Computer Simulation On Structures And Electronic Properties Of Silicon And Germanium And Their Alloys

The design and forecast of the structures and properties of materials aided bycomputers can be achieved through theoretical calculation and numerical simulation bywhich waste of time and mistakes in experiments can be avoided. This field is still in theascendant. There have been many applications in electronics, calorifics and optics for Si,Ge and their alloys due to their excellent properties. Lots of efforts have been contributedto this field up to date. However, there are still many unresolved problems in this fielddue to limitations in preparation method and research techniques. This work will givefurther attention to the phase transition and the changes of electronic and opticalproperties of Si, Ge as well as their alloys and compounds under high pressure.Firstly, the present status and research progress of Si, Ge et. al. and their alloys weresystematically reviewed;The foundation of materials simulation, such as densityfunctional theory, molecular dynamics and electronic energy band theory, was alsoelucidated, for the purpose of investigations of Si, Ge, GeSn and SiC ets.Secendly, Then, the structural evolution of pure Si and Ge was studiedsystematically, especially for the electronic structure and pressure effects on the crystalstructure of Ge R8 phase. The transition pressure and the bulk moduli were obtained. Theresults show that the R8 phase is a semimetal with a band overlap between conductionand valence band edges about 0.7 eV near Z point, similar to that of amorphous Si, thebond length decrease along with the increase of pressure. At the same time, the orbitalcomponents of density of states for Ge R8 are also studied.Thirdly, we investigate the effect of strain on conduction and valence bandsespecially of the variation of direct and indirect band gap values and this is comparedwith the energy bands without strain. The component dependence was given of the bandstructure of SiGe alloy. A series of high pressure phases of SiGe alloys were obtained, forwhich the transition pressures were given. The phase transition pressure is 12.3 GPa fromZB to β-Sn, 17.8 GPa from β-Sn to Imma and 44.5 GPa from Imma to sh respectively.Meanwhile, the component dependence of the metastable phases was also involved, andcritical composition 0.95 was obtained.Fourthly, we studied the crystal structures, the electronic energy bands and theoptical properties of GeSn alloy. The optical transitions were analieed in terms of theintraband and interband transitions in the electronic bands for the GeSn alloy. Thecomponent dependence was given of the band structure of Ge1-xSnx alloy, the band gapenergy decreases along with the increase of composition x. Pressure induced band gapchanges in Ge1-xSnx alloy were characterized by direct band gap at Γ point to indirectband gap at L point transition under pressure 4.7 GPa and indirect band gap at L point toindirect band gap at ?1 point transition under pressure 6.3 GPa.Finally, optical properties for some common polytypes of SiC such as 3C-SiC,4H-SiC and 6H-SiC were studied on the basis of detailed summary of polytypes of SiC.For example, the high pressure induced phase transition of 3C-SiC was also investigated,and the phase transition pressure 69 GPa was obtained, the previous results aboutsymmetry of intermediate phase were corrected by space group Imm2.The present results are expected to be helpful for further experiental researched onSi,Ge and their alloys and compounds in the respects of non-equilibrium phase transition,formation of metastable phases, properties and applications of these metastable phases,etc.