Papers by Keyword: Wurtzite

Abstract: By using first-principle density-functional theory (DFT) calculations supplemented with symmetry analysis, we investigated the effect of thin-film thickness on the electronic properties of non-polar ( ZnO-wz and GaN-wz. We find that the electronic band structures of thin-film non-polar ( surface with layer variations from two until ten bilayers show the Rashba spin splitting. Importantly, we revealed that this splitting is found to be strongly anisotropic observed in the valence band maximum (VBM) around the Γ point. We clarified the origin of the anisotropic spin splitting in the electronic band structures by considering the point-group symmetry (PGS) of the present system. We found that the changes of the PGS from C_6v (for polar [0001] direction) to C_s (for non-polar ( direction) are responsible for inducing the anisotropic of the spin splitting. To further confirm the anisotropic splitting, we calculated the Rashba spin splitting parameters for different directions of the k-path. We found that these parameters are different in magnitude for a different direction of the k-path indicating the anisotropic spin splitting quantitatively, which is consistent well with symmetry analysis. Finally, we conclude that the observed Rashba spin splitting in the wurtzite surface structure is promising for spintronics applications.

Abstract: The present article reports the growth mechanism of zinc oxide (ZnO) nanowires grown on silicon substrate pre-coated with ZnO buffer layer by thermal evaporation method. ZnO nanowires are grown for different growth time of 0, 30, 90 and 120 mins with controlled supply of Ar and O₂ gas at 650 °C. The structural, morphological and crystallinity properties of ZnO nanowires are analyzed by field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) spectroscopy, high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). FESEM images infers that, the nanowires growth is driven by self-catalysed vapor-liquid-solid mechanism, where the buffer layer serve as nucleation site. EDX spectra show the uniform composition and purity of ZnO nanowires. A strong (002) peak is detected in XRD spectra which indicates that the preferred growth orientation of the nanowires is toward the c-axis with a hexagonal wurtzite structure. The HRTEM microscopic graphs confirm the growth of nanowire along the preferred [0001] axis. Based on the analysis of grown ZnO nanowires, the probable growth mechanism is schematically presented.

Abstract: Introduction of Mn²⁺ ions into ZnO in the form of Zn_(1-x)Mn_xO (0.00≤x≤0.25) has been done by means of coprecipitation method at low temperature using Zn(CH₃COO)₂·2H₂O, Mn(CH₃COO)₂·4H₂O, HCl, and NH₄OH as starting materials. The XRD analysis showed that the produced Zn_(1-x)Mn_xO (0.00≤x≤0.09) samples were crystallized in single phase of wurtzite with hexagonal structures. Besides the wurtzite, the presence of the secondary phase of hetaerolite ZnMn₂O₄ with tetragonal structures was detected in the samples having 0.10≤x≤0.25. The nanometer-sized Zn_(1-x)Mn_xO crystals obtained from XRD analysis were well confirmed by SEM and TEM images. The electron diffraction data showed that the secondary phase formed even for 0.01 and 0.10 Mn-doping samples were ZnMn₂O₄ and MnO₂. The VSM data indicate that the paramagnetic properties of Mn doping occurred at 0.00≤x≤0.06 and 0.10≤x≤0.25 as well as superparamagnetic properties occur in Mn doping 0.07≤x≤0.09. The most interesting fact in this study was the formation of secondary phases in all Mn-doped ZnO samples, even for the smallest x of 0.01.

Abstract: Wurtzite three-dimensional ZnS nanostructure has been synthesised under low temperature (180°C) via solvothermal method in mixed solvents of ethylenediamine and ethylene glycol. The phase structure, morphology and optical properties have been characterized by X-ray (XRD), scanning electron microscope (SEM) and photoluminescence spectra (PL). Through the change of reaction time and the mixed solvent ratio, the properties of three-dimensional ZnS have been discussed in this paper.

Abstract: The present study aims to investigate the influence of Coring glass substrate temperature on the topography, deposition rate, crystal structure, optical, and electrical properties of ZnS thin films produced by magnetic radio frequency sputtering method. From plain view SEM micrographs, the pebble structure has shown in all ZnS thin films deposited at various substrate temperatures. Through higher substrate temperature, smaller ZnS grains can be obtained in the present study. From XRD analysis, ZnS thin film exhibits hexagonal Wurtzite structure. When thickness of ZnS thin film arrive 300nm, optical transmission rate can be above 85% regardless of substrate temperature and gets optical energy barrier of 3.9 eV. From electrical measurement, the variation of resistivity with temperature exhibits a linear relationship for ZnS thin film.

Abstract: We demonstrate how growth parameters may be adopted to produce morphologically controlled high-quality indium phosphide (InP) nanowires suitable for optoelectronic device applications. Growth temperature, V/III ratio, and catalyst particle size have a significant effect on the morphology, crystallographic quality, and optical properties of the resulting nanowires. Significantly, we find that higher growth temperatures or higher V/III ratios promote the formation of wurtzite (WZ) nanowires while zinc-blende (ZB) nanowires are favourable at lower growth temperatures and lower V/III ratios. Results also show that InP nanowires grow preferably in the WZ crystal structure than the ZB crystal structure with increasing V/III ratio or decreasing diameter. This causes a blue-shift in the bandgap as growth temperature increases. These results show that careful control of growth temperature, V/III ratio and catalyst size are crucial for obtaining InP nanowires of a specific crystal structure needed for device applications.

Abstract: This paper describes Transmission Electron Microscopy studies of the structural changes of GaN1-xAsx alloys grown by Molecular Beam Epitaxy at low temperatures on Al2O3 substrate. We found that by lowering the growth temperature increasing amount of As can be incorporated in GaN1-xAsx forming a single phase alloy. For the low As content a columnar growth of wurtzite structure is observed but for increasing As in the range of 0.170.75 the layer becomes amorphous. Increase in Ga flux at low growth temperature (about 200°C) leads to columnar alloys with As content >75% with a cubic structure. In addition to the structural changes monotonic change of the band gap is also observed with the As content in the alloy. The amorphous alloy is stable up to annealing at temperatures not higher than 600°C. Annealing at higher temperature leads to phase separation of GaAs:N and GaN:As confirmed by Z-contrast electron microscopy.

Abstract: We investigate critical thicknesses of InGaN epilayers grown on GaN substrates with the growth-plane not being the c-plane. In particular, we focus on non-polar orientations with growth planes being the m- and a-planes. We have taken into account the proper hexagonal symmetry of wurtzite GaN. We have found that there is only a small difference in the critical thickness for the cplane and the a-plane material; however, in the case of the m-plane material, we predict a quite different behaviour along the (in-plane) c-axis and the perpendicular (in-plane) a-direction.

Abstract: An extreme thin SiC buffer and Ga2O3 layer were deposited on silicon substrate sequentially with a r.f. magnetron sputtering system. Then the sample was annealed in the ambiance of ammonia at high temperature. Nanowires were found when the sample was tested with scanning electron microscopy (SEM). The composition of the nanowires is found to be GaN when the sample was tested with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). A nanowire was observed with transmission electron microscopy and it was even and uniform, with diameter of about 60nm. And the nanowire can be testified of wurtzite single crystal structure by electron diffraction (ED) analysis attached to the TEM. The high-resolution transmission electron microscopy (HRTEM) analysis to the nanowire indicates that the nanowire was single crystal with very good quality.