Effects of Structural Defects on Diode Properties in 4H-SiC
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Effects of Structural Defects on Diode Properties in 4H-SiC B.J. Skromme, K.C. Palle, M.K. Mikhov, H. Meidia1, S. Mahajan1, X.R. Huang2, W.M. Vetter2, M. Dudley2, K. Moore3, S. Smith3, and T. Gehoski3 Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, AZ, 85287-5706, U.S.A. 1 Department of Chemical and Materials Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, AZ, 85287-6006, U.S.A. 2 Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY, 11794-2275, U.S.A. 3 Physical Sciences Research Laboratory, Motorola, Inc., Tempe, AZ, 85284, U.S.A. ABSTRACT The influence of two types of structural defects on Schottky diodes in 4H-SiC is investigated. First, a structural transformation that takes place during oxidation or annealing of heavily ndoped wafers is studied using X-ray diffraction, high resolution TEM, secondary electron imaging (SEI), photoluminescence, and electrical measurements on Schottky diodes. Lamellae of 3C SiC with six bilayers are introduced primarily in the heavily doped substrates, and to a lesser extent in the lightly doped epilayers. Lowered Schottky barrier heights result and are spatially correlated to the 3C phase. The 3C lamellae exhibit charge contrast in SEI images where they intersect the surface. Second, we study isolated screw dislocations and show preliminary evidence that they are not the main factor controlling Schottky barrier height and ideality in Schottky diodes in lower-doped 4H-SiC wafers. INTRODUCTION The high breakdown field and reasonable electron mobilities of 4H-SiC make this material very attractive for power electronic devices. However, structural defects often limit performance and yield, especially for large area devices [1]. Here, we focus on the effects of two types of structural defects on the electrical properties of Schottky diodes, including stacking faults induced by thermal oxidation or annealing, and isolated 1c screw dislocations. Our previous work has shown that thermal oxidation of substrates with ~3x1019 cm–3 n-type doping and lightly doped epilayers causes pronounced surface buckling, related to a high density of dislocations imaged by synchrotron-based white-beam X-ray topography (SWBXT) [2]. The distorted regions showed a ~0.74 eV reduction in band-edge photoluminescence (PL) emission energy and a ~0.47 eV reduction in Schottky barrier height for three different metals, independent of the metal work function. We proposed thin, cubic lamellae as an explanation for these results [2]. Independent work by Okojie et al. on the opposite structure, heavily n-type epilayers on lightly p-doped substrates, showed TEM images of stacking faults and thin cubic lamellae limited mainly to the heavily-doped epilayer, along with cathodoluminescence (CL) at energies slightly above the 3C band gap [3, 4]. Subsequent work by Liu et al. showed a similar effect in heavily doped substrates with no epilayers [5
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