To meet the needs of the aerospace propulsion and space power communities, the high temperature electronics program at the Lewis Research Center is developing silicon carbide (SiC) as a high temperature semiconductor material. This program supports a major element of the Center's mission - to perform basic and developmental research aimed at improving aerospace propulsion systems. Research is focused on developing the crystal growth, characterization, and device fabrication technologies necessary to produce a family of SiC devices

Matus, Lawrence G.

Seng, Gary T.

English

NASA Technical Reports Server

N92-22528
IV - HIGH-TEMPERATURE ELECTRONICS
Lawrence G. Matus and Gary T. Seng
In recent years, the aerospace propulsion and space power communities
have acknowledged the growing need for electronic devices that are capable of
sustained high-temperature operation. Applications for high-temperature elec-
tronic devices include engine ground test instrumentation such as multiplexers,
analog-to-digital converters, and telemetry systems capable of withstanding hot
section engine temperatures in excess of 600 °C. Similarly, engine-mounted
integrated sensors could reach temperatures which exceed 500 =C, while uncooled
operation of control and condition monitoring systems in advanced supersonic
aircraft would subject electronic devices to temperatures in excess of 300 °C
(ref. IV-l). Hypersonic vehicles will ultimately pose even more severe demands
on electronic devices and sensors.
In addition to aeronautics, there are many other areas that would benefit
from the existence of high-temperature electronic devices. Space applications
include power electronic devices for the Freedom space station, space plat-
forms, and satellites. Since power electronics require radiators to shed waste
energy, electronic devices that operate at higher temperatures would allow a
reduction in radiator size. Other applications on earth include deep-well
drilling instrumentation, power electronics, and nuclear reactor instrumenta-
tion and control.
To meet the needs of the applications mentioned previously, the high-
temperature electronics program at the Lewis Research Center is developing sil-
icon carbide (SIC) as a high-temperature semiconductor material. This program
supports a major element of the Center's mission: to perform basic and devel-
opmental research aimed at improving aerospace propulsion systems. Research
is focused on developing the crystal growth, characterization, and device fab-
rication technologies necessary to produce a family of SiC devices.
BENEFITS OF SILICON CARBIDE AS A SEMICONDUCTOR MATERIAL
Since the desired operating temperature for some of the applications men-
tioned approaches 600 °C, it is clear that a new semiconductor material will
need to be developed. On the basis of the inherent solid-state properties of
silicon, the maximum temperature at which a silicon device could theoretically
operate is about 300 °C. Gallium arsenide could theoretically operate at 460 °C,
but has not proven operationally stable at this temperature. In comparing
potential candidate materials for high-temperature semiconductor devices, SiC
stands out not only because of its excellent high-temperature electronic prop-
erties but also because it is a very stable ceramic material up to temperatures
of 1800 °C. The maximum operating temperature for a semiconductor is deter-
mined by the forbidden bandgap energy. The temperature limit is reached when
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the number of intrinsic carriers, thermally excited across the energy gap,
approaches the number of extrinsic carriers. This temperature (when expressed
as the absolute temperature) is roughly proportional to the energy bandgap.
By using the same criteria as applied to silicon, SiC could theoretically
be employed at temperatures as high as 1200 °C. A more reasonable, shorter
term goal is to produce electronic devices capable of 600 °C operation. SiC is
characterized by excellent physical and chemical stability making it suitable
for long-term use in high temperature, corrosive environments. The combination
of the material's high thermal conductivity and high breakdown field provides
the potential for improved power system electronics and for increasing the
number of devices per unit area. Those properties which determine the high-
frequency characteristics of semiconductor devices also appear to be excellent
for SiC and superior to those of silicon or gallium arsenide. The properties
and benefits of SiC are summarized in table IV-I.
SILICON CARBIDE CRYSTAL GROWTH
Until recently, there was no process whereby single crystals of SiC with
sufficient size, purity, and perfection could be grown reproducibly. SiC does
not melt at any reasonable temperature and pressure, so this rules out the
growth-from-melt technique commonly used to obtain other semiconductor single
crystals such as silicon and gallium arsenide. Historically, vapor phase
growth processes have proven to be the most successful method for producing
SiC crystals. Early research was done on SiC crystals that were a by-product
of the industrial process for making abrasives. In the industrial process,
SiC is formed at 2500 °C by the reaction of silica and coke. At this tempera-
ture, gas pockets form within the SiC reaction product. The SiC sublimes and
then condenses on the walls of pockets located at cooler parts of the reaction
product. Occasionally, isolated SiC crystals are produced within these pockets
during the production process. The larger and better crystals were selected
for research purposes.
In 1955, Lely developed a laboratory version of the industrial sublima-
tion process and was able to produce rather pure SiC crystals. Encouraged by
the Lely process, NASA Lewis and other laboratories pursued the development of
SiC semiconductor devices during the 1960's and early 1970's (refs. IV-2 to
IV-6). Though SiC devices were demonstrated above 400 °C, by the early 1970's
the Lely process and other processes had not matured to the point where high-
quality large-area crystals could be grown reproducibly. Since crystal sub-
strates are crucial to device fabrication, interest in SiC waned. From 1973
to 1980, there was very little effort in the U.S. on SiC. However, research
did continue in Japan and in Europe during this period.
In 1980, because of the increased need for high-temperature electronics
in advanced turbine engines, NASA Lewis again embarked on a high-temperature
electronics program. The emphasis again has been on developing SiC. The prob-
lem regarding the crystal growth of SiC is rooted in the fact that SiC crystals
can take on many different structural forms called polytypes. In most cases,
SiC crystals grown by sublimation techniques are a mixture of different poly-
types. Since each SiC polytype has its own electronic properties, sublimation-
grown SiC crystals usually contain heterojunctions and possess unpredictable
electronic properties. To favor growth of a single polytype of SiC, epitaxial
growth on a host crystalline substrate from gases containing silicon and carbon
was hypothesized. The host crystal _mparts its crystalline regularity to the
234
thin growing layer. Since silicon is available in perfect, large, and low-cost
crystals, manyattempts at the heteroepitaxial growth of SiC on Si were made.
These efforts were largely unsuccessful because of the large lattice mismatch
that exists between Si and SiC. (The SiC lattice is 20 percent smaller than
the Si lattice.)
Large-area heteroepitaxial growth of cubic SiC on Si was finally achieved
at the NASALewis Research Center in 1983 by using a chemical vapor deposition
(CVD) process (ref. IV-7). In recognition of this achievement, an IR-100 award
was presented to NASALewis. Recently, a research program utilizing the NASA
Lewis SiC crystal growth process has been established at HowardUniversity(ref. IV-8).
Crystal growth takes place at atmospheric pressure in a fairly conven-
tional horizontal CVDsystem. A complete system description is given in
reference IV-9. The chemical vapor deposition reaction system is illustrated
schematically in figure IV-I. To grow single-crystal cubic SiC, first an
electronic-grade silicon substrate is placed on an rf-heated graphite susceptor.
The essential step in the growth process is a rapid temperature ramp from near
room temperature to a growth temperature of 1360 °C in the presence of a hydro-
carbon gas. The NASALewis process uses propane as the carbon-containing gas.
During the first 2 min of growth, a single-crystal cubic SiC film about 20 nm
thick is produced on the Si substrate. After this initial SiC growth, silane
gas is added to provide a silicon source and to initiate the final step, the
bulk growth of cubic SiC to the desired thickness. During this time, the
cubic SiC layer grows at a rate of 3 to 4 _m/hr.
A photograph of the chemical vapor deposition system is shown in fig-
ure IV-2. The reaction tube and associated plumbing are in the hood (center).
The control valves and flow controllers reside in the vented area directly to
the right. In the right foreground are the manual and computer control systems,
while in the left foreground is a mass spectrometer used to monitor system gases.
The rf generator is not shown.
Cubic SiC is a transparent, yellow crystal which fractures into regular
rectangular pieces. In the photograph of figure IV-3, the silicon substrate
has been chemically removedfrom a 16-_m-thick crystal. Although visually, the
material appears to be of relatively good quality, in actuality a high density
of defects exists in the crystal. Certain defects can adversely affect the
electrical properties of SiC devices. During the past five years, muchprog-
ress has been madein understanding problems associated with crystal growth,
but much research remains to be done in this area.
CHARACTERIZATIONF SiC FILMS
Initially, the transition from the Si substrate to the SiC epitaxial
layer was thought to occur by meansof a thin buffer layer or transition layer
of the order of 20 nm thick (ref. IV-7). However, high-resolution transmission
electron microscopy (TEM) has demonstrated that the SiC/Si interface is abrupt
with no transition region (ref. IV-IO). The cubic SiC films do contain a large
235
density of defects that include interfacial twins, stacking faults, and anti-
phase disorder (ref. IV-II). The defect density in the films is greatest near
the SiC/Si interface and decreases with distance away from the interface.
A particular type of lattice defect, called antiphase boundaries (APB's),
was reported to be present in all SiC films grown on Si at NASALewis
(ref. IV-12). The APB's can be decorated by chemical etching, sputter etching,
wet oxidation, or cubic SiC growth in the presence of diborane (ref. IV-13) and
have the appearance of irregular-shaped boundaries as shownin figure IV-4(a).
APB's form in the initial stages of growth on the Si substrate when SiC islands
of opposite phase nucleate and grow together. Across the APB, the chemical
bonding is between like atoms (i.e., Si-Si or C-C), instead of the normal Si-C
bond between neighboring atoms.
Normally, in epitaxial growth on Si, the surface of the Si substrate is
oriented precisely parallel to an atomic plane, for example, the (001) plane.
It has been found in the growth of gallium arsenide on Si that APB's are elimi-
nated by orienting the substrate slightly off-axis from the (001) plane. This
technique was applied to SiC growth at NASA Lewis with the following results:
for Si substrates that were tilted i ° to 4 ° from the (001) plane, all APB's
were eliminated from the subsequent films grown. In addition, the resultant
SiC films were smoother by a factor of 2 to 3 than the films grown on on-axis
substrates (ref. IV-13). The micrograph in figure IV-4(b) shows the results
of using an off-axis Si substrate for SiC growth. Further work is needed to
determine the effect on electrical properties and the performance of devices
fabricated from these films. Also, the optimum tilt angle and the direction
of optimum tilt have yet to be determined.
Additional SiC material characterization is provided by photolumines-
cence, Raman, Rutherford backscattering, cathodoluminescence, and DLTS tech-
niques (refs. IV-14 to IV-17).
Electrical characterization to determine electrical properties of the
cubic SiC films is an important and necessary evaluation step if high-quality
SiC films are to be achieved. Room-temperature Hall measurements were made on
cubic SiC films grown at NASA Lewis using the van der Pauw configuration
(ref. IV-18). Ohmic electrical contacts consisted of sputtered tantalum fol-
lowed by sputtered gold. Detailed analysis of the Hall measurements indicated
that unintentially doped films were always n-type and that the films were heav-
ily compensated. It is believed that all present-day cubic SiC films grown at
NASA Lewis and elsewhere are compensated. The significance of compensation is
the degradation of device quality. Compensated semiconductors have many ion-
ized impurities (positive for donors and negative for acceptors) embedded in
the crystal lattice, and these ions serve as scattering centers for the moving
charge carrier. This increases the impurity scattering and reduces the total
mobility of the charge carrier compared to uncompensated material with the same
density of free carriers. Identification and eventual elimination of the com-
pensating impurities remains a research goal.
SiC DEVICE FABRICATION TECHNOLOGY
Devices fabricated from NASA Lewis-grown SiC have been characterized and
compared to conventional silicon devices. Shown in figure IV-5 are the current-
voltage curves for ion-lmplanted SiC and Si diodes. Note that at 300 aC the
Si diode is rendered useless (shorted) while the SiC retains its rectification
236
properties. The poorer quality of the SiC diode characteristics at room tem-
perature is due to the unoptimized design of the device, the fabrication proc-
ess, and the crystal quality (ref. IV-19).
The availability of SiC substrates now allows NASA Lewis to place
increasing emphasis on device fabrication. In-house research is pursuing the
fabrication of an in situ grown junction diode. Device technology is also being
developed through outside activities at two separate research laboratories.
North Carolina State University (NASA grant NAG3-782) is concentrating on
fundamental SiC device fabrication utilizing, when possible, technology compat-
ible with the silicon industry. Three device structures are being pursued at
NCSU: Schottky barrier diodes, ion implanted diodes, and a metal-_nsulator-
semiconductor field effect transistor (MISFET). The National Bureau of Stan-
dards (Contract C-30007-K) is developing a device quality insulating layer,
oxide or nitride, for SiC. A quality insulating layer is of paramount impor-
tance for integrated circuit technology. Earlier research had examined the
quality of an oxide insulating layer deposited onto the SiC surface (refs. IV-20
and IV-2l).
Although SiC can easily survive the 600 =C temperature requirement for
the NASA Lewis program, an electrical contact material must be developed which
also can survive. Successful metallization (i.e., electrical contact) requires
a metal that adheres well to the SiC surface and possesses a chemically stable
metal/SiC interface. Additionally, low contact resistance and chemical stabil-
ity of the metallization scheme at high temperatures are also necessary. In
the study of SiC metallization, extensive information concerning the SiC sur-
face properties has been learned (refs. IV-22 and IV-23). Methods to create a
carbon-rich or silicon-rich SiC surface prior to metallization have been deve-
loped (refs. IV-24 and IV-25). Metals that are refractory carbide-formers or
silicide-formers are currently being tested at 600 °C for long periods of time
(refs. IV-26 and IV-27).
FOCUS OF CURRENT AND FUTURE SiC RESEARCH
Current and future program plans are outlined in figure IV-6. Near-term
studies will be performed primarily through in-house and grant research
efforts. Computer modeling of the chemical vapor deposition process for SiC
crystal growth has been initiated as an in-house project based on a recently
completed SBIR contract, NAS3-23891 (ref. IV-28). The goal is to improve the
SiC crystal growth through the identification and understanding of the impor-
tant chemical reactions involved in the growth process.
Ultimately, to achieve our goals of integrated circuits and sensors, the
scope of the high-temperature electronics program must be expanded into the
area of device packaging. At the same time, however, the basic crystal growth
and characterization research must continue to receive appropriate attention.
237
CONCLUDING REMARKS
The development of semiconductor materials does not occur overnight.
First germanium, then silicon and then gallium arsenide, for example, have come
to the marketplace after years in the laboratory and countless dollars spent
for development costs. The history of SiC as a hlgh-temperature semiconductor
has been one of high expectations followed by disappointment. Recent advances
in the crystal growth of SiC and the increased knowledge of the bulk material
properties of the grown SiC are cause for renewed enthusiasm. It should be
noted that foreign research programs are more aggressively pursuing the devel-
opment of SiC technology than are those in the U.S., and although the develop-
ment of SiC falls into the category of high-risk research, the future looks
promising and the potential payoffs are tremendous. SiC now appears ready to
emerge as a useful semiconductor material.
TABLE IV-I. - BENEFITS OF SILICON CARBIDE AS SEMICONDUCTOR MATERIAL
Property Benefit
High operating temperature
Excellent stability
High thermal conductivity and
high breakdown Field
Excellent high-frequency
properties
600 °C electronics
Sustained use in hostile environment
Improved power electronics and increased
device packing density
Superior high-frequency devices
238
CHEMICALVAPORDEPOSITIONREACTIONSYSTEM
/- WATER-COOLED
/ REACTIONTUBE /-RF COILS
/ /
/ /
/ 000000000
_,l .....................
GRAPHITE
SUSCEPTOR
QUARTZSUPPORT ....
/-SILICON SUBSTRATE H2
/
/ SIH4
FLOW C3H8
HCL
000000000
CD-87-29389
Figure IV-I. - Chemical vapor deposition reaction system.
CD-87-29390
Figure IV-2. - Chemical vapor deposition system.
239 BLACK AND V.,'HIT_ FHOiOGRAPN
Figure IV-3. - Silicon carbide crystal.
CD-87-29391
ON-AXIS (001) Si SHOWING OFF-AXIS (001) Si SHOWING
DEFECT (ANTIPHASE BOUNDARIES) ELIMINATION OF DEFECT
C0-87-2963 I
Figure IV-4. - Scanning electron micrographs of SiC grown on silicon (Si).
240
ORIGINAL PACE
BLACK AND WHITE F'HOTOGRAPH
SILICON
CARBIDE
20 #A/DIVISION
SILICON
ROOM TEMPERATURE 300 °C
CD-87-29392
2 V/DIVISION
Figure IV-5. - Temperature characteristics of SiC and Si diodes.
b
CURRENT:
• CONTINUE PRESENT CVD STUDIES TO IMPROVE CRYSTAL QUALITY
• STUDY ALTERNATE CRYSTAL GROWTH METHODS
• CONTINUE DETAILED CHARACTERIZATION STUDIES
• CONTINUE DISCRETE DEVICE FABRICATION TECHNOLOGY RESEARCH
• CONTINUE CVD MODELING STUDIES
FUTURE:
• STUDY ALTERNATE CRYSTAL GROWTH METHODS
• CONTINUE DETAILED CHARACTERIZATION STUDIES
• DEMONSTRATE SIMPLE INTEGRATED-CIRCUIT TECHNOLOGY
• DEVELOP DEVICE METALIZATION AND PACKAGING TECHNIQUES
• DEMONSTRATE INTEGRATED-SENSOR TECHNOLOGY
CD-87-29393
Figure IV-6. - Focus of current and future Lewis SiC research.
241
jC',R!_lt' ,,',L ['AG_.
BLACK AND WHITE FHO1-OG_APFt



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