The Structure And Electronic Properties Of Silicon / Germanium Heterojunction Nanowires

Semiconductor nanowires have been giving an increasing effect in the area of optoelectronics and microelectronic devices for they are different from the bulk by their many outstanding features which the bulk don't own. This paper mainly studies Si nanowires, Ge nanowires and Si/Ge core-shell heterostructure nanowires with the first principle calculation software CASTEP, theoretically exploring its electronic properties in order to provide theoretical bases for future fabrications of nano-size devices.In the first section, we get three findings. Firstly, we find that dangling bonds saturated by H atoms can help to eliminate dangling bonds'effect on Si nanowires electronic properties by our analysis. Secondly, it is found that the band gap of nanowires with diameters less than 1.3nm are direct bandgap and the they are greater than that of bulk Si. The growth direction and diameter of nanowires have significant effect on the band structure based on analysis of properties of H passivation Si nanowires such as band structure, density of state. Thirdly, with the diameter of 1.1nm Si nanowires in (001) direction doped in the N(P)type by phosphorus atom(boron atoms), we find that N(P)type doping can make the conduction (valence) band below(above) the Fermi level and reduce the band gap.In the second section, as a comparison with Si nanowires and the basis of Si/Ge heterostructure nanowires, we analysize the properties of H passivation Ge nanowires such as band structure, density of states. The results show that nanowires with diameters less than 1.3nm along (001), (110) directions are direct bandgap semiconductor, and Ge nanowires along (111) directions are indirect bandgap semiconductor. The bandgap of Ge nanowires is greater than that of bulk Ge. The band structure can be influenced by the growth direction and diameter too. Doping on Ge nanowires has similar impacts on Si nanowires.In the third section, we analyze the structure and electronic properties of (001)-oriented Si/Ge core-shell heterostructure nanowires. The results show that compressively strained Ge and tensile strained Si exist both in Si-core/Ge-shell and Ge-core/Si-shell nanowire. Heterostructure interface has the largest strain which is influenced by diameters and Dcore/Dshell. Si/Ge core-shell nanowires of diameters less than 1.3nm are direct bandgap semiconductor. Dcore/Dshell has little effect on the band structure which is influenced significantly by diameters. After calculating the band offset, orbit and population analysis,we find that electron (hole) in the bottom (top) of the conduction (valence) band of Si (Ge) core can be confined in the core well, which are influenced by diameter and Dcore/Dshell. We identify the near-gap electronic states which are separated within the core or the shell region, makes it possible for a dopant to generate carriers in a different region. Therefore, the Si/Ge core-shells nanowires have a wide range of applications in the field of optoelectronics and high speed microelectronic devices.