NASA Technical Memorandum 87269 Compensation in Epitaxial Cubic SiC

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. 35 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