Guiding Principles toward SIC-MOSFET for Future Power Device Applications
K. Shiraishi, K. Chokawa, S. Kato, C. Shinnei, and K. Kamiya
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan
E-mail : shiraishi@comas.frsc.tsukuba.ac.jp
Introduction
Silicon Carbide (SiC) has long time been thought as one of the most promising power device material that substitutes silicon
devices. It is well known that SiC/SiO2 interfaces formed by thermal oxidation of SiC contain many interface trap sites [1]. As for
trap sites at SiC/SiO2 interfaces, carbon interstitials induced by SiC oxidation is thought as a plausible origin, and carbon interstitial
related defects have intensively been studied [2]. It has been reported that these defects form energy levels in the SiC band gap and
they can trap charges during MOSFET operation. The origin of the carbon interstitial defects is thought as follows. SiC release
carbon atoms during oxidation the same as in the case of the emission of Si interstitials during Si oxidation [3,4]. Origins of above
mentioned defects are still under discussion.
In this study, we theoretically propose new origins of SiC/SiO2 interface-related defects. Our proposed defects are caused by only
oxidation induced strain or unique characteristics of SiC conduction band bottoms which wave functions of SiC conduction band
bottoms are nearly free electron (NFE) like states [5].
Methods
In the calculations, we prepared a H-terminated 4H-SiC(0001) surface, and investigated the atomistic oxidation processes of this
surface by the first principle calculation based on the density functional theory (DFT). We used a repeated slab model with 2×2
surface unit cell. Our used slab model contains a 9 atomic layers of SiC and a 10 Å vacuum region. Wavefunctions are expanded by a
planewave basis set with a cutoff energy of 36 Ry, and 2×2×2 k-points are used for the Brillouin zone integration. Structural
optimizations were performed until all the atomic forces were less than 10-3 Ht/a.u..
Results and Discussion
We have found three types defects related to SiC/SiO2 interfaces as follows. (1) Oxidation strain induced C-C bond: Second
nearest neighbor C atoms form a bond due to oxidation induced strain. This C-C bonds induces strain around it, and band gap
modulation occurs due to the NFE states modulation by strain (Fig.1). (2) Second nearest neighbor Si-Si interaction induced by
process strain. This Si-Si interaction form defect levels in 4H-SiC. However, defects do not appear in 3C-SiC. This is due to the
characteristics of NFE states (Fig.2). (3) Intrinsic potential fluctuation at SiC/SiO 2 interfaces due to the position dependent
modulation of NFE states when SiC/SiO2 interfaces are formed (Not shown).
Based on the above results, we can naturally propose a recipe for reducing interface defects of SiC MOSFET. Our proposed
recipe is that oxidation process should be minimized for high quality SiC-MOSFET.
Cut
切断
Cut
切断
Conclusions
We have found three types of intrinsic origins of SiC/SiO2 interfaces states when SiC/SiO2 interfaces are formed, and we have
concluded that oxidation process should be minimized for high quality SiC-MOSFET.
References
[1] H.Yano et al.: IEEE Trans. Electron Devices 3 (1999) 504.
[2] F. Devynck et al.: Phys. Rev. B 84, (2011) 235320.
[3] H. Kageshima and K. Shiraishi: Phys. Rev. Lett. 81 (1998) 5936.
[4] Z. Ming et al.: Appl. Phys. Lett. 88 (2006) 153516.
[5] Matsushita et al. Phys. Rev. Lett. 108 (2012) 246404.
切断
Cut
切断
Cut
Cut
切断
Bulk
切断
Cut
Fig.1: Schematic views of new types of
C-C bond formation.
Oxidized
(a)
(b)
Fig.2: (a) Defect level of 4H-SiC originated from second nearest
neighbor Si-Si bond. (b) Schematic band diagram of second
nearest neighbor Si-Si bond formation.