Bulk modulus and microhardness of tetrahedral semiconductors

Using the plasma oscillations theory of solids, simple relations have been proposed for the calculation of bulk modulus (B) and microhardness (H) of group IV, II-VI, III-V, I-III-VI₂ and II-IV-V₂ semiconductors with tetrahedral structure. We find that B=K₁ (?ω_p)^2.3333 and H=K₂ (?ω_p)^2.3333−K₃, where K₁, K₂ and K₃ are the constants. The numerical values of K₁, K₂ and K₃ are respectively, 0.141, 0.036 and 12.895 for group IV, 0.109, 0.0037 and 0.782 for II-VI, 0.125, 0.0202 and 5.743 for III-V, 0.109, 0.0065 and 1.160 for I-III-VI₂, and 0.125, 0.0359 and 15.310 for II-IV-V₂ semiconductors. The calculated values of B and H are compared with the experimental values and the values reported by different workers. Reasonably good agreement has been obtained between them.     

Microhardness and bulk modulus of binary tetrahedral semiconductors

Using the plasma oscillations theory of solids, the microhardness and bulk modulus of binary tetrahedral semiconductors have been calculated. New relations between plasmon energy and these parameters have been proposed. Our calculated values of microhardness and bulk modulus from two different equations are in excellent agreement with the experimental values and the values reported by other workers. In the present model, no experimental data are required except the plasmon energy, and one can predict the value of these parameters in the case of unknown semiconductors where experimental measurements have not yet been made due to a lack of availability of experimental melting points, dielectric constants or bond lengths.     

Optical properties of the filled tetrahedral semiconductors LiZnX (X=N, P, and As)

We present first principles calculations of the electronic and the optical properties of the filled tetrahedral compounds LiZnN, LiZnP, and LiZnAs performed with the full potential linearized augmented plane wave method within the local density approximation. The origin of the small gap of LiZnN is attributed to the strong p-d coupling in this compound. The assignments of the structures in the optical spectra and band structure transitions are discussed in detail. The predicted values of the dielectric constants for LiZnP and LiZnAs are close to those of the binary compounds GaP and GaAs, respectively.     

Excitonic properties of nanostructure semimagnetic semiconductors

Magneto-optical properties of semimagnetic semiconductors with nanometer-scale structures (nanostructures) are described. Superlattices of the CdTe/Cd_{1 − x}Mn_xTe and Cd_{1 − x}Mn_xTe/ZnTe systems and microcrystals of Cd_{1 − x}Mn_xSe were grown by epitaxy and sputtering methods. These semimagnetic semiconductor nanostructures show remarkably enhanced magneto-optical responses in the optical absorption, the luminescence and the dynamics of the confined excitons. The results are interpreted by the quantum-confined excitonic states interacting with the magnetic ion spins involved in the nanostructures.     

Structural and elastic properties of the filled tetrahedral semiconductors LiZnX (X=N, P, and As)

We report on first-principles study of the structural and elastic properties of the Nowotny-Juza filled tetrahedral compounds LiZnX (X=N, P, As) using the full-potential linearized augmented plane wave method within the local density approximations. Our results indicate that the energetically favourable α-LiZnX materials are slightly softer than their binary analogous GaX and the sound speeds are quantitatively similar for LiZnAs and GaAs.     

Birefringence of optically uni-axial ternary semiconductors

Refractive indices and birefringences for some representative uni-axial crystals such as II- SiP₂ and II- GeP₂ have been determined over a wide range of wavelengths by the use of first-principles electronic structure calculations. First, the calculated refractive indices are fitted usually by a generalized Sellmeier equation which consists of several oscillation terms involve in its parameters more direct information about material such as electronic transitions or resonance wavelengths. Then, in contrast to all other semiconductors under discussion our spectra show a negative birefringence for CdSiP₂ in agreement with the experimental data, and they exhibit a considerable dispersion near the band gap.     

Temperature and pressure behaviour of narrow-gap semiconductors including galena

The structural high pressure and temperature investigation of narrow-gap semiconductors (lead chalcogenides) has been performed in the present article. A realistic approach for room temperature and high temperature study of narrow-gap semiconductors has been used. It is examined that the present compounds are more stable in NaCl-phase and they transform to CsCl-phase at high pressure. In the present article, the phase transition pressures and volume collapses of lead chalcogenides have been investigated at room and high temperatures. Phase transition pressures have been reported at high temperature range from 0 to 1200 K. Elastic and anharmonic constants have also been reported at room temperature. A structural study of the narrow-gap semiconductors have been carried out using the realistic model including temperature effect. The temperature and pressure behaviour of elastic constants for the present compounds have also been discussed. Furthermore, various mechanical and thermo dynamical properties like modulus of elasticity, Debye temperatures etc. are also presented.     

Bond-stretching and bond-bending force constant of binary tetrahedral (AB and AB) semiconductors

In this Letter we present the two expressions relating the bond-stretching force constant (α in N/m) and bond-bending force constant (β in N/m) for the A^IIIB^V and A^IIB^VI semiconductors with the product of ionic charges (Z₁Z₂) and nearest neighbor distance d (Å). Interatomic force constants of these compounds exhibit a linear relationship when plotted on a log-log scale against the nearest neighbor distance d (Å), but fall on different straight lines according to the ionic charge product of the compounds. A fairly good agreement has been found between the observed and calculated values of the α and β for binary tetrahedral semiconductors.     

Electron-phonon interaction in tetrahedral semiconductors

Considerable progress has been made in recent years in the field of ab initio calculations of electronic band structures of semiconductors and insulators. The one-electron states (and the concomitant two-particle excitations) have been obtained without adjustable parameters, with a high degree of reliability. Also, more recently, the electron-hole excitation frequencies responsible for optical spectra have been calculated. These calculations, however, are performed with the constituent atoms fixed in their crystallographic positions and thus neglect the effects of the lattice vibrations (i.e. electron-phonon interaction) which can be rather large, even larger than the error bars assumed for ab initio calculations.Effects of electron-phonon interactions on the band structure can be experimentally investigated in detail by measuring the temperature dependence of energy gaps or critical points (van Hove singularities) of the optical excitation spectra. These studies have been complemented in recent years by observing the dependence of such spectra on isotopic mass whenever different stable isotopes of a given atom are available at affordable prices. In crystals composed of different atoms, the effect of the vibration of each separate atom can thus be investigated by isotopic substitution. Because of the zero-point vibrations, such effects are present even at zero temperature (T=0).In this paper, we discuss state-of-the-art calculations of the dielectric function spectra and compare them with experimental results, with emphasis on the differences introduced by the electron-phonon interaction. The temperature dependence of various optical parameters will be described by means of one or two (in a few cases three) Einstein oscillators, except at the lowest temperatures where the T⁴ law (contrary to the Varshni T² result) will be shown to apply. Increasing an isotopic mass increases the energy gaps, except in the case of monovalent Cu (e.g. CuCl) and possibly Ag (e.g. AgGaS₂). It will be shown that the gaps of tetrahedral materials containing an element of the first row of the periodic table (C,N,O) are strongly affected by the electron-phonon interaction. It will be conjectured that this effect is related to the superconductivity recently observed in heavily boron-doped carbon.     

On the electron energy spectrum in heavily doped non-parabolic semiconductors

An attempt is made to present a simple theoretical analysis of the energy-wave vector dispersion relation of the conduction electrons in heavily doped non-parabolic semiconductors forming band tails. We observe that the complex energy spectrum in doped small-gap materials whose unperturbed conduction band is described by the three band model of Kane is due to the interaction of the impurity atoms in the tail with the spin-orbit splitting constant of the valence band (Δ), For band-gap (E_g)<Δ the imaginary part predominates which tails in to the conduction band. For the opposite inequality the real part comes in to play which tails in to the split-off band. In the absence of the band tailing effect, the imaginary part of the complex energy spectrum vanishes and the same is also true for doped two-band Kane-type and parabolic energy bands respectively. The present formulation helps us in investigating the Boltzmann transport equation dependent transport properties of degenerate semiconductors and are expected to agree better with experiments. The well-known results of unperturbed three and two band models of Kane together with wide-gap parabolic energy bands have been obtained as special cases of our generalized analysis under certain limiting conditions.     

Bondlength distortion of atomic substitutions in semiconductors

The distortion in bondlength is calculated for a wide range of impurities, both isovalent and heterovalent, in all I-VII, II-VI and HI-V compound semiconductors and in elemental semiconductors, silicon and germanium. Universal parameter tight binding (UPTB) theory of Harrison combined with the spring constant model is used. The results agree well with earlier theoretical estimates and also with the available extended X-ray absorption fine structure (EXAFS) data. A systematic behaviour of the bondlength distortion of impurities in semiconductors with respect to (i) the difference in covalent energy of the host-impurity bond, (ii) the difference in electronegativity between the host and that of the impurity atom and (iii) the change in covalency of the bond when the impurity replaces the host is observed.     

Electrical property and phase transition of CdSe under high pressure

In situ electrical resistivity measurement of CdSe was performed under high pressure and moderate temperature using a diamond anvil cell equipped with a microcircuit. With the pressure increasing, a sharp drop in resistivity of over two orders of magnitude was observed at about 2.6 GPa, it was caused by the transition to the rock-salt CdSe. After that, the resistivity decreased linearly with pressure. However, in different pressure range, the decreasing degree was obviously different. This attributed to the different electron structures. By fitting to the curve of pressure dependence of resistivity in different pressure range, the relationship of the band gap to pressure was given and the metallization pressure was speculated to be in the range of 70-100 GPa. The temperature dependence of resistivity showed that in the experimental temperature and pressure range the resistivity had a positive temperature coefficient.     

Computational materials design of filled tetrahedral compound magnetic semiconductors

Based on first-principles calculations within the density functional theory, materials design of filled tetrahedral compound magnetic semiconductors is proposed. By using the Korringa–Kohn–Rostoker coherent potential approximation, electronic structures of Mn-doped LiZnAs, LiZnP and LiZnN are calculated. First, by estimating free energy, phase diagrams of these systems are predicted. It is shown that these systems are phase separating systems and favor spinodal decomposition. However, by introducing Li vacancies, spinodal decomposition is strongly suppressed and Mn can be doped up to high concentration. Moreover, the introduced Li vacancies induce ferromagnetic interaction between Mn and thus we can expect high Curie temperature (T_C) in these systems. To see the chemical trend, electronic structure and T_C of Li(Zn, Cr)As are also calculated.     

Thermal hysteresis and memory effects in TIInS2

The dielectric susceptibility of layered TlInS₂ was studied in the temperature range of successive phase transitions. Thermal hysteresis was observed in the incommensurate phase. It was shown that after annealing the crystal at a fixed temperature within the incommensurate phase, the existing temperature interval of this phase reveals noticeable broadening. The thermal memory effect is discussed using a defect density wave model.     

Structural,elastic and thermodynamic properties under pressure and temperature effects of MgIn2S4 and CdIn2S4

A density functional-based method is used to investigate the structural, elastic and thermodynamic properties of the cubic spinel semiconductors MgIn₂S₄ and CdIn₂S₄ at different pressures and temperatures. Computed ground structural parameters are in good agreement with the available experimental data. Single-crystal elastic parameters are calculated for pressure up to 10 GPa and temperature up to 1200 K. The obtained elastic constants values satisfy the requirement of mechanical stability, indicating that MgIn₂S₄ and CdIn₂S₄ compounds could be stable in the investigated pressure range. Isotropic elastic parameters for ideal polycrystalline MgIn₂S₄ and CdIn₂S₄ aggregates are computed in the framework of the Voigt–Reuss–Hill approximation. Pressure and thermal effects on some macroscopic properties such as lattice constant, volume expansion coefficient and heat capacities are predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account.     

Pure anomalous Hall effect in nonmagnetic zinc-blende semiconductors

We report the experimental observation of the pure anomalous Hall effect (AHE) in nonmagnetic zinc-blende semiconductors without application of the external magnetic fields. The AHE without any contribution from the ordinary Hall current originates from nonequilibrium magnetization induced by spin-polarized electrons generated by the circularly polarized light (σ). We measure the pure AHE as a function of the external bias, crystal temperature and pumping σ-photon energy. The results of their dependences are discussed.     

Time-resolved IR spectroscopy of quantum-optics in semiconductors

Time-resolved IR spectroscopy is a powerful non-destructive technique for probing electron dynamics and plasmonics in semiconductors. We present recent experiments in which intense IR laser pulses are used to induce “quantum-optical” phenomena, including gain without population inversion and slow light, in semiconductor nanostructures. The potential advantages of IR Synchrotron radiation to probe these systems are discussed.