Traps at the SiC/SiO 2 -Interface
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Traps at the SiC/SiO2-Interface Gerhard Pensl1, Michael Bassler1, Florin Ciobanu1, Valeri Afanas’ev2, Hiroshi Yano3, Tsunenobu Kimoto3 and Hiroyuki Matsunami3 1 Institute of Applied Physics, University of Erlangen-Nürnberg, Staudtstrasse 7, D-91058 Erlangen, Germany. 2 Laboratory for Semiconductor Physics, University of Leuven, B-3001 Leuven, Belgium. 3 Department Electronic Science and Engineering, Kyoto University, Yoshidahonmachi, Sakyo, Kyoto 606-8501, Japan. ABSTRACT The density of interface states Dit at SiC/SiO2 interfaces of different SiC polytypes (4H-, 6H- and 15R-SiC) is monitored and the origin of these states is discussed. The hydrogenation behavior of interface states in the temperature range from 250°C to 1000°C is studied by C-V and G-V investigations. The strong increase of Dit close to the 4H-SiC conduction band is attributed to defects located in the oxide (so-called "Near Interface Traps"). INTRODUCTION The progress in the development of SiC-based devices is due to the improvement of the crystal quality of SiC-substrates, to the capability of growing high-quality epitaxial SiC layers by the chemical vapor deposition (CVD) technique and to the advantage that the device processing can partially be taken from the silicon technology, e. g. the formation of thin insulating films by thermal oxidation [1, 2]. In comparison with Si/SiO2-MOS structures, the density of interface states Dit at SiC/SiO2 interfaces is increased in the whole band gap by at least two orders of magnitude [3]. These raised values of Dit are assumed to originate predominantly from carbon precipitates at the interface, which are formed during the oxidation process [3, 4, 5, 6, 7]. Moreover in 4H-SiC MOS structures, it is experimentally demonstrated that Dit close to the 4H-SiC conduction band edge increases much steeper than in the case of 6H- or 15R-SiC [5, 7, 8]. This increase of Dit occurs independently whether the oxide layer is grown in dry or wet ambient. The high density of shallow states is probably responsible for the low channel mobility in 4H-SiC MOS transistors prepared on Si (0001) faces [9, 10]. There is evidence that these energetically shallow states are caused by so-called "Near Interface Traps (NITs)", which have been monitored by photon stimulated electron tunneling (PST) [7, 11]. These traps are located in the oxide about 15Å to 20Å from the interface and 2.77eV below the SiO2-conduction band edge. Because of the fact that the valence band edges of different SiC polytypes are energetically aligned with respect to the SiO2-conduction band edge [4] and the 4H-SiC band gap is larger than the band gaps of the 6H and 15R polytypes, NITs lie energetically within the band gap of the 4H-SiC polytype. There NITs, which act as acceptors, can capture free electrons from the MOS channel and can, in addition, reduce the channel current by Coulomb scattering. In this paper, we summarize our present knowledge on interface states at 4H-, 6Hand 15R-SiC MOS structures and report new results on NITs obtained from conductance/ capa
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