Effect In GaAs Research Articles

Femtosecond investigation of the hot-phonon effect in GaAs at room temperature

Slowing down of the hot electron-lattice thermalization by generation of nonequilibrium phonons (hot-phonon effect) is investigated in GaAs at room temperature using high-sensitivity femtosecond single- and double-wavelength absorption saturation techniques. Measurements were performed for different amplitudes of the hot-phonon effect by changing either the individual excess energy of the photoexcited carriers or their density in the range 7\ifmmode\times\else\texttimes\fi{}${10}^{15}$--4\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. After an initial regime dominated by nonequilibrium LO-phonon k-space redistribution, the long-time delay (\ensuremath{\ge}3 ps) electron thermalization is found to occur with a characteristic time of \ensuremath{\sim}1.9 ps, independent of the total energy initially injected into the carriers and essentially reflecting the LO-phonon energy relaxation. This is in agreement with numerical simulations of the coupled carrier-phonon relaxation dynamics, indicating that energy transfers to holes are responsible for this slight reduction of the thermalization time compared to the LO-phonon lifetime (identified with the dephasing time ${\mathit{T}}_{2}$/2\ensuremath{\sim}2.1 ps). \textcopyright{} 1996 The American Physical Society.

Analysis of space-charge wave effects in GaAs:EL2.

We theoretically study the characteristics of low-frequency space-charge waves in semi-insulating GaAs:EL2 crystals. The calculations are based upon the literature data on the EL2 center; they allow for competition of electrons and holes photoexcited in the spectral region (0.8--1.6) \ensuremath{\mu}m and nonlinearity of electron transport due to the field-increased capture cross section. The obtained characteristics define the enhancement factor of the photorefractive response as a function of applied field and fundamental grating period, and specify the conditions for generation of spatial subharmonics. The main conclusion is that for n-type GaAs one can expect well pronounced space-charge wave effects in the region of moderate (\ensuremath{\lesssim}1 kV/cm) applied fields. We also analyze the experimental conditions necessary to detect these effects. \textcopyright{} 1996 The American Physical Society.

Dose-rate effects in GaAs investigated by discrete pulsed implantation using a focused ion beam

The dependence of the retained lattice damage upon dose rate was investigated by focused ion beam (FIB) implantation of 210 keV Si++ into GaAs at room temperature. The as-implanted and postannealed states were characterized by ion channeling and Hall-effect measurements, respectively. Dose-rate effects arise from stabilizing interactions between populations of defects produced by different ions, and these experiments were designed to probe the time constants of those interactions. In the context of dose-rate experiments, direct-write FIB represents a much more controllable means of implantation over conventional broad beams since the exact timing of dose delivery may be precisely defined and varied. In this work, the final implanted dose was achieved by the successive application of individual flux pulses of constant intensity but of varying duration td and repetition period tr. A consistent trend toward a greater concentration of displaced atoms directly after implantation and a higher sheet resistance after annealing was observed for longer td and for shorter tr. This effect did not manifest itself simply in terms of the average current density, Javg∝td/tr. Furthermore, it was observed on all time scales accessible in this experiment, suggesting that the important self-annealing mechanisms have a wide range of time constants, from less than 1 μs to more than 1 s. A heterogeneous model of damage nucleation is discussed whereby the defect track of an individual ion event self-anneals until it is overlapped by a following event.

Bonding effects in GaAs: measurements of the 222/−2−2−2 Bijvoet ratio

Bonding effects in GaAs: measurements of the 222/−2−2−2 Bijvoet ratio

Weak anti-localisation in 2-D 100-electron gases in GaAS

2-dimensional electrons in GaAs has a strong spin-orbit interaction, which leads to anti-localisation. The origin of this spin-orbit effect in GaAs lies in the band structure, where an intrinsic spin splitting is rooted in the lack of inversion symmetry. The weak localization at small magnetic fields clearly shows this. We have studied this experimentally at various temperatures in 2 different MBE grown GaAlAs/GaAs structures: A triangular 100 oriented quantum well and a symmetrical 100 oriented quantum well.

Selection Rules for Light Scattering by Folded Acoustic Phonons in Superlattices Grown in Arbitrary Directions

The propagation of acoustic waves in superlattices grown in arbitrary directions is considered, and a general formalism for obtaining wave velocities, selection rules and efficiency of light scattering by folded acoustic phonons is presented. Results based on nine different directions of the phonon wavevector are tabulated. Some errors found in the literature are corrected. The piezoelectric contributions to the phonon frequencies and scattering intensities are analysed. It is shown that their influence is the greatest for propagation along [111]. Changes in the elastic stiffness and Pockels coefficients due to the piezoelectric effect in GaAs are shown to be of the order of 0.1% and 1%, respectively.

Raman-scattering probe of anharmonic effects in GaAs

A comparative study of anharmonic effects in various structural forms of GaAs, namely crystalline, disordered and ion-implanted, and pulse laser annealed (PLA), using temperature-dependent Raman scattering, is reported for various phonon modes over the temperature range 10--300 K. The disordered and PLA samples are found to have greater anharmonicity than crystalline GaAs. The localized vibrational mode in PLA GaAs shows shorter relaxation time than the LO-phonon mode.

Hall effect over integral filling factors in GaAs/AlxGa1-xAs quantum wires.

Two-terminal resistance (R) measurements are used to investigate the low-temperature (T1.4 K) Hall effect in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wires with a geometrical width smaller than 400 nm, in order to eliminate the possible invasive influence of (Hall) voltage leads in standard four-terminal (Hall-bar-type) configurations. Experiment shows that although R assumes a temperature- and length-independent value near integral filling factors, i.e., ${\mathit{R}}_{\mathit{x}\mathit{x}}$\ensuremath{\rightarrow}0, the remaining Hall resistance R=${\mathit{R}}_{\mathit{x}\mathit{y}}$ exhibits a finite slope vs B unlike two-dimensional systems. The results suggest that long disordered electronic wires can exhibit an ordinary Hall effect in the vicinity of integral filling factors.

Photorefractive effect in GaAs at low temperature: influence of the metastable state of the EL2 defect

We present here a theoretical and experimental analysis of photorefractive two-beam coupling in undoped GaAs as a function of temperature. Three major features are experimentally observed, firstly, a change of sign of the photorefractive beam coupling gain around 150 K, secondly, an enhancement of the space charge field by a factor 2 compared to the diffusion field and, finally, a strong peak of the absorption grating amplitude around 150 K. A photorefractive model is established that includes the metastable state of the EL2 defect with its optical properties (optical generation and optical recovery). It predicts all observed features correctly and is in good agreement with the experimental data.

Temperature enhancement of the photorefractive effect in GaAs due to the metastable state of the EL2 defect.

We present a theoretical and experimental analysis of photorefractive two-beam coupling in undoped GaAs as a function of temperature. The theoretical treatment includes the metastable state of the EL2 defect with its optical properties (optical generation and optical recovery) in the photorefractive model. Three major features are predicted by this model: First, a change of the sign of the photorefractive beam coupling gain around 150 K; second, an enhancement of the space-charge field by a factor of 2 compared to the diffusion field; and, finally, the appearance of a strong peak due to an absorption grating around 150 K. All these features are actually observed experimentally with a good correlation between experimental data and theoretical simulation.

The Gunn Effect: Instability of the Steady State and Stability of the Solitary Wave in Long Extrinsic Semiconductors

A linear stability analysis of the stationary solution of a one-dimensional drift-diffusion model used to describe the Gunn effect in GaAs is performed. It is shown that for long semiconductor samples under do voltage bias conditions, and small diffusivity, the steady state may lose stability via a Hopf bifurcation. In the limit of infinitely long samples, there is a quasicontinuum of oscillatory modes of the equation linearized about the steady state that a acquire positive real part for voltages larger than a certain critical value. The linear stability of the solitary wave characteristic of the Gunn effect is proved for an idealized electron velocity curve in the zero diffusion limit.

A comparative study of the Kirk effect in GaAs and Si bipolar junction transistors

The Kirk effect in GaAs BJTs has been compared with that of Si BJTs using a PISCES-IIB simulator. The simulation results have shown that, due to the high electron mobility and velocity overshoot effect of GaAs, the Kirk current density of GaAs BJTs is about two times larger than that of comparable Si BJTs. It is shown that the saturation velocity model of the Kirk effect is very accurate for Si BJTs, but not for GaAs BJTs. A modified structure GaAs BJT with n +− n − collector layer, which is generally believed to have a higher Kirk current, was also studied. We have found for that case that a retarding field is formed at the n +− n − interface and that the Kirk current does not increase at all.

Fermi-edge singularities in InGaAs and GaAs quantum wires

Strong Fermi-edge singularities (FES) were observed in the magnetoluminescencc spectra of' InGaAs quantum wires of electronic widths of L ww ≈ 500 Å. These quasi-l-dimensional (1D) many-electron/ one-hole correlation effects cause a large shift in spectral weight from the band-edge to the conduction Fermi energy, when compared with unpatterned (2D) structures. While the FES effects in the InGaAs quantum wires arc associated with spatially direct transitions, we have also observed related effects in GaAs, where a Schottky gate bias is needed to induce the effect due to the dominantly spatially indirect nature of the transitions.

The piezoelectric effect of GaAs used for resonators and resonant sensors

The piezoelectric effect in GaAs is investigated for potential use in sensor and resonator applications. Flexural resonant vibrations of GaAs tuning-fork structures have been excited at 33 kHz, with measured performance in excellent agreement with predictions. Longitudinal vibrations can equally well be activated. High Q-values at atmospheric pressure (Q=570000 at 4 MHz) have been measured for thickness shear vibrations of (100) GaAs wafers, using lateral electric field excitation. The temperature and stress dependences of the resonance frequency are important features when designing GaAs sensors. The temperature dependences of the frequency range from -59 ppm degrees C-1 to -48 ppm degrees C-1, and the stress dependences are at least 200 ppm MPa-1 and 39 ppm MPa-1 for tensile stress of flexural and shear vibrations, respectively. The semiconducting nature of GaAs affects the piezoelectric behaviour only at high temperatures. The successful experiments open up many possibilities of new resonator and resonant sensor concepts.

Novel self-electro-optic device using bulk Franz–Keldysh effects in an n+-GaAlAs-GaAs-Ag asymmetric Fabry–Perot device structure

We report a novel n+-Al0.3Ga0.7As/GaAs/Ag asymmetric Fabry–Perot optical modulator based on bulk Franz–Keldysh (FK) effects in GaAs. The modulator exhibits high on/off contrast (≳120:1) and high reflectivity (33%). Using a pair of these devices in series, we demonstrate a bistable FK self-electro-optic device (FK-SEED) operating at 872.5 nm with on/off contrast ratio of 87:1 and 27% reflectivity under 11-V bias. The negative differential responsivity necessary for SEED operation is provided through a combination of ‘‘normally off’’ mode of the Fabry–Perot and the negative electroabsorption at the wavelength of operation.

Electron and neutron radiation-induced order effect in gallium arsenide

Electron (7 MeV) and neutron (1 MeV equivalent fluence damage in silicon) radiation effects in GaAs grown by the metallorganic chemical vapor deposition method are investigated. One series of samples was intentionally undoped, and another was doped n-type to 2.5*10/sup 15/ Si/cm/sup 3/. The fluences ranged from 10/sup 10/ to 6*10/sup 15/ cm/sup -2/ for electron irradiation and from 10/sup 12/ to 3*10/sup 15/ cm/sup -2/ for fission spectrum neutron irradiation expressed as 1 MeV equivalent fluence in silicon. The radiation damage was characterized by low-temperature photoluminescence (PL) measurements using 1.58 eV laser excitation, deep level transient spectroscopy and transport measurements. The observed decrease of trap concentration accompanied with an increase in PL intensity at lower fluences, an increase in the density of traps at higher fluences, and a fluence-dependent oscillatory PL intensity for acceptor levels indicate radiation-induced order at low fluences following by nonuniform reorganization of defects with radiation in GaAs. >

Pressure and composition effects on the gap properties of AlxGa1−xAs

Effects of pressure and composition on the gap properties of AlxGa1−xAs randomly disordered alloys are investigated theoretically and experimentally. The analogy between pressure and alloying effects is explored. Special attention is given to the direct-to-indirect gap transition region, where the gap sensitivity to both pressure and composition variations is shown to be strongly enhanced. A 30% decrease in luminescence intensity is produced on a x=0.46 sample under applied pressure of 0.7 kbar, while a similar effect in GaAs would require pressures of several tens of kbar. Calculations are performed within the small crystal approach using 64-, 216-, and 1000-atoms basic cluster sizes, with periodic boundary conditions. Convergence of the calculated properties with the cluster size is discussed.

Electric-field-induced exciton-linewidth broadening in short-period GaAs/GaxAl1-xAs superlattices.

We have investigated the Wannier-Stark effect in GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As superlattices under electric fields by photocurrent spectroscopy measurements in the range of temperatures 10--300 K. The linewidth of the 0h Stark-ladder exciton was found to increase significantly along with an increase in peak intensity when the electric field increases. We present a mechanism based on an enhanced interface roughness scattering of electronic states due to Wannier-Stark localization in order to explain this increased broadening with electric field. This electric-field-related scattering mechanism will weaken the negative differential conductance effects in superlattices predicted by Esaki and Tsu.

Phonon Images of Crystals for Different Sources

In our previous papers, we have considered the energy and quasi-momentum focusing patterns [1] (hereafter referred to as I) and the image of crystalline GaAs obtained by the phonon-drag effect in GaAs—AlGaAs heterostuctures [2]. All of these patterns were obtained for sources of pulses of monochromatic phonons. Τhe duration time of these pulses was taken as the shortest of the characteristic times • in the considered problem. However, most experiments are performed not for pulses of monochromatic phonons but for heat pulses. Additionally, the time of duration of pulses may not be the shortest time characterizing the generation, propagation and detection of pulsed phonon beams. Therefore our approach should be generalized. In this paper we shall show that the kinetic approach, based on the use of the Boltzmann equation, can be used for the description of experiments with the use of heat pulses. Since mechanisms for the generation of phonon beams seem not yet properly understood we construct the source terms of the Boltzmann equation using arguments based on dimensional analysis and imposing suitable normalization conditions [1, 2, 3]. Let us briefly describe the experiment performed by Huebener and collaboration [4, 5] and Dietsche et al. [6]. All of these experiments were performed at an ambient temperature Τ much lower than the Debye temperature θD. Thus the thermal phonons were frozen out. Huebener et al. studied the energy focusing patterns of [001] oriented single crystalline Ge [4] and GaAs samples in the form of wafer discs. The diameters of these discs were several times greater than their thickness (1-2 mm). Τhe top

Fermi-edge singularities in InxGa1-xAs and GaAs quantum wires.

Strong Fermi-edge singularities (FES) were observed in the magnetoluminescence spectra of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wires. The distinct shift of spectral weight and peaks at the Fermi energy are strongly temperature dependent, broadening and diminishing in intensity by T=30 K. The FES features shifted diamagnetically in applied magnetic fields, indicative of excitonlike bound states, while the rest of the electron gas was Landau quantized giving also a measure of the typical electronic wire widths of ${\mathit{L}}_{\mathrm{ww}}$\ensuremath{\approxeq}500 \AA{}. While the FES effects in the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wires are associated with spatially direct transitions, aided by hole localization, we have also observed related effects in GaAs, where a Schottky-gate bias is needed to induce the effect due to the dominantly spatially indirect nature of the transitions.