constant
B = Schottky barrier height
RON = specific on-resistance
Figure 3-24 Forward volatge drop VF VS VB for wide band gap
3.6 Vertical and Lateral GaN Rectifiers on Free-Standing GaN Substrates
Although the GaN-based power rectifiers on sapphire substrate show impressive
results, there are still numerous short-comings in these devices, including higher reverse
leakage current than expected from thermionic emission, high forward turn-on voltages,
negative temperature coefficients for reverse breakdown voltage, non-uniformities and
the low thermal condicutivity of the sapphire substrate.
Recently there have been initial reports of reverse recovery characteristics of GaN
Schottky rectifiers fabricated on free-standing substrates. Those substrates have the
advantages of higher thermal conductivity than sapphire and the potential for higher
forward current densities and reverse breakdown voltages than lateral rectifiers fabricated
in insulating substrates.
We investigated the effect of contact dimension and current flow direction (lateral
versus vertical) on the on-state resistance and breakdown voltage of Gan Schottky
rectifiers fabricated on free-standing GaN substrates. There is a dramatic effect of contact
diameter on VB, with the latter ranging from 6 to 700 V as the diameter was decresred
from 7 mm to 75 |tm. At the lower end ofthios range the on-state resistance (RON) are
exceptionally low (1.71-3.01 mQ.cm-2), producing maximum figure-of-merit (VB2/RoN)
above 100 MW-cm2.
The 200 |tm thick GaN quasi-substrates were grown by hydride vapor phase
epitaxy on sapphire substarte, lifted-off by laser heating and then etched and polished as
shown in Figure (3-25). The measured n-type doping concentration was ~1017 cm-3. Mg+
implantation at 5x1014 cm-2, 50 keV, followed by annealing was used to create 30 |tm
Figure 3-25 Free-standing GaN substrate grown by HVPE.
diameter p-guard rings at the edge of the Schottky contacts. The rectifying contact
diameter was 75 |tm for the small-area device and 7mm for the large-area devices. On
these latter structures the Schottky metal was extended over a SiO2 layer deposited by rf
(13.56 MHz) plasma enhanced chemical vapor deposition using SiH4 and N20 as the
precursors. Full-area back ohmic contacts were placed on the N-face using e-beam
evaporation of Ti/Al/Pt/Au. On the small-area devices we also placed ohmic contacts on
X 1,000 irPn/i
2 10.00] rn/iv
a l Iate. loi
X L000 m/diw+
Z 30.000 *M/lW
Figure 3-26 AFM images showing Ga- (front surface, top) and N-
(backside surface, bottom) terminated surfaces.
p -guard ring
p -guard ring
-200 tm undoped GaN(HVPE)
p -guard ring
-200 Ltm undoped
p -guard ring
Figure 3.27. Schematic of 7 mm contact diameter (top) and 75 |tm
contact diameter (bottom) rectifiers.
M I M
the top (Ga-face) surface so that we could compare results from the lateral and vertical
geometries. The top Schottky contacts were e-beam deposited Pt/Ti/Au in both large and
small area devices. In the latter case, the Schottky-ohmic metal spacing was 30 |tm.
Schematics of the completed structures are shown in Fig. 3-27. Current-voltage (I-V)
characteristics were recorded on an HP 4145B parameter analyzer at 25 C for the
forward part of the characteristics, while Tetronix 370A curve tracer was used for the
reverse characteristics measuement.
Figure 3-28 shows the I-V characteristic from the large area rectifers. The reverse
breakdown voltage (VB) is only -6 V and is obviously far below anything of practical
use. The on-state resistance (RON) was 3.4 Q-cm2 for these devices. The low VB is in
stark contrast to the values achieved in smaller devices, as described below. Since the
defect density in the quasi-substrate was ~105 cm-2 as measured by combined photo-
chemical etching and atomic force microscopy, the large area rectifiers are highly likely
to include one or more defects. Hsu et al. found that reverse bias leakage in GaN
Schottky diodes occurred primarily at defects and dislocations. The figure-of-merit
VB2/RoN had a value of 10.7 W-cm-2 for the large area rectifiers while maximum current
of -500mA could be achieved before sample heating became a problem.
I-V characteristics from the small-area rectifers, measured in the lateral geometry
are shown in Fig. 3-29. The VB was -250 V, with an excellent RON of 1.7 mQ-cm2. This
on-state resistance is the lowest reported for any GaN rectifers and shows that continued
improvements in surface cleaning and contact technologies for this materials system have
led to a rapid maturation of our understanding of how to process these devices. The value
of VB2/RN was 36.5 MW-cm-2. Note that remarkable improvement in the electrical
-8 -6 -4 -2 0 2
Fig. 3.28 Effect of Schottky-ohmic contact gap spacing on
VB for GaN and Al0.25Ga0.75N rectifiers.
characteristics in the small-area rectifiers relative to the large devices fabricated on the
same material. The forward turn-on voltage, VF, defined as the forward bias at which the
current density was 100 A-cmn2, was 1.8 V. This is roughly half of what has been
reported previously for GaN Schottky rectifersd on heterepitaxial layers. The forward
turn-on voltage for Schottky rectifiers is given by
VF = nkT/e(ln[J, / A**T2] +nB + RN -VF 3-6
Where n is the ideality factor, k is Boltzmann's constant, T the absolute diode
temperature, e the electronic charge, IF the forward current density at VF, A** is the
Richardson constant, 4B the barrier height (-1.1 eV in this case for Pt on n-GaN) and RON
the on-state resistance. One of the reasons for the much lower VF in these quasi-bulk
rectifers is the small RON value to heteroepitaxial devices. The ideality factors were -2 in
the large area rectifers, indicating that recombination was the dominant current transport
mechanism. In the small-area rectifers, n values was -1.5, which is consistent with
diffusion currents being dominant. These results can be explained by the relative
probabilities for having defects in the active region of the rectifiers for the different
Figure 3-30 shows the I-V characteristics from the small-area diode measured
from top-to-bottom, i.e. through the GaN substrate, rather than in the lateral geometry
employed for the data of Figure 3-29. The VB in the vertical geometry was -700 V,
while Ron was 3.01 mQ-cm2, leading to a figure-of-merit of 162.8 MW-cm-2. This VB is
close to the expected maximum for the drift region doping concentration of 1017 cm-3.
The forward turn-on voltage was still -1.8 V, which is close to the minimum expected for
a GaN rectifier with a VB of 700 V and Assuming a barrier height of 1.1 eV. The ratio
VB/VF is -389, a record for GaN rectifers, and the forward current density could be
pushed above 1000 A-cm-2. A plausible explanation for the large improvement in VB in
the vertical geometry may be not only in the larger thickness in this direction (200 tim)
compared to the 30 |tm spacing between Schottky and ohmic contacts in the lateral
direction, but also in the fact that the vertical depletion mode would minimize surface
breakdown problems. The reverse current at bias value close to VB was proportional to
-30 -20 -10 0
- o ft 1 1 I I - - - - - - -
-250 -200 -150 -100 -50
I-V characteristics from 75 |tm contact diameter
GaN rectifiers measured in lateral geometry.
-30 -20 -10 0
Figure 3.30. I-V characteristics from 75 |tm contact diameter GaN
rectifiers measured in vertical geometry.
.. . ............................... -o -- ---- ----i - -
I I I I
contact diameter, suggesting that the surface is playing a strong role in the origin of the
One can expect major improvements in VB in quasi-bulk GaN rectifiers as the
background doping is decreased. For example, a 200 |tm thick sample with a doping of
1015 cm-3 (which is quite feasible by reducing the background Si and O content, or by
appropriate compensation) would have a predicted VB of ~1.5x104 V. These would have
application for power control system in the 13.8 kV class.
In summary, the size and geometry dependence of GaN Schottky rectifiers on
quasi-bulk substrate has been investigated. The reverse breakdown voltage increases
dramatically as contact size is decreased and is also much larger for vertically-depleting
devices. The low on-state resistances produce high figure-of-merits for the rectifiers and
show their potential for applications involving high power electronic control systems.
GALLIUM NITRIDE P-I-N POWER RECTIFIERS
4.1 Comparison of GaN p-i-n and Schottky Rectifiers Performance
Schottky and p-i-n diodes are employed as high-voltage rectifiers in power
switching application. To suppress voltage transients when current is switched to
inductive loads such as electric motors, these diodes are placed across the switching
transistors. The advantage of simple metal-semiconductor diodes relative to p-n junction
diodes is the faster turn-off because of the absence of minority carrier storage effects and
lower power dissipation during switching. Wide bandgap semiconductors such as GaN
offer additional advantages for fabrication of diode rectifiers, including much higher
breakdown voltages and operating temperatures. There is much interest in developing
advanced switching devices and control circuits for CW and pulsed electrical sub-
systems in emerging hybrid-electric and all-electric vehicles, more-electric airplanes and
naval ships and for improved transmission, distribution and quality of electric power in
the utilities industry. Eventually one would like to reach target goals of 25 kV stand-off
voltage, 2 kA or higher conducting current, forward drop less than 2% of the rated
voltage and maximum operating frequency of 50 kHz.
Figure 4-1 shows a SIMS profile ofH and other background impurities (along
with intentional Si doping) in an MOCVD-grown p-i-n diode structure (left), together
with the Mg profile in the structure (right). Notice once again that the H decorates the Mg