Effect of lattice deformation and phase transitions on the electronic spectra of TlGaS<Subscript>2</Subscript>, TlGaSe<Subscript>2</Subscript>, and TlInS<Subscript>2</Subscript> layered semiconductors

The effect of the lattice deformation on the electronic spectra of TlGaS₂, TlGaSe₂, and TlInS₂ layered semiconductor crystals is analyzed. It is shown that changes in the band gap of these semiconductors due to thermal expansion and a change in the composition under hydrostatic or uniaxial pressure can be described within a unified model of the deformation potential. The main feature of this model is the inclusion of deformation potentials with different signs, which is characteristic of other semiconductors with a layered structure. An analysis of the lattice deformation of the studied semiconductors in terms of the proposed model of the deformation potential has revealed that, in the immediate vicinity of the phase transitions, the crystal lattice under pressure undergo an unusual deformation.     

Use of the integer and non-integer n-dimensional Debye functions in computing thermal expansivity,with applications to Si and Ge semiconductors

In the present study, a novel method is proposed to accurately calculate the thermal expansivity of semiconductors using the analytical expression obtained for the integer and non-integer n-dimensional Debye functions. The proposed approach is novel as it uses the non-integer n-dimensional Debye functions for the accurate calculations of the thermal expansivities of semiconductors. As an example of the application, the calculation is performed for the thermal expansivities of the Si and Ge semiconductors. Analysis using computer simulation proved that the formula was in excellent agreement with the results reported in the literature.     

Raman scattering in ferromagnetic semiconductors under crossed fields

The influence of crossed electric and magnetic fields on the intraband resonant contribution to Raman scattering is discussed. It is shown that in the presence of these fields, the Raman efficiency for one magnon process in ferromagnetic semiconductors exhibits two resonant peaks which are dependent on the ratio βE/Hβ.     

An analysis of phonon emission as controlled by the combined interaction with the acoustic and piezoelectric phonons in a degenerate III–V compound semiconductor using an approximated Fermi–Dirac distribution at low lattice temperatures

Compound semiconductors being piezoelectric in nature, the intrinsic thermal vibration of the lattice atoms at any temperature gives rise to an additional potential field that perturbs the periodic potential field of the atoms. This is over and above the intrinsic deformation acoustic potential field which is always produced in every material. The scattering of the electrons through the piezoelectric perturbing potential is important in all compound semiconductors, particularly at the low lattice temperatures. Thus, the electrical transport in such materials is principally controlled by the combined interaction of the electrons with the deformation potential acoustic and piezoelectric phonons at low lattice temperatures. The study here, deals with the problem of phonon growth characteristics, considering the combined scattering of the non-equilibrium electrons in compound semiconductors, at low lattice temperatures. Beside degeneracy, other low temperature features, like the inelasticity of the electron–phonon collisions, and the full form of the phonon distribution have been duly considered. The distribution function of the degenerate ensemble of carriers, as given by the heated Fermi–Dirac function, has been approximated by a simplified, well-tested model. The model which has been proposed earlier, makes it much easier to carry out analytically the integrations without usual oversimplified approximations.     

Application of optical parametric oscillators to photoacoustic studies in semiconductors

The results of the application of an optical parametric oscillator to photoacoustic studies of semiconductors are reported. The investigation of the photoacoustic signal, waveform and amplitude in Ge, Si, GaAs, GaSb in the 450–1770 nm range allows to make conclusions about the contribution of thermoelastic and deformation mechanisms to pressure pulse formation. At shorter wavelengths, the usual thermoelastic mechanism of acoustic-signal generation is prevailing. At longer wavelengths, for photon energies close to the energy gap, the efficiency of pressure generation in semiconductors with negative deformation potential (Ge, GaAs, GaSb) grows up sharply because of the transition to volume absorption and turning on the deformation mechanism. The experimentally measured values of the photoacoustic pressure are in good agreement with the result of quantitative estimations.     

Electron–phonon-induced spin relaxation in InAs quantum dots

We have calculated spin-relaxation rates in parabolic quantum dots due to the phonon modulation of the spin–orbit interaction in the presence of an external magnetic field. Both deformation potential and piezoelectric electron–phonon coupling mechanisms are included within the Pavlov–Firsov spin–phonon Hamiltonian. Our results have demonstrated that, in narrow gap materials, the electron–phonon deformation potential and piezoelectric coupling give comparable contributions to spin-relaxation processes. For large dots, the deformation potential interaction becomes dominant. This behavior is not observed in wide or intermediate gap semiconductors, where the piezoelectric coupling, in general, governs the spin-relaxation processes. We have also demonstrated that spin-relaxation rates are particularly sensitive to the Landé g-factor.     

The effect of additional Si and SiGe layers on the confinement potential of GeMn diluted ferromagnetic semiconductor

In this work we analyze the spin-polarized charge density distribution in the GeMn diluted ferromagnetic semiconductors (DFS). The calculations are performed within a self-consistent k·p method, in which the exchange correlation effects in the local density approximation, as well as the strain effects due to the lattice mismatch, are taken into account. Our findings show that the extra confinement potential provided by the barriers and the variation of the Mn content in the DFS are responsible for a separation between the different spin charge densities, giving rise to higher mobility spin-polarized currents or high ferromagnetism transition temperatures systems.     

Phonon-assisted Auger recombination in Si with direct calculation of the overlap integrals

The overlap integrals are calculated for Si by means of the full zone double group k · p-method whereas they are usually roughly estimated. The results confirm that phonon-assisted Auger recombination is the predominating radiationless recombination mechanism in indirect band gap semiconductors. This holds not only for highly doped materials but also for electron- hole drops.     

Organic photovoltaic materials and devices

Organic photovoltaic solar cells bere an important potential of development in the search for low-cost modules for the production of domestic electricity. We review the principles and techniques needed for their development: organic semiconductors, their transport properties and photophysical characteristics, photovoltaic molecule and polymer structures, device technologies, electrical and optical behaviour of the cells, state of the art, limitations and perspectives. Despite some recent record efficiencies, research on organic solar cells is still in its infancy when stability and efficiency have to be compared with the performances of silicon cells. A nominal 10% solar efficiency is the research target for the next few years. To cite this article: J.-M. Nunzi, C. R. Physique 3 (2002) 523–542.     

Photothermal and photoacoustic effects in semiconductors and semiconductor structures

A theory of photothermal and photoacoustic effects is developed, on which the contactless diagnostics of semiconductors and semiconductor structures are based. Photothermal and photoacoustic effects are characterized quantitatively by the variable temperature of the specimen surface being exposed and by its shift. These quantities are computed in this paper for a homogeneous semiconductor and a semiconductor with a p-n junction with electron transfer processes, heat liberation as a result of thermalization and charge carrier recombination and their passage through the potential barrier as well as nonthermal deformation mechanisms due to nonequilibrium carrier interaction with the lattice in terms of the deformation potential and the reverse piezoeffect taken into account. It is shown that the surface temperature and shift (particularly the phase of these responses) carry information about such semiconductor characteristics as the charge carrier lifetime, the surface recombination rate, the deformation potential constants, the depth of p-n junction location, the height of its potential barrier, etc.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 119–131, June, 1987.     

Local-field corrections to the static dielectric function of semiconductors: A model study

We present results for the macroscopic static dielectric function at small wave vector q for semiconductors, including the local-field corrections (LFCs). We have used the Penn model for our study. Our calculations demonstrate that LFCs depend on the parameters characterizing a semiconductor. Our calculations are in agreement with the calculations based on more detailed band structures.     

Interference effects in the electrical conductivity tensors of doped polar semiconductors

In this paper, we develop a general procedure, based on the Kubo formula, for finding the frequency- and wavevector-dependent electrical conductivity tensor for arbitrary polarizations of the applied electric field. This procedure gives careful consideration to both the electron and the ion contributions to the current density. We find that, in addition to the standard electron and ion contributions to the conductivity tensor, there are contributions arising from the quantum interference between the electrons and the ions. These interference effects, which will affect the infrared absorption of a material, are similar to the interference effects observed by Cerdeira, Fjeldly, and Cardona (Solid State Commun.13 (1973), 325–328; Phys. Rev. B8 (1973), 4734–4745; 9 (1974), 4344–4350) in Raman scattering from p-type Si. We then evaluate these cross terms for simple models of a variety of semiconductors. We find that these interference effects are finite-q effects, with the actual dependence of the cross terms on the wavevector q being sensitive to the symmetry of the crystal. We also find that, in principle, these cross terms may be quite large near the TO-phonon frequency ω_TO. The cross terms are evaluated for both n-type and p-type semiconductors, and it is suggested that they are probably most important for p-type materials. However, we also find that the basic structure of these terms is very similar in the two cases.     

Effect of an intense laser field on the magnon damping in free carrier magnetic semiconductors

The influence of an intense radiation fieldE₀ on the magnon damping in free carrier magnetic semiconductors is discussed. It is shown that the effect of the intense field is to give a drift velocity v₀ = eE₀/mΩ to the carriers such that when v₀ exceeds the phase velocity of the spin waves the electrons are more able to emit magnons than they absorb and as a consequence the magnon population grows with time.     

Temperature and magnetic field dependence of the absorption edge and the conduction band width of ferromagnetic semiconductors

We investigate the shift of the absorption edge and the behaviour of the conduction band width of ferromagnetic semiconductors as functions of temperature T and an external field B, respectively. Numerical results are given for EuO and EuS. As a consequence of electron-magnon scattering processes the band width of EuO is enhanced by more than 14% in the temperature region: 40 K…T…80 K. The external field tries to surpress this effect.     

Hall coefficient factor in polar semiconductors

The Hall coefficient factor in polar semiconductors is calculated avoiding any serious approximation in the calculation. Lattice scattering (polar optical, acoustic deformation potential and piezoelectric scattering), combined lattice and ionized impurity scattering, and the effect of a nonparabolic conduction band are considered. Results are given both in the limit of vanishing magnetic field and for arbitrary values of the magnetic field. Furthermore our results are compared with experimental values reported previously.     

Laser-induced conductivity of semiconductors at low temperatures

We consider the negative conductivity of electrons in semiconductors excited by a picosecond laser pulse at low temperatures, due to the inelastic electron-phonon collisions. For the first time, the dependence of the deformation potential on the phonon wave number is taken into account. This dependence significantly changes the region of negative electron conductivity as a function of the phonon temperature. The text was submitted by the authors in English.     

Dependence of vibrational density of states and Raman spectra on size in non-polar and polar nano-crystalline semiconductors

The vibrational density of states (VDOS) of Si, diamond, SiC, and InSb clusters has been calculated using the cluster model for various cluster sizes. The results show that the peak frequency of optical-like bands of VDOSs of non-polar nano-crystalline (NC) semiconductors varies with size, but that of polar NC semiconductors does not vary with size. We attribute the origin of this different behavior of non-polar and polar NC semiconductors to different interactions in the optical-like modes of them. That is, the deformation potentials for the non-polar NC semiconductors and electrostatic potentials for the polar NC semiconductors. According to the amorphous crystal (aC) model, Raman spectra are directly related to VDOS. In this Letter, it is verified that the aC model can be applied to NC semiconductors. Calculated VDOS are compared with observed Raman spectra of corresponding samples, which show agreement.     

Deformation electron-phonon coupling in disordered semiconductors and nanostructures

We study the electron-phonon relaxation (dephasing) rate in disordered semiconductors and low-dimensional structures. The relaxation is determined by the interference of electron scattering via the deformation potential and elastic electron scattering from impurities and defects. We have found that in contrast with the destructive interference in metals, which results in the Pippard ineffectiveness condition for the electron-phonon interaction, the interference in semiconducting structures substantially enhances the effective electron-phonon coupling. The obtained results provide an explanation to energy relaxation in silicon structures.     

Energy of free and bound excitons in GaAs in a magnetic field

We have measured in photoluminescence the energy shift of free, weakly and tightly bound excitons in high purity GaAs in magnetic fields up to 12T. In the weak binding limit we find a surprising result: the mass obtained from the magnetic shift is the electron-heavy hole reduced mass in contrast to the results obtained from k · p calculation in zincblende type semiconductors in zero magnetic field. In the tight binding limit the mass deduced from the magnetic shift is the effective electron mass.     

Experimental determination of the constants of absolute volume deformation potentials at band edges in semiconductors

A method is proposed for determining the constants of absolute volume deformation potentials at edges of the conduction and valence bands in semiconductors. This method is based on (i) the volume-concentration effect, (ii) the concept that the energy of deep-lying strongly localized impurity centers does not depend on the hydrostatic pressure, and (iii) the use of experimental data on the electrical resistivity and Hall coefficient. For Ge, GaAs, InAs, and InSb semiconductors, the constants of absolute volume deformation potentials at edges of the conduction and valence bands are determined from our results and data available in the literature.