First-principles Prediction Of Novel Hexagonal Silicon Crystals And The Study Of Photoelectric Properties

Silicon has huge reserves in earth and it is high-stable,non-toxic and environment-friendly.Crystalline silicon is very important to human being that it is the cornerstone of semiconductor industry and the material pioneer for photovoltaic industry.The widely-used silicon in solar cells is the diamond-structure with indirect band gap feature,which affected the efficiency of sun-light absorption.To improve the conversion efficiency of solar-energy,possible silicon allotropes with direct or quasi-direct band gap features are highly expected in both theoretical and experimental works in recent years.According to the four-connected bonding features of silicon,we employ our recently developed RG~2 code to search potential silicon allotropes with hexagonal symmetry.The systematic results are discussed in this thesis,including four new silicon allotropes as well as their structures,stabilities,electronic and optical properties calculated from first-principles calculations.(1)Two new silicon crystal structures with hexagonal symmetry and low-energy are discovered by RG~2,which are named as Hex-193 and Hex-194,respectively.Their structures,stabilities,electronic and optical properties are systematically investigated by the density functional theory base first-principles method.The results show that Hex-193 and Hex-194 are low-energy silicon allotropes with positive dynamical stability and mechanical stability.Hex-193 and Hex-194 are confirmed to be quasi-direct band gap silicon crystals with proper band gaps for solar-cell applications.They possess excellent ability for sun-light absorption in comparison with the traditional diamond-like silicon and most of the previously proposed allotropes.(2)Two new hexagonal crystal structures with chiral features,namely Hr152 and Hl154 are discovered by RG~2 and investigated by first-principles method.The results confirm that Hr152 and Hl154 are dynamically stable and mechanically stable as carbon,silicon,gemanium and tin.They are semiconductors with band gaps of 4.88eV,1.54 eV,1.08 eV and 0.68 eV for carbon,silicon,gemanium and tin,respectively.As silicon,Hr152 and Hl154 show excellent ability in sun-light absorption exceeding than all the previously reported silicon allotropes.