Electrical and optical investigation of the position of vanadium related defects in the 4H and 6H SiC bandgaps
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Abstract Hall effect, deep level transient spectroscopy, optical absorption, and optical admittance spectroscopy were employed to determine the position of the vanadium acceptor and vanadiumnitrogen complex in vanadium- and nitrogen-doped 4H and 6H SiC. Hall effect results indicate that the acceptor position in 4H(6H) SiC is 0.80(0.66) eV beneath the conduction band edge. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 806 meV and a capture cross section of 1.8x10-'6 cmr2 The optical absorption measurements proved that the acceptor level investigated is related to isolated vanadium, and therefore the vanadium acceptor level. Based upon DLTS and SIMS measurements, the maximum solubility of vanadium in SiC was determined to the 3x10'" crn 3 . An examination of polarized light experiments indicates that vanadium also complexes with another element to form electronic(at 5000 cm-') and vibrational absorption(at 683 cm-') bands. While the other constituent cannot be identified, evidence suggests that nitrogen is a likely candidate. This complex introduces a deep level at Ec-0.78 eV as determined using optical admittance spectroscopy.
I. Introduction Due to its wide band gap and inherent properties, silicon carbide has been sought out as a material
which can be employed in unique conditions, where other semiconductors would fail. Silicon carbide is specifically well suited to both high temperature and high power applications. However, due to the extreme conditions associated with the production of suitable wafer-sized boules of this material, the incorporation of impurities is required to facilitate its utility. Deep levels are required to elucidate the desired properties which would otherwise be present in pure silicon carbide. One element which has been identified as having the potential of producing such a level is vanadium. It has been studied with electron paramagnetic resonance (EPR)', photo-EPR1 , and infrared absorption'. From these measurements, vanadium has been found to occupy silicon substitutional sites in the three following charge states: positive (3d°), neutral (3d'), and negative (3d), and therefore produces two levels in the SiC bandgap. The donor level (0/+) has been shown3 to reside near the middle of the band gap (at E,-1.35 eV). In the absence of all other considerations, vanadium appears to be the most promising element for the production of high resistivity silicon carbide. However, one must not only look at isolated defects which are created when an element is incorporated, but also the different complexes formed as well as the viability for their formation when examining a potential deep dopant. We report on the discernment of a complex which can be attributed to the pairing of vanadium and nitrogen. Prior to this examination of this vanadium complex, an discussion of the vanadium acceptor level is undertaken.
507 Mat. Res. Soc. Symp. Proc. Vol. 423 ©1996 Materials Research Society
II. Sample Preparation/Experimental Setup For this study, SiC samples were s
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