Ohmic Contacts to Wurtzite Silicon Carbide Using Polarization Technology
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1246-B07-15
Ohmic Contacts to Wurtzite Silicon Carbide Using Polarization Technology Choudhury Jayant Praharaj previously with University of Utah, Salt Lake City, 84112 USA
Abstract We present theoretical calculations for ohmic contact technology to wurtzite Silicon Carbide using thin Indium Gallium Nitride and Aluminium Indium Nitride cap layers. Spontaneous and piezoelectric polarization in Indium Gallium Nitride and Aluminium Indium Nitride cap layers gives rise to bound interface sheet charge density of the order of 1013 electrons per cm2, and built-in electric fields of the order of MV per cm. For Si-face p-type SiC, the large compressive strain in very thin Ga face InGaN and AlInN cap layers results in negative interface sheet charge densities and much lower tunneling widths for holes compared to bulk contacts. For C-face ptype SiC, pseudomorphic N face nitride layers yield no benefit over bulk contacts since positive interface sheet charge densities repel holes and give higher contact resistances. However, thick, relaxed cap layers lead to spontaneous polarization-based negative charge densities that attract holes, leading to lower contact resistances. The contributions of the heavy, light and split-off band holes to the total tunneling flux is taken into account in our calculations. All tunneling probabilities are calculated within the Wentzels-Kramers-Brillouin approximation. The presence of the appropriate cap layers leads to several orders of magnitude improvement in tunneling transmission probabilities, and in many cases, makes ohmic technology to SiC feasible. While the major difficulties are encountered for contacts to p-type SiC, the effect of cap-based polarization technology on contacts to n-type layers is also discussed in this paper. Our calculations have relevance to contact technology for bipolar devices built from wide band-gap wurtzite semiconductors like heterojunction bipolar transistors, light-emitting diodes and lasers.
Introduction Large Schottky barrier heights and relatively high hole effective masses lead to very low hole tunneling probabilities in wide band-gap p-type semiconductor ohmic contacts. The large spontaneous and piezoelectric polarization in wurtzite wide band gap semiconductors [ 1 ] can be used to engineer the band profiles of metal-semiconductor barriers in order to obtain improved tunneling characteristics. We present theoretical calculations that demonstrate that the carrier flux is constituted primarily by the split-off holes even though the actual number of such holes is a small fraction of the total number of holes. A two orders of magnitude improvement is seen in the transmission probabilities for a 10 วบ thick indium gallium nitride cap layer for holes very close to the valence band edge, compared to a barrier without any indium gallium nitride cap. The effect of InGaN cap layers on contact resistances to GaN has been reported in [ 2 ]. The calculations in this paper extend the studies to InGaN and AlInN cap layers on Silicon Carbide.
Polarization charges at I
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