Non-centrosymmetric Semiconductors Research Articles

Effect of non-centrosymmetry on Stark broadening in bulk indium antimonide crystal

Stark shift and self-energy correction in a non-centrosymmetric semiconductor have been studied by using the time-dependent perturbation technique and considering the diagonal elements of the transition dipole moment operator to be finite. The Stark shift obtainable from the Rabi frequency shows a time dependence. The analysis is applied to a sample of InSb duly irradiated by a near-resonant laser and it is found that the magnitude of Stark shift decreases while the magnitude of self-energy correction increases due to the incorporation of non-centrosymmetry.

Stimulated Raman and Brillouin Scattering Processes in Centrosymmetric Semiconductor Plasmas

A simple analytical treatment based on the hydrodynamical model of plasmas is developed to study both steady-state and transient stimulated Raman and Brillouin scattering processes in centrosymmetric or weakly noncentrosymmetric semiconductors. Gain constants, threshold-pump intensities, and optimum pulse durations for the onset of Raman and Brillouin instabilities are estimated. The qualitative behaviour of transient gain factors is found to be in agreement with the experimental and other theoretical observations. The analysis explains satisfactorily the competition between stimulated Raman and Brillouin processes in the short-and long-pulse duration regimes.

The Study of Dispersion Parameter and Excitation Intensity of Acoustic Wave in Semiconducting Plasma

Dispersive characteristics and pump excitation intensity are analytically investigated in transversely magnetised piezoelectric and deformation potential coupling noncentrosymmetric semiconductor, by assuming that origin of the nonlinear interaction lies in the second-order optical susceptibility arising from the nonlinear induced polarization (current density). By the real part of susceptibility, the dispersive parameter and the threshold value of the pump electric intensity are obtained with wave number, magnetic field, scattered angle. The phenomenon of self-defocusing of the signal in the prevailing case is found to be a consequence of the negative dispersive characteristics exhibited by the acoustic waves.

Two-photon transitions in systems with semiconductor quantum dots.

Optical interband and intraband transitions in semiconductor quantum dots (QD's) are analyzed theoretically. It is found that three-dimensional confinement essentially modifies intraband matrix elements of the electron-photon interaction in the QD's as compared to the bulk materials. This effect is quite important for the multiphoton processes. It is shown that two competitive types of two-photon transitions with different selection rules are in the QD's of noncentrosymmetric semiconductor and amplitudes of these transitions differently depend on light polarization, electron and hole effective masses, and QD radius as well. Analytical expressions for the two-photon absorption coefficient are derived for the strong confinement regime, taking into account the size distribution of the nanocrystals. \textcopyright{} 1996 The American Physical Society.

Modulation instability in optically strained magnetoactive semiconductors

The modulational instability of an intense laser beam is investigated in a doped semiconductor by use of four-wave parametric coupling theory. The crystal is subjected to an intense magnetic field along the direction of the input laser field. The origin of the modulational instability process is assumed to lie in the third-order nonlinear optical susceptibility of the medium. The effects of the optically induced electromechanical and acousto-optical strain are considered. These include piezoelectric and deformation potential coupling in noncentrosymmetric (NCS) semiconductor systems and electrostrictive and acousto-optic strain in centrosymmetric (CS) systems. The threshold intensity required for inciting the instability process and the subsequent modulation amplification process are investigated. The effect of the carrier-density modulation that is due to doping is also considered. The electron collision frequency is also found to modify the output profile of the modulated wave significantly. The enhanced diffraction of the modulated wave that is due to the finite second-order susceptibility retards the amplification process in a NCS medium, resulting in a lower growth rate of the signal as compared with that for a CS medium. The growth rate of the unstable mode can be enhanced by an increase in the carrier density of the doped semiconductor. The external magnetic field also appreciably enhances the growth rate of the signal at lower threshold fields.

Optical second harmonic generation studies of the nature of the oxide-covered and clean c(4 × 4) and (2 × 4) reconstructed GaAs(001) surfaces

We report the application of the “epioptic” technique of optical second harmonic generation (SHG) to the study of surface chemical processes occurring at the technologically important (001) surfaces of the non-centrosymmetric compound semiconductor GaAs. SHG has been used to monitor the thermal removal of surface contamination from an GaAs(001) substrate mounted in a conventional molecular beam epitaxy (MBE) chamber. Rotational anisotropy plots of the reflected SH signal recorded at near-normal incidence indicate that the oxide layer is not fully disordered and possesses some structural anisotropy, i.e. some preferred net surface orientation. In addition the rotational anisotropy plots were observed to change in both intensity and apparent phase between the oxide-covered surface and the clean reconstructed surfaces. Following heating to 850 K under the background As 2 flux present in the chamber, the SH signal from the clean, reconstructed surface was approximately 5 × weaker than that observed for the contaminated surface and was subsequently found to be insensitive to the reconstruction change between the (2 × 4) and c(4 × 4) surfaces, in agreement with theoretical predictions for surfaces having 2mm and 4mm symmetry, respectively. These data are discussed in terms of the difficulties encountered when attempting to observe a surface SH response from a non-centrosymmetric substrate. The sensitivity of the technique towards the thin oxide overlayer suggests that for overlayers which are likely to possess net oriented in-plane dipoles due to the structural mismatch with the substrate, an SH response may be observed in preference to a bulk response when a normal incidence geometry is employed.

Optical second harmonic generation studies of the nature of the oxide-covered and clean c(4 × 4) and (2 × 4) reconstructed GaAs(001) surfaces

We report the application of the “epioptic” technique of optical second harmonic generation (SHG) to the study of surface chemical processes occurring at the technologically important (001) surfaces of the non-centrosymmetric compound semiconductor GaAs. SHG has been used to monitor the thermal removal of surface contamination from an GaAs(001) substrate mounted in a conventional molecular beam epitaxy (MBE) chamber. Rotational anisotropy plots of the reflected SH signal recorded at near-normal incidence indicate that the oxide layer is not fully disordered and possesses some structural anisotropy, i.e. some preferred net surface orientation. In addition the rotational anisotropy plots were observed to change in both intensity and apparent phase between the oxide-covered surface and the clean reconstructed surfaces. Following heating to 850 K under the background As 2 flux present in the chamber, the SH signal from the clean, reconstructed surface was approximately 5 × weaker than that observed for the contaminated surface and was subsequently found to be insensitive to the reconstruction change between the (2 × 4) and c(4 × 4) surfaces, in agreement with theoretical predictions for surfaces having 2mm and 4mm symmetry, respectively. These data are discussed in terms of the difficulties encountered when attempting to observe a surface SH response from a non-centrosymmetric substrate. The sensitivity of the technique towards the thin oxide overlayer suggests that for overlayers which are likely to possess net oriented in-plane dipoles due to the structural mismatch with the substrate, an SH response may be observed in preference to a bulk response when a normal incidence geometry is employed.

On aspects of modulational amplification in noncentrosymmetric and centrosymmetric semiconductors

A comparison of the modulational growth rates arising due to transverse modulational instability has been made in noncentrosymmetric (NCS) and centrosymmetric (CS) semiconductors. The retardation effect due to the noncentrosymmetricity of the medium has been investigated using an electromagnetic approach. The threshold field E 0 th required to obtain a modulational amplification process has also been estimated. Both the modulational gain factor and E 0 th are found to be strongly dependent on the carrier concentration of the medium. The retardation effect due to finite Д (2) in NCS crystals is found to play a more significant role in the case of a lightly doped medium. The modulational gain constant is found to undergo absorption in the strongly dispersive regime of the acoustic mode in the case of NCS crystals, while it grows appreciably under similar conditions in a CS medium.

Bulk and surface second-harmonic generation in noncentrosymmetric semiconductors

The use of optical second-harmonic generation as a surface probe for noncentrosymmetric media is demonstrated. By exploiting the difference in bulk and surface symmetry of a GaAs(001) crystal, separate determination of bulk and surface nonlinear optics responses is possible if suitable polarization directions are selected and sub-bandgap excitation is used. The detection scheme makes use of the interference between bulk and surface SH electric fields, and provides both amplitude and phase information.

Determination of structure factor phase invariants from non-systematic many-beam effects in convergent-beam patterns

The determination of structure factor phase invariants and magnitudes from non-systematic many-beam effects in convergent-beam patterns is discussed. Non-centrosymmetric semiconductors are investigated with emphasis on the phase problem. Two-dimensional simulations using three or more beams show that a three-phase invariant may be determined from each of the numerous line intersections observed provided the three-beam coupling is non-negligible. The use of two-dimensional energy-filtered patterns for intensity registration is suggested, and an example using this technique is given.

Photomechanical vibration of thin crystals of polar semiconductors

It was found that thin crystals of polar (non-centrosymmetric) semiconductors constitute a new type of photo-sensitive system in which incident illumination is converted into mechanical energy: thus, illumination-induced elastic deformation (bending) was observed on thin (00.1) CdS and (111) GaAs crystals; furthermore, by employing chopped light the crystals were excited to their resonant vibration (photomechanical vibration); the dependence of the amplitude of this vibration on the energy of the incident radiation was found to be similar to the dependence of the surface photovoltage on the energy of the incident radiation (surface photovoltage spectrum). The present findings are consistent with a model based on light-induced modulation of the piezoelectric surface stresses.

Surface piezoelectric effect in non-centrosymmetric semiconductors: Cds

It was found that mechanical bending of CdS wafers with the (00.1) orientation causes pronounced changes in the contact potential difference. The changes were of the order of one volt. This effect was attributed to the polarization induced in the depletion layer by the mechanical stress. On this basis a model was developed which accounts for the experimental results.