Active Defects in 4H–SiC MOS Devices

Abstract

The research findings presented in this thesis have provided several key contributions towards a better understanding of the SiC–SiO2 interface in SiC MOS structures. The electrically active defects directly responsible for degrading the channel-carrier mobility in 4H–SiC MOSFETs have been identified and a novel technique to detect these defects in 4H–SiC MOS capacitors has been proposed and experimentally demonstrated. With a better understanding of defects at the SiC–SiO2 interface two alternative gate oxide growth processes have been proposed to overcome the practical limitations associated with current NO-nitridation techniques in high-volume, production based oxidation furnaces. This work therefore contributes to the wider research effort towards improving the performance of SiC MOSFETs in several ways. The following paragraphs summarise the key conclusions that have been obtained as a result of this study.

Electrically Active Defects and the Channel-Carrier Mobility (Chapter 3)

A critical review of defects at the SiC–SiO2 interface exposed a few key discrepancies in both the current understanding of the dominant defects responsible for channel-carrier mobility degradation in 4H–SiC MOSFETs and in the current approach to characterise and evaluate the SiC–SiO2 interface. Firstly, it was recognised that the Shockley-Read-Hall statistical model, based on thermally activated transport for traps spatially located at the semiconductor-oxide interface, cannot be directly applied to describe the transfer mechanism between free conduction band electrons and the shallow NITs near EC. This implication tends to suggest that the NITs near EC in SiC MOS structures cannot be accurately examined using traditional MOS characterisation techniques that are based on this statistical model. Secondly, in accordance with the studies conducted by Saks et. al. [1-3], it was realized that channel-carrier mobility degradation in 4H–SiC MOSFETs is primarily due to the significantly reduced free electron density in the inversion channel. In light of this understanding, the interfacial defects that actively trap channel electrons under strong inversion conditions were considered to be dominant in these devices as opposed to the NITs near EC that are typically examined using conventional MOS characterisation techniques on N-type MOS capacitors in depletion. To further support this hypothesis, a theoretical analysis of the inversion carrier concentration using the charge sheet model was conducted to demonstrate that the NITs with energy levels corresponding to strong inversion are of key importance to the channel-carrier mobility.