Effect of disorder on direct and indirect band gaps of semiconductor alloys

A simple analysis of the effect of disorder on different band edges of ternary III–V alloys is performed in terms of the symmetry properties of Bloch functions. The effect of disorder on the X₁ conduction band minimum is shown to be strongly dependent on the valence of the alloyed element, in agreement with available experimental data. The implications for indirect gaps and for the calculations of crossover compositions are briefly discussed.     

Multiphoton transitions at the direct and indirect band gaps of gallium phosphide

The non linear photoconductivity of GaP has been studied using three-photon and two-photon excitation by a neodymium and a ruby laser. The non linear cross sections for three- and two-photon transitions corresponding to the direct gap and for two photon-phonon assisted ones have been obtained. The results are compared with theoretical calculations.     

The quasienergy spectrum of semiconductors with direct band gaps in the laser field

Explicit analytical expressions are derived for Floquet states and the quasienergy spectrum of semiconductors with zinc-blende structure and direct band gaps, taking into account the real symmetry properties of the degenerate valence bands.The author would like to thank Academician professor Nguyen Van Hieu for suggesting the problem and for valuable discussions.     

Pressure coefficients of band gaps in semiconductors

A first principles pseudopotential method within the local density approximation is used to calculate the pressure coefficients for band gaps in several diamond and zincblende semiconductors. Despite the underestimation of the band gaps found in almost all local density calculations, the predictions for the linear pressure coefficients and the critical pressures for conduction band inversion are consistent with experiment for all cases considered.     

Diluted II-VI oxide semiconductors with multiple band gaps

We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.     

Excitons in direct band gap cubic semiconductors

Perturbation is applied to study of the Wannier-Mott excitons in direct band gap cubic semiconductors with a fourfold degenerate highest valence band. The fine structure of exciton energy levels is investigated. General formulae are derived for the matrix elements of the perturbation. From these expressions it is straightforward to obtain values of the fine structure splittings of the energy levels and the wave functions of corresponding staes in any order of perturbation theory. A comparison with results of previous works is made.     

Modulation of direct and indirect gaps in piezotransmission experiments

Piezomodulation spectroscopy of direct and indirect transitions in GaAs_1-xP_x is investigated. It is shown that, due to the different values of the deformation potential of the Γ_1c and X_1c minima, the strain induced modulation of these transitions are out of phase.     

Green's function approach in the theory of interaction of exciton-polaritons in semiconductors with direct band gaps

The renormalization procedure of perturbation theory is applied to study polariton effects in the optical processes involving photons with energies near that of an exciton: resonant Raman scattering, resonant electronic Raman scattering, and resonant high — order optical harmonic generation. To demonstrate the general method and the computation technique we consider in this work a simple model with the following properties: (a) The semiconductor has a direct band gap with allowed electrical dipole transition and with nondegenerate valence and conduction bands. (b) We consider only the photons in a definite polarization state; they are in resonance with one bound state of an electron-hole pair, and we take into account only the transition between this exciton state and the photon in the given polarization state. We work in the framework of the Green's function approach and we shall see that this approach provides for the inclusion of damping effects in a relatively rigorous manner.     

Electronic band gaps of semiconductors as influenced by their isotopic composition

The present paper focuses on the renormalization effects of the band gaps in the electronic band structure of the elemental semiconductors traced to zero-point vibrations. Electron-phonon interaction and volume changes (in combination with anharmonicity) are the underlying microscopic mechanisms, both dependent on M^−1/2, M being the average isotopic mass. Thus isotopically controlled crystals offer an extraordinary opportunity to test the theoretical predictions with a variety of spectroscopic techniques. The paper discusses the theoretical predictions and their experimental verifications, exploiting derivative and photoluminescence spectroscopy. Illustrative examples on Si and Ge, drawn from the investigations of the authors, are presented.     

The nature of temperature dependence of energy gaps in layer semiconductors

The temperature dependences of direct and indirect energy gaps in layer semiconductors GaS, GaSe and GaS_xSe_1?x are investigated in the temperature range 5–150 K. The nonmonotonous behaviour of E_g(T) dependences is observed in these crystals. It is shown that the effect of thermal expansion cannot in itself explain the observed anomalies. A new model of electron-phonon interaction explaining the E_g(T) behaviour in layer crystals is proposed.     

Transverse wave band gaps and longitudinal wave band gaps in solid phononic crystals

Based on the properties of transverse (divergenceless) waves and longitudinal (irrotational) waves, we divided the transverse wave modes and longitudinal wave modes from the mixed eigen modes in solid phononic crystals. By investigating the transverse wave and longitudinal wave band structures at low frequency, we found that transverse bands and longitudinal bands exhibit different behaviors in solid systems including spherical scatterers. Phononic crystal with a large density ratio of solid spheres to the background can guarantee both the large longitudinal and large transverse band gap, but solid spheres with a small ratio of longitudinal wave velocity to transverse wave velocity can only help to enlarge the longitudinal band gap, and do not help to enlarge the transverse band gap.     

Effects of finite temperatures,valence bands and energy gaps on the indirect exchange interaction in intrinsic semiconductors

The indirect exchange interaction between localized magnetic moments via virtual excitations from the valence band in intrinsic semiconductors is calculated taking into account the temperature, energy gaps, finite valence bands and effective masses. The inclusion of finite temperature effects changes significantly the phase and magnitude of the oscillations. For small gap semiconductors the oscillatory character and the possibility of ferro-magnetic as well as anti-ferromagnetic coupling are obtained. The exchange interaction oscillates with temperature suggesting interesting applications of this model.     

Recombination in electron-hole droplets in polar and non-polar indirect band gap semiconductors and carrier concentration power law

The dependence of the recombination relaxation rate on the carrier concentration, n, is examined in detail. It is found that in polar indirect band gap semiconductors the phonon-induced recombination rate varies as $\text{n}{}^{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}$ at low temperatures. This is a new power law. On the other hand, in the non-polar materials, the relaxation rate varies as a linear combination of n², $\text{n}{}^{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}$ and $\text{n}{}^{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}$ terms. We find the experimental evidence for the occurence of $\text{n}{}^{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}$ and $\text{n}{}^{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}$ contributions for the first time.     

A shortcut to band renormalization in semiconductors with a finite temperature plasma

It is shown that the band renormalization in semiconductors containing a plasma can be calculated by combining Lindhard's screening theory with electrostatic energy considerations in a two-test-charge model. The calculations for finite temperatures involve two numerical integrations in reciprocal space. The excitonic ground state below the Mott density is also calculated. The results agree well with experiments on GaAs and with calculations based on more elaborate microscopic theories.