After many years of promise as a high temperature semiconductor, silicon carbide (SiC) is finally emerging as a useful electronic material. Recent significant progress that has led to this emergence has been in the areas of crystal growth and device fabrication technology. High quality single-crystal SiC wafers, up to 25 mm in diameter, can now be produced routinely from boules grown by a high temperature (2700 K) sublimation process. Device fabrication processes, including chemical vapor deposition (CVD), in situ doping during CVD, reactive ion etching, oxidation, metallization, etc. have been used to fabricate p-n junction diodes and MOSFETs. The diode was operated to 870 K and the MOSFET to 770 K

Matus, Lawrence G.

Powell, J. Anthony

English

NASA Technical Reports Server

NASA Technical Memorandum 103655
Sill co n C arbid e; h::gemic:on dUc_0 r_
for Space Power Electronics _ _
J. Anthony Powell and Lawrence G. Matus _
Lewis Research Center
Cleveland, Ohio ......
Prepared for the ..................
8th Symposium 6n Space Nuclear P0v_r Systerns -
cosponsored by the University of New Mexico, National Aeronautics and
Space Administration, Strategic Defense Initiative Organization,
U.S. Department of Energy, and U.S. Air Force
Albuquerque,-N_wMexi66; Ja_ 6-10; i99-i ...........
[U/LN 
(NASA-TM-103o55)
Sgt_ iCCINDUC TOR FOR
(NASA) a p
SILICON CAR_iDE, A
SPAC_ POWER CL_CTeJNICS
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https://ntrs.nasa.gov/search.jsp?R=19910005537 2020-03-19T19:26:20+00:00Z

SILICON CARBIDE, A SEMICONDUCTOR
FOR SPACE POWER ELECTRONICS
J. Anthony Powell and Lawrence G. Matus
NASA Lewis Research Center
21000 Brookpark Road
Cleveland, OH 44135
Abstract
After many years of promise as a high temperature semiconductor, silicon
carbide (SIC) is finally emerging as a useful electronic material. Recent
significant progress that has led to this emergence has been in the areas of
crystal growth and device fabrication technology. High quality single-crystal
SiC wafers, up to 25 mm in diameter, can now be produced routinely from boules
grown by a high temperature (2700 K) sublimation process. Device fabrication
processes, including chemical vapor deposition (CVD), in situ doping during
CVD, reactive ion etching, oxidation, metallization, etc. have been used to
fabricate p-n junction diodes and MOSFETs. The diode was operated to 870 K and
the MOSFET to 770 K.
INTRODUCTION
In recent years, there has been a growing need for electronic components
capable of operating at high temperatures for extended periods of time. The
desired operating temperature in some applications approaches 900 K, which is
well beyond the capability of currently available semiconductor devices.
Silicon carbide possesses unique properties which make it suitable for such
applications (Harris and Yang 1989). Because of these properties, NASA Lewis
is developing a new family of semiconductor devices based on SiC for aerospace
propulsion and power applications. This paper reviews pertinent SiC properties
and describes recent advances in SiC semiconductor technology at NASA Lewis
and elsewhere.
PROPERTIES
An unusual aspect of SiC is that its crystal structure exhibits a form of
one-dimensional polymorphism called polytypism. The many polytypes of SiC
differ from one another only in the stacking sequence of double layers of Si
and C atoms. This paper considers only the two most common polytypes, a cubic
structure known as 3C-SiC or B-SiC, and a hexagonal structure known as 6H-SiC.
An appreciation of the potential of SiC can be gained by examining Table i,
which is a comparison of its properties with diamond and GaP (two other
contenders for high temperature semiconductor applications) and the two most
common commercially available semiconductors, Si and GaAs. In the SiC column
of the table, the quantities in parenthesis pertain to 6H SiC. The maximum
operating temperature was calculated relative to that of Si by assuming a
maximum for Si of 600 K and multiplying this temperature by the ratio of
bandgaps. The maximum for diamond is imposed by a phase change.
Silicon carbide does not melt at any reasonable pressure, but does sublime
at temperatures greater than 2100 K. Below 1800 K, its physical stability is
TABLE 1. Comporlsen of Semiconductors
Propar_y
Send,cO (eV)
at 300 K
Maximum operGting
tempercture (K)
Mert;ng point (K)
Si GoAs
1.4
76O
1SIO
Fair
8500
600 t 400
.3 .4
1.5 .5
2
12.8
7
.5
6O0
1690
Ph ysicol stebil;ty Good
Electron rnobiIity
R.T., cm2/V-s 1400
Hole mobility
R.T., cm2/V-s
Breakdown voltage
Eb , 106 V/cm
Therrn¢l conductivity
c_, W/cm-'C
Sat. eFec. drift vel.
v(sot), 107 crn/s 1
D;electric const., K 11.8
Re[otive Z j 1
Re!ot;ve Z K 1
2 v2(sot)Zj O< Eb
GGP #-SIC(6H s_c?
2.2
2.3 (2.9)
1200
12S0 ( ) 580)
Subl;rnes
1740 > 2100
Fo;r Excellent
1000
350 (600)
100 40
-- 4
.8 5
2.5
11.1 9.7
-- 1100
-- 6
ZKCx o_[v(sat)/K]1/2
Diamond
55
14oo(?)
Phase
change
Very gooc
2200
1600
10
20
2.7
5.5
8100
32
ORIGINAL PAGE IS
OF POOR QUALITY
excellent and its stability in an oxidizing atmosphere gives it an edge over
diamond. We believe that long term stability at high temperatures will be a
problem with the more common III-V compounds, such as GaAs and GaP. Thus, SiC
has a significant advantage where long-term reliable operation is a
requirement. For high temperature power applications, the advantages of SiC
and diamond are illustrated by their much higher values of breakdown electric
field and thermal conductivity. Another potential area of application for SiC
(and diamond) is high frequency. Two Figures of Merit of semiconductors for
high frequency applications are the Johnson (Zj) and Keyes (ZK). Relative
values (to that of Si) for these quantities illustrate the high potential of
SiC and diamond. While SiC is advancing rapidly, the difficulty of diamond
crystal growth casts doubt over its near-term commercial semiconductor
development.
CRYSTAL GROWTH
An essential element of a viable SiC semiconductor device technology is the
availability of large-area substrates on which to grow single-crystal SiC
films for device fabrication. For many years, the only substrates available
were small irregular-shaped SiC crystals that were grown by a sublimation
process, known as the Lely process. During the 1980s, much progress was made
in growing single-crystal 3C-SiC films on single-crystal Si substrates
(Nishino et al. 1983). But the 20% lattice mismatch between SiC and Si
resulted in a large defect density in the SiC films, making the films
unsuitable for device fabrication. A process, known as the modified
sublimation process, has been developed to grow large single-crystal SiC
boules from which wafers can be sliced. In Fig. I, a 6H-SiC wafer (grown by
Cree Research, Inc.) illustrates the extent of improvement over the small
Lely-grown crystals. The following sections describe SiC boule and epitaxial
film growth processes.
],.'ET",C.... , 2j l • , 41.[ I11 IJIiltl!llll[lflllillllll{!! []!l![i!lil 
FIGURE i.
Left: 6H-SiC Wafer
Obtained from Cree
Research, Inc.
Right: Typical Lely-
Grown 6H-SiC Crystals
Boule Growth
The modified sublimation process for growing SiC boules was pioneered by
Tairov and Tsvetkov (1978) in Russia. In this process, nucleation takes place
on a SiC seed crystal located at one end of a cylindrical growth cavity. A
temperature gradient is established within the cavity such that the
polycrystalline SiC is at 2700 K and the seed crystal is at 2500 K. At these
temperatures SiC sublimes from the polycrystalline material and condenses on
the cooler seed crystal. Growth takes place on the seed crystal in an
atmosphere of argon at 200 Pa. Boules of single polytype 6H-SiC can be grown
with growth rates up to 4 mm/h and sizes greater than 25 mm in diameter and 20
mm long. This process is now also being used by groups in the U. S., Germany,
and Japan.
Epitaxial Film Growth
Although a variety of epitaxial growth processes has been used to produce
SiC films on SiC substrates, we believe that the most promising ks the CVD
process; hence, we discuss only this process in this paper.
At a CVD growth temperature of 1750 K, the orientation of the 6H-SiC
substrate determines the polytype of the grown film. If the growth surface is
within about 1° of the (0001) plane, then a 3C-SiC film will form, if the
angle is greater than 1.5 °, then 6H-SiC will form (Matsunami 1989). Generally,
6H-SiC substrates with tilt angles of 3 ° to 4 ° are used to grow 6H-SiC films.
In the NASA CVD growth (Powell 1990), substrates were cut from 3.5 ° 6H-SiC
wafers obtained from Cree Research, Inc. The CVD system utilized a water-
cooled horizontal growth chamber operating at atmospheric pressure. The
carrier gas was hydrogen, and the sources of Si and C were silane and propane,
ORIGINAL PAGE IS
OF POOR QUAL/Ty
respectively. After an initial etch of the SiC substrate with hydrogen
chloride at 1450 K, the substrate temperature was increased to 1730 K for film
growth. Typically, a 4-_m/hr growth rate was achieved for the 6H-SiC films.
DEVICE TECHNOLOGY
Much SiC device technology has been developed over the last several decades
and much can be borrowed from Si technology. To fabricate devices, a variety
of processes are used to introduce controlled amounts of dopants (i.e. donors
and acceptors), and to oxidize, etch, and metallize the material. This section
reviews some of these processes and describes two SiC devices, a p-n junction
diode and a MOSFET, that were fabricated at NASA Lewis.
Controlled Dopinq
Either A1 or B can be used as p-type dopants in SiC, but A1 is preferred
because its ionization energy is less (257 meV vs 735 meV in 3C-SiC). Nitrogen
is electrically active in SiC and is preferred as the n-type dopant because of
its high solubility and relatively low ionization energy (54 meV in 3C-SiC).
These dopants can be introduced during CVD growth.
In commercial Si technology, diffusion and ion implantation are the key
processes used to introduce dopants during device fabrication. Diffusion is
not practical with SiC because diffusion coefficients are negligible below
2100 K. In fact this is one of the strengths of SiC; dopants don't move at
elevated temperatures. Ion implantation has been successfully applied to SiC.
Recent work by Edmond et al. _ (1987) demonstrated that implanting at high
temperatures (approx. 800 K), followed by annealing at 1500 K, resulted in
improved structural and electrical properties.
Oxidation, Etchinq, and Metall_zation
Silicon carbide can be oxidized similar to Si. The oxide can be used for
insulation, passivation, and as masks for ion implantation. Also, thermal
oxidation has been used successfully to produce the gate dielectric in the
fabrication of SiC field-effect transistors (FETs).
During device fabrication, etching processes for removing portions of the
semiconductor are required. In the past, molten salts and gases (e.g. CIz/O 2
mixture) at high temperatures served as etchants for SiC. Recently reactive
ion etching (RIE) has been used very successfully for SiC. Clean RIE-etched
SiC surfaces with a high degree of anisotropic etching can be achieved.
Since a major use of SiC devices will probably be in high temperature
environments, the development of suitable metallization for electrical
contacts is crucial. Below 700 K, the problem may not be very difficu!t, A_n
alloy of Au with a few percent of Ta has been used for n-type, and A1 has been
used for p-type at these temperatures. At higher temperatures, a multi-layer
structure will probably be required. The goal of the various layers will be to
provide an ohmic contact, while also providing a diffusion barrier to prevent
intermixing of materials. Various metal carbides and silicides are being
investigated.
4
P-N Junction Diode
The first high-temperature prototype SiC device fabricated at NASA Lewis was
a p-n junction diode. The junction structure was produced by first growing a
nitrogen-doped n-type 6H-SiC film followed by an aluminum-doped p-type film.
Photolithography and RIE were used to fabricate an array of mesa diodes. After
passivating the junction boundary with SiO2, metal contacts were then applied
to the p and n regions. Excellent diode characteristics were observed up to
870 K, the highest temperature measured, as shown in Fig 2. The forward
voltage drop at room temperature for 20 ma was 12 volts, but decreased to 3.4
volts at 870 K. The leakage current at -300 volts was 48 pa at 870 K.
I (200
RM TEMP
.... i' I
V (i00 V/DIV)
uA/DIV) I (200 uA/DIV>
V (IV/DIV)
870 K
l, I
V (I00 V/DIV) V (IV/DIV)
FIGURE 2. Current-Voltage (I-V) Curves for a 6H-SiC Junction Diode.
MOSFET
Another prototype SiC device fabricated at NASA Lewis was a MOSFET. For this
device, the sequence of 6H-SiC films grown on the 6H-SiC substrate consisted
of (I) an n-type buffer layer, (2) a p-type isolation layer, and (3) a 0.7-
pm-thick n-type channel layer. An array of MOSFETs were then fabricated from
this structure using many additional steps that included photolithography,
oxidation, and meta!lization. The source-drain I-V curves of one of the
MOSFETs are given in Fig. 3. An excellent characteristic of the curves is that
the saturated room-temperature drain currents are horizontal out to i00 volts.
The early stage of development of this device is illustrated by some
characteristics of the curves. For example, the non-linear I-V behavior near
the origin is most likely due to non-ohmic contacts. Also, the MOSFET does not
shut off with 20 volts applied to the gate. The effect is greater at 770 K as
expected. This is probably due to non-optimized oxide and channel thicknesses.
CONCLUSIONS "
Silicon carbide semiconductor technology was just a promise for many years,
but has accelerated rapidly over the last several years. The devices
fabricated at NASA Lewis and elsewhere demonstrate that excellent performance
can be achieved. We want to emphasize that much development and optimization
remain. However, we believe that SiC can be an enabling electronic technology
for many of the ambitious plans that man has for both earth and space.
Eu_
6
u
C
rlm Temp Q
E
V, = -5V/STEP
c
e_
I 1 i I
DRAIN VOLTAGE (10 V/DIv)
770 K
V, = -5/STEP
I Ll _ J l I I l l I
DRAIN VOLTAGE (5 VlDIv)
FIGURE 3. Source-Drain Current Voltage (I-V) Curves for a 6H-SiC Depletion-
Mode MOSFET. Note Change in Horizontal Scale.
Acknowledgments
The work described herein was performed at the NASA Lewis Research Center
under internal funding support.
References
Edmond, J. A., S. P. Withrow, W. Wadlin and R. F. Davis (1987) in Proc. Mats.
_es. Soc. Symp., Vol. 77, 193.
Harris, G. L. and C. Y.-W. Yang, eds. (1989) Amorphous and Crystalline Silicon
Carbide, Springer Proceedings in Physics, Vol. 34, Springer-Verlag,
Heidelberg.
Matsunami, H., K. Shibahara, N. Kuroda, and S. Nishino (1989) in Amorphous and
Crystalline Silicon Carbide, Springer Proceedings in Physics, Vol. 34, G. L.
Harris and C. Y.-W. Yang, eds., Springer-Verlag, Heidelberg.
Nishino, S., J. A. Powell, and H. A. Will (1983) "Production of Large-Area
Single-Crystal Wafers of Cubic SiC for Semiconductor Devices," AppI. Phys.
Lett., 42(5): 460-462.
Powell, J. A. et al. (1990) "Growth of High Quality 6H-SiC Epitaxial Films on
Vicinal (0001) 6H-SiC Wafers," Appl. Phys. Lett., 56(15): 1442-1444.
Tairov, Y. M. and V. F. Tsvetkov (1978) "Investigation of Growth Processes of
Ingots of Silicon Carbide Single Crystals," J. Cryst. Growth, 43: 209-212.
6
Report Documentation PageNalional Aeronaulics and
Space Administration
1, Report No. 2. Government Accession No, 3, Recipient's Catalog No,
NASA TM- 103655
5. Report Date4. Title and Subtitle
Silicon Carbide, a Semiconductor for Space Power Electronics
7, Author(s)
J. Anthony Powell and Lawrence G. Matus
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
6. Performing Organization Code
8. Performing Organization Report No.
E-5846
10. Work Unit No.
505-62-50
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the 8th Symposium on Space Nuclear Power Systems cosponsored by the University of New
Mexico, National Aeronautics and Space Administration, Strategic Defense Initiative Organization, U.S.
Department of Energy, and U.S. Air Force, Albuquerque, New Mexico, January 6-10, 1991.
16. Abstract
After many years of promise as a high temperature semiconductor, silicon carbide (SIC) is finally emerging as a
useful electronic material. Recent significant progress that has led to this emergence has been in the areas of
crystal growth and device fabrication technology. High quality single-crystal SiC wafers, up to 25 mm in diameter,
can now be produced routinely from boules grown by a high temperature (2700 K) sublimation process. Device
fabrication processes, including chemical vapor deposition (CVD), in situ doping during CVD, reactive ion
etching, oxidation, metailization, etc. have been used to fabricate p-n junction diodes and MOSFETs. The diode
was operated to 870 K and the MOSFET to 770 K.
17. Key Words (Suggested by Author(s))
Silicon carbide
High temperature electronics
Semiconductor
18. Distribution Statement
Unclassified - Unlimited
Subject Category 76
119. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages
Unclassified Unclassified 9
NASAFORM1626OCT86 *For sate by the National Technical Information Service, Springfield, Virginia 22161
22. Price*
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