First principles calculations of Cd and Zn chalcogenides with modified Becke–Johnson density potential

In this work, we present first principles calculations based on a full potential linear augmented plane-wave method (FP-LAPW) to calculate structural and electronic properties of CdX and ZnX (X = S, Se, Te) based II–VI chalcogenides. First principles calculations using the local density approximation (LDA) and the related generalized gradient approximation (GGA) lead to a severe underestimate of the band gap. The proposed model uses various exchange–correlation potentials (LSDA, GGA and MBJLDA) to determine band gaps and structural properties of semiconductors. We show that using modified Becke–Johnson (MBJLDA) density potential leads to a better agreement with experimental data for band gaps of Cd and Zn based semiconductors.     

Fundamental and transport properties of ZnX,CdX and HgX (X = S,Se, Te) compounds

We have performed ab initio self-consistent calculations based on full potential linear augmented plane-wave method (FP-LAPW) with the local density approximation (LDA) and generalised gradient approximation (GGA) to investigate the relativistic effects on the structural, electronic, transport and optical properties of II–VI compounds. We mainly show that the stabilisation (destabilisation) of s, p*(p) orbital energies reduces the lattice parameters of II–VI compounds, the band gaps and the effective masses. This, however, induces strong spin–orbit splitting of heavier II–VI compounds.     

Pressure-dependent dynamical properties of Zn-based II–VI semiconductors

The effect of pressure on optical phonons and polaron properties in ZnS, ZnSe, and ZnTe II–VI compound semiconductors has been investigated. The calculations are performed in the framework of ab initio pseudopotential approach based on the density functional perturbation theory. At zero pressure, a reasonable degree of agreement is generally found between our results and data available in the literature. It is found that when pressure is increased the phonon modes at Г in the Brillouin zone are shifted towards high energies. The pressure dependence of features such as Fröhlich coupling parameter, the Debye temperature of the longitudinal optical phonon frequency and the polaron effective mass showed that the polaron properties are sensitive to the pressure effect.     

Spin dynamics and quantum transport in magnetic semiconductor quantum structures

Quantum structures derived from magnetic semiconductors serve as a powerful arena within which to study the interplay between quantum electronics and thin film magnetism. In particular, the semiconductor aspects of these flexible systems allow direct access to the electronic spin degrees of freedom using both magneto-optical as well as magneto-transport probes. Here we provide an overview of recent developments in the experimental study of II–VI magnetic semiconductor quantum structures, with particular emphasis on the dynamical behavior of field-tunable electronic spin states and spin-dependent quantum transport.     

Ferromagnetism in transition-metal-doped II–VI compounds

We report systematical density-functional calculations of ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductor. Some of them are half-metallic at their optimized cell volumes. The effect of atomic position optimization (and cell volume re-optimization) on the electronic structures is to widen the energy bands near the Fermi energy and to reduce the density of states there. As a result, the Fermi level moves upward and the energy gap of the minority-spin bands at the Fermi energy becomes narrower. Therefore, the half-metallic gaps are reduced, two of them even being closed. These compounds are compatible with the II–VI semiconductors, and could be useful in spin-based electronics.     

Hyperfine Interactions and Magnetism of 3d Transition-Metal-Impurities in II–VI and III–V Compound-Based Diluted Magnetic Semiconductors

The electronic structure of II–VI and III–V compound-based diluted magnetic semiconductors is calculated based on the local density approximation (LDA) using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. The magnetism of 3d transition-metal-atom-doped ZnO, ZnS, ZnSe, ZnTe, GaN, GaAs is investigated from first-principles. It is suggested that the double exchange mechanism stabilizes the ferromagnetism in these DMSs. In order to obtain microscopic information on the electronic structure of transition-metal-impurities in semiconductors, the hyperfine field of respective impurities in each host material is calculated. It is found that the agreement with the experimental values is not good, probably because the LDA is not sufficient to describe the core states of transition metals. However, it is suggested that the hyperfine fields clearly reflect the local magnetic moments for 3d impurities. This revised version was published online in September 2006 with corrections to the Cover Date.     

Effect of electron–phonon interaction on surface states in wurtzite nitride semiconductors under pressure

A variational theory is proposed to study the electronic surface states in semi-infinite wurtzite nitride semiconductors under the hydrostatic pressure. The electronic surface state energy level is calculated, by taking the effects of the electron–Surface–Optical–phonon interaction, structural anisotropy and the hydrostatic pressure into account. The numerical computation has been performed for the electronic surface state energy levels, coupling constants and the average penetrating depths of the electronic surface state wave functions under the hydrostatic pressure for wurtzite GaN, AlN and InN, respectively. The results show that electron–Surface–Optical–phonon interaction lowers the electronic surface state energy levels. It is also found that the electronic surface state energy levels decrease with the hydrostatic pressure in wurtzite GaN and AlN. But for wurtzite InN, the case is contrary. It is shown that the hydrostatic pressure raised the influence of electron–phonon interaction on the electronic surface states obviously. The effect of electron–Surface–Optical–phonon interaction under the hydrostatic pressure on the electronic surface states cannot be neglected, in specially, for materials with strong electron–phonon coupling and wide band gap.     

Recent advances in superconductivity of covalent superconductors

A summary and critical analysis are given of recent results on superconductivity in diamond, Si, SiC, III–V and IV–VI semiconductors, as well as graphite and fullerites intercalated with alkali metals. Superconductivity in III–V semiconductors is questioned, and proposals are outlined for further enhancement of the transition temperature in diamond and intercalated graphite.     

Recent developments in the optical spectroscopy of II–VI compound semiconductors

This review emphasises the information on the binding energies and the identities of electrically impurity and defect centres in II–VI semiconductors which may be obtained from optical spectra. Those shallow centres which may promote useful electrical conductivity are of particular interest. They contribute the richly structured near gap (edge) luminescence, containing weak to moderate phonon coupling and therefore very accessible information about the energy states of the different centres. It is shown that improved material and spectroscopic techniques have yielded considerable information about donors and, particularly shallow to moderately deep acceptors in ZnTe, a key II–VI semiconductor. Persistent impurities such as Li and Cu exert a greater influence on the properties of this semiconductor than formerly supposed. This influence is traced to other II–VI semiconductors, particularly ZnSe. Both the Cu_Zn and P_V acceptors appear to be much deeper in ZnSe than in ZnTe due to lattice relaxation, although evidence for a shallow P-related centre, probably a complex, is presented. Finally, some comments are made about the energy states of centres involving native defects in these compounds, particularly the V_Zn-donor associate centre, responsible for the self-activated luminescence.The author is grateful to D. Bensahel, R. N. Bhargava, S. G. Bishop, H. G. Grimmeiss, N. Magnea, J. E. Nicholls, J. Pautrat, D. J. Robbins, H. Tews and H. Venghaus for permission to use some of their data in this paper. He is also grateful to several of these and other colleagues. cited in the references for discussions of some of the problems described here.     

Spectroscopic investigation of buried interfaces and liquids with soft X-rays

When, in general, two entities interact, they do it by forming an interface. The properties of such interfaces are determined not only by the properties of the two interface partners, but also to a large degree by the peculiarities of the interface-formation process itself. This is of particular importance in solid-state devices composed of two or more different materials. Unfortunately, the investigation of such interfaces is very difficult for two reasons. First they are, by their nature, buried. Secondly, interfaces generally form a thin layer within a larger ensemble and thus give very weak signals. Nevertheless, a few experimental techniques are available to study such buried interfaces. This report demonstrates that a combination of soft-X-ray spectroscopies (X-ray emission, photoemission, and X-ray absorption) is extremely well suited for this task. As examples, the electronic and chemical properties of several material systems are discussed, including II–VI semiconductors, Cu(In,Ga)(S,Se)₂ thin-film solar cells, organic thin films, and liquids. PACS 82.80.Ej; 73.20.-r; 68.35.Fx     

First-principles studies of structural,mechanical, electronic,optical properties and pressure-induced phase transition of CuInO2 polymorph

Using the first-principles density-functional theory within the generalized gradient approximation (GGA), we have investigated the structural, elastic, mechanical, electronic, and optical properties and phase transition of CuInO₂. Structural parameters including lattice constants and internal parameter, pressure effects and phase transition pressure were calculated. We have obtained the elastic coefficients, bulk modulus, shear modulus, Young's modulus and Poisson's ratio. We find that two phases of CuInO₂ are indirect band gap semiconductors (F–Γ and H–Γ for 3R and 2H, respectively). Optical properties, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function and optical conductivity have been obtained for radiations of up to 30 eV.     

Phase transition,electronic and optical properties of III-Sb compounds under pressure

III-V semiconductors are the backbone of optoelectronic industry. Here, we have performed first principle calculations to investigate the structural, electronic and optical properties of III-Sb (III = B, Al, Ga, Sb) compounds under the effect of pressure. The structural phase transition from zincblende to rocksalt phases is determined by the common tangent of the two E–V curves. The obtained results are in good agreement with the available literature. Compounds make electronic transition from semiconductors to metals under pressure. The calculated band structure in zincblende structure was compared with experimental and theoretical findings. Optical properties including real and imaginary parts of the complex dielectric function, frequency-dependent reflectivity and optical conductivity are explained to characterize the optical nature of these compounds in both phases.     

Optical properties of III-Mn-V ferromagnetic semiconductors

We review the first decade of extensive optical studies of ferromagnetic, III-Mn-V diluted magnetic semiconductors. Mn introduces holes and local moments to the III–V host, which can result in carrier mediated ferromagnetism in these disordered semiconductors. Spectroscopic experiments provide direct access to the strength and nature of the exchange between holes and local moments; the degree of itineracy of the carriers; and the evolution of the states at the Fermi energy with doping. Taken together, the diversity of optical methods reveal that Mn is an unconventional dopant, in that the metal to insulator transition is governed by the strength of the hybridization between Mn and its p-nictogen neighbor. The interplay between the optical, electronic and magnetic properties of III-Mn-V magnetic semiconductors is of fundamental interest and may enable future spin-optoelectronic devices.     

Optical properties of highly excited direct gap semiconductors

The electronic excitations in direct gap semiconductors interact strongly with the photon field. We discuss both the experimental and the theoretical aspects of the optical properties of these materials under strong optical excitation. We distinguish between intermediate excitation levels at which the electronic excitations form a dense system of excitons and excitonic molecules and very high excitation levels at which a degenerate electron-hole plasma occurs. The optical spectra of dense excitonic systems, which are mainly observed in copper halides and II–VI compounds, are shown to be determined mainly by the interaction processes between excitonic molecules, polaritons and free carriers. The optical properties of the electron-hole plasma, which has been observed in II–VI and especially in III–V compounds, can be understood only by taking into account many-body effects, such as dynamical screening of the Coulomb interactions, plasmon-assisted transitions and excitonic enhancement.     

Bilinear optical susceptibilities of potassium dihydrogen phosphate and of III–V,II–VI and I–VII semiconductors

The approximate orbital-approach of Jha and Bloembergen is used to calculate the non-dispersive part of the bilinear optical susceptibility,χ ⁽²⁾, for various compounds. Using bonding and antibonding states of the molecular orbital theory it is shown thatχ ⁽²⁾ satisfies a simple relation in terms of other measurable physical quantities. This relation is used to calculateχ ⁽²⁾ for various III–V, II–VI and I–VII semiconductors, both with cubic zinc blende structure and with hexagonal wurtzite structure. The same procedure is used to obtainχ ⁽²⁾ for potassium dihydrogen phosphate (KDP). The calculated values are compared with the experimentally observed values and it is found that the present model gives excellent results for II–VI compounds and for KDP.     

Some empirical relations for electron mobility in II–VI compound semiconductors

Electron mobility has been calculated in a number of binary II–VI compound semiconductors using a displaced Maxwellian distribution function and taking the various scattering mechanisms into consideration at different lattice temperatures and for various amounts of ionized impurity concentrations. It is observed that the low field mobility values can be expressed by a cubic power relationship with lattice temperature and with ionized impurity concentration using a least mean square fit technique with an accuracy better than 5 per cent. Similarly, the field dependence of mobility can also be expressed as a power series of the applied electric field. It is suggested that these equations can be profitably used for a quick estimation of mobility values as a check on experiments and also as sufficiently accurate formulae for simulation and modelling purposes.     

Fir magnetospectroscopy on (CuIn)1?xMn2xTe2

The paramagnetic resonance of Mn²⁺ in the diluted magnetic semiconductor (CuIn)_1–xMn_2xTe₂ was observed in a pulsed magnetic field up to 15 T at temperatures ranging from 4.2 K to 45 K. The temperature dependence of the line width of the paramagnetic resonance in (CuIn)_1–xMn_2xTe₂ resembles the behaviour of other diluted magnetic semiconductors. A Dzyaloshinski-Moriya exchange constant of approximately 0.62 K was derived. This value fits well with the values reported for II–VI based diluted magnetic semiconductors [1], if we consider the larger degree hybridization of the 3d electrons with the band electrons in chalcopyrite semiconductors.     

Influence of quantization of band states on the thermoelectric power in II–VI semiconductors

Summary An attempt is made to present a simplified analysis of the thermoelectric power in II–VI semiconductors in the presence of quantization of band states under different physical conditions. The thermopower decreases with increasing electron concentration in different manners, which is a direct consequence of band state quantization. We have plotted the thermopower with various other physical variables and the same power exhibits different types of oscillations in low-dimensions, respectively. The well-known results for parabolic energy bands have also been obtained as special cases of our generalized formulations under certain limiting conditions. In addition, the theoretical results are in quantitative agreement with the experimental data.     

Structural properties of ZnS/GaAs epilayers grown by atomic-layer epitaxy

ZnS epilayers were grown on (1 0 0) semi-insulating GaAs substrates using an atmospheric pressure metal-organic chemical vapor deposition (CVD) technique under the atomic-layer epitaxy (ALE) mode. Atomic force microscopy (AFM) and photoluminescence (PL) measurements were carried out to find the effect of the II–VI ratio of the 30-nm thick ZnS epilayers and to investigate the thickness-dependent characteristics of ZnS epilayers with the thicknesses of 30 and 100 nm. While the II–VI ratio-dependent ZnS quality is consistent regardless of the measurement, the thickness-dependent epilayer quality is quite contrary depending on the measurement. This difference demonstrates the non-uniform distribution of the strain–relaxation in the ZnS epilayer along the depth.     

Powder Pattern Hyperfine Spectroscopy of Shallow- Donor Muonium Centres

The hyperfine spectroscopy of muonium in II–VI semiconductors is reviewed, suggesting that whereas hydrogen is a deep-level defect in ZnS, ZnSe and ZnTe, it constitutes a shallow donor in ZnO, CdS, CdSe and CdTe. Shallow and deep states coexist in CdTe. Using new data for ZnO, it is shown that the principal values of the muonium hyperfine tensor may be obtained with equal facility from measurements in longitudinal or in transverse magnetic field, and from samples that are polycrystalline powders or single crystals. Spin density on the central muon in the shallow states correlates with the electron binding energy or donor depth. This revised version was published online in September 2006 with corrections to the Cover Date.