Hall measurements on four n-type cubic SiC films epitaxially grown by chemical vapor deposition on SiC substrates are reported. The temperature dependent carrier concentrations indicate that the samples are highly compensated. Donor ionization energies, E sub D, are less than one half the values previously reported. The values for E sub D and the donor concentration N sub D, combined with results for small bulk platelets with nitrogen donors, suggest the relation E sub D (N sub D) = E sub D(O) - alpha N sub N sup 1/3 for cubic SiC. A curve fit gives alpha is approx 2.6x10/5 meV cm and E sub D (O) approx 48 meV, which is the generally accepted value of E sub D(O) for nitrogen donors in cubic SiC

Alterovitz, S. A.

Haugland, E. J.

Matus, L. G.

Segall, B.

English

NASA Technical Reports Server

.-
NASA Technical Memorandum 87269 
Compensation in Epitaxial 
Cubic SiC Films 
B. Segall 
Case Western Reserve University 
Cleveland, Ohio 
and 
S.A. Alterovitz, E.J. Haugland, 
and L.G. Matus 
Lewis Research Center 
Cleveland, Ohio 
March 1986 
NI\S/\ 
; NASA-TM-87269 19860015796 
\.~-------~ 
lIBRAR'1 COpy 
.APR 4 1986 
LANGLEY RESEARCH CENTER 
LIBRARY. NASA 
HA!.'PTON, VIRGINIA 
1111111111111 1111 IIIII 1111111111 11111 11111111 
NF01S12 
https://ntrs.nasa.gov/search.jsp?R=19860015796 2020-03-20T15:28:35+00:00Z
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N 
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COMPENSATION IN EPITAXIAL CUBIC SiC FILMS 
B. Segal] 
Case Western Reserve University 
Physics Department 
Cleveland, Ohio 44106 
and 
S.A. Alterovitz, E.J. Haugland, and L.G. Matus 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
ABSTRACT 
Hall measurements on four n-type cubic SiC films epitaxially grown by 
chemical vapor deposition on (100) SiC substrates are reported. Detailed 
analyses of the temperature dependent carrier concentrations indicate that the 
samples are highly compensated (>90 percent), contrary to the assumption of no 
compensation made in previous studies of similarly prepared SiC films. Donor 
ionization energies, ED, are found to be less than one half the values 
previously reported. The values for EO and the donor concentration NO' 
combined with results for small bulk platelets with nitrogen donors, suggest 
1/3 the relation EO(NO) = EO(O) - a NO for cubic SiC. A curve fit 
-5 gives a ~ 2.6xl0 meV cm and EO(O) ~ 48 meV, which is the generally 
accepted value of EO(O) for nitrogen donors in cubic SiC. 
Cubic silicon carbide (B-SiC), a wide bandgap semiconductor with high 
electric breakdown field, high saturated drift velocity, and high thermal 
conductivity, is an attractive electronic material for high temperature, high 
1 power and/or high frequency applications. These potentialities 
unfortunately have been difficult to realize due to the unavailability of 
reasonably large crystals. The recent development of a technique for growing 
large area crystal films on (100) Si substrates has renewed interest in cubic 
SiC. 2 Others have since reported similar results. 3- 5 The temperature 
dependence of carrier concentration in cubic SiC films has been measured using 
Hall effect4 and cyclotron resonance techniques. 6 The results of these 
studies were analyzed assuming that these n-type cubic SiC films were 
uncompensated. A similar assumption was also made in a study of cubic SiC 
platelets. 7 The authors find that the conductivity is due to the presence 
of donors with ionization energies, EO' roughly equal to 40 to 50 meV. The 
donor has been identified as nitrogen, which, in small high purity crystals is 
. 8 found to have an ionization energy, EO' of 54 meV. 
In the present work we report Hall measurements on four film samples with 
mobilities as high as any previously reported. In our analysis, which focuses 
on the carrier concentrations neT), we find, contrary to the assumption made 
467 in previous work, , , that the material is highly compensated. We also 
find that EO is much less than one-half the values quoted earlier. Thus, 
our determination of the values of the key parameters characterizing the 
electrical properties, namely the donor and acceptor concentrations NO 
and NA, and Eo' differ markedly from those resulting from the 
noncompensation assumption. However, we do find evidence identifying the 
donors as nitrogen. 
The samples investigated were prepared by the method described 
earlier. 2 All samples were roughly square with sides approximately 5 mm and 
2 
thicknesses from 4.6 to 16.9 ~m. Electrical contacts were made on each 
corner, in a square pattern and 2.5 mm apart, by sputter deposition of Ta/Au. 
Measurements of the resistivity and Hall coefficient were made with dc current 
9 
using the van der Pauw technique. Appropriate averages were taken over all 
possible current and magnetic field reversals and all van der Pauw 
configurations to eliminate extraneous effects. Corrections for contact size 
and position were not made. Magnetic fields up to 13 kG were used. The 
temperature was varied in a gas flow cryostat and stabilized to within 0.02 K. 
The carrier concentrations n(T) were obtained from the Hall 
coefficient. Experimental results for three of the four samples studied are 
shown as points in Fig. 1. One of the samples (number 438), which was fairly 
thick (16.9 ~m) and hence capable of being separated from the substrate, was 
also subjected to high temperature (300 K < T < 800 K) measurements on a 
different apparatus. Results for this sample are shown in Fig. 2. As so~e 
measure of the quality of the samples, we note that the room temperature 
mobilities for sample numbers 340, 437, 438, and 503 are 270, 310, 305, and 
245, cm2/V sec, respectively. The data for T ~ 50 K exhibit a flattening 
out, which apparently is associated with impurity band conduction. The van 
der Pauw ratio was quite constant over the relevant temperature range, with 
deviations of ~1 percent from the mean value for samples 340 and 503 and about 
+10 percent for samples 437 and 438. 
While the rough linearity of log n(T) for T> 50 K makes the assumption 
n n exp[-Eo/2kT] tempting, a thorough and unbiased analysis is required, 
especially since ED is small. Here we use the appropriate relation 
n(n + NA) Nc 
No - NA - n = S- exp (-EO/kT) ( 1 ) 
which is accurate for a single level and nondegenerate carriers. Relevant 
information regarding the conduction band minima obtained from luminescence 
3 
Zeeman stud1es8 show that they l1e along <100> axes, presumably on a ~l 
band. The nondegeneracy of ~l 1mp11es tha\the spin degeneracy factor s 
1s 2. The conduct10n band density of states Nc is Nc = 
312 -3 2M(2~dkT) h where M is the number of equivalent minima and 
md is the "density of states effective mass" given by (m~mll.)1/3 
with mt and mil. being the transverse and longitudinal effective 
masses. The values mt = 0.241 mo and mil. = 0.611 mo determined 
1 by cyclotron resonance measurements, which are within a few percent of 
those found in the magneto-optical studies,8 yield md = 0.346 mo. 
Since symmetry requires that the end of the <100> axis be an extremum, it is 
very reasonable to expect the minima to be at the zone boundary and thus M 
= 3. The fitting of n(T) is obtained by suitable adjustments of ED, 
NO' and NA. 
Our fits of the data are shown by the solid curves in figs. 1 and 2: 
They clearly provide very satisfactory representations of the data except for 
the temperature range exhibiting impurity band conduction, which, of course, 
must be treated separately. The values for ED, NO' and NA yielding 
these curves are given in Table I. 
The first point evident from the tabulation is that the samples are 
highly compensated with NA/No > 0.9. The second is that the values of 
ED are much smaller than (less than one-half) the published film values and 
the generally accepted value for the nitrogen donor (z50 meV). It is 
reasonable here to raise the question of the uniqueness of our fits and, in 
particular, of the possibility of achieving a comparably good fit assuming no 
compensation over the relevant range of temperature T. Considering the 
different T-dependence resulting from the assumption of no compensation, that 
seems unlikely. Indeed, our attempts at achieving such fits, as illustrated 
by the dashed curve in Fig. 2 for sample number 438, bear out that 
4 
expectation. The dashed curve is required to agree with the experimental data 
in the "saturation" region and in the middle~of the region exhibiting "freeze 
out." 
and 
. 
17 -3 The values obtained for the dashed curve are NO ~ 1.6xlO cm 
ED ~48 meV, the latter being close to the value for nitrogen 
donors. The agreement, however, is clearly much less satisfactory than for 
the solid curves, with deviation reaching roughly 40 percent. While different 
requirements could be imposed, agreement nearly as good as that provided by 
solid curves does not seem possible. The above value of ED is more than 
twice (closer to three times) that obtained with compensation. This fact is 
due to the T-dependence of 
values of ED. 
N which has an appreciable effect for low 
c 
Another point evident in the tabulation is the variation of ED with 
NO' That variation, while admittedly small because of the limited range of 
impurity concentration in our samples, is nevertheless significant when the 
present values are combined with those obtained by Aivazova et al. 10 from a 
study of small platelets of cubic SiC. In Fig. 3 we plot all these energies, 
ED, versus No' Aside from some scatter, the ED points lie on a curve 
which extrapolates to the value of the nitrogen donor at NO ~ 0 and 
vanishes at NO = NO' slightly above those of our samples. 
Such a curve can be represented by the relation used for shallow 
acceptors in Sill and shallow donors in Ge,12 
ED (NO) = Eo(O) - a N~/3 (2) 
-5 Here Eo(O) ~ 48 meV and a ~ 2.6xlO meV cm. Noteworthy is the 
12 -5 
closeness of our a to that reported for donors in Ge (2.34xlO meV 
cm) and to the value we obtained by fitting Eddolls· curve14 for shallow 
-5 donors in GaAs (2.5xlO meV cm). 
Additional confirmation for our results follow from the condition 
Eo(N
o
) = 0 namely, that the average interimpurity distance equal twice the 
5 
effect1ve Bohr rad1us of the donor. In the effect1ve mass approx1mat10n we 
Th1s agrees sat1sfact9rily with the value 
4 
f1nd NO ~ 1019 cm-3 
18 -3 6.3x10 cm 1nd1cated by the curve. F1nally, since the magnitude of 
EO(O) agrees sat1sfactor1ly w1th the accepted value of the nitrogen donor at 
d1lute concentrat10n8 and since A1vazova et al. 10 have ident1f1ed their 
donors as n1trogen by ESR measurements, the above correlat10n suggests the 
same identif1cation for the donors in our samples. Considering the 
preparation conditions for the present samples, nitrogen contamination seems 
reasonable. 
In conclusion, we have found a h1gh degree of compensation and a large 
impurity concentration induced reduction of the n1trogen donor depth 1n all 
samples. This enhanced understand1ng of the present state of cub1c S1C films 
should be helpful 1n the1r development for electronic app11cations. 
ACKNOWLEDGMENT 
Thanks are due to J.A. Powell for supplying the cubic S1C samples used in 
this study and to H.A. W1ll who sputter-deposited the electr1cal contacts on 
the samples. 
6 
REFERENCES 
1. W.C. Nieberding, Ind. Res. Dev., II (9) .]48 (1983): J.D. Parsons, R.F. 
Bunshah and O.M. Stafsudd, Solid State Technol., 28 (11) 133, (1985). 
'. 
2. S. Nishino, J.A. Powell and H.A. Will, Appl. Phys. Lett., 42, 460 (1983). 
3. A. Addamiano and P.H. Klein, J. Cryst. Growth, 70, 291 (1984). 
4. K. Sasaki, E. Sakuma, S. Misawa, S. Yoshida, and S. Gonda, Appl. Phys. 
Lett, 45, 72 (1984). 
5. H.P. Liaw, and R.F. Davis, J. Electrochem., Soc., 111, 3014 (1984). 
6. R. Kaplan, R.J. Wagner, H.J. Kim, and R.F. Oavis, Solid State Commun., 
22" 67 (1985). 
7. A. Rosengreen, Mater. Res. Bull., 4, S355 (1969). 
8. P.J. Oean, W.J. Choyke and L. Patrick, J. Lumin., 12,299 (1977). 
9. L.J. van der Pauw, Philips, Res. Rep., 13, 1 (1958). 
10. L.S. Aivazova, S.N. Gorin, V.G. Sidyakin, and 10M. Shvarts, Sov. Phys'. 
Semicond. (Engl. Transl.), 11, 1069 (1977). 
11. G.L. Pearson, and J. Bardeen, Phys. Rev.,]i, 865, (1949). 
12. P.P. Debye and LM. Conwell, Phys. Rev., 21,693, (1954). 
13. D.V, Eddolls, Phys. Status Solidi, 17, 67 (1966). 
7 
TABLE I. 
Sample NO. NA. ED. 
1018 cm- 3 1018 cm- 3 meV 
340 2.79 2.50 13.2 
437 1. 76 1.60 17.2 
438 2.00 1.83 14.5 
502 2.15 1.97 14.6 
rt'\ 
I 
E 
u 
C 
1018 
SAMPLE 
NUMBER 
0 340 
0 437 
0 503 
1017 
1016 
o 
o 
1015L-______ -L ________ ~ ______ ~L_ ______ ~ __ ~ __ ~~ ______ ~ 
o 5 10 15 20 25 30 
l000/T K-1 
. 
Figure 1. - Carrier concentration n versus lOOO/T for three cubic SiC samples. The points are 
experimental results while the continuous lines are calculated fits using compensation. 
C'<'I 
I 
E 
u 
c· 
50 
40 
> 30 
Q) 
E 
CI 
l.LI 20 
10 
1015 '---___ ---L-____ -'---___ ---L-____ ~~ __ ___L~ _ _=..._::J 
o 5 10 15 20 30 
1000lT, K-1 
Figure 2. - Carrier concentration n versus 1000lT for a self-supporting sample (number 438), 
The points are experimental results while the continuous and dashed lines are calculated 
fits using compensation and noncompensation, respectively. 
1017 
NO' cm-3 
o AIVAZOVA, ET AL 
• PRESENT WORK 
Figure 3. - Donor activation energy ED versus donor density No. The continuous line is given by 
equation (2) using EO(O) = 48 meV and a = 2.6xlO-5 meV cm. 
1. Report No. 2. Government Accession No. 
NASA 1M-81269 
4. Title and Subtitle 
Compensat1on 1n Epitax1a1 Cub1c S1C Films 
7. Author(s) 
B. Segall, S.A. A1terovitz, E.J. Haugland, and 
L. G. Matus 
9. Performing Organization Name and Address 
National Aeronaut1cs and Space Admin1stration 
Lewis Research Center 
Cleveland, Ohio 44135 
12. Sponsoring Agency Name and Address 
National Aeronaut1cs and Space Administration 
Wash1ngton, D.C. 20546 
15. Supplementary Notes 
3. Recipient's Catalog No. 
5. Report Date 
March 1986 
6. Performing Organization Code 
506-4422 
8. Performing Organization Report No. 
E-2918 
10. Work Unit No. 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
lechn1ca1 Memorandum 
14. Sponsoring Agency Code 
B. Segall, Case Western Reserve Un1vers1ty, Physics Department, Cleveland, Ohio 
44106; S.A. Alterovitz, E.J. Haugland, L.G. Matus, Lewis Research Center. 
16. Abstract 
Hall measurements on four n--type cub1c SiC films epitaxia11y grown by chemical 
vapor deposition on (100) SiC substrates are reported. Detailed analyses of the 
temperature dependent carrier concentrations indicate that the samples are highly 
compensated (>90 percent), contrary to the assumption of no compensation made in 
previous studies of s1m11ar1y prepared SiC f1lms. Donor ion1zation energies, 
ED, are found to be less than one half the values previously reported. lhe 
values for ED and the donor concentration NO, comb1ned with results for 
small bulk ~late1ets with nitrogen donors, suggest the relation EO(NO) = 
EO(O) - a NO/3 for cubic SiC. A curve fit gives a = 2.6xl0- 5 meV cm 
and EO(O) = 48 meV, wh1ch is the generally accepted value of EO(O) for 
nitrogen donors in cubic SiC. 
17. Key Words (Suggested by Author(s» 
Compensation 
Epitaxial films 
Silicon carbide 
19. Security Class if. (of this report) 
Unclassified 
18. Distribution Statement 
Unclassified - unlimited 
SlAR Category 76 
20. Security Classlf. (of this page) 
Unc lass ified 
21. No. of pages 
'For sale by the National Technical Information Service, Springfield, Virginia 22161 
22. Price' 
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Compensation in epitaxial cubic SiC films

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