Polytypes and stacking faults in C, Si, Ge and SiC

Abstract

The goal of my master’s thesis is to find a theoretical explanation for mechanical failure of SiC in power electronic devices. This is accomplished using first-principles density functional theory calculations to determine the energetics of stacking faults in SiC and related group IV semiconducting crystals (C, Si and Ge) for comparison. SiC, unlike other group IV semiconductors, exhibits numerous polytypical forms such as 3C, 4H and 6H, which differ only in the stacking sequence along the (111) direction of its diamond (3C) structure. First, we established that in all the above semiconductors, the relevant slip system is always the on the glide plane. Secondly, the stacking fault energies of SiC polytypes are much smaller than those of the elemental semiconductors. All these estimates agree well with experiment, wherever the measured values are available. The generalized stacking fault energy surfaces determined for the glide planes of C, Si, Ge, 3C-SiC, 4H-SiC and 6H-SiC were used in statistical thermodynamical analysis to get relative stability of the faulted crystal with respect to the perfect crystal as a function of temperature. We demonstrated that SiC is quite different from the elemental semiconductors: above a certain Tc, the faulted SiC crystal becomes more stable than the perfect crystal, proving that there should be rapid expansion of the stacking faults in SiC if the operating temperatures in high power devices exceed Tc. To identify the fundamental difference between SiC and other semiconductors, we used the Axial Next Nearest Neighbour Model (ANNNI) to map the stacking fault energies of these semiconductor crystals. We find that the ANNNI model captures the energetics of all these semiconductors accurately. The nearest neighbour parameter in this model is found to be significantly smaller in SiC than in other systems. Using modern crystal growth techniques it is possible to grow various polytypes of any semiconductor. The stacking fault energies in such atomically engineered ix structures can be predicted easily using ANNNI model. We present our theoretical predictions of the stacking fault energies of such crystal structures. We also prove that 6H- polytype of any semiconductor would always have very low stacking fault energy. Our results are expected to be useful in estimating the plastic behaviour of these novel polytypes. In short, the thesis explains the phenomenon of stacking fault expansion seen in hexagonal polytypes of SiC. Our work has resulted in ANNNI models for various semiconductors that can be used to predict stacking fault energies in atomically engineered semiconductor structures.