Characterization of Vanadium-Doped 4H-SiC Using Optical Admittance Spectroscopy
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Vanadium is an important dopant in SiC because it gives rise to donor levels near the middle of the bandgap which can be used to make the material semi-insulating, and semiinsulating material has many applications as a substrate material for high-power electronics. However, conventional means of characterizing electronic levels in the bandgap of the material require very high temperatures, in the neighborhood of 650-800 'C, in order to move the Fermi level to midgap and cause ionization of the V donors. The technique of Optical Admittance Spectroscopy permits the ionization of the midgap donors using light of the appropriate energy, and thus avoids the need for high temperatures. Using this technique we have examined several specimens of V-doped and high-resistivity 4H-SiC. We have identified levels previously associated with V, and new levels we attribute to Ti. Pinning of the Fermi level in some specimens was verified by high-temperature Hall effect measurements. SIMS measurements were used to determine impurity concentrations. IR absorption measurements were correlated with the Ti, V, and Cr concentrations determined by SIMS.
High-power, high-temperature applications of semiconductors require a stable platform or substrate. This substrate should be able to conduct heat very well and conduct electricity very poorly. Semi-insulating 4H-SiC is such a material. SiC can be made semiinsulating by very close compensation of residual impurities, or by intentionally adding elements that give rise to electrical levels near the middle of the bandgap in quantities sufficient to pin the Fermi level there. One of the most popular elements for this purpose is vanadium. However, it is almost impossible to add just V because there are usually impurities present in the graphite parts of the growth chamber comprised of other transition metals (e.g. Ti, Cr. Zr). If their concentration is small enough, they are of little consequence, but if their concentration becomes large, it may be the impurities that affect the electrical characteristics of the semiconductor as much as the intentional dopant. For the most part these impurities have been assumed to be electrically insignificant, even though Ti has2 been determined to give rise to an isoelectronic center in SiC, as well as forming Ti-N pairs.1" 253 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
It is difficult to determine the position in the bandgap of energy levels that are near midgap because of the extremely high temperatures that are required to ionize these elements in conventional thermal characterization experiments. What is needed is a technique that does not require high temperatures. One such technique is Optical Admittance Spectroscopy (OAS). This technique utilizes the response of a Schottky diode to the excess charge created when electrons are excited from either the valence band to an acceptor, or from a donor to the conduction band. Other transitions are possible depending on the charge state of the impurity, but th