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Produced by the NASA Center for Aerospace Information (CASI)
NASA Tech► _' ,

al Memorandum 83514 %`-

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LWSri ^ ^ W

^i

Silicon Carbide, a High

Temperature Semiconductor

;RASA-TM-83514) SILICCr CAIEIIE, A HIGH N84- it9=2

TEMPERATURE SEMICONDUC'ICR (NASA) 7 p

HC A02/MF A01 CSCL 20L

Unclas

G3/76 42F1C

J. Anthony Powell

Lewis Research Center

Cleveland, Ohio

Prepared for the

Cleveland Electronic Conference (CECON '83)

sponsored by the Institute of Electrical and Electroti s Engineers, Inc.

Cleveland, Ohio, October 4-6, 1983

L
SILICON C ARB IDE, A HIGH TEMPERATURE

SEMICONDUCTOR

i. Anthony Powell

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio

ABSTRACT

Electronic applications are described that

would benefit from the availability of high temperature semiconductor devices. Comparisons art

made among potential materials for these devices

and the problems of each are discussed. Recent

progress in developing silicon carbide as a high

temperature semiconductor is described.

INTRODUCTION

In recent years, there has been a grcN ­:

need for electronic components capable of operating at high temperatures for extended periods of

time. 1 The desired operating temperature in som

applications approaches 600 * C, which is well

beyond the capability of currently availabla eamiconductor devices. In this paper, a few of these

applications will be discussed and then some potential high temperature semiconductors will be

compared. Progress in the development of silicon

carbide (SiC) as a high temperature semiconductor

will be described.

APPLICATIONS

Since the NASA Lewis Research Center is primarily involved with aerospace R&D, applications

related to this field will be emphasized in Ws

paper.

A major potential user of high temperature

semiconductors is jet (or turbine) engines.2 The

reasons are much the same as for automobile

engines where there in a need for on-board electronics to rinitor a^,d control the engine. For

turbine engines, this need can ke divided into two

areas: first, ground testing during the development phase uf a new engine, and second, flight

condition monitoring and c-ntrol systems after the

engine becomes operational.

During the ground testing of a new turbine

engine, tests are often run with the engine

heavily instrumented with sensors of many types.

of particular interest are measurements on the

rotating part of the turbine. This creates a

problem in that there usually is not sufficient

room to connect all these sensors to the outside

world by means of wires and slip rings. So, for

each test, only a fraction of the sensors can be

connectad. Tests must then be repedted with a

different set of sensors until sufficient information is obtained from all the sensors. This repeated building up and tearing down of an engine

to connect sensors is very expensive and timeconsuming. This problem could be eliminated if a

multiplexer could be installed on the turbine

disc. Unfortunately, current electronic devices

cannot operate in the 600' C environment of the

turbine disc.

After an engine becomes operational, monitoring and control systems are highly desirable in

order to reduce maintenance, improve engine performance and improve fuel efficiency. Such electronic systems woitld have to operate for long

periods of time in a temperature environment that

will reach 300' C in advanced supersonic engines,

and the trend is to design the engine to run even

hotter for sake of engine efficiency. In order to

satisfy the extreme reliability requirements of

flight electronics, the electronic devices will

have to be capable of cperating much higher than

this expected 300* C environment for purposes of

accelerated life testing.

We have concluded that an operating temperature of 600 * C is a reasonable goal for devices

that will be used for either ground testing or

flight applications.

In addition to turbine engine applications,

there are other applications that could benefit

from high temperature electronics. In space, the

thermal environment can be a problem for electronics where there is a significant amount of power

dissipated. So, satellite power systems in

general, and communication satellites in particular, are two potential applications. Another

obvious space application would be planetary

probes. Other applications on earth include down

hole or deep-well drilling instrumentation, power

electronics, and nuclear reactor instrumentation

and control.

C014PARISON OF MATERIALS

Since the temperature limits of available

devices are exceeded in many of the above applications, new semiconductor materials will have to be

developed. Six materials were selected and some
09'!NRAt PAGE M

OF p0OR QUALITY

Table 1. A Covarlson of Sae Electronic Materials.

Maximse

landwD Carrier Physical Thermal Operating

Material (eV) Mobility Stabi ► ItY CenductlVlt y

Temp . (CC)

Silicon 1.1 Goad Good Good 300

6olllus 1.4 Excellent Fair Fair 460

Arsenide

Gallium 2.2 Fair Fair Fair V5

Rwsohlde

Cubic 23 Fair Excellent Very 60od 925

Silicon

Carbide

6M 2.9 Fair Excellent Very 600d 1240

Silicon

Ccrblde

Diamond 5.5 Good ) 1 ) Very Good Excellent 1100

pertinent properties of these materials are

assessed in Table I. There are other materials

that could have beeen included but the six

selected are either commercially available or

offer much potential for future development. They

also present a wide range of operating temperature

capability.

The first material listed in Table I is the

most commonly used semiconductor, silicon; it provides a basis of comparison for the other materials. The next two on the list, gallium arsenide

and gallium phosphide, are representative of the

many III-V compounds and their alloys. Two silicon carbide crystalline forms are included because

they are both potentially useful high temperature

semiconductors but have significantly different

electronic and optical properties. Diamond is

listed because of its long-term potential.

The maximum operating temperature for a semiconductor is determined by the forbidden energy

bandgap. The temperature limit is reached when

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. In Table I, a

maximum temperature of 300° C was assumed for

silicon, and based on the ratio of bandgaps, the

maximum operating temperature was calculated for

the other materials. Diamond is a special case in

that a phase change around 1100° C determines its

maximum temperature. These temperatures should be

used only as a starting point since other factors

also affect the maximum temperature, such as the

type of device and the length of service required

for a given application.

Other material properties of major importance

for the operation of a semiconductor at high temperatures are the carrier mobility, physical stability, and thermal conductivity. The thermal

conductivity is of particular importance if much

power is to be dissipated in a device.

In addition to the properties listed in Table

I, there are other considerations in selecting a

semiconductor material. For example, the technology base for silicon is so far ahead of the

other materials that there must be compelling

reasons for choosing any other material. In the

case of the Lewis program for developing electronics for advanced turbine engines, the target

operating temperature was set at 600° C. This

ruled out silicon. Gallium arsenide and gallium

phosphide are also commercially available, and

gallium phosphide meets the Lewis temperature

goal. However, the stability of this material

(and the other III-V compounds) was not considered

sufficient for reliable operation for long periods

at 600° C.

A major disadvantage of the remaining three

materials in Table I has been the lack of reproducible processes for growing the large-area

single-crystal substrates that are needed for

commercial production of devices. This is still a

problem with diamond, but considerable progress

has been made recently on the crystal growth of

SiC. The remaining part of this paper will review

the technology of SiC and describe this recent

progress.

SOME BACKGROUND ON SiC

Silicon carbide is commonly known as a

ceramic and as an abrasive material. Crystals

that were a byproduct of the industrial process

for making sandpaper grit have been studied for

many years. These studies have shown that SiC

grows in many different crystalline forms, called

polytypes. The cubic and 6H are two of the most

common of these polytypes. Another aspect of SiC

is that it doesn't melt at any reasonable pressure

and temperature, but rather sublimes at temperatures above 1800° C at one atmosphere. So, most

crystal growth processes involve a vapor phase

approach. In the industrial process, SiC is

formed at 2500° C by the reaction of silica anc

coke. At these temperatures, gas pockets fora

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 are selected

for research purposes.

In 1955, Lely 3 developed a laboratory version of the industrial sublimation process and was

able to produce rather pure SiC crystals. Enccuraged by the Lely process and support from the

military and NASA, SiC research mushroomed.

Justification at that time was the supersonic

transport and nuclear-power systems for space

applications. This research continued for a

period of nearly twenty years to the extent that

three international SiC conferences were held4-6

with many papers presented. During the late sixties and early seventies, NASA Lewis also had an

active SiC research effort.

Around 1973, the programs that funded this

SiC research were phased out. Unfortunately, the
ORIGINAL PAGE 18

07 POOR QUALITY

Lely process and other processes had not been

developed to the point where high-quality largearea crystals could be grown in any reproducible

way. Since crystal substrates are crucial to

device fabrication, interest in SiC waned, and

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.

SiC POLYTYPES

In Table II is a list of a few of the known

polytypes of SiC. The only difference in the

crystal structure of the various polytypes is in

the sequence of stacking of double layers of silicon and carbon atoms. The relative orientatiins

of these double layers with respect to the other

double layers is the key factor. A given double

layer can occupy one of three possible orientations, which are arbitrarily designated as A, B,

and C. The cubic SiC polytype, frequently called

B-SiC, has the stacking sequence ABCABC, and so

forth. The Ramsdell notation is also commonly

used to describe the polytypes. This consists of

a number followed by a letter. The number designates the number of double layers in a stacking

sequence before it repeats. The letter designates

the crystal structure. Thus, we have 3C for cubic

SiC. All of the polytypes, except the cubic polytype, are known as alpha SiC. The most common of

these is 6H. Among the alpha polytypes, the H

stands for hexagonal and the R star.'_^ for rhombohedral.

Figure .l is a photo of some alpha SiC crystals that were were produced by the Lely process.

Most of the crystals are a mixture of polytypes,

and since each polytype has its own electronic

properties, th% crystals are unsuitable for device

fabrication. The coloration, from black to clear,

is determined by the level of impurities in the

crystals. Cubic SiC, with a smaller bandgap than

the alpha polytypes, is intrinsically yellow in

color. Until recently, available cubic SiC crystals were even smaller and poorer in quality than

the alpha crystals shown in Fig. 1.

The crystal growth situation for SiC has

dramatically changed within the last year. Two

new reproducible growth processes have been developed: one for cubic SiC at NASA Lewis ? and one

for 6H SiC at Siemens 8 in West Germany. Both of

these processes promise to provide the large-area

single-crystals that are necessary for device production, so both will be briefly described in the

following sections.

The reason for interest in both the cubic and

6H polytypes is that each may have advantages over

the other in specific applications. Since the 6H

energy bandgap is significantly larger than cubic,

6H has the potential for a higher maximum operating temperature. Also, blue light-emitting diodes

have been fabricated from 6H, 9 but not from

cubic because of the smaller bandgap. A potential

advantage of cubic SiC over 6H in some applicaTable 11, Some SIC Polytyoes

RunsdeII Stacking

Notodon SeauenCR

(cubic or beta) 3C ABC...

(aloha) 2H AS...

4H ABAC...

6H ABCACB.

15R ABCBACABACBCACB.

tions is based on the higher carrier mobility that

has been reported for cubic. 10 This may lead to

particular cubic SiC devices whose performance may

be superior to similar 6H SiC devices.

GROWTH OF CUBIC SiC

For many years, researchers have tried to

grow cubic SiC single-crystal films on silicon

single-crystal wafers because silicon wafers have

the same crystal structure and are readily available. These SiC films could then serve as substrates for device fabrication. Unfortunately,

the difference in atomic spacing between silicon

and SiC ( about 20%) usually leads to SiC layers

that are polycrystalline and thus unsuitable for

this purpose.

The Lewis developed process 7 has solved

this lattice mismatch problem. The approach uses

chemical vapor deposition ( CVD) to grow SiC on the

silicon wafer. As described below, an initial

layer is grown in a manner that minimizes the

effects of lattice mismatch.

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Fig. 1. Some typical Lely-grown a-SiC crystals.
PATER- COOL IO

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6RAPHI TI.

SUSCEPIOR SILICON SUBSTRATE

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QUARTZ SUPPORT

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Fig. 2. Schematic diagram of reaction chamber for

the cubic SiC crystal growth process.

) 

ORf01NI L PAGE is

OF POOR QUALfTY

The CVD system is of conventional design with

special attention given to system cleanliness and

reproducibility of growth conditions. The source

of silicon and carbon are silane and propane,

respectively, in a purified hydrogen carrier gas.

Deposition takes place at atmospheric pressure in

a horizontLl water-cooled quartz reaction tube.

The silicon substrate is placed on an rf-heated

graphite susceptor as shown in Fig. 2.

The process of crowing the SiC layer consists

of three steps as shown in Fig. 3. The etching

step cleans the substrate pri g to growth. The

initial layer step provides a 20 nm thick layer of

SiC that is necessary for the subsequent singlecrystal growth step. With the conditions listed

for the crystal growth step, the growth rate of

the SiC single-crystal layer is about 4 L,m/h.

The process has consistently yielded high

quality single-crystal layers with areas of

several cm 2 and thicknesses up to 34 um. The

silicon substrate can be removed by chemical etching leaving a thin SiC wafer, as shown in Fig. 4.

INITIAL

LAYER CRYSTAL GROWTH

1400

ETCHING

PROCESS

1200

Fig. 4. A 16- M-thick cubic SiC wafer (broken into

five pieces).

X-ray diffraction measurements and electron

channeling patterns obtained from the cubic SiC

w;.fers verify that they are high quality single

crystals. Hall measurements have yielded electron

mobilities up to 380 cm2/v-s.

GROWTH OF 6H SiC

The recently reported Siemens process e con

reproducibly grow large bulk, crystals of 6H SiC.

The technique is a modification of the Lely process and a Russian-developed process involving

growth on a SiC seed crystal. 11 Polycryst-illine

SiC is sublimed at one end of a hollow growth

chamber and deposited on the seed crystal located

at the other end. Important parameters in the

growth process are: (1) a temperature nradient

with a temperature of 2400 0C at tha polycrystalline SiC, decreasing to 2200 o C at the 6H SiC

seed crystal, and (2) the chamber pressure which

is reduced to 2 mbar during crystal growth.

Growth rates of 4 mm/h are achieved; single crystals of 6H SiC up to 24 mm in length and 20 mm in

diameter have been grown. Substrates for device

fabrication con be obtained by slicing these bulk

crystals. with controlled aluminum doping of

these crystals, olue light emitting diodes were

fabricated with the highest quantum efficiency for

SiC reported so far.8

SiC DEVICE TECHNOLOGY

Because of the long history of SiC research,

much device technology already exists. 12,13 The

technology required for device fabrication includes (1) the introduction of specific dopant

elements, (2) the formation of junctions and

device structures, (3) the formation of passivating and insulating layers, (4) contacting, and (5)

packaging.

Silicon carbide can easily be doped either n

or p type by introducing appropriate doping

31H "C1 I I.01N21HBI

I

.041 S 1 144 N C3% 2 . C21

10 MIN. 2 MIN. 1 TO 6 HOURS

TITIE

Fig. 3. Conditions for growing a cubic SiC

single-crystal layer on a silicon Fingle-crystal

wafer.

L J
ORIGINAL PAGE 13

OF POOR QUALITY

material during crystal growth for either th3 sublimation or CVD process. For example, during CVD,

p-type SiC can be grown by adding the doping gas,

diborane (924) . 7

Techniques that have been used successfully

to fabricate junctions in SiC include both

vapor l4,15 and liquid phaseg epitaxy, diffusion, 13 and ion implantation 16 . Of these

three approaches, epitaxial techniques have been

the most successful. As in the doping of SiC substrate material, the addition of appropriate

material during the epitaxial growth is used to

product the desired doped profile (e.g., a p-n

junction). Temperatures required for epitaxial

techniques range from 1320 OC14 to 1800oC17.

Diffusion for doping purposes in SiC requires

temperatures in excess of 1800 0C. At these

temperatures, material stability and control of

jm.ction profiles becomes a problem, so this technique is not recommended. Ion implantation may

become a useful doping technique in SiC but so far

has not been used very much.

Silicon carbide surfaces can be passivated in

much the same way as used for silicon. The surface can be thermally oxidized in an atmosphere of

wet or dt-f oxygen at around 1000oC 17,18 The

oxide layer can also be used as a mask during the

fabrication of mesa-type devices, such as SiC

bipolar transistors 17 and blue light-emitting

diodes9 . In these processes, windows are

created in the oxide using photolithographic techniques; the SiC is then shaped by gaseous etching

in a mixture of C1 2 , 02 , and Ar at around

1000oC.

Contacting and packaging for 6000C SiC

devices will probably be a difficult task. Careful selection of materials will have to be made in

order to prevent harmful interdiffusion of elements over long periods of tire. Ohmic contacts

for both n- and p-type SiC have been developed

that are suitable for lower temperatures

(<4000C); these contacting materials include

gold/tantalum alloys and nickel.12

As in the development of silicon and GaAs,

SiC device technology will not mature .vern-.ght.

But, considering the recent advances _n crystal

growth and the feasibility of practical devices

that has been demonstrated, SiC finally appears to

be emerging as a useful semiconductor.

REFERENCES

1. Special Issue on High-Temperature Electronics, I.E.E.E. Transactions on Industrial

Electronics, vol. IE-29, no. 2, pp.

101-165, May, 1982.

2. W. C. Nieberding and J. A. Powell, "HighTemperature Electronic Requirements in Aeropr.;pulsion Systems," Ref. 1, pp. 103-106.

3. J. A. Lely, "Darstellung von Einkr13t&llen

von Siliziumcarbid and Beherrschung von Art

and Menge der eingebauten Verunreinigungen,"

Ber. Dt. Kerm. Gas., vol.32, pp. 229-231,

1955.

4. Silicon Carbide, a High Temperature Semiconductor. J. R. O'Connor and J. Smiltens,

Eds., New York:Pergamon. 1960.

5. Silicon Carbide-1968. H. K. Henisch and R.

Roy, Eds., New York: Pergamon, 1969.

6. Silicon Carbide-1973. R. C. Marshall, J. W.

Faust, and C. E. Ryan., Eds., Columbia,

S.C.:University of South Carolina Press, 1974.

7. S. Nishino, J. A. Powell, and H. A. Will,

"Production of Large-Area Single-Crystal

Wafers of Cubic SiC for Semiconductor

Devices," Appl. Phys. let., vol. 42, no. 5,

pp. 460-462, March 1, 1983.

S. G. Ziegler, P. Lanig, D. Theis, and C.

Weyrich, "Single Crystal Growth of SiC Substrate Material for Blue Light Emitting

Diodes", I.E.E.E. Trans. on Electron Devices,

vol. ED-30, no. 4, pp. 277-281, April, 1983.

9. W. von Muench and W. Kurzinger, "Silicon

Carbide Blue-Emitting Diodes Produced by

Liquid-Phase Epitaxy," Solid-State Electronics, vol. 21 pp. 1129-1132, 1978.

10. W. E. Nelson, A. Rosengreen, R. W. Bartlett,

F. A. Halden, and L. E. Marsh, "Growth and

Characterization of -Silicon Carbide Single

Crystals," Final Report, Contract No. AF

19(628)-4190, Stanford Research Institute,

Menlo Park, California, February 1967.

11. Y. M. Tairov and V. F. Tsvetkov, "General

Principles of Growing Large-Size Single Crystals of Various Silicon Carbide Polytypes,"

J. Cryst. Growth, vol. 52, pp. 146-150, 1981.

12. E. Pettenpaul, W. von Muench, and G. Ziegler,

"Silicon Carbide Devices", Inst. Phys. Conf.

Ser., No. 3, pp. 21-35, Solid State Devices,

1979, Edited by H. Weiss, Bristol:Institute

of Physics, 1980.

13. R. B. Campbell and H. S. Berman, "Electrical

Properties of SiC Devices," Ref. 5, pp.

5211-5222.

14. J. A. Powell and H. A. Will, "Epitaxial

G.owth of 6H SiC in the Temperature Range

1320-13900C," J. Appl. Phys., vol. 44, No.

11. November 1973.

15. R. W. Brander, "Epitaxial Growth of SiC

Layers," Ref. 6, pp. 8-24.

16. O. J. Marsh, "Ion Implantation in SiC," Ref.

6, pp. 471-485.

17. W. von Muench and F. Hoeck, "Silicon Carbide

Bipolar Transistor," Solid-State Electronics,

vol. 21, pp. 479-480, 1978.

18. A. Suzuki, K. Mameno, N. Furui, and H.

Mateunami, "C-V Characteristics of SiC

Metal-Oxide-Semiconductor Diode with a

Thermally Grown Si0 2 Layer," Appl. Phys.

Lett., vol. 39, No. 1, pp. 89-90, July 1,

1981.

-,