Ralph Bray

Saturation of impurity photoconductivity in n‐GaAs with intense YAG laser light

We report the observation of saturation of impurity photoconductivity in semiconducting n‐GaAs illuminated by a Q‐switched Nd:YAG laser with light intensity up to 3.5 MW/cm2. From the saturation of conductivity we determined the concentration of defects to be ≈1016 cm−3. These form the well‐known 1.2‐eV absorption band and are presumably Ga vacancies. The curve of photoconductivity versus light intensity can be fitted with a simple model, which yields the electron recombination coefficient. The relation of the absorption band to nonlinear optical studies in GaAs at 1.06 μ is reviewed.

Optical Probing of Inhomogeneities in n‐GaAs with Applications to the Acoustoelectric Instabilities

A study is presented of the intimate relationship between current instabilities of acoustoelectric origin in n‐GaAs at 77°K and the inhomogeneity in the Ohmic resistivity of the samples. In order to correlate various aspects of the acoustoelectric instabilities with the sample inhomogeneities, two optical probing techniques were developed for determining the Ohmic‐resistivity profile with good spatial resolution, ⪝0.1 mm. One involves the measurement of the local photoconductance in the sample, which is related to the local dark resistivity; the other method involves the measurement of the change in optical transmission produced by the local thermal shift in the intrinsic absorption edge when the sample is heated by a high current pulse. The change in optical transmission is related to the local resistivity. The acoustic flux distribution in the sample is determined by yet another optical probe, utilizing the modulation in transmission produced by the high acoustic‐energy density in the domain. Several as...

Optical and magnetic surface studies on germanium

GOLD-DOPED germanium at liquid nitrogen temperature has been very useful for studying surface behavior.(l) The high bulk resistivity of the material ~10s Q cm at 85”K, permits sensitive detection of surface conductive and galvanomagnetic properties. At the low temperature, communication between the “bulk” and “slow” surface states becomes so ineffective that changes in surface potential, induced by external fieldeffect, or illumination are very stable. This makes possible detailed measurements of surface properties at various values of the surface potential. Illumination(s) of the surface of the samples at 80°K with intense white light produces, in addition to the usual fast bulk photoconductivity, a very slowly decaying component which may be called a retentive conductance, G,. An appreciable fraction of this conductance remains even several days after the light is shut off. Gr may be built up to values several orders of magnitude higher than the initial dark conductance, Go. G, depends on the integrated flux which has fallen on the sample, and may be built up for hours without

ERRATA: Modulation of Optical Absorption at the Intrinsic Edge by Acoustoelectric Domains in n‐GaAs

The enhancement of optical absorption by propagating acoustoelectric domains is observed in n‐GaAs at 77°K. The spectral dependence of this absorption corresponds to an exponential broadening of the intrinsic absorption edge. Several possible mechanisms for this effect are considered in terms of the electric field and the acoustic flux present in the domain.

RESONANT AND ROUND‐TRIP GAIN FOR ACOUSTOELECTRIC DOMAINS IN n‐InSb

Resonant and round‐trip acoustoelectric gain are attainable in the high‐mobility, piezoelectric semiconductor n‐InSb, upon application of transverse magnetic fields. These features are demonstrated in the context of acoustoelectric domain formation and acoustoelectric instabilities. The elucidation of the basic features is greatly simplified by the use of constant‐current pulses, which eliminate oscillatory effects.

The role of reflectivity in the scattering of light from surface ripples

Abstract Some earlier theories of scattering of light from surface ripples involved the reflectivity from an unperturbed surface as a fundamental factor in the scattering intensity. This formulation leads to substantial errors in the relative strengths of the Stokes and anti-Stokes intensities, and in the dependence of scattering intensity on polarization of light incident near the Brewster angle. Experimental results are presented to pinpoint the above errors and to demonstrate the validity of new formulations based on correctly considering the optical boundary conditions at a rippled surface.

Collision-narrowing of Raman spectrum for spin-density fluctuations of electrons in n-GaAs

Abstract The inelastic light-scattering spectrum for single-particle excitations in n-GaAs has been found to be considerably narrower than the Gaussian form usually predicted for a Maxwell-Boltzmann energy distribution. In this paper, we compare the contribution to the narrowing for two factors: collisions and the momentum-dependence of resonant enhancement terms. The analysis is restricted to the spin-density fluctuation mechanism, which has been found to be best suited for probing the electron distribution.

Broadband generation of acoustic surface waves on GaAs by conversion from acoustoelectrically amplified bulk waves

This paper describes the first observation of the generation of surface waves in a broad frequency band (0.2<f<1.0 GHz) by conversion from bulk acoustic waves. The latter are obtained by acoustoelectric amplification from thermal background in GaAs samples at 300 K. The surface waves are generated in the form of narrow propagating domains. Both group and phase velocity measurements, carried out on the principal crystallographic surfaces, served to identify the surface waves.

Raman probe studies of Nd: YAlG laser generated non-equilibrium excitations in GaAs

Abstract A survey is presented of the non-equilibrium excitations in GaAs that can be probed by Raman scattering. Intense Q-switched Nd:YAlG laser pulses are used both for generating the non-equilibrium excitations at low temperature, and for in-situ Raman scattering probe measurements. Particular emphasis is placed on the search for non-equilibrium, zone-edge slow TA phonons. We discovered that the difference frequency combination spectrum involving slow TA phonons is surprisingly coincident with the acceptor excitation line spectrum for the Zn impurity. The discrimination between the non-equilibrium phonon population and a non-equilibrium hole population on minority acceptors in n-GaAs is discussed.

Linearity test for Brillouin scattering in reflection and transmission modes for intense acoustic flux

Abstract Brillouin scattering intensity provides a linear measure of acoustic intensity for thermal equilibrium phonon populations. We present here a linearity test for scattering from intense acoustic flux such as is generated in acoustoelectrically amplified domains in GaAs and CdS. The test depends on superposition of oppositely propagating domains and does not require independent calibration of acoustic intensity. Two separate light scattering mechanisms were tested, (a) the bulk phonon-induced surface ripple mechanism in reflection, and (b) the bulk elasto-optic mechanism in transmission.