xAs Material Research Articles

Comparison of the magneto-optical properties of the semi-parabolic well with those of the parabolic and rectangular wells under the combined influences of aluminum concentration and hydrostatic pressure

In this paper, we report the comparison of the magneto-optical transport properties of the semi-parabolic potential quantum well with those of the parabolic and rectangular potential quantum wells under the combined influences of the aluminum concentration, the temperature, the hydrostatic pressure, and the Landau levels by using the methods of the quantum field theory for many-particle systems in statistical physics and the profile to calculate analytically the conductivity tensor expression, the magneto-optical absorption power expression of the optically detected magneto-phonon resonance (ODMPR) oscillation, and the full width at half maximum (FWHM) of the ODMPR peak, respectively, in quantum wells with three above different confining potential shapes for typical GaAs/AlxGa1−xAs material. The results obtained from the present study show that (i) the ODMPR peak position undergoes a red-shift as the aluminum concentration and the hydrostatic pressure increase while it experiences a blue-shift as the temperature augments for all three above confining potential shapes; (ii) the FWHM as functions of the barrier's Al-concentration, the system's temperature, and the hydrostatic pressure for all three confining potential shapes is presented to compare; (iii) the dependence of the FWHM on the barrier's Al-concentration and the hydrostatic pressure for the Landau levels for three confining potential shapes also is taken into account in detail; (iv) our present theoretical calculations are found to be consistent with previous experimental calculations.

Density functional theory study on the properties of Cu-doped GaAs

The electronic structure and optical properties of the sphalerite structured CuxGa1−xAs (x = 3.125%, 6.25% and 12.5%) were studied based on the first-principles method of density functional theory. The results show that the CuxGa1−xAs materials exhibit direct band-gaps, from valence band top to guide band low (solid blue line) i.e., 1.30 eV, 0.82 eV, 0.97 eV. It contains impurity levels in the middle, reaching the second transition. The calculated results also show that the energy required for electrons to jump from valence band to impurity levels, i.e., 0.42 eV, 0.36 eV, 0.31 eV. In addition, higher impurity levels resulted from copper doping reduces GaAs band-gaps in a great extent, which is beneficial to the transition and migration of electrons. At the same time, the static dielectric constant and refractive index of the doped system escalate with the increase of Cu doping ratio. Moreover, the absorption spectrum is red-shifted and the absorption of light is enhanced. In the system doped with 6.25% Cu, the electrons in Cu-3d orbital form a strong localization in the energy range of −3.9 eV ~ −1.8 eV. Indeed, the least influence on the complex dielectric function of the [010] is achieved when the ratio of doped Cu decreases to 3.125%, which makes the change of optical properties of the material in this crystal direction smaller than those in [100] and [001] orientations. The CuxGa1−xAs transition layer between GaAs devices and copper wires can substantially improve the electrode contact and subsequently the photoelectric characteristics of GaAs photoelectric devices.

First principles study on the structural stability and optoelectronic properties of InxGa1−xAs materials with different Indium component

The III–V ternary semiconductors InxGa1−xAs (0 ≤ x ≤ 1) have attracted a great deal of interests in electronics and optoelectronics due to their superior transport properties, high-efficiency thermoelectric applications and so on. In this work, the optoelectronic properties of bulk InxGa1−xAs materials and the phase transition of InxGa1−xAs nanowires (NWs) at different Indium (In) component have been investigated by first-principles calculation using density functional theory. Different calculation models have been chosen to simulate the (2 × 2 × 1) supercells of pure bulk InxGa1−xAs structures and the most stable one has been adopted for further calculation. NWs models of In0.25Ga0.75As and In0.75Ga0.25As have been employed to represent the In-rich and Ga-rich structures, respectively. Our calculation results have clearly shown that the In-rich nanostructure show more zinc-blende characteristic while the wurtzite phase exhibit dominating role in the Ga-rich NWs. The band gap of bulk models decreases with increasing the In component, which is well consistent with the theoretical formula. With the In component increasing, the lower valence band shifts toward high energy region and the peak value increases, while the absorption edge and peak value move to lower energy side and the static dielectric constant increases. The metal reflective properties emerge in certain photon energy range. Our results offer the guidance to make use of the different properties of InxGa1−xAs materials at different In component.

Experimental investigation of loss and gain characteristics of an abnormal InxGa1-xAs/GaAs quantum well structure

In this Letter, the loss and gain characteristics of an unconventional InxGa1−xAs/GaAs asymmetrical step well structure consisting of variable indium contents of InxGa1−xAs materials are measured and analyzed for the first time, to the best of our knowledge. This special well structure is formed based on the indium-rich effect from the material growth process. The loss and gain are obtained by optical pumping and photoluminescence (PL) spectrum measurement at dual facets of an edge-emitting device. Unlike conventional quasi-rectangle wells, the asymmetrical step well may lead to a hybrid strain configuration containing both compressive and tensile strains and, thus, special loss and gain characteristics. The results will be very helpful in the development of multiple wavelength InGaAs-based semiconductor lasers.

Material quality frontiers of MOVPE grown AlGaAs for minority carrier devices

In this study, secondary ion mass spectroscopy of oxygen, deep level transient spectroscopy and power dependent relative photoluminescence are compared regarding their ability to resolve differences in AlxGa1−xAs material quality. AlxGa1−xAs samples grown with two different trimethylaluminum sources showing low and high levels of oxygen contamination are compared. As tested in the growth of minority carrier devices, i.e. AlxGa1−xAs solar cells, the two precursors clearly lead to different device characteristics. It is shown that secondary ion mass spectroscopy could not resolve the difference in oxygen concentration, whereas deep level transient spectroscopy and photoluminescence based measurements indicate the influence of the precursor oxygen level on the material quality.

Equilibrium Lattice Relaxation and Misfit Dislocations in Step-Graded In x Ga1−x As/GaAs (001) and In x Al1−x As/GaAs (001) Metamorphic Buffer Layers

The inclusion of metamorphic buffer layers (MBLs) in the design of lattice-mismatched semiconductor heterostructures is important in enhancing reliability and performance of optoelectronic and electronic devices through proper control of threading dislocations; threading dislocation can be reduced by allowing the distribution of the misfit dislocations throughout the MBL, rather than concentrating them at the interface where substrate defects and tangling can pin dislocations or otherwise reduce their mobility. Compositionally graded layers have been particularly used for this purpose and in this work we considered heterostructures involving a step-graded InxGa1−xAs or InxAl1−xAs epitaxial layer on a GaAs (001) substrate. For each structure type, we present minimum energy calculations including (i) the surface and (ii) average in-plane strain and (iii) the misfit dislocation density profile with various grading coefficients (thickness and indium composition variation). In both types of structures, the average in-plane strain and misfit dislocation density profile scale with the average grading coefficient, but InxAl1−xAs structures with a greater average elastic stiffness constants exhibit slightly higher average compressive in-plane strain (absolute valued) which is associated with higher misfit dislocation densities. However, the rate of change in the normalized relaxation percentage per unit thickness of each step with respect to the lattice mismatch of the step is lower in the InxAl1−xAs material system. The difference of the in-plane strain is small (<3%), however, so that these material systems are virtually interchangeable in terms of their mechanical behavior (<5.1% change in elastic constants).

Electric field and shape effect on the linear and nonlinear optical properties of ellipsoidal finite-potential quantum dots

In this work, the optical properties of ellipsoidal finite-potential quantum dot with an on-center shallow hydrogenic impurity in the presence of an external electric field have been studied. A variational procedure in the framework of perturbation theory was employed within the effective-mass approximation. Numerical results on typical GaAs/AlxGa1−xAs materials show that, besides incident light intensity, the electric field and dot shape have an important influence on the peak positions and magnitudes of linear and third-order nonlinear absorption coefficients and refractive index changes.

Mutual alloying of XAs (X = Ga, In, Al) materials: Tuning the optoelectronic and thermodynamic properties for solar energy applications

In the present work we did mutual alloying of the versatile XAs (X=Ga, In, Al) materials in order to improve their efficiency and enhance their range of technological applications using state of the art first principles method. We investigate the structural, electronic and thermodynamic properties of Ga1−xAlxAs, Ga1−xInxAs and In1−xAlxAs for x=0.25, 0.50, and 0.75. Calculations have been performed using the density functional theory (DFT) as implemented within the full potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method. For exchange and correlation energy treatment, we employed the local density approximations (LDA) as proposed by Wang and Perdew and the generalized gradient approximation (GGA) from Perdew et al. proposed. To calculate the accurate band structure, recently modified Becke Johnson (mBJ) potential was suggested as an alternative. Our calculations show a linear fall in the lattice constant in contrast to linear rise in bulk moduli of Ga1−xAlxAs and In1−xAlxAs with the increase of Al concentration. However the change of indium concentration in Ga1−xInxAs is displaying a reverse effect. The energy band gap of Ga1−xAlxAs and In1−xAlxAs was found to be increased, where a crossover from direct to indirect band gap has been observed with the increase of Al concentration. This direct to indirect crossover was found at 93.4% of Al concentration for Ga1−xAlxAs and at 84.63% of Al concentration for In1−xAlxAs. The effect of the mutual alloying of XAs materials on the thermodynamic properties is comprehensively reported.

Bandwidth control of paired photons generated in monolithic Bragg reflection waveguides

Bragg reflection waveguides are considered as monolithic sources of frequency correlated photon pairs generated using spontaneous-parametric down-conversion in a AlxGa1-xAs material system. The source described here offers unprecedented control over the process bandwidth, enabling bandwidth tunability between 1 nm and 450 nm while using the same wafer structure. This tuning is achieved by exploiting the powerful control over the waveguide dispersion properties afforded by the phase-matching technique used. The offered technology provides a route for realizing electrically pumped, monolithic photon pair sources on a chip with versatile characteristics.

Interface phonons in cylindrical quantum dot heterostructure

Abstract The top interface optical (TIO) and side interface optical (SIO) phonon modes of a cylindrical GaAs/ AlxGa1−xAs quantum dot are derived within the framework of dielectric continuum approximation. Results reveal that, in the case of taking the “two-mode” behavior of the AlxGa1−xAs material into account, there exist eight branches of TIO phonon modes and four branches of SIO phonon modes. The dispersion frequencies of TIO or SIO phonon modes sensitively depend on the Al mole fraction x in the AlxGa1−xAs material. With increasing wavevector q ( κ ), the frequency of each TIO (SIO) mode approaches one of the two frequency values of the single AlxGa1−xAs heterostructure.

Photoreflectance determination of the wetting layer thickness in the InxGa1−xAs∕GaAs quantum dot system for a broad indium content range of 0.3–1

We have investigated a set of InxGa1−xAs∕GaAs quantum dot structures grown by solid source molecular beam epitaxy for a wide range of In content ranging from 30% to pure InAs∕GaAs dots. It is well known that in a self-assembled growth mode, the dots are formed on a thin layer of the InxGa1−xAs material, which is called wetting layer (WL). The WL thickness is driven by the strain, i.e., lattice mismatch between the layer and substrate materials. Usually, the WL quantum well is not optically active in emission type of experiments (the whole radiative recombination goes through the dot states) and even if so, the heavy hole ground state transition is probed only. In order to detect all the possible transitions, e.g., transitions related to light hole and possible higher order heavy hole states (including those transitions, which are nominally parity forbidden), we have used modulation spectroscopy in a form of photomodulated reflectivity measurements. This is an absorptionlike method, which has been proven to be highly sensitive to even very low intensity transitions in low-dimensional semiconductor structures. The aim of this work is to determine in a contactless optical manner the thickness of the wetting layer as a function of indium content starting with very low content and low-strain structures (at the limit of self-assembled dot creation) up to typical InAs dots on gallium arsenide with the effective wetting layer as thin as 1.5 ML. The observed optical transitions have been identified based on energy level calculations for thin rectangular wells and using effective mass approximation in an envelope function approach in which the well width has been treated as a fitting parameter to the experimental data.

High-efficient carbon-doped InGaAs/AlGaAs/GaAs quantum well lasers

The carbon-doping behaviors of carbon tetrabromide (CBr4) in AlxGa1−xAs material system are studied by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). Doping level dependence on growth temperature, and V/III ratio is established. C-doped InGaAs/AlGaAs/GaAs separate confinement laser structure (∼940nm) are grown and demonstrated high quality with low-threshold current density. High-performance monolithic laser diode arrays are fabricated. A high slope efficiency of 1.17W/A and power efficiency of 62% are achieved, which is much better than the performance of Zn-doped structure of the same design. Therefore, carbon doping is demonstrated to have unique advantages over Zn-doping for high-power semiconductor laser applications.

The Role of the Thermal Oxide in GaAs-Based Photonic Bandgap Waveguide Microcavities

Recent work on optical microcavities is concisely reviewed in this article. Optical microcavities can be fabricated using III–V compound semiconductors by introducing a large periodic refractive-index variation along the axis of GaAs waveguides. Strong optical confinement in the microcavities is achieved by oxidizing the underlying III–V material and further optimized by preferentially etching away the oxide to form an air-bridge structure. The III–V thermal oxide is obtained from the wet thermal oxidation of high Al-content AlxGa1–xAs material. The critical role of the thermal oxide in the design and fabrication of the waveguide microcavities is outlined and the characterization of the properties of the thermal oxide salient to the fabrication of the devices is described. Finally, preliminary transmission data demonstrating the successful realization of these microcavities are presented.

The Temperature and Pressure Dependence of Electron Transport in Plastically Relaxed InxGa1?xAs

Electron transport in relaxed InxGa1—xAs materials has been investigated as a function of temperature (4.2 to 300 K) and pressure (0 to 8×108 Pa) for a wide range of alloy compositions. A dramatic decrease in both the mobility and carrier concentration in a limited alloy composition range x = 0.30 to 0.60 has been observed. The results were analyzed using an iterative solution of the Boltzmann equation (ISBE) based on the usual scattering mechanisms. Both the measured temperature and pressure dependence of mobility and carrier concentration together with the ISBE calculations indicate that the deep levels are responsible for the sharp decrease both in mobility and carrier concentration in the alloy composition range x = 0.30 to 0.60. For the x = 0.51 and 0.60 layers an additional scattering mechanism having a temperature dependence of ∼T2.6 has been observed, which is attributed to electron scattering from the deep levels generated in association with the layer relaxation.

Growth of GaxIn1−xAs quantum wire heterostructures by the strain-induced lateral-layer ordering process

GaxIn1−xAs quantum wire (QWR) arrays were grown in situ by molecular beam epitaxy. A lateral composition modulation was achieved in the GaxIn1−xAs material system via the strain-induced lateral layer ordering process occurring spontaneously when (GaAs)n/(InAs)n short-period superlattices are grown on (100)-oriented on-axis InP substrates. The resulting quasiperiodic modulation of the GaxIn1−xAs composition, with a periodicity of 300 Å, offers a lateral quantum confinement along the growth plane. The quantum confinement in the growth direction is provided by sandwiching the thin (∼100 Å) laterally ordered GaxIn1−xAs layer between layers of bulk Al0.24Ga0.24In0.52As. The formation of the QWR array is confirmed by cross-sectional transmission electron microscopy and polarized photoluminescence measurements.

Optical dispersion of AlxGa1−xAs at crystal growth temperature

We propose a scheme to calculate the optical dispersion of III-V compounds at crystal growth temperature, based on the Afromowitz’ formula and the shift of the lowest direct band gap energy. The thermal behaviors of the major parameters are extracted from their respective physical meanings. This model is applied to AlxGa1−xAs materials and the calculated results compare quite well with the experimental data. The validity of our model is also tested by comparing the static dielectric constant of GaAs with that of known empirical formula in the temperature range from 200 to 600 K.

Self-consistent simulation of quantum wires in periodic heterojunction structures

We simulate the low temperature confinement of electrons in quantum wires formed at the corners of periodic saw-tooth structures consisting of layers of AlxGa1−xAs and GaAs materials. A very efficient self-consistent procedure for the solution of Schrödinger’s and Poisson’s equations, including shifted periodic boundary conditions, is used to obtain the wave functions and energy levels in the quantum well. Self-consistency is very important to obtain an accurate result; in particular, the precise details of the potential at the corners must be resolved. The formation of a quantum wire at the corners of the GaAs layer is found to be very sensitive to the physical parameters of the neighboring AlxGa1−xAs layers. We find that to avoid sizeable electron population in the GaAs region between the corners, asymmetric doping is necessary, which normally results in electron confinement only at every other corner.

Application of ‘‘critical compositional difference’’ concept to the growth of low dislocation density (&amp;lt;104/cm2) InxGa1−xAs (x≤0.5) on GaAs

Multilayer epitaxial structures consisting of InxGa1−xAs layers of various compositions were grown on GaAs substrates by the molecular beam epitaxy technique. Dislocation evolution and residual strain in these heterostructures were studied using cross-sectional transmission electron microscopy (XTEM) and high-resolution x-ray diffraction analyses, respectively. The multilayer heterostructures were designed such that the compositional difference between two adjacent InxGa1−xAs layers in the stack was less than a critical compositional difference of Δx=0.18, taking partial lattice-relaxation into account. XTEM studies of the stacked structures indicated dislocation evolution to be confined to the GaAs substrate and the InxGa1−xAs layers underlying the top InxGa1−xAs layer in the stack, the top InxGa1−xAs layer being essentially dislocation-free. This phenomenon is attributed to a monotonic increase in the yield strength of InxGa1−xAs at the appropriate growth temperatures with increasing values of x. Such behavior appears to persist up to an InxGa1−xAs composition of approximately x=0.5, whereupon a further increase in composition results in dislocation evolution in the top layer of the stack. It is postulated that the yield strength of InxGa1−xAs decreases with increasing values of x beyond x=0.5. Extremely low dislocation density InxGa1−xAs material was grown on GaAs using the stacked structure approach as evidenced by etch pit analysis. For example, dislocation densities of 1–2×103/cm2 and 5–6×103/cm2 were recorded from In0.35Ga0.65As and In0.48Ga0.52As top layers, respectively. Such InxGa1−xAs alloys would be potentially suitable for the fabrication of photonic devices operating at 1.3 μm (x=0.35) and 1.55 μm (x=0.48).

Facet modulation selective epitaxy−a technique for quantum-well wire doublet fabrication

The technique of facet modulation selective epitaxy and its application to quantum-well wire doublet fabrication are described. Successful fabrication of wire doublets in the AlxGa1−xAs material system is achieved. The smallest wire fabricated has a crescent cross section less than 140 Å thick and less than 1400 Å wide. Backscattered electron images, transmission electron micrographs, cathodoluminescence spectra, and spectrally resolved cathodoluminescence images of the wire doublets are presented.

Strain Relief in InxGa1−xAs/GaAs Multiple Layer Systems

ABSTRACTIn a recent paper[l] we presented the notion of a “critical composition” of InxGa1−xAs (x=0.18) which when exceeded results in threading dislocation evolution in InxGa1−xAs (x ≥ 0.18) material grown on GaAs. For InxGa1−xAs compositions below x=0.18, threading dislocation evolution occurs in the GaAs substrate material but not in the InxGa1−xAs epitaxial layer material, this phenomenon being attributed to a yield strength mismatch in favor of InxGa1−xAs at the epilayer growth temperature.In this paper we describe recent results concerning an extension of this work to the study of strain relief in multiple layer systems. It was found that large x (x-0.5) InxGa1−xAs/GaAs layers can be grown having extremely low dislocation densities (&lt;104/cm2 ) by step increasing the InxGa1−xAs composition at a series of InxGa1−xAs/InyGa1-yAs heterointerfaces such that a “critical composition difference” is not exceeded at each heterointerface. High resolution x-ray diffraction analysis was used to determine the extent of relaxation in each layer of these multilayer systems. As evidenced by cross-sectional TEM analysis, dislocations propagate only in the underlying material at each heterointerface for suitably selected compositional differences which results in the top InxGa1−xAs layer in such multiple layer schemes having a low dislocation density. It is to be noted that conventional methods for blocking dislocations such as strained layer superlattices and compositionally graded layers were not employed in our multilayer systems.