Trends in the band-gap pressure coefficients and bulk moduli in different structures of ZnGa2S4, ZnGa2Se4 and ZnGa2Te4

We have performed a first-principles investigation for the family of compounds ZnGa2X4 (X = S, Se, Te). The properties of two possible structures, defect chalcopyrite and defect famatinite are both calculated. We reveal that ZnGa2S4 and ZnGa2Se4 have direct band gaps, while ZnGa2Te4 has an indirect band gap. The local density approximation band gaps are found to be very different in two structures, while the lattice parameters and bulk moduli are similar. We extend Cohen’s empirical formula for zinc-blende compounds to this family of compounds. The pressure coefficients are calculated and metallization pressures are discussed. We find that agi remains fairly constant when the group-Ⅵ element X is varied in ZnGa2X4 (Ⅱ-Ⅲ2 -Ⅵ4 ).     

Synthesis of single crystal CuAlS2 nanowires via a low temperature direct polyol route

The direct polyol method is used to prepare copper aluminium sulfide CuAlS₂ nanowires. The lattice constants of CuAlS₂ nanowires calculated from powder X-ray diffraction data indicates chalcopyrite structure. The CuAlS₂ nanowires are uniformly in shape and their dimensions are about 50 nm in diameter and several of micrometers in length. The Photoluminescence (PL) spectrum of CuAlS₂ nanowires show a sharp absorption edge at 358.96 nm and a strong near band edge emission at 3.46 eV. The direct energy gap Eg of the sample has been calculated as 3.48 eV, that corresponding to the photoluminescence study. A possible formation mechanism of copper aluminium sulfide is proposed.     

Interband optical transition strengths in SiO2, GeO2, SnO2 and TiO2

An analysis of interband optical transition strengths for a series of MO₂ oxides is presented which gives clear evidence for a direct proportionality between transition strengths (as measured by the dispersion energy) and nearest-neighbor cation coordination number. It is also found that neither the extent of O2p orbital delocalization nor the d-electron configuration has a strong influence on transition strengths suggestin that these strengths can be viewed, to a good approximation, as intrinsic properties of valence s,p-electrons which are largely independent of band structure details.     

Electronic energy band calculation in β-PbO2 comparison with SnO2 and GeO2 energy band structures

The energy band structure of β-PbO₂ is determined semi-empirically by the Kohn-Korringa-Rostoker method. It is believed to be the first PbO₂ calculated band structure that appears in literature. The direct band gap value that is obtained directly from this method is 2.7eV. It is lowered to the value of 1.6 eV which seems to be the most likely value of β-PbO₂ energy gaps that were already obtained. This energy band structure is compared with those of other oxides whose lattices belong to the same space group (D¹⁴_4h).     

Fabrication of p-CuGaS2/n-ZnO:Al heterojunction light-emitting diode grown by metalorganic vapor phase epitaxy and helicon-wave-excited-plasma sputtering methods

Greenish-white electroluminescence (EL) was observed from the heterojunction light-emitting diodes (LEDs) composed of p-type (001) CuGaS₂ chalcopyrite semiconductor epilayers and preferentially (0001)-oriented polycrystalline n-type ZnO thin films. The CuGaS₂ layers were grown on a (001) GaP substrate by metalorganic vapor phase epitaxy and the ZnO films were deposited by the surface-damage-free helicon-wave-excited-plasma sputtering method. The n-ZnO/p-CuGaS₂ LED structure was designed to enable an electron injection from the n-type wider band gap material forming a TYPE-I heterojunction. The EL spectra exhibited emission peaks and bands between 1.6 and 2.5 eV, although their higher energy portions were absorbed by the GaP substrate. Since the spectral lineshape resembled that of the photoluminescence from identical CuGaS₂ epilayers, the EL was assigned to originate from p-CuGaS₂.     

First-principles study of rocksalt MgxZn1− xO: band structure and optical spectra

ABSTRACT

The present work deals with electronic band structure and derived optical spectra of Mg_xZn_1?xO in the hypothetical rocksalt structure. The computations are performed using full-potential linearised augmented plane wave method. The exchange–correlation potential is described using the Wu-Cohen and Tran-Blaha modified Becke–Johnson generalised gradient approximation (TB-mBJ-GGA). The calculated lattice parameter deviates by less than 1% from experiment showing a net improvement when compared with previous calculations. Moreover, its variation with respect to x does not violate Vegard's law. The TB-mBJ-GGA approach improves the magnitude of the fundamental band gap with respect to experiment. The rocksalt Mg_xZn_1?xO is found to be an indirect gap semiconductor for x?=?0, 0.25, 0.50 and 0.75 and a direct gap semiconductor for x?=?1. The nature of the gap for rocksalt Mg_xZn_1?xO is still in controversy and further investigations are required in this respect. The optical spectra of Mg_xZn_1?xO are analysed and discussed. Our findings yield values of 1.55 and 1.25 for the static refractive index and 2.4 and 1.55 for the static dielectric constant for rocksalt ZnO and rocksalt MgO, respectively.     

Synthesis of new pure ferromagnetic semiconductors: MnGeP2 and MnGeAs2

We have synthesized two new chalcopyrite compounds: MnGeP₂ and MnGeAs₂. Total energy calculations predicted that both compounds are indirect semiconductors with band gaps of 0.24 and 0.06 eV, respectively. Both compounds exhibit room-temperature ferromagnetism with T_C∼320 and 340 K for MnGeP₂ and MnGeAs₂, respectively, based on magnetization and resistance measurements. We have also observed the anomalous Hall effect, indicating polarization of the carriers.     

First-principles prediction of half-metallic ferromagnetism in Cd(TM)O2 (TM=Cr, Mn, Fe, Co, Ni) compounds

We report the electronic structure of Cd(TM)O₂ (TM=Cr, Mn, Fe, Co, Ni) in the chalcopyrite structures. From this study we find that Cd(TM)O₂ is a half-metallic ferromagnetic compound. From the energy consideration we find that Cd(TM)O₂ is more stable in chalcopyrite structure rather than in rock salt structure. A careful analysis of the spin density reveals the ferromagnetic coupling between the p-d states and the cation dangling-bond p states, which is believed to be responsible for the stabilization of the ferromagnetic phase. The calculated heat of formation, bulk modulus and cohesive energy are reported.     

Donor-acceptor pairs and excitons recombinations in AgGaS2

Silver thiogallate (AgGaS₂) is a ternary semiconductor which crystallizes in the chalcopyrite structure. Silver thiogallate has been widely used in different applications for its interesting physical properties: wide transparency range (from 0.5 to 12 μm), high non-linear optical coefficient combined with good mechanical properties.The direct band gap in this compound is of about 2.7 eV and emissions due to free and bound excitons had been observed. Photoluminescence spectrum is also characterized by a wide emission band centred at 496 nm (2.50 eV) due to donor-acceptor pairs recombination (DAP).We performed photoluminescence (PL) measurements exciting with the third harmonic (3.5 eV) of a Nd:YAG laser from room temperature down to 10 K at different excitation power.In this work, we report the dependence of the photoluminescence features of AgGaS₂ on the excitation power at various temperatures: ionization energy of defects are estimated on the basis DAP theoretical model and of thermal quenching of the photoluminescence; evidences of non-radiative processes competitive to DAP is also presented.     

Influence of proton irradiation on the photoluminescence spectra of zinc oxide modified by ZrO2 and ZrO2·Y2O3 nanopowders

ZnO powder photoluminescence spectra at 360-660 nm modified and unmodified by ZrO₂, ZrO₂Y₂O₃ nanopowders before and after 100 keV proton irradiation were investigated. It was found that introduction of nanoparticles led to ultraviolet band intensity decrease and to visual spectrum band intensity increase. Extinction of intensity occurs under the effect of protons in both bands of luminescence. Decomposition of spectra into elementary defects and analysis of their area change during modification and irradiation were carried out.     

Effective ionic charges in CdGa2Se4 and CdGa2S4

Available experimental data (Raman and Infrared) on the optic phonon spectra in defect chalcopyrite crystals CdGa₂Se₄ and CdGa₂S₄ are used to determine the dynamical ionic effective charges. The results are compared with those found in other ternary compounds as spinels and chalcopyrites. An emperical correlation between the normalized Szigeti charges and the optical dielectric constant is discussed.     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Modification in structural and optical properties of ZnO, CeO2 doped Al2O3-PbO-B2O3 glasses

The structural and optical analysis of glasses is carried out by XRD, FTIR, density and UV visible spectroscopic measurement techniques. XRD results have confirmed the glassy nature of the samples. The FTIR spectral analysis reveals that with the combined presence of ZnO and CeO₂ contents in Al₂O₃-PbO-B₂O₃ glasses, more BO₃ groups are transformed into BO₄. The optical analysis reveals that optical band gap energy decreases more for CeO₂-ZnO-Al₂O₃-PbO-B₂O₃ glasses (from 2.28 to 1.84 eV). The presence of CeO₂ and ZnO in the glass samples causes more compaction of the borate network due to the formation of more BO₄ groups and the presence of ZnO₄ groups, which results an increase in density, refractive index and decrease of molar volume.     

Zn incorporation and (CuIn)1−xZn2xSe2 thin film formation during the selenization of evaporated Cu and In precursors on Al:ZnO coated glass substrates

CuInSe₂ thin films with typical 1.0 eV gap energy and tetragonal chalcopyrite structure have been obtained on soda–lime glass substrates by the reaction of sequentially evaporated Cu and In layers with elemental selenium vapor, at 500 °C in flowing Ar. When analogous deposition and reaction processes were performed on Al:ZnO coated glasses, some increment in the band gap energy and diminution in the crystalline interplanar spacings have been detected for the resulting films with an extent that depends on the Cu/In atomic ratio of the evaporated precursor layers. This fact has been related to Zn incorporation into the selenized film, with quaternary (CuIn)_1−xZn_2xSe₂ compound formation that is influenced by the presence of copper selenide phases during the reaction process. Such deductions are supported by the optical, structural and compositional characterizations that have been performed comparatively on samples prepared by selenization of evaporated metallic precursors with two different Cu/In ratios (0.9 and 1.1) on bare and Al:ZnO coated glass substrates.     

First-principles calculations of the optical band-gaps of ZnxCd1−xO alloys

Using first-principles calculations, we investigated the structural and electronic properties of two binaries: ZnO in wurtzite structure and CdO in wurtzite and rock-salt structures. In addition several compositions with various ordered structures of Zn_xCd_1−xO alloys were studied within the theory of order–disorder transformation. The full potential linearized augmented plane wave method was used and the d orbitals of Zn and Cd were included in the valence bands. In this investigation of alloying ZnO with CdO, the fundamental band-gap of the alloys is shown to be direct and to decrease versus the Cd composition.     

Structural characterization of Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O3 templates by P-MBE

In this study, structural properties of epitaxial Ga-doped Mg_0.1Zn_0.9O layers grown on ZnO/α-Al₂O₃ templates by plasma-assisted molecular beam epitaxy have been investigated by high-resolution transmission electron microscopy (HRTEM), and high resolution X-ray diffraction (HRXRD). From analysis of the diffraction pattern, the monocrystallinity of the Mg_0.1Zn_0.9O layer with hexagonal structure is confirmed. The orientation relationship between Mg_0.1Zn_0.9O and the template is determined as (0 0 0 1)_Mg0.1Zn0.9O(0 0 0 1)_ZnO(0 0 0 1)_Al2O3 and [ [ ]_ZnO[ . The density of dislocations near the top surface layers measured by plan-view TEM is about 3.610¹⁰ cm⁻², one order of magnitude higher than the value obtained for ZnO layers on α-Al₂O₃ with a MgO buffer. Cross-sectional observation revealed that the majority of threading dislocations are in the [0 0 0 1] line direction, i.e. they lie along the surface normal and consist of edge, screw, and mixed dislocations. Cross- sectional TEM and X-ray rocking curve experiments reveal that most of dislocations are edge dislocations. The interface of Mg_0.1Zn_0.9O and ZnO layers and the effect of excess Ga-doping in these layers have been also studied.     

SiO2 and Al2O3 valence band density of states from self-deconvolution of L23VV Auger spectrum

The self-deconvolution of L₂₃VV Auger spectra of SiO₂ and Al₂O₃ has been carried out. The transition density functions obtained are compared with the local density of states (LDOS) of the valence band near the surface, as given by other techniques (XPS, UPS, XES) and also by theory. A fair agreement in the number and peak positions of valence band is produced. These compounds with MgO constitute an oxide series of increasing ionicity and the effects of initial hole localization in the transition density function are discussed.     

Optical absorption spectrum of Cu2O-CaO-P2O5 glasses

Homogeneous CaO-P₂O₅ and Cu₂O-CaO-P₂O₅ glasses were prepared using a melt-quenched method under controlled conditions. The binary glasses were found to be colourless and transparent while the ternary glasses changed from light green to dark green as the Cu₂O content increased. From the absorption edge studies, the values of the optical band gap, E_opt and Urbach energy, ΔE were evaluated. The position of the absorption edge and hence the optical band gap were found to depend on the glass composition. Analysis of the optical band gap shows that for the binary glasses, the value increases as the content of CaO decreases, while for the ternary glasses, the value of the optical band gap increases as the content of the Cu₂O decreases. The density of the glasses was also measured and was found to increase with the increase in CaO and Cu₂O contents.     

Optical and acoustic properties of TeO2/WO3 glasses with small amount of additive ZrO2

The optical and acoustic properties of tellurite glasses in the system TeO₂/ZrO₂/WO₃ have been investigated. The refractive index at different wavelengths and the optical spectra of the glasses have been measured. From the refractive index and absorption edge studies for prepared glasses, the optical parameter viz; optical band gap (E_opt), Urbach energy, (ΔE), dispersion energy, E_d, and the average oscillator energy, E₀, have been calculated. Sound velocities were measured by pulse echo technique. From these velocities and densities values, various elastic moduli were calculated. The variations in the refractive index, optical energy gap and elastic moduli with WO₃ content have been discussed in terms of the glass structure. Quantitatively, we used the bond compression model for analyzing the room temperature elastic moduli data. By calculating the number of bonds per unit volume, the average stretching force constant, and the average ring size we can extract valuable information about the structure of the present glasses.