Robert N. Sacks

Heterojunction acoustic charge transport devices on GaAs

We have demonstrated a new type of buried‐channel acoustic charge transport device in which charge is transported in an (Al,Ga)As/GaAs/(Al,Ga)As heterojunction channel. Traveling‐wave potential wells, associated with a surface acoustic wave (SAW) propagating on the 〈100〉 surface of a GaAs crystal, transport electrons at the SAW velocity by means of a large‐signal acoustoelectric interaction. Heterojunction acoustic charge transport (HACT) delay lines have been fabricated, and the transport of charge demonstrated. Charge packets with up to 16×106 electrons/cm were measured in a delay 1.4 μs long. The HACT device is much simpler, the transport channel is more reliably produced (by molecular beam epitaxy or metalorganic chemical vapor deposition), and the device has potential for higher dynamic range when compared to the previously developed acoustic charge transport technology. This new device type is useful for the implementation of high‐speed monolithic signal processors.

Photoreflectance of GaAs and Ga0.82Al0.18As at elevated temperatures up to 600 °C

We report a modulation spectroscopy experiment on GaAs and Ga0.82Al0.18As at elevated temperatures. Using the contactless electromodulation method of photoreflectance, the direct gaps (E0) of these materials have been observed from 77 K to 600 °C. The latter temperature is comparable to molecular beam expitaxy, metalorganic chemical vapor deposition, growth temperatures, etc. Our results are at the highest temperature yet reported for E0 (GaAs) in a reflectance experiment and the first observation of E0 (Ga1−xAlxAs) at elevated temperatures. From the latter, the Varshni coefficients [Physica 34, 149 (1967)] for Ga0.82Al0.18As were determined.

Raman scattering determination of free‐carrier concentration and surface space‐charge layer in 〈100〉 n‐GaAs

We have used Raman scattering as a nondestructive, contactless method for determining not only the free‐carrier concentration N but also the width of the space‐charge layer in 〈100〉 n‐GaAs with 4×1017 cm−3≤N≤1×1019 cm−3 using as an excitation several different wavelengths of an Ar+ laser. By comparing the intensity at different wavelengths of the uncoupled longitudinal optic phonon mode originating in the space‐charge layer with the signal from a piece of undoped 〈100〉 material, it is possible to experimentally evaluate the width of the depletion layer Ls. We find that there is very good agreement between the experimental values and those obtained from a generalized theory for both degenerate and nondegenerate materials. Thus, these experimental results demonstrate that for 〈100〉 III‐V semiconductors, Raman scattering can be used as a contactless method to determine the width of the space‐charge region for carrier concentrations up to 1×1019 cm−3.

Highly doped GaAs:Si by molecular beam epitaxy

Highly doped (N++) GaAs:Si with n up to 1.8×1019 cm−3 has been grown by molecular beam epitaxy at a ‘‘normal’’ growth rate of ∼0.8 m/h−1. These layers have been studied by Raman spectroscopy, van der Pauw–Hall measurements, and capacitance‐voltage plotting. They show no appreciable surface accumulation or diffusion of donors into low‐doped layers grown on top of them, and thus should be suitable as buffer layers for n/N+ devices as well as aiding in the production of low resistivity ohmic contacts. Resistivity of these layers has a lower limit of 4.75×10−4 Ω cm occurring at about 1.2×1019 cm−3. The possibility of this being an intrinsic lower limit to the resistivity of GaAs is discussed.

Miniband dispersion of the confined and unconfined states of coupled multiple quantum wells

Abstract In the photoreflectance spectra at 300K of a series of GaAs/Ga0.82Al0.18As multiple quantum wells with different barrier widths we have observed for the first time evidence for coupling between multiple wells (miniband dispersion) in several confined transitions above the fundamental transition. In addition, a number of unconfined transitions also exhibit coupling effects. Excellent agreement is found between experiment and a Bastard model calculation.

Heterojunction acoustic charge transport device technology

A brief description of the general acoustic charge transport (ACT) device and the operation of ACT devices is given. The application of GaAs-AlGaAs heterojunctions to ACT technology and the design of heterojunction ACT (HACT) devices is discussed. The performance characteristics of experimental HACT devices are presented. It is shown that Nyquist rate bandwidths with rolloffs less than 3dB can be obtained at signal output taps, and that transport currents of 100 mu A can be carried by the SAW with less than 4 mW/ lambda acoustic drive power. In addition, heterojunction FETs (field-effect transistors) with gain up to 10 GHz have been fabricated on HACT substrates, illustrating the compatibility of integrated circuitry with the HACT device substrate.<<ETX>>

Symmetry forbidden LO-phonon Raman scattering in heavily doped 〈1 0 0〉 n-GaAs

Abstract We have investigated symmetry forbidden longitudinal optic Raman for various polarization configurations at 4579A from the 〈1 0 0〉 surface of heavily doped n-GaAs caused by the strong surface-fields and high impurity levels. We have evaluated the magnitude and phase of the coefficient of the electric-field induced term as well as the magnitude of the coefficients of the impurity-induced factor.

A 3.35 microsecond HACT transversal filter

A heterojunction acoustic charge transport (HACT) transversal filter with a 3.35- mu s integration time is reported. This is the longest acoustic charge transport device reported to date. The device operates at an acoustic clock frequency of 144 MHz and at an acoustic power of 100 mW. The output signal is generated by 480 equally weighted taps. The multiple output tap structure results in the device exhibiting 12 dB of gain when embedded in a 50- Omega system. Measurements of the 1-dB compression point, the third-order intercept, and the noise floor show the dynamic range of the device to be in excess of 80 dB over the 300-kHz bandwidth of the filter.<<ETX>>

The Relaxation of In x Ga 1-x As/GaAs Strained Multilayers

In this paper we report on the kinetics of strain relaxation in GaAs/In x Ga 1-x As/GaAs/AlAs (0.05 2 areas by plan-view TEM. The thickness of the In x Ga 1-x As layers studied was 36.4 nm and four compositions were chosen so that the critical thickness predicted by strain energy considerations was exceeded. Due, however, to sluggish dislocation nucleation and glide kinetics at the deposition temperature, the as-grown misfit dislocation densities were well below the predicted level for fully relaxed films. We have studied the rate at which these metastable strained films relax as a function of post-growth annealing time and temperature.

Piezospectroscopy of a GaAs/Ga0.73Al0.27As single quantum well structure: Piezoelectric effects

Abstract We have investigated the effects of large uniaxial stress (⇀T) along [100] (≈10 kbar) and [110] (≈16 kbar) on the photoreflectance spectra at 300K of a (001) GaAs/Ga0.73Al0.27As single quantum well structure. For ⇀T‖[100] the stress-induced shifts can be explained using only deformation potential theory while for ⇀T‖[100] there also are effects due to the piezoelectric field generated along the growth direction. However, the magnitude of this field is considerably smaller than predicted by theory, indicating the presence of screening charges. Also the hydrostatic (a) and shear (b) deformation potentials of the direct gap of Ga0.73Al0.27As have been evaluated.