Selected growth of cubic and hexagonal GaN epitaxial films on polar MgO(111)

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1 x 1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.     

Off‐resonance Raman analysis of wurtzite CdS ground to the nanoscale: structural and size‐related effects

Different techniques were used to follow the transformation of CdS platelets during grinding and under hydrostatic pressure. X‐ray diffraction and transmission electron microscopy revealed that the platelets included zinc blende (cubic) CdS nanodomains dispersed in a wurtzite (hexagonal) single‐crystalline matrix. Extended grinding led to a decrease of the grain size and to a progressive transformation of the hexagonal stacking into a cubic one. The same phase transition was observed up to 9 GPa under hydrostatic pressure. Off‐resonance Raman spectra collected at different stages of the transition led us to connect band groups that were usually overlooked (in resonance conditions) or considered separately. They all probe the stacking disorder and their intensity can be related to the density of stacking faults. Off‐resonance Raman spectroscopy offers a way of probing the optical properties of CdS (and, more generally, layered semiconductors) as a function of the structure and of the confinement of vibrations by structural defects. Copyright © 2011 John Wiley & Sons, Ltd.     

Electronic structure,mechanical and optical properties of ternary semiconductors Si1-xGexC (X = 0, 0.25, 0.50, 0.75, 1)

The electronic structure of silicon carbide with increasing germanium content have been examined using first principles calculations based on density functional theory. The structural stability is analysed between two different phases, namely, cubic zinc blende and hexagonal phases. The zinc blende structure is found to be the stable one for all the Si_1-xGe_xC semiconducting carbides at normal pressure. Effect of substitution of Ge for Si in SiC on electronic and mechanical properties is studied. It is observed that cubic SiC is a semiconductor with the band gap value 1.243?eV. The band gap value of SiC is increased due to the substitution of Ge and the band gap values of Si _0.75 Ge _0.25 C, Si _0.50 Ge _0.50 C, Si _0.25 Ge _0.75 C and GeC are 1.322 eV, 1.413 eV, 1.574 eV and 1.657?eV respectively. As the pressure is increased, it is found that the energy gap gets decreased for Si_1-x Ge_xC (X?=?0, 0.25, 0.50, 0.75, 1). The elastic constants satisfy the Born – Huang elastic stability criteria. The bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also calculated and compared with the other available results.     

Self-consistent model of nanowire growth and crystal structure with regard to the adatom diffusion

A self-consistent model of growth and structure of semiconductor nanowires is proposed. The crystal phase of group III–V semiconductor nanowires is studied. The critical radius of the transition from the hexagonal wurtzite (WZ) structure to the cubic structure of zinc blende (ZB) type is calculated as a function of parameters of the system of materials and the gaseous medium supersaturation. The model presented here is applicable to both gas-phase and molecular beam epitaxies and allows one to calculate the probability of formation of the WZ and ZB phases under various deposition conditions.     

Hole-mediated stabilization of cubic GaN

We propose here a new approach to stabilize the cubic zinc-blende (ZB) phase by incorporation of impurities into a compound that has a hexagonal wurtzite (WZ) ground state. For GaN, we suggest that this can be achieved by adding 3d acceptors such as Zn, Mn, or Cu because the p-d repulsion between the 3d impurity levels and the valence band maximum is larger in the ZB phase than in the WZ phase. This makes the top of the valence states of the ZB structure higher than that of the WZ structure. As holes are created at the top of the valence states by the impurities, it will cost less energy for the holes to be created in the ZB structure, thus stabilizing this phase. Our first-principles total energy calculations confirm this novel idea.     

Ab initio total energy calculation of the dynamical stability of noble metal carbides

The structural, electronic, vibrational and thermodynamical properties of transition metal carbides RuC, RhC, PdC and AgC are investigated using the plane-wave pseudopotentials method within the generalized gradient approximation (GGA) in the frame of density functional theory (DFT). There is a good agreement between present theoretical and available experimental theoretical data in the case of ground state properties such as lattice parameter and bulk modulus. The electronic band structure of these compounds show that all compounds except RuC in zinc blende phase are metallic in nature. RuC in zinc blende phase is semiconducting in nature with an indirect band gap. The phonon properties of RhC, PdC and AgC are investigated for the first time. The phonon frequencies in the phonon dispersion curves are positive throughout the Brillouin zone for zinc blende RuC and AgC and rocksalt RhC and PdC indicating dynamical stability for these compounds in the said phases. Temperature variation of thermodynamical properties for noble metal carbides are calculated and discussed.     

Why does wurtzite form in nanowires of III-V zinc blende semiconductors?

We develop a nucleation-based model to explain the formation of the wurtzite phase during the catalyzed growth of freestanding nanowires of zinc blende semiconductors. We show that in vapor-liquid-solid nanowire growth, nucleation generally occurs preferentially at the triple phase line. This entails major differences between zinc blende and wurtzite nuclei. Depending on the pertinent interface energies, wurtzite nucleation is favored at high liquid supersaturation. This explains our systematic observation of zinc blende during early growth of gold-catalyzed GaAs nanowires.     

Optical Properties of Zinc Selenide Produced by Isovalent Substitution

The reflectance and photoluminescence spectra of layers of zinc selenide with cubic and hexagonal modifications produced by isovalent substitution are investigated. It is established that the main band in the photoluminescence spectra at 300 K is the edge band associated with the exciton annihilation through inelastic scattering on free current carriers.     

First principles calculations of mechanical properties of cubic 5d transition metal monocarbides

The electronic and elastic properties of cubic 5d transition metal monocarbides in rocksalt, cesium chloride, and zinc blende structures have been studied by first principles calculations. The calculations show that the incompressibility for ReC in cesium chloride structure is even higher than that of diamond under pressure (above 89 GPa). The transformation pressure from zinc blende structure to rocksalt structure takes place at about 47 GPa for PtC. HfC-NaCl, ReC-CsCl, and HfC-ZnS have the smallest metallicity, leading to higher hardness. A valence electron number of 8/cell may be a stable valence shell configuration for 5d transition metal monocarbides in rocksalt and zinc blende structures.     

The source of room temperature ferromagnetism in granular GaMnAs layers with zinc blende clusters

Granular GaAs:(Mn,Ga)As films were prepared by annealing the Ga_0.985Mn_0.015As/GaAs layers at 500 °C or 600 °C. It is commonly accepted that this processing should result in the formation of cubic or hexagonal MnAs clusters, respectively. We demonstrate that such a priori assumption is not justified. If in the as grown sample there are not many defects with the interstitial Mn atoms, only small cubic clusters can be formed even after annealing at 600 °C. Moreover, in a sample containing solely cubic GaMnAs clusters, the Mn ions are ferromagnetically coupled at room temperature. This fact was explained by the existence of GaMnAs solid solution in the clusters, with content of Mn close to 20% (higher than ever found in the layers) as was confirmed by experiment and theory. Extended X‐ray absorption spectroscopy studies excluded the possibility of formation of the hypothetic zinc blende MnAs clusters. Not more than one Mn atom was detected in the second shell around central Mn atom. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,electronic and optical properties of Ca3Bi2: First-principles investigation

In this work by applying first principles calculations structural, electronic and optical properties of Ca₃Bi₂ compound in hexagonal and cubic phases are studied within the framework of the density functional theory using the full potential linearized augmented plane wave (FP-LAPW) approach. According to our study band gap for Ca₃Bi₂ in hexagonal phase are 0.47, 0.96 and 1?eV within the PBE-GGA, EV-GGA and mBJ-GGA, respectively. The corresponding values for cubic phase are 1.24, 2.08 and 2.14?eV, respectively. The effects of hydrostatic pressure on the behavior of the electronic properties such as band gap, valence bandwidths and anti-symmetry gap are investigated. It is found that the hydrostatic pressure increases the band widths of all bands below the Fermi energy while it decreases the band gap and the anti-symmetry gap. In our calculations, the dielectric tensor is derived within the random phase approximation (RPA). The first absorption peak in imaginary part of dielectric function for both phases is located in the energy range 2.0–2.5?eV which are beneficial to practical applications in optoelectronic devices in the visible spectral range. For instance, hexagonal phase of Ca₃Bi₂ with a band gap around 1?eV can be applied for photovoltaic application and cubic phase with a band gap of 2?eV can be used for water splitting application. Moreover, we found the optical spectra of hexagonal phase are anisotropic along E||x and E||z.     

On the yellow-band emission in CdS films

CdS polycrystalline thin films were prepared by the chemical bath deposition (CBD) method on glass substrates. X-ray diffraction (XRD) studies show that the films grow in the cubic zinc-blende crystalline phase. Upon thermal annealing (TA) in Ar+S₂ flux at normal pressure in the temperature range 240–510 °C, the evolution of the transformation into the hexagonal wurtzite phase is observed. This hexagonal crystalline structure is the stable phase. From XRD diagrams the phase transition can be appreciated to occur upon TA at approximately 300 °C. Photoluminescence (PL) data prove that the green-emission band is present for well-defined phases – cubic or hexagonal ones. A second band located at 2.2 eV appears for samples near the transition region. This band at 2.2 eV, called the yellow band, has already been reported to be associated with interstitial Cd atoms. A model for this yellow-band-mechanism formation, arising during the phase transformation, has been proposed based on Frenkel-pair creation. Received: 27 June 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001     

Optical absorption and electron spin resonance of Co2+ in cubic,hexagonal and 4H polytype zinc sulfide

Electron spin resonance and optical absorption measurements are reported for Co²⁺ in structurally pure single crystals of cubic, hexagonal, and 4H polytype ZnS. Co²⁺ spectra corresponding to the four different cation sites expected for these structures were observed, i.e. a cubic center from zinc blende, an axial center from wurtzite, and two different axial centers from the 4H polytype host. The energy of the optical ⁴A₂-⁴T₂ transition and the spin Hamiltonian parameters show characteristic differences for these four centers, which are discussed.     

Structural phase modification in Cu incorporated nanostructured zinc sulfide thin films

Cu incorporated zinc sulfide (ZnS) films are prepared by a RF magnetron sputtering technique and the influence of Cu doping concentration on the structural, morphological and optical properties is systematically analyzed using techniques like grazing incidence X-Ray diffraction (GIXRD), micro-Raman spectroscopy, atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) and UV–vis spectroscopy. XRD examination of the as-prepared films revealed the presence of polycrystalline structure with co-existence of cubic and hexagonal phases in the pure and lower Cu incorporated films. Increase in Cu doping concentration causes a gradual phase transformation from mixed phase to cubic phase. Micro-Raman spectra further confirms the structural phase modifications with the addition of Cu in ZnS. Morphological analysis shows compact distribution of elongated grain geometry with good connectivity and detectable grain boundary in the pure and Cu incorporated films. Increase in Cu incorporation results in the systematic reduction of RMS surface roughness. EDS analysis confirms the incorporation of Cu and surface vacancy defects in the doped films. All the films are transparent in the visible region and band gap calculation by Tauc plot shows that increase in Cu incorporation results in band gap renormalization.     

Pressure-induced phase transition of zinc-blende AlN: An ab initio molecular dynamics study

An ab initio constant pressure technique is carried out to study the pressure-induced phase transition of the zinc blende AlN (aluminum nitride). A first order phase transformation into a rock salt structure is observed in the constant pressure simulations. The transformation is accompanied by an initial tetragonal distortion and a subsequent shearing, similar to that found in the other zinc blende structured materials. This phase transition should occur around 6.2 GPa based upon the enthalpy calculations.     

纳米硫化锌球壳的高压相变研究

 采用同步辐射能量色散X射线衍射（EDEX）技术和金刚石对顶砧高压装置,对纳米硫化锌球壳进行了原位高压X射线衍射实验。最高压力达33.3 GPa。常压下纳米硫化锌球壳为纤锌矿结构和闪锌矿结构共存的混相结构。压力达到11.2 GPa时,纳米硫化锌空心球中的纤锌矿结构全部转变为闪锌矿结构。压力达到16.0 GPa时,发生了由闪锌矿结构向岩盐矿结构的相变,在17.5 GPa和21.0 GPa时分别出现未知峰,33.3 GPa时基本完全转变为岩盐矿结构。两个相变均为可逆相变。     

Volume compression of CuCl to 7 GPa

The unit cell volume of CuCl as a function of pressure has been measured up to 7 GPa (giga Pascals). The compression behaviour is quite normal. The analysis of the compression data gives 40·3±1·5 GPa for the bulk modulus of the zinc blende phase. The zinc blende phase transforms to a tetragonal phase at 5·5 GPa, the volume change associated with the transformation being 12%. A comparison of the bulk modulus of CuCl with those of CuBr and CuI indicates that an anomaly exists in this group.     

Pt掺杂CdS光学性质的第一性原理研究

基于密度泛函理论的第一性原理平面波超软赝势方法,运用Castep计算分析了Pt元素掺杂CdS结构,对本征CdS和掺杂晶体的能带结构、态密度以及光学性质进行了分析对比, 由掺杂前后的结果分析发现：Pt掺杂闪锌矿相CdS产生了新的能带,带隙明显缩小;CdS的吸收边产生红移,禁带宽度变窄,在可见光区具有较大吸收系数,提高了可见光的利用率,表现出较好的可见光光催化活性。     

Pt掺杂CdS光学性质的第一性原理研究

基于密度泛函理论的第一性原理平面波超软赝势方法,运用Castep计算分析了Pt元素掺杂CdS结构,对本征CdS和掺杂晶体的能带结构、态密度以及光学性质进行了分析对比, 由掺杂前后的结果分析发现：Pt掺杂闪锌矿相CdS产生了新的能带,带隙明显缩小;CdS的吸收边产生红移,禁带宽度变窄,在可见光区具有较大吸收系数,提高了可见光的利用率,表现出较好的可见光光催化活性。     

高压下ZnSe的电子结构和光学性质

运用密度泛函理论体系下的平面波赝势(PWP)和广义梯度近似(GGA)方法,利用第一性原理计算了不同的压强下ZnSe晶体闪锌矿结构,得到了它的平衡晶格常数、总能量、电子态密度分布、能带结构、光反射与吸收系数等性质,详细讨论了高压下ZnSe的电子结构,并且结合实验结果定性地分析了高压下的光学性质. 关键词： 闪锌矿结构 态密度 能带结构 密度泛函理论