Acoustoelectric Domains Research Articles

Nature of damped current oscillations in the formation of a static acoustoelectric domain in a n-InGaAs/GaAs quantum-well heterostructure

It is shown experimentally that reflection of the acoustic flux from the sample borders under the conditions of formation of a static acoustoelectric domain in a n-InGaAs/GaAs quantum-well heterostructure in a lateral electric field substantially affects the duration of the incubation period of domain formation and the shape and amplitude of current oscillations.

Current Oscillations under Lateral Transport in GaAs∕InGaAs Quantum Well Heterostructures

The current-voltage (I–V) characteristics were measured and the current-pulse oscilloscope patterns were obtained for multilayer n-InGaAs/GaAs quantum-well heterostructures and n-GaAs epitaxial layers with various doping levels. It is shown that the I–Vcharacteristics flatten at low doping levels in fields of 300–400 V/cm. In more heavily doped samples, current oscillations are initiated with a period corresponding to a drift velocity of (3–3.5)×105 cm/s at E ‖ [110] and are approximately larger at E ‖ [100] by a factor of 1.5. The results obtained are attributed to the formation of, respectively, stationary and moving acoustoelectric domains in the structures. In fields above 1.5 kV/cm, high-frequency Gunn oscillations corresponding to a drift velocity of 1.5 × 107 cm/s were observed.

Self-sustained acoustoelectric and photoelectric oscillations in semiconductor–piezoelectric structures

Existence and possibility of self-sustained oscillations of acoustoelectric and photoelectric voltages in layered piezoelectric–semiconductor structures without any negative differential conductivity and acoustoelectric domains are demonstrated experimentally and theoretically. A joint experimental study of acoustoelectric and photoelectric phenomena in the same structure permits to establish similar electronic processes. Experimental and theoretical results show that nonlinear acoustoelectric and photoelectric voltages with electronic relaxation properties and external feedback are necessary for observing the phenomena. The theory presented explains the experimental results obtained. A criterion for excitation in a soft mode and dynamic characteristics of the self-sustained oscillations under various external conditions, as temperature, illumination and humidity are discussed. Periodic and stochastic modes of oscillation are noted. The quasi-harmonic mode of oscillations described by the Van der Pol equation is very useful and attractive for practical application because of the fact that the oscillation frequency is proportional to the square root of the relaxation time of electronic processes in semiconductors and that it can be precisely measured and digitized. Due to the precise method presented for measuring the relaxation time, new fundamental possible applications to methods of investigation of physical and chemical sensors with a frequency output are noted.

Acoustoelectric domain in piezoelectric semiconductors: Nucleation and properties

A model is proposed for the onset and development of an acoustoelectric domain in piezoelectric semiconductors as the acoustic instability grows. The model not only gives a qualitative description of all the basic nontrivial features of the development of the domain but also permits quantitative estimates of its parameters: the amplitude and duration. The fact that the estimates are close to the experimental values is a testament to the correctness of the model.

Acousto-optic modulation in diffusive semiconductors

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analyzed in a strain-dependent semiconductor crystal. The effect of the diffusion of charge carriers due to the doping of the medium has been investigated using the coupled-mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear current density of the medium. The AO modulation process has been treated as a four-wave parametric mixing process and the effective third-order acousto-optic susceptibility characterizing the instability process has been deduced. The AO modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold characteristics and the steady state growth rates are estimated from the acousto-optic polarization of the medium. Analytical estimation reveals that the modulated beam can be amplified in a dispersionless acoustic wave interaction regime in the presence of enhanced diffusion due to excess charge carriers. The relative magnitude of the pump field at various doping levels exhibits entirely different steady-state gain characteristics. A particular pump field Er exhibits maximum gain in the lightly doped regime while the same field exhibits a minimum due to reverse energy flow from electromagnetic fields to the collision dominated space-charge field in a heavily doped medium. At very high densities with the electron plasma frequency of the medium close to the pump frequency or under the influence of very high (drift) pump field the acoustoelectric domains are washed out due to domination of the drift process over diffusion instability process. This leads either to gain saturation in lightly doped regime or explosive increment of the gain constant in heavily doped regime. The magnitude of the third-order nonlinear optical susceptibility for III-V semiconductors obtained from our theoretical analyses is found to agree well with the previously reported values.

Acoustic Domain Injection Method and Resonant Brillouin Scattering

Acoustoelectric domains consisting of high intensity ultrasonic waves with wide frequency components of 0.1 to 4 GHz are amplified in semiconducting CdS and injected to other semiconductors. The transmission efficiency was found to be 5–90% depending upon the frequency. By making use of the acoustic domain injection method, we measured some acoustic properties: acoustic domain transmission efficiency and attenuation coefficient. Further, we investigated the resonant Brillouin scattering of various semiconductors and determined the spectral dependence of the Brillouin scattering cross sections near the fundamental absorption edge.

Note on the Selection Rule of Resonant Brillouin Scattering in CdS at Room Temperature

The resonant Brillouin scattering has been investinated in CdS at room temperature by using monochromatic acoustic waves generated by ZnO thin film transducer. Resonant cancellation and enhancement are observed in the allowed configuration, whereas no signals are found in the forbidden configuration in contrast to the results observed by the acoustoelectric domains.

The growth of wave discontinuities in piezoelectric semiconductors

The reference coordinate description of the general nonlinear differential equations describing the interaction of finitely deformable, polarizable, n-type semiconductors with the quasistatic electric field is applied in the study of acceleration waves in piezoelectric semiconductors. As a consequence, the mechanical and dielectric nonlinearities are included in the treatment as well as the semiconduction nonlinearity. The general equation for the propagation velocity of the disturbance is obtained as a function of the state of the material immediately ahead of the wavefront. In the special case of plane waves entering a homogeneous steady state, the growth equation for the amplitude of the acceleration wave is determined and, of course, the propagation velocity and coefficients in the growth equation depend on the propagation direction, but otherwise are constant. The relation between acceleration waves and the formation and propagation of acoustoelectric domains is indicated. The solutions of the growth equation indicate the formation of a shock in a finite time for conditions conducive to domain formation except in certain unusual cases possibly occurring with purely transverse acceleration waves. In the course of the treatment the condition for the threshold field for domain formation is determined under quite general circumstances. When the electrical conduction equation, which can be quite general in this treatment, is specialized to the simple form usually employed for anisotropic semiconductors, the aforementioned more general condition reduces to the anisotropic generalization of the well-known elementary result. In addition, the behavior of weak waves is discussed.

Resonant Brillouin Scattering by LA Phonons in CdS

Resonant Brillouin scattering by longitudinal acoustic phonons has been observed for the first time in CdS by using mode converted phonons generated by partial reflection of acoustoelectric domains. Analysis based upon resonant Brillouin scattering theory and piezobirefringence theory shows a good agreement with the present observation, and predicts a weak cancellation at 750 nm.

A lattice attenuation in CdS measured by the ultrasonic injection method

This paper concerns the use of Brillouin scattering measurements to study lattice attenuation in semiconducting CdS crystals. Measured acoustic fluxes are produced by the acoustoelectric domains. In order to measure the attenuation constant, an ultrasonic injection method is applied. A sample is divided into two parts; one part is used as a generation region of the acoustic flux [region (1)] and the other part is used as a propagation region of the injected flux [region (2)]. The acoustic attenuations of various frequencies are measured at region (2). The lattice attenuation is greatly affected by the rise time of an applied pulse in region (1). It is clear that acoustic flux in the acoustic domain which originates from the amplification of the pure thermally excited acoustic flux attenuates as f2 and is in accordance with the Akhieser loss. On the other hand, a shock-excited acoustic flux attenuates as f∼1.5.

Acoustoelectric domain induced transparency in tellurium at 11 μm

A drastic increase of transmission was observed in tellurium at wavelengths near 11 microm for a polarization E ||C, if an acoustoelectric domain is present. This behavior is opposite to the observations made with other semiconductors. It is assumed that the absorption, which is caused by an intervalence band transition, decreases due to carrier heating and increased damping in the field of the acoustoelectric domains. Other effects, such as the Franz-Keldysh-type optical transitions, seem to be of much less importance. From comparison of the experimental results with numerical data of the p-band absorption a carrier temperature of 2000 K and damping of 81 meV are estimated.

Acoustoelectric domain induced transparency of tellurium near 11 micron

In the presence of acoustoelectric domains a drastic decrease of the intervalence band absorption ( p-band) was observed in tellurium. This behaviour is contrary to the increase measured at the long wavelength tail of the fundamental absorption in various semiconductors. The new results are explained by a carrier heating up to 2000 K and an increase of damping in the field of acoustoelectric domains.

Injection of Acoustic Domains into Semiconductors

An analysis of the acoustic wave injection to obtain the transmission and reflection coefficients is given. The coefficients are estimated in the case of a CdS–ZnTe system, where acousto-electric domains excited in CdS are injected into semi-insulating ZnTe. It is found that the injected acoustic waves can be used in various experiments such as the determination of the elastic constants, the attenuation coefficient and the dispersion of the resonant Brillouin scattering cross section. Results of such experiments on the CdS–ZnTe system are reported.

Shock waves and acoustoelectric domains in piezoelectric semiconductors

The theory of shock waves is applied to the one-dimensional version of the general nonlinear electroelastic equations for deformable semiconductors employed in a recent acceleration wave analysis of acoustoelectric domains in piezoelectric semiconductors. Consequently, the mechanical and dielectric nonlinearities are included in the shock wave analysis as well as the semiconduction nonlinearity. Equations are derived for both the propagation velocity and the amplitude of the shock wave as a function of the jumps in certain variables across, and the state of the material immediately ahead of, the wave front. Expressions are obtained for the jumps in other variables across the shock front, such as the electric field, in terms of the amplitude of the shock. Some physical information concerning the evolutionary behavior of certain simplified types of shock is extracted from the rather complicated shock amplitude equation. When the analysis is specialized to the case of infinitesimal shocks, the results reduce to those obtained in the aforementioned analysis of acceleration waves in piezoelectric semiconductors with linear piezoelectric response.

Frequency dependent microwave absorption in acoustoelectric domains in ZnO

Microwave absorption measurements in theX andR† bands were performed in acoustoelectrically excited ZnO crystals at room temperature. In the resistivity range considered a frequency dependence of this absorption was found, confirming the prediction of the theory, that the microwave absorption in acoustoelectrical domains is mainly determined by the diffusion and the dielectric relaxation times. The experimental results were compared with theory, and average microfields of a strength of 15 kV/cm were obtained. These fields are generally high enough to produce excitonic Franz-Keldysh effect in ZnO.

One-dimensional acceleration waves and acoustoelectric domains in piezoelectric semiconductors

The theory of acceleration waves is applied in the analysis of the formation and propagation of acoustoelectric domains in piezoelectric semiconductors. A one-dimensional version of the general nonlinear electroelastic equations for deformable semiconductors recently presented is employed in the analysis. Consequently, the mechanical and dielectric nonlinearities are included in the analysis as well as the semiconduction nonlinearity. Equations are derived for both the propagation velocity and the amplitude of the growing disturbance as a function of the state of the material immediately ahead of the wave front. These rather general results are specialized to the case of a homogeneous steady state ahead of the wave and the condition for the threshold field is determined. In this latter case the wave front propagates with constant velocity and the amplitude equation indicates the formation of a shock in a finite time for conditions conductive to domain formation. When the electrical conduction equation, which can be quite general in this treatment, is specialized to the form usually employed for semiconductors, the aforementioned more general condition for the threshold field reduces to the known result.

Modulation of acoustoelectric domains by intense 1.06-μm laser excitation of electrons in GaAs

Modulation of acoustoelectric domains by intense 1.06-μm laser excitation of electrons in GaAs

Brillouin scattering from acoustoelectric domains in ZnO for various light wavelengths

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A new type of voltage-controlled negative resistance in thin samples of InSb

A new type of voltage-controlled negative resistance is observed in very thin samples of n-InSb in a high electric field in the region of avalanche breakdown (E≳600 V/cm) and a transverse high magnetic field (B≳1 kG) at 77 K. The negative resistance is observed only in thin samples whose thickness is less than about 30 μm regardless of the length. The mechanism of negative resistance seems to be different from proposed effects such as trapping of hot electrons, Gunn effect, and the acoustoelectric domain effect.

Photoconductive probing of acoustoelectric domains with application to optical image scanning

A simple new method is described for probing acoustoelectric domains in n-GaAs through their strong photoconductive response to a focused light beam. This permits a scan of the spatial distribution and propagation of the amplified acoustic flux in the sample. As a further application of the photoconductive response, a narrow propagating domain can be used to reveal an optical image projected onto the sample. The techniques are illustrated here primarily for extrinsic photoexcitation with Nd:YAG laser light.