Splitting Of Valence Band Research Articles

Effect of heat treatment on some of the optical parameters of Cu 9Ge 11Te 80 films

Some optical parameters of as-deposited and annealed Cu 9Ge 11Te 80 films were studied in the visible spectral range using both absorbance and transmittance measurements. The absorption coefficient was computed and the analysis of the data revealed the existence of two optical transition mechanisms. The direct optical energy gap E gd decreases from 1.74 to 1.58 eV as the film annealed to 520 K. Such behavior is attributed at present to unsaturated defects, which increase the density of localized states. The width of the tails of localized states E e were calculated and found to be increased at high annealing temperatures. Films annealed at high temperature ( T a>420 K) show a splitting of valence band by the spin orbit interaction. One may conclude that at high concentrations of Cu ( x=9), it is likely that the glass forming ability decreases. Other optical parameters ( ε i, ε r, n and k) were calculated at different annealing temperatures.

Valence-band ordering and magneto-optic exciton fine structure in ZnO

Using first-principles linear muffin-tin orbital density functional band structure calculations, the ordering of the states in the wurtzite ZnO valence-band maximum, split by crystal-field and spin-orbit coupling effects, is found to be ${\ensuremath{\Gamma}}_{7(5)}g{\ensuremath{\Gamma}}_{9(5)}g{\ensuremath{\Gamma}}_{7(1)},$ in which the number in parentheses indicates the parent state without spin-orbit coupling. This results from the negative spin-orbit splitting, which in turn is due to the participation of the Zn $3d$ band. The result is found to be robust even when effects beyond the local density approximation on the Zn $3d$ band position are included. Using a Kohn-Luttinger model parametrized by our first-principles calculations, it is furthermore shown that the binding energies of the excitons primarily derived from each valence band differ by less than the valence-band splittings even when interband coupling effects are included. The binding energies of $n=2$ and $n=1$ excitons, however, are not in a simple $1/4$ ratio. Our results are shown to be in good agreement with the recent magneto-optical experimental data by Reynolds et al. [Phys. Rev. B 60, 2340 (1999)], in spite of the fact that on the basis of these data these authors claimed that the valence-band maximum would have ${\ensuremath{\Gamma}}_{9}$ symmetry. The differences between our and Reynolds' analysis of the data are discussed and arise from the sign of the Land\'e g factor for holes, which is here found to be negative for the upper ${\ensuremath{\Gamma}}_{7}$ band.

Observation of large optical anisotropy and valence band splitting in AlInAs self-assembled lateral quantum wells

Self-assembled lateral quantum wells in alloys of AlInAs, epitaxially deposited by metal–organic chemical vapor deposition on InP, are studied by transmission electron microscopy, modulation spectroscopy, and photoluminescence. Under particular growth conditions, the growth of a homogeneous layer results in the spontaneous self-assembly of a sequence of quantum wells with quantization axes oriented along the [110] direction. With respect to a homogeneous alloy of similar average composition, the band gap reduction observed is as large as 360 meV. A polarization anisotropy exceeding 90% is observed for the lowest energy transition and a large valence band splitting of 139 meV separates the heavy- and light-hole-like valence bands.

Optical properties of self-assembled lateral superlattices in AlInAs epitaxial layers and AlAs/InAs short-period superlattices

Characterization of self-assembled lateral superlattices in AlInAs epitaxial layers and AlAs/InAs short-period superlattices is presented. These structures are spontaneously generated during the epitaxial growth by metal–organic chemical vapor deposition and molecular beam epitaxy. Transmission electron microscopy reveals the structural details and electro-modulated reflectance is used to characterize the energy and anisotropy of the optical transitions in the lateral superlattices. We demonstrate several properties of these self-assembled structures: (a) the band gap energy can be changed by as much as 350 meV, (b) the polarization anisotropy of the lowest energy transition exceeds 90%, (c) the superlattice axis and the direction of the optical anisotropy can be oriented along two non-equivalent directions in the plane of the substrate, and (d) the valence band splitting between heavy- and light-hole transitions is significant. We discuss the difference between the samples from the two growth techniques. Finally, we theoretically model the electronic states in these lateral superlattices and we demonstrate that the difference in average InAs composition between the well and barrier can be as high as 35%.

Direct evidence for the trigonal symmetry of shallow phosphorus acceptors in ZnSe

Spin-flip Raman scattering has been used to provide direct experimental evidence that shallow phosphorus acceptors in relaxed ZnSe epitaxial layers grown on GaAs lie at sites of predominantly trigonal local symmetry. As a consequence, the energy of the heavy-hole bound exciton is about 0.45 meV less than that of the light exciton. This splitting is significantly smaller than (and of opposite sign and different symmetry to) the valence-band splitting of 3 meV observed due to the macroscopic biaxial tensile strain in the same specimens. The behavior is in complete contrast to that of nitrogen acceptors, for which no trigonal field is observed. The experiments provide confirmation of the behavior predicted by previous pseudopotential total-energy calculations for the shallow acceptor states formed by these two different group-V dopants when substituting at selenium sites.

Density functional electronic structures calculations of GeSe 2

Valence and conduction band electronic structures for amorphous and crystalline GeSe 2 are discussed based on the density functional calculation of the density of states for model clusters. Results are compared with photoemission and inverse photoemission spectra and their peak structures are reproduced very well. We present a full interpretation of these spectra through investigations of partial density of states and molecular orbitals around the bandgap, and reveal that the states around the bandgap are Se 4p lone-pair states and Ge–Se anti-bonding states, and the split band structure of the higher conduction band is made up of the anti-bonding states correspond to the bonding states in the split valence band. The higher conduction band is also revealed to be combined with effects of 4d orbitals and inter-layer interactions.

Direct-Gap Reduction and Valence-Band Splitting of Ordered Indirect-Gap AlInP2 Studied by Polarized Piezoreflectance

The order-dependent direct-gap reduction and valence-band splitting in spontaneously ordered indirect-gap AlInP2 are determined by polarized piezoreflectance spectroscopy at room temperature. The experimentally deduced values of the ratio of band-gap reduction to crystal field splitting for the perfectly ordered alloys, ζ=-ΔEg(1)/ΔCF(1)=1.15, and the ratio of band-gap reduction to the ordering change of spin-orbit splitting between the perfectly ordered and random alloys, ξ=-ΔEg(1)/[ΔSO(1)-ΔSO(0)]=8.24, are in good agreement with the recent theoretically calculated value of 1.13 and 13.5, respectively, by [S. H. Wei and A. Zunger. Phys. Rev. B 57, (1998) 8983] but are much larger than those of the previous report by [M. Schubert et al..: Phys. Rev. B 54 (1996) 17616] using dark-field spectroscopy.

Influence of radiation on the optical parameters of AgInSe2 films

The analysis of optical constants of AgInSe2 thin films has been discussed. That ternary chalcopyrite compound shows an allowed direct transition near the fundamental absorption edge Eg1 in addition to a transition originating from crystal-field split levels Eg2. The optical band gaps of as-deposited films and γ-irradiated films at 0.5, 1.25, 2.75 and 3.5 Mrad were calculated. For the high γ doses (2.75 and 3.5 Mrad) the chalcopyrite film shows a three-fold optical structure near the fundamental edge due to crystal field and spin orbit splitting of the uppermost valence band. The extinction coefficient (k), refractive index (n) and dielectric constants (ε1, ε2) were also calculated for irradiated films.

Physical properties of AgInS2 films prepared by chemical spray pyrolysis

We have prepared AgInS2 thin films by the spray pyrolysis technique. The effects of the growth temperature and the chemical composition of the solution on the structural, optical and electrical properties of the films have been studied. It was found that growth temperatures above 400°C lead to single-phase chalcopyrite type AgInS2 films, but deposition temperatures lower than 400°C lead to mixed chalcopyrite and orthorombic structure films. All the films grown here show n-type conductivity, with room temperature resistivity in the range between 103 and 105 Ω cm. The absorbance derivative spectra of single-phase AgInS2 films revealed two optical energy gaps at approximately 1.87 and 2.03 eV, attributed to the fundamental edge and to the valence band splitting by the crystal field, respectively.

Valence Band Splittings of 15R-SiC Measured using Wavelength Modulated Absorption Spectroscopy

Valence Band Splittings of 15R-SiC Measured using Wavelength Modulated Absorption Spectroscopy

MOCVD growth and characterization of ZnS and Zn 1− xMg xS alloys

The growth of ZnS and Zn 1− x Mg x S alloys on GaAs and GaP substrates oriented 2° off the (1 0 0) in the [1 1 0] direction, using metalorganic chemical vapor deposition, is reported. The photoluminescence of high-quality ZnS samples is presented. Strain effects are considered to explain the valence band splitting, and the energy shift between ZnS/GaAs and ZnS/GaP photoluminescence spectra. For Zn 1− x Mg x S alloys, the composition as well as the optical and structural properties are investigated as a function of the growth parameters.

Magnetic Properties of fcc Fe Thin Films

The magnetic properties of fcc Fe thin films with a thickness of 1–11 monolayers (ML) grown at room temperature on fcc Co, which was prepared more than 15 ML on Cu(001), have been studied by spin-resolved core level and valence band photoelectron spectroscopy. The observed exchange split valence band structures show that Fe films below 11 ML are in a high spin ferromagnetic phase. Based on the line shape analysis of 3s core level spectra with a cluster model calculation and considering the probing depth of photoelectron spectroscopy, we demonstrate that in the 5–11 ML region, only the topmost few layers of Fe film reveal ferromagnetism as observed in the Fe/Cu(001) system.

Fundamental reflection spectrum and the electronic band structure of chlorine-doped CdTe

The effect of chlorine impurity on the fundamental reflection spectrum and the electronic band structure of cadmium telluride crystals has been studied. At the impurity concentration NCl>5.0×1019 cm−3, a peak appears in the reflectance spectra. This peak is due to electron transitions at the X point of the Brillouin zone from the upper split valence band to Cl levels lying 0.05 eV above the Γ minimum of the conduction band. The other features in the reflectance spectra and band structure are explained as being due to the effect of spin-orbit splitting at the X point and to indirect electronic transitions from the Cl levels to the Γ minimum.

Spin–orbit splittings in AlN, GaN and InN

We have calculated the spin–orbit splittings of the top valence bands of AlN, GaN and InN crystallized in the zinc blende structure, using the LMTO–LDA method. Because of the large differences in the atomic splittings of cations and anions, and the core d levels of Ga and In, these materials are ideal to investigate anomalies in the valence band splittings. We have obtained strong deviations from the Δ 1≃(2/3) Δ 0 rule and extremely anomalous dependences of Δ 1 and Δ 0 on volume for GaN and InN. Such effects should also appear in the wurtzite counterparts of these nitride semiconductors.

MAGNETIC PROPERTIES OF fcc Fe THIN FILMS

The magnetic properties of fcc Fe thin films with a thickness of 1–11 monolayers (ML) grown at room temperature on fcc Co, which was prepared more than 15 ML on Cu(001), have been studied by spin-resolved core level and valence band photoelectron spectroscopy. The observed exchange split valence band structures show that Fe films below 11 ML are in a high spin ferromagnetic phase. Based on the line shape analysis of 3s core level spectra with a cluster model calculation and considering the probing depth of photoelectron spectroscopy, we demonstrate that in the 5–11 ML region, only the topmost few layers of Fe film reveal ferromagnetism as observed in the Fe/Cu(001) system.

Splitting of the heavy and light hole bands due to the indium-induced strain in three inch indium-alloyed semi-insulating GaAs substrates

Photoreflectance (PR) spectroscopy, x-ray diffraction (XRD) method, and secondary ion mass spectroscopy (SIMS) were used to observe the radial distribution of the band-edge transition and modification of the band structure due to the variation of indium in 3 in. indium-alloyed semi-insulating GaAs (InxGa1−xAs) grown by the liquid-encapsulated Czochralski method. The data from room temperature PR measurements showed the variation of the transition energy with positions indicating the radial distribution of the indium content across the wafers; indium content being higher around the edge region than the central area. The splitting of the degenerate valence band around the edge region of the wafers was also shown in PR data due to the different indium content in adjacent regions where indium content varies rapidly. The XRD measurements showed the drastic change in the distribution of lattice constant on where the splitting of the heavy and light holes happened and the SIMS analysis was adopted to confirm the distribution of indium content across the wafer. The possible model was proposed from the experimental data.

Differential Absorption Measurement of Valence Band Splittings in 4H SiC

Differential Absorption Measurement of Valence Band Splittings in 4H SiC

An ab initio estimate of correlation effects on the band gap of covalent semiconductors: diamond and silicon

An ab initio scheme, which has previously been used to determine electron-correlation effects on valence-band splittings in semiconductors, is extended to yield the correlation-induced shift of the upper valence-band edge in diamond and silicon. Assuming that the processes of removing/adding one electron from/to the solid causes symmetric correlation effects, this information allows for an estimate of correlation contributions to band gaps. Reasonable agreement with experiment is obtained.

Valence-band splitting and shear deformation potential of diluteGaAs1−xNxalloys

The valence-band splitting in thin ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x} (0.011<~x<~0.033)$ epilayers strained coherently by the GaAs substrate is observed in electroreflectance. This study reveals that the valence-band deformation potential does not follow the linear interpolation of those for GaAs and GaN, but shows a rather strong composition dependence with a surprising bowing in a small composition region of the alloy. These results contradict the currently held view that the conduction band is greatly altered but that the valence band is only weakly perturbed by dilute nitrogen doping of GaAs.

Electro-optical examination of the band structure of ordered InGaAs

Using electroabsorption measurements, we have studied the effects of atomic superlattice ordering on the electronic band structure of InGaAs for different growth parameters. We have observed ordering-induced polarization anisotropy, valence-band splitting and band gap reduction strongest for 550 °C growth and 2°[111]B tilted substrates. Back-folded conduction-band states show an ordering dependent energy shift. The position of the split-off valence-band, however, is almost unaffected. An extension to extremely low growth temperatures exhibits ordering also for 450 °C growth. Atomic force microscopy measurements reveal a temperature-dependent change of InGaAs surface from step-like to island formation.