Mid Infrared Photoconductivity Spectra of Donor Impurities in Hexagonal Silicon Carbide
- PDF / 571,377 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 39 Downloads / 153 Views
IMPURITIES IN HEXAGONAL SILICON CARBIDE R. J. LINVILLE, G. J. BROWN, W. C. MITCHEL, A. SAXLER AND R. PERRIN Air Force Research Laboratory, Materials & Manufacturing Directorate (AFRL/MLPO) 3005 P ST, Wright-Patterson AFB, OH 4533-7707 ABSTRACT Mid-infrared photoconductivity (PC) is a useful technique for identifying and investigating donor and acceptor centers in many semiconductors. This is especially true when the PC results are combined with other measurements such as Hall Effect and DLTS. We report on the first Fourier Transform Infrared (FTIR) photoconductivity spectra for n-type 6H and 4H-SiC. The samples studied had temperature dependent Hall activation energies around 45 meV and 85 meV in the 4H samples, and a single activation energy of 106 meV in the 6H. For the 4H samples, the PC spectra showed an increase in photoresponse between 40 and 47 meV, with another sharp increase at 120 meV. In the 6H-SiC, the photoresponse also had a rapid increase at 120 meV, and at 77 meV in one sample. The photoresponse spectra of the n-type 4H and 6H-SiC samples were distinctly different in the mid-infrared. INTRODUCTION SiC has, in comparison with Si, superior properties regarding high-power, high-frequency and high-temperature electronics. The material has high thermal conductivity, can withstand high electric fields before breakdown and also high current densities. The wide bandgap results in a low leakage current even at high temperatures. The bandgap of SiC depends on the polytype and ranges from 2.4eV (3C-SiC, T=4.2K) to 3.3eV (2H-SiC, T=4.2K).' Nitrogen is the main donor impurity in all the polytypes of SiC 2 and dominates the electrical properties of n-type SiC since it has shallow levels in the upper half of the forbidden gap. Nitrogen substitutes for carbon in the SiC lattice. 2 The activation energy of the nitrogen levels depends on the polytype and the substitutional lattice site involved. In recent years, there have been several studies on identifying the nitrogen levels in 6H and 4H silicon carbide. One technique that has not been used previously to study these energy levels is mid-infrared photoconductivity. The only reported photoconductivity spectra for SiC are for deep levels observed at energies higher than 0.5 eV. Mid-infrared photoconductivity (0.025 to 0.5 eV) can be 34 a sensitive technique for identifying both defect levels and impurities in semiconductors. ' Typically the optical activation energies identified in photoconductivity spectra agree well with the thermal activation energies measured by temperature dependent Hall effect for samples from the same wafer. In this study we compare the results from Hall effect and Fourier transform infrared spectral photoconductivity for several samples of 6H and 4H n-type SiC. EXPERIMENTAL The 4H-SiC and 6H-SiC samples have been grown by physical vapor transport. All of the samples were found to be of the n-type conductivity by Hall effect measurements. The 281
Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
investigated samples were
Data Loading...
