Optical Properties Of GaAs Research Articles

Characterisation of the GaAs-based intermediate band solar cell with multi-stacked InAs/InGaAs quantum dots

We report on both the photovoltaic and the optical properties of GaAs p-i-n solar cell (SC) and GaAs p-i-n SC with multiple InAs quantum dot (QD) layers in the i-region. Current-voltage and impedance spectroscopy measurements have been used to characterise the two SC devices. Refractive index, extinction and absorption coefficients were deduced from spectroscopic ellipsometry. I-V measurements in dark and illumination conditions have been carried out to investigate the photovoltaic effect in these devices. Our results suggested that the presence of trap states could cause the degraded photovoltaic performance of the QD SC device. The C-V characteristics of the QD SC device show forward bias region with negative values of capacitance, which is caused by the filling process of the dots near the junction. The equivalent circuit model consisting of two-series connected RC networks with a series resistance Rs was found to give a good fit to the experimental data.

Photoreflectance characterization of vanadium-doped GaAs layers grown by metalorganic vapor phase epitaxy

We characterized the vanadium-doped (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy. The conductivity of the samples is checked by the Hall effect measurements, and it is n type with electrons concentration ranging from 2×1013 to 2×1017cm−3. We used the photoreflectance technique (PR) to study the effect of vanadium doping on the optical properties of GaAs. Our experimental results show that the built-in electric field F and the broadening parameter Γ0 increase with V-doping concentration. Furthermore, the temperature dependence of the built-in electric field and the broadening parameter of GaAs:V samples have been investigated by PR. It has been observed that these parameters increase with increasing temperature. The decrease of the built-in electric field with decreasing temperature is due to the photovoltage effect.We characterized the vanadium-doped GaAs (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy.

Growth of GaAs1−xBix by molecular beam epitaxy: Trade-offs in optical and structural characteristics

Recent work has shown that Bi incorporation increases during molecular beam epitaxy (MBE) when surface processes are kinetically limited through increased growth rate. Herein we explore how the structural and optical properties of GaAs1−xBix films are modified when grown under conditions with varying degrees of kinetic limitations realized through growth temperature and growth rate changes. Within the typical window of MBE growth conditions for GaAs1−xBix, we compare films with similar (∼3%) compositions grown under conditions of reduced kinetic limitations, i.e., relatively low gallium supersaturation achieved at higher temperatures (∼350 °C) and lower growth rates (∼0.5 μm/h), to those grown farther from equilibrium, specifically, higher supersaturation achieved at lower growth temperatures (∼290 °C) and higher growth rates (∼1.4 μm/h). Both the x-ray diffraction full width at half maximum of the omega-2theta scan and the 300 K photoluminescence intensity increase when samples are grown under less kinetically limited conditions. We interpret these findings in relation to the incorporation of Bi-related microstructural defects that are more readily formed during less kinetically limited growth. These defects lead to enhanced luminescence efficiency due to the spatial localization of carriers.

Physical Properties of Thin Films of GaAs and Al<sub><i>x</i></sub>Ga<sub>1-x</sub>As Grown by Solid-Arsenic-Based MOCVD

We have studied the optical properties of GaAs and AlxGa1-xAs thin films using low-temperature photoluminescence and Fourier transform infrared spectroscopy. The GaAs and its alloys were grown by MOCVD using solid arsenic instead of arsine, as the arsenic precursor. The gallium and aluminium precursors were trimethylgallium (TMGa) and trimethylaluminium (TMAl), respectively. Some difficulties for growing AlxGa1-xAs by solid-arsenic-based MOCVD system are the composition homogeneity of the layers and the oxygen and carbon incorporation during the growth process. The composition homogeneity of the films was evaluated by low-temperature photoluminescence. Infrared measurements on the samples allowed the identification of the residual impurities, which are carbon-substitutional, Ga2O3, molecular oxygen, humidity and two unidentified impurities. Samples grown at temperatures lower than 750°C were highly resistive, independently of the ratio V/III used; the samples grown at higher temperatures were n-type, as it was proved by Hall effect measurements.

Localization effects and band gap of GaAsBi alloys

The structural and optical properties of GaAs1−xBix alloys for x up to 0.108 have been investigated by high resolution X-ray diffraction and photoluminescence (PL). At room temperature (RT), the PL intensity of the GaAs0.97Bi0.03 sample was found to be ∼300 times higher than a GaAs control sample grown at the same temperature (400 °C). PL measurements carried out at 10 K show that when excitation power, Pex was increased from 0.11 to 1140 W cm−2, the PL peak energy blue-shifts by 80 meV while the full-width-at-half-maximum reduces from 115 to 63 meV. However, the PL peak emission energy becomes independent of the excitation power at RT. The results indicate the presence of localized energy states in the GaAs0.97Bi0.03 sample, which trap carriers at low temperatures and that the majority of the carriers become delocalized at RT. Furthermore, the temperature dependent PL also shows an S-shape behavior, which is a signature of localization effects. A theoretical model, which was derived by solving a rate equation was employed. The model successfully reproduces the observed S-shape behavior and the theory fits well with the experimental data. The RT band gap of GaAs1−xBix for x up to 0.108 has been plotted and compared with the literature.

Effects of AlGaAs cladding layers on the luminescence of GaAs/GaAs1−xBix/GaAs heterostructures

The structural and optical properties of GaAs1−xBix quantum wells (QWs) symmetrically clad by GaAs barriers with and without additional confining AlGaAs layers are studied. It is shown that a GaAs/GaAs1−xBix/GaAs QW with x ∼ 4% and well width of ∼4 nm grown by molecular beam epitaxy demonstrates efficient photoluminescence (PL) that becomes significantly more thermally stable when a cladding AlGaAs layer is added to the QW structure. The PL behavior for temperatures between 10 and 300 K and for excitation intensities varying by seven orders of magnitude can be well described in terms of the dynamics of excitons including carrier capture in the QW layer, thermal emission and diffusion into the cladding barriers. Understanding the role of these processes in the luminescence of dilute GaAs1−xBix QW structures facilitates the creation of highly efficient devices with reduced thermal sensitivity and low threshold current.

Quantitative study of localization effects and recombination dynamics in GaAsBi/GaAs single quantum wells

Localization effects on the optical properties of GaAs1−xBix/GaAs single quantum wells (SQWs), with Bi contents ranging from x = 1.1% to 6.0%, are investigated using continuous-wave and time-resolved photoluminescence. The temperature- and excitation density dependence of the PL spectra are systematically studied, and the carrier recombination mechanisms are analyzed. At low temperatures, the time-integrated PL emission is dominated by the recombination of localized electron-hole pairs due to the varying content and clustering of Bi in the alloy. The extracted energy scales fluctuate tremendously when the Bi content is varied with a weak tendency to increase with Bi content. Relatively low energy scales are found for the SQW with x = 5.5%, which makes it a potential candidate for long-wavelength optoelectronic devices.

Photoemission properties of GaAs (100) \beta 2(2 \times 4) and GaAs (100) (4 \times 2) reconstruction phases

Using the first-principles plane-wave pseudopotential method, based on the density function theory, the electron structure and optical properties of GaAs (100) \beta 2(2 \times 4) and GaAs (100) (4 \times 2) reconstructions are calculated. The formation energy of As-rich \beta 2(2 \times 4) reconstruction is minus and the formation energy of Ga-rich (4 \tiems 2) reconstruction is positive; As-rich \beta 2(2 \times 4) reconstruction is stable and Ga-rich (4 \times 2) reconstruction is unstable. Ga-rich (4 \times 2) reconstruction owns lower work function. The electrons at two reconstructions both move into the bulk and form a band-binding region. Both the absorption and the reflectivity of As-rich 2(2×4) reconstruction are smaller than the Ga-rich (4×2) reconstruction. Asrich \beta 2(2 \times 4) reconstruction is more benefit for the movement of photos through the surface to emit photoelectrons.

A GaAs MEMS for AFM and spin injection

We investigate the mechanical and electrical properties of tip-less, GaAs micro-cantilevers on silica supports that are fabricated using a novel assembly approach. The resulting device is compatible with an atomic force microscope (AFM) and takes advantage of the electronic and optical properties of GaAs as well as the mechanical properties of silica. Mechanically, their resonant frequency and quality factor, as well as their AFM imaging capabilities (lateral resolution ∼10–20nm), are comparable to commercial silicon cantilevers despite the absence of micromachined tip. In the same AFM-like configuration, they can also function as novel spin-polarized electron injectors under excitation by a circularly polarized laser from the rear. Surface nitridation of the cantilever and deposition of a hydrophobic thin polymer film on the sample surface are found to stabilize the injected photocurrent, making them potentially useful for a variety of fundamental and applied investigations in atmosphere.

Structural and Optical Properties of GaAs1−xNx/GaAs Grown by Molecular Beam Epitaxy

Structural and Optical Properties of GaAs_1−<i>x</i>N_<i>x</i>/GaAs Grown by Molecular Beam Epitaxy

Optical study of hydrogen-irradiated GaAsN/GaAs heterostructures

The effect of hydrogen irradiation on the optical properties of GaAs1−xNx/GaAs heterostructures was investigated using photoreflectance and reflectance techniques. Systematic measurements performed on both as-grown and hydrogenated samples for N-concentrations ranging from 0.0% to 3.5% and for H-implanted doses from 3 × 1018 to 6 × 1018 ions/cm2 have shown that (a) the H-induced widening of the energy gap is accompanied by a decrease of the refractive index of the H-treated samples with respect to the as-grown ones, resulting in an index mismatch that can be as large as 2% in the subgap spectral region; and (b) the presence of compressive strain in fully passivated GaAsN determines a decrease of the refractive index even below that of GaAs that can be eliminated via moderate thermal annealing. These findings are promising for the development of heterostructures with planar geometry, in which the simultaneous confinement of both carriers and photons, even on a nanometric scale, can be obtained in a single step process.

On optical properties of GaAs and GaAs/AlGaAs core-shell periodic nanowire arrays

Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the effects of size, geometry, height, packing density, and polarization. Energy dependence of the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also the air bands. Simulation of polarization dependence verifies higher absorption with the electric field along the wire axis. Higher absorption at much lower volume compared to thin film, combined with excellent charge transport, make core-shell nanowire arrays excellent candidates for optoelectronics applications.

Influence of ion beam on optical properties of GaAs: Calculation using the fractional‐derivative‐spectrum method

AbstractSemiconductor GaAs (100) was implanted with In+ at a room temperature. Fluences of implanted ions were from 1x 1012 cm‐2 to 3x1013 cm‐2 and the energy was equal to 85 keV. The detectability of the indium content in near surface layer of GaAs has been studied with the use of the secondary ion mass spectrometry (SIMS). The depths distribution of a indium concentration measured by SIMS method ware compared with depth profiles calculated with the help of SATVAL code. It was noticed that calculated profiles are in a good agreement with the measured values. Additionally, the concentration of indium in the near surface layer increased with growing of the implantation fluence and a maximum of these profiles shift in the side of ions exposed surfaces of GaAs. This effect can be explained by self‐diffusion process of target atoms and indium during the implantation. The fractional derivative spectrum (FDS) model in connection with spectroscopic ellipsometry ES was used for particular optical analyses of implanted GaAs substrates. The FDS is especially promising technique when the limitations of the standard treatment by Lorentz shape line analysis occur and it is very helpful to extract basic information on relevant physical quantities from the detected εE2(E) spectra determined by spectroscopic ellipsometry. In particular, using spectroscopic ellipsometry and FDS, the critical points E1 and E1+Δ observed in the εE2(E) spectra in the implanted substrates have been investigated. The differences in complex refractive index and dielectric function (ε=ε1+iε2) between GaAs substrate before and implantation were observed. Moreover, the several nanometres thick oxide layer on GaAs surface was detected. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interface Morphology Investigation of Bonded p-GaAs/p-Si Wafers

The integration of GaAs and Si can combine the superior electrical and optical properties of GaAs with the mechanical and economical advantages of Si. It presents great potential for OEICs applications. In this study, direct wafer bonding was applied to combine bulk p-Si and p-GaAs. Interface morphologies of bonded p-GaAs/p-Si wafers were investigated by TEM.

Optical study of GaAs 1− xSb x layers grown on GaAs substrates by gas-source molecular beam epitaxy

The optical properties of GaAs 1− x Sb x (5.9% ≤ x ≤ 9.7%) layers grown on GaAs substrates by gas-source molecular beam epitaxy were characterized by photoreflectance (PR) and piezoreflectance (PzR). Identification of conduction to heavy-hole (HH) band and conduction to light-hole (LH) band transitions originated from the strained induced valence band splitting have been achieved by comparing the relative intensities of PR and PzR spectra. The results indicate increases of the valence band splitting with increasing of Sb content. The temperature dependences of near band edge transition energies were analyzed using Varshni and Bose–Einstein expressions in the temperature range from 15 to 300 K. The parameters that describe the temperature variations of the near band edge transition energies and broadening function were evaluated and discussed.

Structural and optical properties of GaAs(001) surfaces thermally annealed in dry N2 atmosphere

Thermal annealing behaviors of GaAs(001) surfaces in dry N2 atmosphere have been studied by using spectroscopic ellipsometry (SE), optical microscopy, scanning electron microscopy, ex situ atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) techniques. The SE data indicate that thermal annealing at temperatures T below 500 °C causes no or a little influence on the surface morphology. Microscopic roughening starts to occur at T&amp;gt;600 °C and exhibits a saturated roughness value at ∼800 °C. AFM observation confirms the presence of a microscopically roughened surface overlayer after annealing at T∼600–750 °C and a macroscopically roughened overlayer at above 700 °C. The XPS data indicate the removal of As2O3 oxide after annealing at above 500 °C; however, the Ga2O3 oxide is always observed regardless of annealing temperature, although its peak intensity becomes remarkably large at T∼700 °C and small at T≥800 °C. Annealing at T=1000 °C leads to deliquesced sample surface. The native GaAs oxide is also found to act as a passivation film against annealing-induced PL degradation for T below ∼400 °C.

Optical properties of GaAs1−x N x alloys grown by molecular beam epitaxy

Optical properties of GaAs1−x N x alloys grown by molecular beam epitaxy using GaAs (001) as the substrate have been studied. These include photoluminescence (PL), cathodoluminescence (CL), photocurrent and photomemory effects. The low-temperature (77 K) PL characteristics were measured on samples with 0–0.105% N content. The wide emission band indicates the defective nature of the materials. The widening of the band for materials with increasing nitrogen concentration also suggested that the concentration of defect states in these materials dramatically increased with increasing nitrogen content. The PL and CL spectra for GaAs1−x N x layer 1854 did not show identical characteristics. Some layers showed a very sharp fall in photocurrent at low temperatures, indicating a very sharp photoquenching and an interaction between antisite, interstitial and vacancy defects. The photomemory effect, which causes photoquenching and the transition from the EL2 to the EL2* metastable state, was strongly influenced by the optical exposure and thermal history of the sample.

Dispersion and Modulation of the Linear Optical Properties of GaAs–AlAs Superlattice Waveguides Using Quantum-Well Intermixing

We report on the large modulation of the optical properties of a 14:14 monolayer GaAs-AlAs superlattice waveguide following quantum-well intermixing. Low-temperature photoluminescence measurements illustrate a large 169-meV differential blue-shift obtained between the disordering-suppressed and disordering-enhanced materials. Effective index measurements are presented as a function of polarization, for both the as-grown and disordered material for near-bandedge and half-bandedge wavelengths, which is the wavelength range 775-1550 nm. The largest effective refractive index shift observed was 9/spl times/10/sup -2/ which exceeds that previously reported for disordered AlGaAs ternary multilayer structures, and illustrates the potential of the superlattice for the fabrication of etch-free, planar optical components with large index contrast.

III–V layer transfer onto silicon and applications

AbstractIntegration of GaAs and InP with Si technology presents a huge potential interest. When realised, it will combine the superior electrical and optical properties of GaAs and InP with the mechanical and economical advantages of Si, including large integration density. To obtain such hybrid structures, heteroepitaxial growth has been investigated extensively but, up to now, the resulting devices have limited performance due to a high density of threading dislocations. Bonding technologies have also been developed and III–V optical devices on Si have been already demonstrated. In this paper, we review the application of the Smart CutTM technology which allows large diameter GaAs (or InP) thin film to be transferred onto a full silicon (handle) wafer. This technique is fully compatible with the requirements of microelectronics manufacturing and, both, HEMTs and P‐HEMTs structures have been realised. They exhibit electrical characteristics in close comparison to the prototype structures directly grown on a conventional GaAs or InP substrate. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Smooth p-type GaAs(001) films grown by molecular-beam epitaxy using silicon as the dopant

Atomic force microscopy, Hall effect and photoluminescence measurements were used to investigate the morphological, electrical and optical properties of GaAs(001):Si films obtained by droplet-assisted molecular-beam epitaxy using a peculiar shutter sequence for the delivery of the silicon, gallium and arsenic species. Although silicon is almost exclusively used as a n-type dopant on GaAs(001) substrates, with such growth conditions a p-type character of the GaAs:Si layers was generally detected. The best morphological results were obtained when a single monolayer of gallium and arsenic were alternately deposited and the dopant was evaporated during the gallium cycle, since in that special case no gallium droplets were formed on the substrate, minimizing thus the total roughness of the final surface.