Mistry, Manesh V.; (2021) Modelling the mechanisms of nitridation of SiC based devices during anneals in NH3 and NO gases. Doctoral thesis (Ph.D), UCL (University College London.

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Abstract

The work presented in this thesis is focused on the mechanisms of processes thatoccur during the NO and NH3anneals of 4H-SiC/a-SiO2devices, specifically on thenitridation of performance limiting defects in a-SiO2. All results are found usingdensity functional theory (DFT) and classical molecular dynamics.The first two results chapters of this thesis investigate the interactions of nitricoxide (NO) and ammonia (NH3) with a pristine a-SiO2network. This investigationis important on two fronts, the first is that the oxide used in these devices is of highquality (CVD oxide), and the second is that these molecules have been shown to in-corporate into the oxide and, in some cases, chemically interact with it. Hence onemust understand the interactions of these molecules with the pristine a-SiO2. My re-sults demonstrate that neutral NO molecules only have steric repulsive interactionswith the pristine network and negative NO molecule interacts with the network Siatoms electrostatically. These interactions manifest as higher NO migration barri-ers in the negative charge state compared to the neutral charge state. Ammonia isshown to form similar interstitials but also react with the surface silanol groups toform smaller ammonia fragments, like NH2and NH, which then lead to nitridationseen in elemental studies of such devices.In the next chapter I examine the interaction of NO and NH3fragments withcommon defects in the a-SiO2network. The results in this chapter show how thecharge transition levels (CTLs) of known oxide defects move deeper into the SiC/a-SiO2band gap, on nitridation, leading to the conclusion that the tunnelling proba-bility to these defects decreases due to the large difference in energy between theSiC CBM and the nitridated defect levels. In the final chapter I present the results of simulations of the structure and properties of a SiC/a-SiO2interface as well asthe effects of proximity of the interface for defect properties. Finally, I discuss thecharacter of surface relaxation of the a-face of 4H-SiC.

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