Electrical Properties of n-GaN/p-SiC and n-AlGaN/p-SiC Heterojunction Diodes
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ELECTRICAL PROPERTIES OF n-GaN/p-SiC and n-AlGaN/p-SiC HETEROJUNCTION DIODES B. Luo, J. Kim, R. Mehandru, F. Ren, Department of Chemical Engineering, University of Florida, Gainesville FL 32611, USA; K. P. Lee S.J. Pearton, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA; A.Y. Polyakov, N.B. Smirnov, A.V. Govorkov, E.A. Kozhukhova, Institute of Rare Metals, Moscow, 109017, B. Tolmachevsky, 5, Russia; A.V. Osinsky, P.E. Norris, Corning Applied Technologies, Woburn, MA 01801, USA
ABSTRACT Properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions prepared by HVPE on 4H SiC substrates were studied by means of C-V, C/G-T, C-f, I-V and DLTS. It is shown, in agrrement with earlier publications, that the GaN/p-SiC HJ is staggered type II with ∆Ec=-0.4 eV and ∆Ev=0.6 eV. When changing GaN for AlGaN with Al mole fraction of x=0.25-0.3 the band alignment becomes normal type I with ∆Ec=0.2 eV and ∆Ev=0.6 eV. I-V characteristics of both heterojunctions bear evidence of strong tunneling via defect states, particularly centers with activation energy of 1.25 eV for GaN/4H SiC HJ. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these HJs showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. DLTS measurements on both types of HJs revealed the presence of broad bands whose behavior is indicative of these bands being related to continuous states in the gap, most likely near the nitride/carbide interface.
INTRODUCTION n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions HJs are of great interest for use in GaN/SiC heterojunction bipolar transistors HBTs which show promise for high-temperature, high-speed applications [1]. Previous current-voltage I-V and capacitance-voltage C-V measurements on n-GaN/pSiC diodes prepared both on 4H SiC polytype and 6H SiC polytype have shown that such HJs have type II staggered bandgap alignment with the conduction band edge of the nitride layer located lower than the conduction band edge of the SiC substrate [2-4]. This arrangement should handicap electron injection from the wide-bandgap n-GaN emitter in GaN/SiC HBTs, particularly in the case of HJs with the 4H SiC polytype where the conduction band offset ∆Ec was reported to be quite high, about 1.1 eV [2]. It has been suggested that switching from GaN to AlGaN in HJs should decrease the ∆Ec value and, for a certain unspecified AlGaN composition, the AlGaN/SiC HJs could become normal type I which theoretically should improve the performance of corresponding HBTs [2]. However, it has been shown that tunneling is very much an issue in GaN/SiC HJs on either polytype and that the leakage currents in AlGaN/SiC HJs on 6H polytypes were considerably worse than for the GaN/SiC HJs [5]. Also, the values of conduction and valence band offsets were shown to vary quite substantially for different HJs growth techniques indicating that interface states could play a serious role in determining the band line-up for GaN/SiC or AlGaN/SiC. In what follo
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