Spectral Characterization of Persistent Photo Conductance in SiC
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0911-B05-05
Spectral Characterization of Persistent Photo Conductance in SiC Steven R. Smith1,2, Andrew Evwaraye3, and William Mitchel4 1 Air Force Research Laboratory AFRL/MLPSO, Dayton, OH, 45433-7707 2 University of Dayton Research Institute, Dayton, OH, 45469-0178 3 Physics, University of Dayton, Dayton, OH, 45469 4 Air Force Research Laboratory, AFRL/MLPS, WPAFB, OH, 45433-7707 ABSTRACT The transient photo response of 4H- and 6H-SiC specimens at various wavelengths has been studied using Optical Admittance Spectroscopy. Differences in the transient response were found for excitation and recombination for different specimens in both polytypes. The results indicate that optical excitation of charge carriers to the conduction band is a single transition, but recombination of free carriers from the conduction band is a process that may involve multiple transitions to the ground state mediated by deep centers in the material. Similarities in the persistent photo conductance after excitation from deep centers, and after excitation at above bandgap energies, suggest that carriers from the conduction band can drop through deep centers on the way to the valence band. Annealing studies also indicate that stoichiometry can play a significant role in the PPC effect. INTRODUCTION Persistent Photo Conductance (PPC) is a phenomenon associated with the nonexponential recombination of carriers.[1] The measurement of the AC conductance of a transparent Schottky diode on a semiconductor as a function of time reveals the nature of the conductance decay after illumination. The measurement is accomplished in the same manner that Optical Admittance Spectroscopy (OAS) is accomplished, except that OAS is a steady state measurement of G(λ).[2] This measurement differs from photo conductivity, in that it is a vertical measurement across the depletion region under the diode. In this paper, we describe a spectral approach to this experiment wherein we have illuminated the specimen diode with monochromatic light of various wavelengths. The wavelengths were chosen to correspond to features in the OAS spectrum that were related to either superbandgap excitation, or impurityrelated excitation. We reported on PPC in 6H-SiC, from the perspective of temperature dependence, several years ago.[3-5] These studies on 4H-SiC, principally, approach the problem as a function of wavelength, hence photon energy.
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Figure 1. OAS spectra for two 4H-SiC specimens, (a) and (b), with good stoichiometry, but different impurity content. The shoulder from 2 eV to 3 eV in (a) indicates excitation from compensated acceptors. This feature is absent in 1(b). Figure 1 illustrates OAS spectra of two 4H-SiC specimens. Illumination wavelengths chosen for the PPC experiments were d
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