Stacking Faults in Silicon Carbide
Sammanfattning: This PhD thesis comprises a series of theoretical studies on various stacking faults in silicon carbide polytypes, based on first-principles density functional modelling, which lead to a detailed insight into general electronic properties of stacking disordered system. This work is largely motivated by the discovery in 1999 by ABB Corporate Research of the electronic degradation phenomenon in 4H-SiC p-i-n diodes. The p-i-n diodes gradually degraded in the sense that the voltage drop across the diode, for a constant current, increased gradually with the time of operation. More significantly, the timing of the electronic deterioration was correlated with the occurrence of structural defects, mainly interpreted as stacking faults in the basal planes. In the initial stage of our research, the primal purpose was to fulfil a logical gap: even if stacking faults are created in connection with diode degradation, are they the culprit for degradation? If not, eliminating stacking faults makes no sense. If they do cause degradation, how does it work? However, later on, we encountered the unexpected diverse nature of stacking faults in silicon carbide polytypes.In paper I, we reported the discovery of localized electronic states around stacking faults in silicon carbide. It was found that certain types of stacking faults in 4H- and 6H-SiC can create very clear quantum-well-like structures. Additionally, all geometrically distinguishable intrinsic stacking faults in 3C-, 4H-, and 6H-SiC were recognized.In paper II, the stacking fault energies for all the different stacking faults in 3C-, 4H-, and 6H-SiC as well as Si and diamond were determined.In paper III, a detailed investigation of cubic inclusions in 4H-SiC was performed. Moreover, strong evidence for the rich occurrence of double-stacking fault structure in 4H-SiC was revealed.In paper IV, it was found that a wide variety of electronic properties of stacking faults in 3C-, 4H-, 6H-, and 15R-SiC can actually be classified into three classes according to the dominant factors that determine their electronic properties:1. quantum-well class, 2. spontaneous-polarization class, and 3. electrically-inactive class.In paper V, a microscopic model to account for the effect of the spontaneous polarization on the electronic structures of stacking faults was developed, as well as a convenient notation system to describe a variety of different stacking faults. Some analysis based on a simple rectangular quantum-well model was also done.In paper VI, a theoretical investigation of stacking faults in 15R-SiC was reported. There are as many as five different stacking faults with different properties in this polytype.In paper VII, multiple stacking faults in 6H-SiC were investigated, and some differences from those in 4H-SiC were discussed.In paper VIII, effective masses for two-dimensional electron gases around stacking faults, which are actually very novel two-dimensional quantum structures, were calculated.In paper IX, twin boundaries in 3C-SiC were studied in detail in comparison with those in Si and diamond, and we discovered that interacting twin boundaries which are separated by only two or three Si-C bilayers are actually favourable in energy.
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