GaAs System Research Articles

Fabrication and Characterization of InP-Based Quantum Cascade Distributed Feedback Lasers with Inductively Coupled Plasma Etched Lateral Gratings

We present details of the fabrication and operating characteristics of quantum cascade distributed feedback lasers with lateral gratings. These devices emit light with a wavelength of ∼10 µm and operate with pulsed drive current above room temperature. InP-based material offers significant advantages over the GaAs system for mid-infrared quantum cascade lasers. High performance, single-mode lasers are achieved using InP-based material grown by metal organic vapor phase epitaxy and utilising double-sided lateral gratings. The deeply etched gratings were made possible by the development of a high aspect ratio, multi-stage, inductively coupled plasma (ICP) etch process, using Cl2/Ar and SiCl4/Ar gas mixtures. Threshold current density was measured to be ∼5.5 kA/cm2 at a temperature of 293 K. Side mode suppression ratios >20 dB and a tuning coefficient of -0.067 cm-1 K-1 were observed.

Conduction band offset for Ga0.62In0.38NxAs0.991−xSb0.009∕GaNyAs1−y∕GaAs systems with the ground state transition at 1.5–1.65μm

Conduction band offset for Ga0.62In0.38NxAs0.991−xSb0.009∕GaNyAs1−y∕GaAs systems with different N contents (x=2.2%–3.0% and y=3.1%–4.3% of N) has been investigated by contactless electroreflectance spectroscopy supported by theoretical calculations performed within the effective mass approximation. It has been found that Ga0.62In0.38NxAs0.991−xSb0.009∕GaNyAs1−y quantum wells (QWs) are promising for laser applications from the point of view of carrier confinement since the conduction band offset (QC) for these QWs is between 70% and 75%. In addition, it has been shown that GaNAs∕GaAs interface is type I with QC between 80% and 90%.

Electron correlations in GaAs and AlGaAs superlattices

AbstractWe have calculated the electronic correlation energy and the exchange energy of a system of GaAs lattice using the density functional theory. In 15 different calculations, we are able to find the energy values for different number of Al and Ga atoms. We have performed the calculation of the energy of the Schroedinger equation in three different approximations, namely, the local density approximation (LDA), the generalized gradient approximation with exchange (GGA‐X) and the generalized gradient approximation with exchange and correlations (GGA‐XC). The correlation energy has a peak for the cell which has 9 Ga and 5 Al atoms. In this way, we find that the material for which the composition is in the ratio of Ga:Al = 9:5, has the strongest electron correlations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical designing of semiconductor structure for optothermionic refrigeration

The cooling processes in the system of AlxIn1−xAs∕InP∕AlxIn1−xAs are analyzed with a self-consistent calculation based on the model of optothermionic refrigeration. The refrigeration heat as several W∕cm2 is obtained theoretically with the optimal parameters of the aluminum atom fraction x and the doping density for the system. Due to the small Auger coefficient and the lower carriers’ density in the InP well, the Auger dissipation heat in the present system is smaller than that in the system of AlGaAs∕GaAs∕AlGaAs. Furthermore, the applied bias range for obtaining high cooling heat in the present system is calculated to be larger than that in the system of GaAs. For obtaining larger cooling heat, a double-well system of AlxIn1−xAs∕InP∕AlxIn1−xAs∕InP∕AlxIn1−xAs is designed, in which the maximum cooling heat is predicted as about 50% higher than that in a single-well system of AlxIn1−xAs∕InP∕AlxIn1−xAs.

Fractional quantum Hall effect in graphene

Unlike regular electron spin, the pseudospin degeneracy of Fermi points in graphene does not couple directly to magnetic field. Therefore, graphene provides a natural vehicle to observe the integral and fractional quantum Hall physics in an elusive limit analogous to zero Zeeman splitting in GaAs systems. This limit can exhibit new integral plateaus arising from interactions, large pseudoskyrmions, fractional sequences, even/odd numerator effects, composite-fermion pseudoskyrmions, and a pseudospin-singlet composite-fermion Fermi sea. The Dirac nature of the B=0 spectrum, which induces qualitative changes in the overall spectrum, has no bearing on the fractional quantum Hall effect in the $n=0$ Landau level of graphene. The second Landau level of graphene is predicted to show more robust fractional quantum Hall effect than the second Landau level of GaAs.

Terahertz photoconductivity in GaAs/AlGaAs and HgTe/HgCdTe quantum Hall devices

AbstractWe present measurements of the THz photoconductivity on different quantum Hall systems. GaAs/AlGaAs and HgTe/HgCdTe (MCT) heterostructures with Hall‐bar and Corbino geometry are investigated. A recipe for the preparation of metallic Corbino contacts on MCT is shown. The system is excited by the radiation of a p ‐Ge laser (tunable from 1.7 to 2.5 THz) and the photoresponse (PR) is measured versus the magnetic field B . We observe enhanced PR around integer filling factors (bolometric PR) and cyclotron resonance (CR) effects. Because of the lower effective mass in MCT, the CR in this system appears at a relatively low magnetic field (≈2 T). This is appropriate for possible applications. Finally we present time resolved measurements of the PR on the GaAs system. We find relaxation times from about 10 to over 200 ns, which depend on the geometry, the applied source‐drain voltage and the on mobility of the sample. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fundamental reactions controlling anion exchange during mixed anion heterojunction formation: Chemistry of As-for-Sb and Sb-for-As exchange reactions

As-for-Sb and Sb-for-As anion exchange reactions have been investigated by the exposure of GaSb surfaces to As2 and As4 species and by the exposure of GaAs to Sb2, respectively. The effect of surface temperature, anion soak time, and anion species (either As2 or As4) on the chemistry governing the anion exchange reactions during GaAsySb1−y∕GaSb and GaSbyAs1−y∕GaAs heterostructure formation by molecular beam epitaxy is examined. It is found that when GaSb surfaces are exposed to arsenic, the anion exchange reaction competes with the formation of isoelectronic compounds, AsSby, which form clusters precipitating in the GaAsySb1−y∕GaSb heterostructures. The relative amount of GaAs and AsSby depends on the surface temperature, the As soak time, and on the As species, i.e., As2 or As4. We observe specific process conditions that minimize AsSby formation, yielding more abrupt heterojunction interfaces. In the case of the Sb2∕GaAs system, the Sb-for-As anion exchange does not occur to a significant degree, but surface segregation of antimony is found. A chemical model for the As-for-Sb anion exchange reaction is proposed.

Spin-polarization-induced tenfold magnetoresistivity of highly metallic two-dimensional holes in a narrowGaAsquantum well

We observe that an in-plane magnetic field ($B_{||}$) can induce an order of magnitude enhancement in the low temperature ($T$) resistivity ($\rho$) of metallic 2D holes in a narrow (10nm) GaAs quantum well. Moreover, we show the first observation of saturating behavior of $\rho(B_{||})$ at high $B_{||}$ in GaAs system, which suggests our large positive $\rho(B_{||})$ is due to the spin polarization effect alone. We find that this tenfold increase in $\rho(B_{||})$ even persists deeply into the 2D metallic state with the high $B_{||}$ saturating values of $\rho$ lower than 0.1$\times$h/e$^2$. The dramatic effect of $B_{||}$ we observe on the highly conductive 2D holes (with $B$=0 conductivity as high as 75e$^2$/h) sets strong constraint on models for the spin dependent transport in dilute metallic 2D systems.

Thermodynamic Density of States of Two-Dimensional GaAs Systems near the Apparent Metal-Insulator Transition

We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition--a crossover in the sign of deltaR/deltaT with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a tenfold difference in r(s), the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover.

Band gap discontinuity in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum wells with ⩽x<0.06 studied by contactless electroreflectance spectroscopy

Contactless electroreflectance (CER) spectroscopy has been applied to study optical transitions in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum well (QW) with antimony content varying from 0% to 5.4%. CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions have been matched with those obtained from theoretical calculations. It has been determined that the conduction band offset decreases from ∼55% to ∼45% with the increase in Sb content from 0% to 5.4%. This result demonstrates that the band gap discontinuity for Ga0.9In0.1N0.027As0.973−xSbx∕GaAs system can be simply tuned by a change in antimony content.

Magnetic anisotropy of (Ga,Mn)As films grown on Si (001) substrates

We investigated the anisotropic magnetotransport and magnetic properties of (Ga,Mn)As films on GaAs buffer layers grown on Si (001) substrates. The in-plane magnetoresistance (MR) showed similar dependence on the applied magnetic field at 10K for crystallographically equivalent [110] and [1¯10] directions. On the other hand, the in-plane MRs for [110] and [100] directions in an as-grown sample were observed to have slightly different magnetic field dependence, which disappeared after low-temperature annealing. The behavior observed in this experiment was different from that observed from (Ga,Mn)As∕GaAs systems. This difference was probably related to the competition between uniaxial magnetic anisotropy and cubic magnetic anisotropy induced by the antiphase domains observed for our samples grown on Si (001) substrates.

Small-angle impurity scattering and the spin Hall conductivity in two-dimensional semiconductor systems

An arbitrarily small concentration of impurities can affect the spin Hall conductivity in a two-dimensional semiconductor system. We develop a Boltzmann-like equation that can be used for impurity scattering with arbitrary angular dependence, and for arbitrary angular dependence of the spin-orbit field b(k) around the Fermi surface. For a model applicable to a 2D hole system in GaAs, if the impurity scattering is not isotropic, we find that the spin Hall conductivity depends on the derivative of b with respect to the energy and on deviations from a parabolic band structure, as well as on the angular dependence of the scattering. In principle, the resulting spin Hall conductivity can be larger or smaller than the ``intrinsic value'', and can have opposite sign. In the limit of small angle scattering, in a model appropriate for small hole concentrations, where the band is parabolic and b ~ k^3, the spin Hall conductivity has opposite sign from the intrinsic value, and has larger magnitude. Our analysis assumes that the spin-orbit splitting $b$ and the transport scattering rate tau^{-1} are both small compared to the Fermi energy, but the method is valid for for arbitrary value of b*tau.

Spin Hall Effect in Doped Semiconductor Structures

In this Letter we present a microscopic theory of the extrinsic spin Hall effect based on the diagrammatic perturbation theory. Side-jump and skew-scattering contributions are explicitly taken into account to calculate the spin Hall conductivity, and we show that their effects scale as sigma(xy)SJ/sigma(xy)SS approximately (h/tau)/epsilonF, with tau being the transport relaxation time. Motivated by recent experimental work we apply our theory to n- and p-doped 3D and 2D GaAs structures, obtaining sigma(s)/sigma(c) approximately 10(-3)-10(-4), where sigma(s(c)) is the spin Hall (charge) conductivity, which is in reasonable agreement with the recent experimental results of Kato et al. [Science 306, 1910 (2004)] in n-doped 3D GaAs system.

Spin susceptibility of a two-dimensional electron system in GaAs towards the weak interaction region

We determine the spin susceptibility $\chi$ in the weak interaction regime of a tunable, high quality, two-dimensional electron system in a GaAs/AlGaAs heterostructure. The band structure effects, modifying mass and g-factor, are carefully taken into accounts since they become appreciable for the large electron densities of the weak interaction regime. When properly normalized, $\chi$ decreases monotonically from 3 to 1.1 with increasing density over our experimental range from 0.1 to $4\times10^{11} cm^{-2}$. In the high density limit, $\chi$ tends correctly towards $\chi\to 1$ and compare well with recent theory.

PL Measurements on the Electric Field in a Double Quantum Well Structure of GaAs System

PL Measurements on the Electric Field in a Double Quantum Well Structure of GaAs System

Interaction correction to the longitudinal conductivity and Hall resistivity in high-quality two-dimensional GaAs electron and hole systems

We present a systematic study of the corrections to both the longitudinal conductivity and Hall resistivity due to electron-electron interactions in high quality GaAs systems using the recent theory of Zala et al. [Phys. Rev. B 64, 214204 (2001)]. We demonstrate that the interaction corrections to the longitudinal conductivity and Hall resistivity predicted by the theory are consistent with each other. This suggests that the anomalous metallic drop in resistivity at B=0 is due to interaction effects and supports the theory of Zala et al.

In As Sb ∕ In P quantum dots for midwave infrared emitters: A theoretical study

A theoretical study of the electronic properties of InAsSb quantum dots (QDs) deposited on InP substrate is presented. Unstrained bulk materials present a direct gap between 0.1and 0.35eV suitable for mid-infrared emitters (2–5μm). However, strain and quantum-confinement effects may limit the extension of the emission spectrum of these nanostructures towards the higher wavelengths. Various associations of materials in the barrier are considered. Among the possible associations, InAs0.5Sb0.5∕GaAs0.5Sb0.5 QDs may provide a low-energy emission with a material system similar to the well-known InAs∕GaAs system. Other materials associations such as InAsSb∕InGaAsP∕InP are also studied. Band lineups, optical transitions, optical losses, and effective masses are computed and discussed.

Temperature-Dependent Photoluminescence of Highly Strained InGaAsN/GaAs Quantum Wells (λ= 1.28–1.45 µm) with GaAsP Strain-Compensated Layers

The effects of nitrogen incorporation into the In0.4Ga0.6As1-xNx/GaAs single quantum wells (SQWs), where x = 0.5 and 2%, grown on GaAs substrates by metalorganic chemical vapor deposition (MOCVD) were investigated using photoluminescence (PL) and high-resolution transmission electron microscopy (HRTEM). The evolution of the excitation-dependent PL and PL-peak position with temperature between 10 and 300 K shows that quantum-dot-like states occurred at that high nitrogen incorporation (x = 2%) and were confirmed by an HRTEM image which showed small dark regions about 2–3 nm in size was found in the interface of In0.4Ga0.6As0.98N0.02 and GaAs. Our investigations indicate that high nitrogen incorporation into the In0.4Ga0.6As1-xNx/GaAs system influenced carrier localization and might cause the formation of the dot-like states.

Similarities and differences in two-dimensional metallicity induced by temperature and a parallel magnetic field: Effect of screening

We compare the effects of temperature and parallel magnetic field on the two-dimensional metallic behavior within the unified model of temperature and field dependent effective disorder arising from the screened charged impurity scattering. We find, consistent with experimental observations, that the temperature and field dependence of resistivity should be qualitatively similar in $n\text{\ensuremath{-}}\mathrm{Si}$ MOSFETs and different in $n$-type GaAs two-dimensional (2D) systems, with the $p$-type 2D GaAs system being somewhat intermediate. Based on our calculated results we critically comment on the expected similarities and differences between temperature and field dependent carrier transport properties in various dilute 2D semiconductor systems.

Crack healing during molecular-beam-epitaxy growth of GaP∕GaAs thin films

A crack-healing phenomenon occurring during epitaxial growth of GaP films on a GaAs substrate was studied by transmission electron microscopy. The process is driven by a decrease in the surface energy of the cracked film. The results indicate that the fundamental mechanism operating during healing is the deposition and diffusion of Ga and P atoms onto the crack surface in the GaP lattice, combined with self-diffusion of GaAs within the crack tip in the GaAs substrate. This process is not fully completed in the GaP∕GaAs system; unhealed crack tips located in the GaAs substrate always remain in the structure. Development of cracks and subsequent crack healing during film growth lead to a decrease in residual stress in the film. New cracks are formed at an equilibrium spacing which increases with increasing film thickness. A modified expression for predicting the relation between crack spacing and film thickness in epitaxial films is proposed.