Table Of ContentPreface
There has been a resurgence of interest in wide bandgap semicon-
ductors in recent times, for two important classes of applications, namely
blue/green light emitters and high power/high temperature electronics. For
the first set of applications, ZnSe andG aN are leading the race, duet o their
direct bandgaps which give them a huge advantage over the indirect gap SiC.
NichiaC hemical Industries of Japan, led by the efforts of Shuji Nakamura,
announced its first blue liGgahNt -emitting diode product in 1993, followed by
quantum well blue and green devices in 1995/1996. Currently, more than 01
million blue LEDs are sold per month. Attention has turned to work on high
brightness (15 lumens per watt) white LEDs using a blue LED to excite a
yttriuaml uminum garnet phosphor. These may have applications in white-
lighting situations, with the advantage ofmuch longer lifetimes than incandes-
cent bulbs. Nichia has also demonstrated continuous-wave blue-violet laser
diodes with room-temperature lifetimes acceptable for commercial applica-
tions. The active layer in these devices is InGaN with A1GaN cladding layers.
The LEDs are useful for full-color displays, whereas the main commercial
laser application is high density optical storage on CD-ROMs.
A notable feature of current GaN light-emitter technology is the
fact that all devices are currently grown heteroepitaxially on ,3021A SiC,
or magnesium aluminate substrates. The resulting high defect density
(109-10 ~~ cm 2) does not appear to affect the light output of LEDs, but can
VII io
viii Preface
cause problems in laser diodes through the migration of the p-contact metal
along dislocations which may short-out the p-nj unction. For this reason,
there have appeared some novel lateral overgrowth techniques on SiO 2
patterned GaN templates that produce defect-flee regions above the
masked areas. The active regions of the laser diodes are then processed in
these areas, with the result that their lifetimes under high current operation
are much longer than in devices grown in a blanket (non-patterned) fashion.
While II-VI based laser diodes were the first to be demonstrated,
their lifetimes are currently in the several hundred hours range and are
limited by the ease of defect generation and migration in thesseo ft materials.
A typical laser diode structure consists of ZnSe/ZnMgSSe/ZnSSe/ZnCdSe/
ZnMgSSe/ZnSe layers grown on GaAs substrates by Molecular Beam
Epitaxy. In this materials system, Metal Organic Chemical Vapor Deposi-
tion lags somewhat, due to poorer control over precursor purity and in-situ
thickness control. This is in contrast to the GaN system, where MOCVD
seems to have an advantage over MBE for photonic devices because of
higher quality material due to the higher growth temperature.
For the second major class of applications, high power electronics,
SiC is by far the most mature, with diamond and GaN as other candidates.
Diamond actually has the most appropriate material parameters, but prob-
lems with producing large single crystals and lack ofn-type dopability have
retarded its progress. GaN has the advantage of the availability of
heterostructures and excellent transport properties, but has relatively poor
thermal conductivity. SiC has excellent thermal conductivity, demonstrated
breakdown voltages of severalk V and more well-developed substrates and
device processing techniques. To date, the highest rfpower (850 W per mm
at 850 MHz CW) was demonstrated by a 4H-SiC MESFET, and the highest
total power (450 W pulsed at 600 MHz) produced by a SiC static induction
transistor. A SiC power module containing four SITs has demonstrated a 1
kW capability at 600 MHz. The basic driving force is the requirement for
electronics in adtomobiles, aircraft, and ships that can function directly on
engines to lower the weight and cost of control functions. As an example,
it is estimated that approximately 800 pounds couldb e eliminated on an F-
61 fighter jet if current mechanical, hydraulic, and pneumatic systems were
replaced with advancepdo wer electronics. Si-based electronics is limited to
-- 100~ for reliability reasons, requiring active cooling systems. NASA and
other agencies have needs for advanced electronics capable of operation at
600~ for temperatures <300~ it is likely that Silicon-on-Insulator technol-
Preface ix
ogy will prevail. SiC has advantages over Si for power devices because of
reduced parasitics, faster switching speed, and reduced thermal manage-
ment. A key requirement is for SiC substrate micropipe and dislocation
densities to be reduced from current levels. Several groups have demon-
strated a positive temperature coefficient breakdown behavior in SiC, an
important proof that SiC can have acceptable reliability. SiC has a thermal
conductivity exceeding that of Cu, and is inert in virtually all chemicals. It is
also radiation-hard with high saturation drift velocity. Initial technology
insertions include HDTV receivers, power conditioning, and high power
microwave applications operating at room temperature atmospheric ambi-
ents with high internal junction temperatures. SiC exists in many different
polytypes, named according tthoe stacking sequence of Si and C atoms. For
example, 6H material has a hexagonal lattice with an arrangement of six
different Si and C layers before the pattern repeats itself. More than 200
different polytypes of SiC exist, and the exact physical properties depend on
the crystal structure. The most common polytypes are 6H, 4Ha,n d 3C. Initial
work focused on 3C material because of its excellent transport properties.
Emphasis is now being placed on the 4H and 6H polytypes. For these latter
materials, single crystal substrates are commercially available (up to 3"~
currently). Additionally, the use of vanadium compensatpiroond uces semi-
insulating substrates which are of interest for microwave power devices.
4H-SiC has a substantially higher carrier mobility than 6H-SiC (800 to 370
cmVV.s). In the latter polytype, there is an inherent mobility anisotropy that
degrades conduction parallel thet o crystallographic c-axis. Moreover, highly
conductive 4H substrates are available for vertical power devices. There is
also excellent progress on heteroepitaxy of 3C-SiC on both low-tilt-angle 6H
substrates and on S .i
In this volume, we have brought together experts in the fields of
growth, processing, and characterization of wide bandgap semiconductors.
The coverage includes growth and contacts oflI-VI compoundsp,r ocessing
techniques for SiC, GaN and diamond, and materials analysis of all wide gap
semiconductors. We believe this is a very useful volume, as a valuable
reference tool, both for people just entering this field, and for established
researchers.
Gamesville, Florida Stephen J. Pearton
January,2 000
Contributors
Jeffrey B. Casady Berthold Hahn
University State Mississippi University of Regensburg
ippississiM ,etatS ippississiM Germany Regensburg,
Wolfgang Faschinger Paul H. Holioway
University of Wurzburg University of Florida
Germany Wurzburg, ,ellivseniaG adirolF
Joseph R. Flemish Tae-Jie Kim
scigidanA University of Florida
Warren, New Jersey ,ellivsemaG adirolF
Wolfgang Gebhardt Jewor W. Lee
University of Regensburg University of Florida
Germany Regensburg, ,ellivsemaG adirolF
Donald R. Gilbert Stephen J. Pearton
University of Florida University of Florida
,ellivseniaG adirolF ,ellivseniaG adirolF
xi
xii Contributors
Randy J. Shul John M. Zavada
Sandia National Laboratories US Army Research Office
Alburquerque, New Mexico Research Triangle Park, North
aniloraC
Rajiv K. Singh
University of Florida John C. Zolper
,ellivseniaG Florida Sandia National Laboratories
Albuquerque, New Mexico
Robert G. Wilson
Hughes Research Laboratories
California Malibu,
1
Doping Limits and
B a n d g a p Engineering in
Wide Gap l l - V l
C o m p o u n d s
gnagfloW regnihcsaF
1.0 INTRODUCTION
Figure 1 shows the energy gap of wide gap II-VI compounds as a
function of the cubic lattice constant. In addition to the "classical" II-VI
compounds ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe, the range of avail-
able materials has been considerably enlarged in the last few years by the
introduction of new compounds that form a metastable zincblende modi-
fication. The first metastable materials reported were manganese eom-
pounds,[ ]1 which were soon followed by the introduction of magnesium
compounds with similar lattice constants and energy gaps.[ ]2 All of these
compounds crystallize naturally in a hexagonal or NaCl-structure and are
insulators. Only when they are grown epitaxially on substrates with a
zincblende structure do they form semieonducting zincblende modifica-
tion. In addition, the wide gap II-VI spectrum has recently been further
extended by the epitaxial realization of beryllium compounds.P] In con-
trast to the Mn- and Mg-compounds, these materials crystallized in the
Wide Bandgap Semiconductors
zincblende structure even as bulk materials, but were only recently in-
tensely investigated, partly due to their high toxicity.
5,4 -I " - ' " I ~t, .... SgM'' I ' ' ' I ' ' I " " I '
4,0
eS
eTgM
SnM
> 3,5 II
eTa
o. 3,0 r~I
eTeB
~ 2,5 eTnZ
= 2,0
5,1 eSdC
eTdC
1~0 4,5 I ' ' 6,5 I ' 8,5 i " 6,0 I " 2,6 'I '~ 6,4 I' ' 6,6
Lattice Parameter [/~ ]
Figure .1 Energy gap of wide gap II-VI compounds as a function of their lattice constant.
Altogether, wide gap II-VI materials cover a considerable part of the
light speemma; from near infrared (CdTe) into the ultraviolet region (MgS,
BeSe). One advantage is that nearly the entire range of compounds with
different energy gaps exists at a given lattice constant, a necessary condition
for the fabrication of heterojunction devices. These properties make II-VI
materials very interesting for applications as light emitters and detectors in
the blue and ultraviolet range.
However, the application of these interesting properties has long
been hindered by the limited dopability of these materials. Until 1990,
CdTe (with an energy gap close to that of GaAs) was the only material that
could be doped n- and p-type, while compounds with a larger gap could
only be doped either n-type (e.g., ZnSe and ZnS) or p-type (e.g., ZnTe).
All attempts to dope the other way led to strong compensation, so that no
devices could be realized, although the material could be fabricated in
outstanding crystalline quality.
This picture changed drastically in 1990 when nitrogen plasma was
introduced as a p-dopant which allowed p-doping levels around 8101 cm 3-
and Limits Doping pagdnaB gnireenignE 3
for ZnSe. ]5114[ Suddenly, blue emitting devices came into reach and within
a few months the first blue ZnSe based laser was reported.[ ]6 This device
operated only at liquid nitrogen temperature and with a veryh igh threshold
voltage. Soon the introduction of graded ZnSe/ZnTe contacts[7][ ]8 and
ZnMgSSe cladding layers for waveguiding[ ]9 led to low threshold voltages
and operation at room temperature. Room temperature cw operation for
more than 100 hours has also been achieved.[ ~l A typical device structure
is given in Fig. .2 The entire device, except the active zone, is lattice
matched to the GaAs substrate. The active zone consists of CdZnSe, which
is surrounded by ZnSSe waveguide- and ZnMgSSe cladding-layers. A typical
emission spectrum of a diode fabricated in our lab is shown in Fig. 3, while
the LI-characteristics are depicted in Fig. 4. As can be seen, threshold
current densities as low as 340 A/cm 2 are obtainable even for simple gain
guided structures without facet coating,[ ]11 while the lowest reported value
for index guided structures with facet coating is 240 A/cm2.[ ]21 Threshold
voltages well below V 5 have been obtained.[ MT These devices are already
nearly comparable to III-V lasers from an optical and electrical point of
view. Therefore, the main issue for device optimization is, at the moment,
the increase of the device lifetime, which seems to be limited primarily by
stacking faults originating at the GaAs-ZnSe-interface.[ ]41
Despite these impressive successes, one should note that the device
structure given in Fig. 2 is a compromise governed by the obtainable
doping. The use of CdZnSe as the active zone implies a number of
disadvantages. Ipnr inciple, it would be desirable to use ZnSe as the active
zone. Its energy gap is really in the blue; it is a binary material with much
narrower spontaneous emission linewidth;[ ]51 and it is nearly lattice matched
to GaAs, so that the width of the active zone could be optimized without
limits imposed by the critical thickness. However, the maximum doping
level in the ZnMgSSe cladding decreases strongly with an increasing
energy gap, ]61[ limiting the energy gap of the cladding to values below
about 2.9 eV at room temperature. For room temperature operation with
low threshold currents, an energy difference of about 400 meV is needed
between the cladding and the active zone to prevent carrier overflow and
to allow reasonable light guiding.[ ]71 As a consequence, the emission
energy of the device is around 2.5 eV, which is more in the green than in
the blue range of the spectrum. This energyc orresponds to a Cd content in
the active zone of about 25%. Since the lattice constant of CdSe is
considerably larger than that of ZnSe, this high Cd content, in turn,
seriously limits the thickness of the active zone to values below 5 nm.
Wide Bandgap Semiconductors
Gold
' ..... I sulator
i " " :. '~ ZnSe/ZnTe Graded Contact
" " ZnMgSSe:N (p-Cladding)
~///~///////~//////~//~~ Zn S S e: N (p-Wave i u g de )
~ / ~ Z ~ , . . ~ CdZnSe Quantum Well
.............. ZnSSe:CI (n-Waveguide)
ZnMgSSe:C1 (n-Cladding)
~-- GaAs:Si Substrate
. J
Figure .2 Layer sequence of a ZnSe based blue-green laser .erutcurts
" ' I I '" 'I ' I " I ' "
o..-I
r~
r.
.... .._J
I..--4
I , ~I , I , I ~. I a I (cid:12)9
525 526 527 528 529 530 531
Wavelength (nm )
Figure .3 Stimulated emission spectrum of a blue green .resal
Doping Limits and Engineering Bandgap
I ' ' ' I' ' I
Pulsed Operation
~3
E
2
f~
(cid:14)9
I
""
(cid:14)9 f~
0 I (cid:12)9 I , | i , _ (cid:12)9 (cid:12)9 (cid:12)9 |
0 200 h,J 400 600
Current Density ( A/cm 2 )
erugiF 4. Light output of a blue-green laser as a function of current density.
As a consequence, in spite of the use of nitrogen plasma, the device
potential of II-VI compounds is still seriously restricted by compensation
phenomena. An understanding of the processes leading to compensation is
therefore of major importance. In the following, we will give a review on
ab initio attempts to understand and describe compensation in widegap II-VI
materials, and then describe, in more detail, a phenomenologieal model
that is able to account for most of the experimentally observed doping
limitations. This model will be further extended to give some insight into
microscopic mechanisms leading to compensation during nitrogen dop-
ing, and will be used to explain important properties of contacts to p-ZnSe.
