Hexagonal (2H) germanium is found to be a direct bandgap semiconductor, showing the potential of efficient light emission. Based on 2H–Ge, the structure and electronic properties of 2H–SiGe alloys are studied in detail by hybrid functional calculations. By varying the Si content of the 2H–SiGe alloys, the bandgap is found to be direct for Si contents smaller than 0.35. We find that the key factor in determining the indirect-to-direct transition of the band structures for 2H–SiGe alloys originates from the variation of lattice constant. Furthermore, the Si-rich 2H–SiGe alloy can be changed from indirect to direct bandgap by strain engineering. Furthermore, we consider the effective electron masses (me), band alignments with several oxides, optical absorption properties, and vacancy formation energies of 2H–SiGe alloys, which show that the direct-gap 2H–SiGe alloys have the potential for optoelectronic applications.
The performance of SiC MOSFETs is limited by many defects at the SiC/SiO2 interface. However, there are no fully consistent atomic models of these defects or how their large densities arise. We show how the high heat of formation of SiO2 causes a selective oxidation of Si in SiC, leaving carbon clusters in SiO2. We consider chemical potentials across the interface from both the thermochemical and oxidation kinetics viewpoint. SiO2 native defects give states too far in energy from the SiC band edges, while defects in bulk SiC have too high a formation energy. Only carbon clusters have a low enough formation energy to give sufficient defect densities to account for the mobility loss, and experimental evidence for them is discussed.
In this paper, a single-phase extended-boost quasi-Z source (EqZS) cascaded multilevel inverter (CMI) for photovoltaic power systems is proposed. A phase-shifted pulse width amplitude modulation strategy based on dual switching frequency modulation (DFM-PWAM) for the inverter is also presented. Compared to the traditional quasi-Z source inverter, the EqZS network can significantly increase the dc-link voltage and achieve a higher voltage gain. Due to the inherent double-line-frequency (2ω) voltage and current ripples in the single-phase EqZS inverter (EqZSI), an accurate equivalent model based on the EqZSI module is established to design rational parameters of the EqZS network, which can limit the 2ω ripples in the required range. However, large capacitance and inductance are needed to restrain 2ω ripples through this method, which results in a bulky impedance-source network. Higher switching frequency can reduce the size of the impedance-source network but will increase the switching losses due to hard-switching. The proposed DFM-PWAM technique can not only keep a compact impedance-source network but also mitigate the switching frequency effect on the switching losses. Finally, simulation and experimental results validate the feasibility of the proposed topology and modulation technique and the correctness of theory.
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