First-Principles Study Of Structural Stability And Electronic Properties Of ?-? Semiconductor Nanowires

?-? semiconductor nanowires present large potential in nanoscale electronic and optoelectronic devices due to their outstanding physical and chemical properties as well as the compatibility with modern technology of silicon.Because the transport properties of electronic devices and optical absorption and emission of optoelectronic devices are mainly determined by electronic properties of materials,thus the realization of ?-? nanowires in various devices requires the understanding of their fundamental structural and electronic properties.Owing to size confinement and the large surface/volume ratio,?-? nanowires exhibit different and novel structural and electronic properties as compared to that of their bulks.Furthermore,the structural and electronic properties of ?-? semiconductor nanowires are affected by their size,crystal phase,crystal facet,and composition in the nanoscale range.Hence,it is difficult to explore the surface morophlogy and electronic properties of ?-? nanowires only by the experimental technologies.In the thesis,size,surface,crystal effects on structural and electronic properties of ?-? semiconductor nanowires,including InP,GaP,and GaAs,are explored using the first-principles calculations within density-functional theory.Our aim is to explore the microcosmic mechanism for controlling structural and electronic properties of ?-? nanowires,and provide the theoretical guidance for the controlled growth and applications of ?-? semiconductor nanowires in experiment.The main content and conclusions in the thesis are as follows:1.We first study systematically the crystal structure and electronic properties of ?-? semiconductors(In P,GaP,and GaAs)with four different crystal phases(2H,3C,4H and 6H phases).The calculated results suggest that the band gap of GaP gradually decreases and there is an indirect-to-direct band-gap transition with the transition of crystal phase from zinc-blende phase(3C)to the wurtzite phase(2H).In contrast,electronic structures of InP and GaAs present an opposite trend.Namely,their band gaps increase with the transition of crystal phase from zinc-blende phase(3C)to the wurtzite phase(2H).Moreover,InP and GaAs keep a direct band-gap characteristic in all four crystal phases.2.Then we investigate surface and size effect on the structural stability and electronic properties of wurtzite InP nanowires with different side-facets.Our reaseach results suggests that side facets of wurtzite InP NWs pefer to the low-index,nonpolar {1(?)00 } and {11(?)0 } facets,and the distribution of side facets and surface morophology of these nanowires are strongly related to their surface energies.The surface-energy calculation indicates that {1(?)00 } facet has the lower surface energy than the {11(?)0 } facet,which is responsible for the surface of wurtzite InP nanowires with { 1(?)00 } facets,in good agreemeet with experimental observations.The calculation of electronic structures indicates that the band gap of InP NWs increases with the decrease of diameters.Moreover,the nanowire facted by high-ratio {11(?)0 } facets will have the larger band gap when the diameter of nanowires is larger than 3 nm,while the nanowire facted by high-ratio {1(?)00 } facets will have the larger band gap when the nanowire diameter is smaller than 3 nm.Our results imply the possibility to tune the band gap of wurtzite InP nanowires by controlling their size and side-facets.3.We investigate detailedly the crystal-facet and crystal-phase effect on the structural and electronic properties of GaP nanowires.The calculated results suggest that the stability of GaP nanowires depends on the competition between the crystal phase and crystal facet effects: the former dominates the stability of larger-sized nanowires,which is responsible for the formation of stacking faults and twin defects in zinc-blende nanowires,while the stability of ultrathin nanowires is mainly determined by the latter that leads to the formation of wurtzite nanowires.The above mechanism can be applied to explain the growth of ?-? semiconductor nanowires in experiment.For electronic properties of GaP nanowires,including the band structure characteristic and band gap,are also sensitive to both the nanowire size and the crystal phase.Moreover,we provide a quantitative relationship between the band gap of GaP nanowire polytypes and their diameter,and the calculated band gaps agree well with the experimental data.Our results suggest that electronic structures of ?-? semiconductor nanowires can be tuned by their size and crystal phases.4.Finally,we systermatically investigate the core/shell composition and size effects on electronic band structures and the spatial confinement of carriers in GaAs/GaP core-shell nanowires.Our results demonstrate that the passivation of surface GaP shell layer can suppresse surface states of GaAs-core nanowires,resulting in an intrinsic semiconductor characteristic.The further study indicates that the band gap,band-structure characteristic,and the carrier spatial confinement of core-shell nanowires strongly depend on their size,the core/shell composition,and the crystal phase.All wurtzite core-shell nanowires are direct band-gap semiconductors.Unlike the direct band gap in wurtzite core-shell nanowires,there is an indirect-to-direct band-gap transition in zinc-blende core-shell nanowires with increasing composition.The spatial charge-density distribution of band-edge states indicate that the smaller-sized GaAs/GaP core-shell nanowires(d <5 nm)have a pseudo-type-I band alignment,and the futher increase of nanowire diameter will induce a transition from a pseudo-type-? band alignment to the type-? one where both electrons and holes are localized in the GaAs core.The findings provide a theoretical guidance for engineering electronic structures of GaAs/GaP nanowires and their applications in high-speed electronic devices.