High Purity GaAs Research Articles

Manifestations of classical size effect and electronic viscosity in the magnetoresistance of narrow two-dimensional conductors: Theory and experiment

We develop a classical kinetic theory of magnetotransport of 2D electrons in narrow channels with partly diffusive boundary scattering and apply it to description of magnetoresistance measured in the temperature interval 4.2-30 K in long mesoscopic bars fabricated from high-purity GaAs quantum well structures. Both experiment and theory demonstrate a number of characteristic features in the longitudinal and Hall resistances caused by the size effect in two dimensions owing to the high ballisticity of the transport. In addition to the features described previously, we also reveal a change in the slope of the first derivative of magnetoresistance when the cyclotron orbit diameter equals to half of the channel width. These features are suppressed with increasing temperature as a result of the electronic viscosity due to electron-electron interaction. By comparing theory and experiment, we determine the characteristic time of relaxation of angular distribution of electrons caused by electron-electron scattering.

Fast-Neutron Detectors Based on Surface-Barrier GaAs Sensors with an Ultrahigh-Molecular-Weight Polyethylene Converter

Surface-barrier structures based on high-purity epitaxial GaAs layers with an ultrahigh-molecular-weight polyethylene converter were tested as fast-neutron detectors. The α-particle spectra and the response to fast neutrons were measured under exposure to a 241Am−Be source. The detectors demonstrated a high neutron-detection efficiency (1.22 × 10−3 counts/neutron) and an acceptable ratio of the signal to γ‑ray background (approximately 40). The possibility of using such detectors at temperatures as high as 120°C was shown.

Detectors on the Basis of High-Purity Epitaxial GaAs Layers for Spectrometry of X and Gamma Rays

The characteristics of detectors of soft-X and γ rays based on high-purity epitaxial GaAs layers are discussed. The characteristics of detectors with different rectifying contacts are compared, that is, those with a Schottky barrier and a p–n junction. The spectral characteristics of the manufactured detectors that were obtained under the irradiation by 57Co and 241Am sources at different bias voltages and in a photovoltaic mode and the simulation results using the Geant 3.21 software package are presented.

Giant permanent dipole moment of two-dimensional excitons bound to a single stacking fault

We investigate the magneto-optical properties of excitons bound to single stacking faults in high-purity GaAs. We find that the two-dimensional stacking fault potential binds an exciton composed of an electron and a heavy-hole, and confirm a vanishing in-plane hole $g$-factor, consistent with the atomic-scale symmetry of the system. The unprecedented homogeneity of the stacking-fault potential leads to ultra-narrow photoluminescence emission lines (with full-width at half maximum ${\lesssim 80~\mu \text{eV} }$) and reveals a large magnetic non-reciprocity effect that originates from the magneto-Stark effect for mobile excitons. These measurements unambiguously determine the direction and magnitude of the giant electric dipole moment (${\gtrsim e \cdot 10~\text{nm}}$) of the stacking-fault exciton, making stacking faults a promising new platform to study interacting excitonic gases.

GaAs detectors with an ultra-thin Schottky contact for spectrometry of charged particles

For the first time, samples of particle detectors based on high-purity GaAs epilayers with an active area of 25 and 80mm2 and an ultra-thin Pt Schottky barrier were fabricated for use in the spectrometry of charged particles and their operating characteristics were studied. The obtained FWHM of 14.2 (for 25mm2 detector) and 15.5keV (for 80mm2 detector) on the 5.499MeV line of 238Pu is at the level of silicon spectrometric detectors. It was found that the main component that determines the energy resolution of the detector is a fluctuation in the number of collected electron–hole pairs. This allows us to state that the obtained energy resolution is close to the limit for VPE GaAs.

Dimensional crossover of free exciton diffusion in etched GaAs wire structures

We report on low-temperature spatially resolved photoluminescence spectroscopy to study the diffusion of free excitons in etched wire structures of high-purity GaAs. We assess the stationary diffusion profiles by the free exciton second LO-phonon replica to circumvent the inherent interpretation ambiguities of the previously investigated free exciton zero-phonon line. Moreover, strictly resonant optical excitation prevents the distortion of the diffusion profiles due to local heating in the carrier system. We observe a dimensional crossover from 2D to 1D exciton diffusion when the lateral wire width falls below the diffusion length.

Correct determination of low-temperature free-exciton diffusion profiles in GaAs

We report on low-temperature spatially resolved photoluminescence (SRPL) experiments to study the diffusion of free excitons in a $1.5\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-thick layer of high-purity epitaxial GaAs. Extending previous SRPL experiments, we analyze the stationary diffusion profiles detected on the second LO-phonon replica of the free exciton. This allows us to circumvent the inherent interpretation ambiguities of the free-exciton zero-phonon line. Moreover, a spatially resolved line shape analysis of the $(FX)\ensuremath{-}2\ensuremath{\hbar}{\mathrm{\ensuremath{\Omega}}}_{\text{LO}}$ replica provides direct experimental access to the pump-induced exciton temperature profile. We demonstrate that only resonant optical excitation prevents the buildup of a temperature gradient in the carrier system, which otherwise severely distorts the stationary and time-resolved free-exciton diffusion profiles.

Spatially Resolved Thermodynamics of the Partially Ionized Exciton Gas in GaAs.

We report on the observation of macroscopic free exciton photoluminescence (PL) rings that appear in spatially resolved PL images obtained on a high purity GaAs sample. We demonstrate that a spatial temperature gradient in the photocarrier system, which is due to nonresonant optical excitation, locally modifies the population balance between free excitons and the uncorrelated electron-hole plasma described by the Saha equation and accounts for the experimentally observed nontrivial PL profiles. The exciton ring formation is a particularly instructive manifestation of the spatially dependent thermodynamics of a partially ionized exciton gas in a bulk semiconductor.

Acoustic measurements of the stripe and the bubble quantum Hall phase

We launch surface acoustic waves (SAW) along both the and the directions of a Hall bar and measure the anisotropic conductivity in a high purity GaAs two-dimensional electron system in the quantum Hall regime of the stripe and the bubble phases. In the anisotropic stripe phase, SAW propagating along the ‘easy’ direction sense a compressible behavior (finite resistance) which is seen in standard transport measurement only if current flows along the ‘hard’ direction. In the isotropic bubble phase, the SAW data show compressible behavior in both directions, in marked contrast to the incompressible quantum Hall behavior seen in transport measurements. These results challenge models that assume that both the stripe and the bubble phase consist of incompressible domains and raise important questions about the role of domain boundaries in SAW propagation.

Excitonic Refractive Index Change in High Purity GaAs Modulator at Room Temperature for Optical Fiber Communication Network

Excitonic Refractive Index Change in High Purity GaAs Modulator at Room Temperature for Optical Fiber Communication Network

Atomic Force Microscopy Studies on GaAs/In Bilayers Deposited on Si (100)

GaAs/In bilayers were prepared by RF Magnetron Sputtering in an Ar atmosphere on Si (100) substrates using high purity (95.95%) GaAs (100) and In targets. The growth temperatures were 300oC and 580oC for the high purity targets of In and GaAs, respectively. Three samples were prepared: the deposition time (td) for the GaAs layers was fixed to 30 minutes, while varied for the In layers from td = 10, 15, and 20 minutes. The morphological and optical studies of the samples were made by means of Amplitude Modulation Atomic Force Microscopy (AM-AFM). In order to analyze and correlate surface morphology and alloy composition properties, the as-prepared samples were cleaved along the [001] direction and subsequently studied by AM-AFM-micrographs. From topographic images, a statistical study of the roughness and grain size was made. Additionally, cross sectional AM-AFM-micrographs were performed for each sample, where the phase channel, which is sensitive to the material properties of the specimen, was of particular interest.

Fast neutron detector based on surface-barrier VPE GaAs structures

Surface-barrier structures based on thin high-purity GaAs epilayers with a polyethylene converter were studied as fast neutron detectors. A continuous Schottky barrier with a large area of 5 × 5 mm2 to the high-purity GaAs epilayers was fabricated using a Pt/TiN/Au metallization system. Results of measurement of electric parameters and α-particle spectra are presented. The fast neutron spectra from the 241Am-Be source measured by the recoil proton method for various thicknesses of the polyethylene converter and various operating biases are shown as well. The proposed detectors have shown high neutron detection efficiency of 1.30·10−3 puls./neutr. and an acceptable signal-to-background ratio (at level of 50) as well as the capability to operate efficiently without any external bias.

Radiation detectors fabricated on high-purity GaAs epitaxial materials

Epitaxial GaAs material shows a great potential in X-ray spectroscopy and radiography applications due to its high absorption efficiency and low defect density. Fabrication of pixel radiation detectors from high-purtity epitaxial GaAs has been developed further. The process is based on mesa etching for pixellisation and sputtering for metallization. The leakage currents of processed pad detectors are below 10 nA/cm2 at a reverse bias of 100 V and decrease exponentially with the temperature. Measurement with transient current technique (TCT) shows that electrons have a trapping time of 8 ns. Good spectroscopic result were obtained from both a pad detector and a hybridized Medipix GaAs detector.

Electric field effects on exciton in the shape of transmission spectra in high-purity GaAs at room temperature

The electric field effects on the shape of transmission spectra associated with exciton including continuum band transition in ultra-high-purity GaAs have been theoretically studied at room temperature. We have calculated the Stark red shift, linewidth broadening of exciton, height of exciton peak and extinction ratio with applied electric field. The theoretical results are compared with experimental and found the close agreement between them.

Proposed Model of Electric Field Effects in High-Purity GaAs at Room Temperature

We have proposed a new model for the calculation of excitonic electroabsorption based on modified previously reported models for bulk structure. The excitonic absorption spectra in high purity GaAs have been theoretically studied in the presence of electric field at room temperature (RT). The Stark shift, linewidth broadening of exciton and extinction ratio have been calculated as a function of electric field. For the validity of our model we have compared with experimental result.

Effect of electric field on exciton in high‐purity GaAs epilayer measured at room temperature

The excitonic electroabsorption has been investigated for a high-purity GaAs epilayer of 10 µm thickness at room temperature and clear red-shift (≃1.44 nm) of the excitonic absorption peak has been observed. An extinction ratio over 10 dB has been obtained with an applied voltage of 11 V, which is nearly five times larger than the theoretical estimation without considering the exciton. This may be the first observation for of a surface normal structure with a polyaniline as a transparent Schottky electrode. The insertion loss is estimated to be 3 dB. This relative low driving voltage for a bulk configuration without quantum wells is due to high-purity GaAs.

Coherent spin dynamics of donor bound electrons in GaAs

We report experimental studies of coherent spin dynamics of donor-bound electrons in high-purity GaAs by using transient differential transmission. The donor-bound exciton transitions, which are not visible in the linear absorption spectrum, are spectrally resolved in the nonlinear differential transmission spectra. The spin beats in the transient differential transmission response, arising from electron spin precession in an external magnetic field, are investigated with the pump and probe coupling to various donor-bound exciton transitions. The spectral dependence of the spin beats provides important information on the polarization selection rule for the underlying donor-bound exciton transitions. The polarization selection rules deduced from these experiments indicate that contributions from higher-energy donor-bound exciton transitions can severely limit the effectiveness of optical spin control using mechanisms such as polarization-dependent optical Stark shifts.

Ultrahigh-Purity Undoped GaAs Epitaxial Layers Prepared by Liquid Phase Epitaxy

Ultrahigh-purity (UHP) GaAs epilayers have been grown reproducibly by liquid phase epitaxy (LPE) with conventional solution baking in a H2 environment. We have succeeded in achieving a high-purity (HP) n-type GaAs epilayer with a low electron carrier concentration of 5.84×1012 cm-3 and a high mobility of 312,000 cm2 V-1 s-1 at 77 K, which is the highest value ever reported. Photoluminescence (PL) analyses at 15 K for both UHP n-type and p-type GaAs epilayers were also performed. We have clarified that the PL intensity of an HP GaAs epilayer has a strong correlation with the mobility measured by Hall measurements at 77 K. The electron carrier concentration of the UHP GaAs epilayer also agrees very well with that obtained from capacitance–voltage (C–V) measurements. We also showed that the recyclic growth process used in this study effectively improves the purity of the epilayers because the mobility at 77 K is enhanced approximately threefold and the ionized carrier concentration decreases threefold compared with their initial values. We also demonstrated that the recyclic growth process is effective in eliminating impurities such as carbon and silicon by analyzing the PL spectra.

High Purity GaAs Nanowires Free of Planar Defects: Growth and Characterization

We investigate how to tailor the structural, crystallographic and optical properties of GaAs nanowires. Nanowires were grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. A high arsine flow rate, that is, a high ratio of group V to group I I I precursors, imparts significant advantages. It dramatically reduces planar crystallographic defects and reduces intrinsic carbon dopant incorporation. Increasing V/III ratio further, however, instigates nanowire kinking and increases nanowire tapering. By choosing an intermediate V/III ratio we achieve uniform, vertically aligned GaAs nanowires, free of planar crystallographic defects, with excellent optical properties and high purity. These findings will greatly assist the development of future GaAs nanowire-based electronic and optoelectronic devices, and are expected to be more broadly relevant to the rational synthesis of other III-V nanowires.

Characterization of high-purity arsine and gallium arsenide epilayers grown by MOCVD

Impurities present in the metal organic chemical vapor deposition (MOCVD) process gases and precursors can have a significant effect on the performance of III–V compound semiconductor devices. High-purity arsine purified using chemical, adsorption and distillation techniques, has been characterized for impurities by using high sensitivity gas analysis methods and low temperature photoluminescence (PL) of GaAs epilayers. Permanent gas, hydrocarbon and dopant impurities can all be removed using these purification methods to below the detection limit of instrumentation (low nmol mol −1–pmol mol −1, depending on method). Capability to remove water vapor to single digit nmol mol −1 levels is also demonstrated and cylinder depletion studies show that gas-phase arsine, with consistently low H 2O, can be delivered from the cylinder, even well after phase break. Low temperature PL measurements are made on 10 μm GaAs/GaAs grown with three different arsine sources. Well-resolved near-band emission characteristics of high-purity n-type GaAs is obtained with high-purity distilled arsine. PL of epilayers grown with less pure arsine show the presence of Ge as well as elevated levels of Mg and Zn, incorporated from the trimethylgallium. The incorporation of O from an arsine cylinder containing H 2O at 200 nmol mol −1 results in reduced full width at half maximum (FWHM) of the near-band emission and decreased ( D 0, X) and ( F, X) intensity, highlighting the importance of minimizing H 2O impurity.