Band Gap Energy Of GaAs Research Articles

Analysis of mixed optical transitions in dilute magnetic AlAs/GaAs/GaMnAs quantum wells grown on high substrate index by molecular beam epitaxy

In this study, we report the influence of high-index substrate orientation on the optical properties of dilute Mn containing a single quantum well (QW) structure grown by molecular beam epitaxy (MBE). The electronic band structure of the samples is calculated with finite element methods (FEM). Photomodulated reflectance (PR) spectroscopy is employed to determine the optical transition energies at room temperature. PR experimental results are analyzed with the Third Derivative Functional Form (TDFF) signal function. TDFF fitting shows that the optical transition starts below the fundamental GaAs bandgap energy for the samples grown on (311) B and (411) B-oriented substrates. Comparing TDFF fitting and calculated optical transition energies with FEM show that the optical transition occurs in type-I and type-II as direct and indirect transitions, respectively. Type-I and -II optical transitions are addressed concerning the electronic band structure of the sample grown on oriented substrates.

Outdoor performance of GaAs/bifacial Si heterojunction four-terminal system using optical spectrum splitting

Bifacial photovoltaics and multijunction systems represent the most promising alternatives to overcome the theoretical limit of single-junction Si photovoltaics, and these solutions can also be combined to achieve higher performances. This work investigates the outdoor performance of a bifacial four-terminal photovoltaic system based on combining a III-V semiconductor with the silicon heterojunction technology. By exploiting the wide band gap energy of GaAs, the bifaciality of the Si heterojunction and the spectrum-splitting capability of dichroic mirrors, an optimal voltage match between mini-modules of the two solar cells was achieved, with an open-circuit voltage mismatch of 4% of the average value. In this study, we demonstrate the full functionality of bifacial operation with a 17% increase of power conversion efficiency throughout the day compared to monofacial operation. Furthermore, we show that though the solar spectrum is subjected to strong variations from morning to afternoon, which lead to changes of the ratio of the GaAs to the Si mini-module short-circuit currents up to 43% throughout the day, the variation of the overall system power conversion efficiency remains quite limited, less than 16% of the maximum value thanks to the efficient coupling of the two mini-modules.

MOVPE Growth of AlInP–InGaP Distributed Bragg Reflectors

This article discusses metal–organic vapor phase epitaxy growth of all-phosphide (AlInP–InGaP) distributed Bragg reflectors (DBRs) and their applications for thin flexible multijunction solar cell devices. DBRs with a peak reflectance of 884 nm—close to the GaAs bandgap energy—were grown. This all-phosphide DBR was monolithically integrated with a dual-junction (DJ) InGaP/GaAs solar cell. The DJ cell with a DBR exhibited a 3% increase in short-circuit current density ( J sc), in comparison with the DJ without a DBR. Both open-circuit voltage ( V oc) and fill factor ( FF) values of the solar cell did not change with the DBR integration.

A simple and efficient device configuration applicable in high-performance solar cells with limited material requirements

Efficient device configurations with simple structure and limited requirements for active materials, including quality and quantity, are of particular interest in solar cells. In this paper, a ‘U’ pool-shaped nanowall array is proposed and investigated regarding light management and photoelectric conversion behaviors based on gallium arsenide (GaAs). It was found that excellent omnidirectional light trapping can be realized in a broad structural parameter range. A photocurrent density (Jph) up to ~29.0 mA cm−2, i.e. ~93.5% absorption of incident photons with energy larger than 1.42 eV, i.e. the bandgap energy of GaAs is predicted at AM 1.5G illumination for the optimized array with an effective thickness of less than 350 nm. As a distinct comparison, a 2000 nm thick flat GaAs film only delivers Jph of ~19.8 mA cm−2 under the same illumination. Photoelectric simulations predict that to achieve a rationally high power conversion efficiency, bulk lifetime longer than 10−7 s for minority carriers in the base material, appropriate surface passivation of reducing the recombination velocity to the order of ~102 cm s−1 and contact resistance between electrodes and emitters lower than 10 Ω cm2 are required.

Intrinsic magneto-optical spectra of GaMnAs

In the spectrum of reflection magnetic circular dichroism (MCD) of Ga1−xMnxAs, the E0 peak energy, which is assigned to the band gap of GaMnAs, is higher than the band gap energy of GaAs. In the past, this blue shift was attributed to the Moss-Burstein effect, in which the Fermi level moves toward lower energy in the valence band as the Mn concentration x increases; however, this picture is inconsistent with the impurity-band conduction picture, which has been verified in recent studies. Here, by measuring reflection MCD spectra of Ga1−xMnxAs thin films (x = 1%, 2%, and 8%) with various thicknesses, we derive the off-diagonal element of the dielectric tensor of GaMnAs and obtain the intrinsic MCD spectra of GaMnAs that are free from optical interference. We find that optical interference is significantly strong even in the extremely thin (10–100 nm) GaMnAs films and that the E0 peak of the intrinsic MCD spectra is located close to the band gap energy of GaAs even in highly Mn-doped GaMnAs. This is consistent with the recent understanding of the band structure of GaMnAs; the Fermi level exists in the impurity band in the band gap regardless of x.

Carrier dynamics in GaAs photonic crystal cavities near the material band edge.

We measure fast carrier decay rates (6 ps) in GaAs photonic crystal cavities with resonances near the GaAs bandgap energy at room temperature using a pump-probe measurement. Carriers generated via photoexcitation using an above-band femtosecond pulse cause a substantial blue-shift of three time the cavity linewidth for the cavity peak. The experimental results are compared to theoretical models based on free carrier effects near the GaAs band edge. The probe transmission is modified by nearly 30% for an estimated above-band pump energy of 4.2 fJ absorbed in the GaAs slab.

Optimisation of GaAs nanocrystals synthesis by laser ablation in water

Laser ablation of a gallium arsenide (GaAs) wafer immersed in distilled water was carried out using the fundamental wavelength of a high frequency Nd:YAG laser with 240 ns pulse duration. Rate of nanoparticles generation through laser ablation for various amounts of laser pulse energies (0.4–0.94 mJ) and repetition rates (400–2000 Hz) were studied and a maximum ablation rate of 19.6 µgr/s was obtained. Formation of the pure GaAs nanocrystals (NCs) is confirmed using TEM micrograph and X-ray diffraction analysis. Band-gap energy of generated GaAs NCs is calculated by Tauc method to be between 2.48 and 2.60 eV which is larger than the band-gap energy of bulk GaAs. The band-gap energy of NCs is increased by increasing the energy of laser pulses and is decreased by increasing the pulse repetition rate.

Microscopic origin of compressive strain in hydrogen-irradiated dilute GaAs1−yNyalloys: Role of N-Hncenters withn>2and their thermal stability

The addition of a small percent of nitrogen to GaAs causes a large reduction of the band gap energy and a tensile strain within the lattice. The further addition of H at 300 \ifmmode^\circ\else\textdegree\fi{}C causes a recovery of the band gap of the N-free GaAs host. Concomitantly, tensile strain turns into compressive strain. Upon reduction of hydrogenation temperature, high-resolution x-ray diffraction studies show now a remarkable increase of compressive strain, while photoluminescence measurements show that the recovered band gap energy of GaAs does not change. Infrared measurements indicate that several N-H${}_{n}$ centers are formed in addition to the well-established H-N-H center ($n$ $=$ 2), which accounts for the band gap recovery. The vibrational properties of the corresponding deuterium centers provide clues to the microscopic structures of these centers. Furthermore, theory shows that the center with $n$ $=$ 2 is robust when additional H is added in its vicinity and remains as a core of likely N-H${}_{n}$ defects.

Electrical properties of C60 and Si codoped GaAs layers

C60 uniformly doped GaAs and C60, Si codoped GaAs layers are grown by a migration enhanced epitaxy method. C60 doped GaAs layers show a single and sharp diffraction peak in x-ray diffraction and only an LO phonon peak is confirmed, indicating that the crystalline quality is fairly good. All of the C60 doped GaAs layers have highly resistive characteristics, and C60, Si codoped GaAs layers show n-type conductivity only when the Si concentration is of the same order or greater than the total carbon concentrations. At low temperatures the conductivity of the C60, Si codoped GaAs layers increases with exposure to light whose energy is below the GaAs bandgap energy. The electron concentrations and mobilities of the layers are confirmed to be increased under illumination by wavelengths between 900 and 1100 nm. These results imply that the electron transitions from the valence band to the trap levels and from the trap levels to the conduction band occur simultaneously as if the traps act as intralevels. As a result, the carrier concentrations are enhanced in the same way they would be if the excitation was above the GaAs bandgap energy.

Luminescence of GaAs nanowires consisting of wurtzite and zinc-blende segments

GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This result is consistent with the assumption that the band gap energy of wurtzite GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, and transition energies exceeding E(g,ZB) are possible for very thin ZB segments. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile as these can effectively bind holes in the ZB segments. From cathodoluminescence measurements of single GaAs NWs performed at room temperature, we deduce a lower bound of 55 meV for the difference E(g,WZ)-E(g,ZB).

Axial pn-junctions formed by MOVPE using DEZn and TESn in vapor–liquid–solid grown GaAs nanowires

We report on axial pn-junctions in GaAs nanowires. The nanowires were grown by MOVPE on (1 1 1)B GaAs substrates using the vapor–liquid–solid mechanism in combination with Au seed particles. At the low growth temperature of 400 °C any additional growth on the nanowire sidewalls can be excluded such that a pure axial pn-junction is realized. p-Type doping was provided by diethyl zinc, while tetraethyl tin was introduced for n-type doping. The impact of dopant supply was investigated both on structural properties and on carrier density. The carrier type was independently verified by processed nanowire metal–insulator FETs. The lengths of the whole pn-GaAs nanowires reach up to 20 μm while their diameters are up to a few 100 nm, as defined by the Au seed particles used. The pn-GaAs nanowire device exhibits diode-like I– V characteristics and strong electroluminescence. While the reverse current is in the low pA-regime, the forward current reaches a few μA, limited by the n-doped side. A diffusion voltage V D =1.4 V is determined, which corresponds to the GaAs band gap energy. To our knowledge this is the first axial GaAs pn-diode realized in a single GaAs nanowire.

Optical characterization of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under As2 and As4 fluxes

The authors have investigated the optical properties of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under different experimental conditions, mainly by changing the molecular species of the As beam stemming from a cracker cell at different temperatures. By low-temperature photoluminescence (PL) spectroscopy, they observed that using As2 instead of As4, the crystal quality of the ternary alloy improves significantly since narrower lines in the near-band-gap energy of AlGaAs were obtained when the temperature of the As-cracker zone increased. The experimental position of the energy of the ternary band gap transition as a function of temperature fits the Varshni and Viña functional forms satisfactorily, while it is difficult to assess the data for those samples grown employing As4. It was found that the As2 molecular beam reduces the creation of donor acceptor levels in AlGaAs, as compared with As4. The PL spectra line shape in the GaAs band gap region shows lines associated with band-to-band transition, exciton-bound to neutral-point defects (d,X), and conduction-band to neutral carbon-acceptor (C,A0). The intensity of these transitions was found to be independent of the As molecular-beam species; nevertheless, the photoreflectance spectra of the samples show line shape changes close to the GaAs band gap energy. This region has been related to the electron mobility of two-dimensional electron-gas systems.

Application of pulsed laser annealing to ferromagnetic GaMnAs

In this experimental and theoretical work we focus on the technique of pulsed laser annealing applied to the metastable ferromagnetic semiconductor GaMnAs. Analytical heat-flow calculations are used to illustrate the position and time-dependent temperature distribution during the whole laser annealing process. Such heat-flow calculations will also play an indispensable role for the preparation of other new metastable diluted ferromagnetic semiconductors by ion implantation and subsequent laser annealing. The structural, magnetic, and magnetotransport properties of ferromagnetic GaMnAs have been probed in dependence on the annealing parameters, e.g., the number of laser pulses and the pulse length. Annealing with a single KrF laser pulse of 30 ns and $0.26\text{ }\text{J}/{\text{cm}}^{2}$ with the photon energy above the GaAs band gap energy leads to similar magnetic properties like annealing with a single 3 ns Nd:YAG laser pulse with the photon energy below the GaAs band gap energy. We observed that possibly due to Mn diffusion and decreasing hole concentration, several laser pulses degrade the structural and magnetic properties of GaMnAs. Our results reveal the largest saturation magnetization in Mn-implanted GaAs annealed with a single KrF laser pulse.

Luminescence of GaAs/AlGaAs core–shell nanowires grown by MOVPE using tertiarybutylarsine

The luminescence properties of GaAs/AlGaAs core–shell nanowires grown by metalorganic vapor phase epitaxy (MOVPE) on (1¯ 1¯ 1¯)B GaAs using tertiarybutylarsine, trimethylgallium and trimethylaluminium are reported. Untapered kink-free GaAs nanowires with average diameters around 50–70 nm were grown at 400 °C by the vapour–liquid–solid method; to this purpose, colloidal Au nanoparticles were used as metal catalyst. Al 0.33Ga 0.67As shells were grown at 650 °C around GaAs nanowires by conventional MOVPE, with thickness ranging in the 70–160 nm interval. Low-temperature photoluminescence (PL) and high spatial resolution cathodoluminescence (CL) measurements were performed, respectively, on dense ensembles of core–shell nanowires (still on their original substrates) and single nanowires; comparison between secondary electron and monochromatic CL images of single nanowires led to spatially resolve the major CL emissions. The low-temperature luminescence of nanowires above the GaAs band-gap energy consists of three contributions: (i) the 1.997 eV band-edge (excitonic) emission of the Al 0.33Ga 0.67As shell, followed (in CL spectra) by a more intense GaAs-like LO-phonon replica; (ii) a broad weaker band at ∼1.90 eV, ascribed to a donor–acceptor pair recombination associated to residual Si donors in the AlGaAs; and (iii) a dominant and very broad band at ∼1.67 eV, due to the spatially indirect recombination between electrons in the core and holes in the shell. Comparison between CL and PL spectra suggests that each nanowire has a slightly different GaAs core emission, its peak energy varying in the 1.46–1.49 eV interval.

Study of optical properties of GaAsN layers prepared by molecular beam epitaxy

We have grown GaAsN layers (with nitrogen concentration between 1.2% and 3.2%) on GaAs(1 0 0) substrates by molecular beam epitaxy (MBE) using a radio frequency (RF) plasma nitrogen source, and solid sources for Ga and As. The growth temperature was varied from 420 to 600 °C, and the GaAsN growth mode was in situ monitored by reflection high-energy electron diffraction (RHEED). The optical properties of the layers were studied by photoreflectance spectroscopy (PR) and phase modulated ellipsometry (PME). For the growth temperature of 420 °C the films grew in a three-dimensional (3D) mode as indicated by the appearance of transmission spots in the RHEED patterns. In contrast, GaAsN layers grown at higher temperatures presented a two-dimensional (2D) growth mode. These GaAsN layers are pseudomorphic according to high-resolution X-ray diffraction (HRXRD). The PR spectra of all samples exhibited Franz–Keldysh oscillations (FKO) above of the GaAs band gap energy. From these oscillations we obtained the built-in internal electric field intensity ( F int) at the GaAsN/GaAs interface. In the low-energy region of the PR spectra we observed the transitions associated to fundamental band gap of the GaAsN layers. The variation of the GaAsN fundamental band gap obtained by PR as a function of the N content was explained according the band anti-crossing model (BAC). On the other hand, the E 1 and E 1+Δ E 1 critical points were obtained from the analysis of spectra of the imaginary part of the dielectric function obtained by PME. We observed a shift of these critical points to higher energies with the increase of N content, which was explained by a combination of strain and alloying effects.

Physical property analysis of C-doped GaAs as function of the carrier concentration grown by MOCVD using elemental arsenic as precursor

This work presents the characterization of GaAs layers grown in a metallic-arsenic-based-MOCVD system. The gallium precursor was the compound trimethylgallium (TMG) and elemental arsenic as precursor of arsenic. The most important parameters of the growth process include the substrate temperature and the composition of the carrier gas; an N 2 + H 2 gas mixture. The influence of carbon doping on the optical and electrical properties of GaAs layers have been studied by photoluminescence (PL) spectroscopy, Photoreflectance (PR) and Hall Effect measurements. To carry out doping with carbon in the range of around 10 16 to 10 20 cm -3 , it was necessary to modifying the hydrogen activity in the reacting atmosphere with the control of the N 2 +H 2 , mixture which was used as carrier gas. The PL response of the samples is strongly dependent on the growth temperature and showed mainly two radiative transitions, band-to-band and band-to C-acceptor. PR spectra present transitions associated to GaAs. Besides, short period oscillations near the GaAs band-gap energy are observed, interpreted as Franz-Keldysh oscillations associated to the hole-ionized acceptor (h-A - ) pair modulations. For investigating the chemical bonds of impurity-related species in the GaAs layers optical absorption was measured using a FT-IR spectrometer. Device quality GaAs layers have been grown in a broad range of growth temperatures.

Study of the homoepitaxial growth of GaAs on (631) oriented substrates

We have studied the GaAs growth on (631) oriented substrates by molecular beam epitaxy (MBE). Different samples were prepared by varying the growth temperature and the III/V equivalent pressure ratio. We observed by atomic force microscopy a high density of hilly like features elongated towards the [−5,9,3] direction formed during the MBE growth. The growth temperature dependence of the hillock length and width follows an Arrhenius-type behavior with activation energies of 1.4 and 0.5eV, respectively. The hillock formation is discussed in terms of adatom diffusion anisotropy and diffusion barriers. Employing photoreflectance spectroscopy we found a splitting of the GaAs band gap energy transition that increases with the hillock density.

Optical characterization of InGaAsN/GaAsN/GaAs quantum wells with InGaP cladding layers

Optical properties of InGaAsN/GaAs and InGaAsN/GaAsN/GaAs quantum well structures with InGaP cladding layers were studied by photoreflectance at various temperatures. The excitonic interband transitions of the InGaAsN/GaAsN/GaAs QW systems were observed in the spectral range above hν= E g(InGaAsN). The confinement potential of the system with strain compensating GaAsN barriers became one step broader, thus more quantum states and larger optical transition rate were observed. A matrix transfer algorithm was used to calculate the subband energies numerically. Band gap energies, effective masses were adopted from the band anti-crossing model with band-offset values adjusted to obtain the subband energies to best fit the observed optical transition features. A spectral feature below and near the GaAs band gap energy from GaAs barriers is enhanced by the GaAs/InGaP interface space charge accumulation induced internal field.

Photo-induced anomalous Hall effect in GaAs:MnAs granular films

We investigated the temperature-dependent Hall resistance and magnetization of the semiconductor/ferromagnetic hybrid structure of GaAs:MnAs granular films under dark and laser irradiated conditions. Under the laser irradiation with the energy above the band gap energy of GaAs, negative large Hall resistance due to the anomalous Hall effect was observed below the blocking temperature T b of the MnAs clusters. The intrinsic photo-induced magnetization of the MnAs clusters was, however, hardly observed. These results indicate that the blocking phenomena correlate with the photo-induced anomalous Hall resistance of the GaAs:MnAs granular films. Therefore, the magnetic interaction between the photo-induced carriers and the blocked magnetization of the MnAs clusters plays an important role in the photo-induced phenomena in the GaAs:MnAs granular films.

Experimental investigation of the CMN matrix element in the band anticrossing model for GaAsN and GaInAsN layers

The temperature dependence of bandgap energy of GaAs 0.98N 0.02 and Ga 0.95In 0.05As 0.98N 0.02 compounds has been analysed in order to determine the C MN matrix element of the band anticrossing model. We have found that the element equals 2.48 and 2.60 eV for GaAs 0.98N 0.02 and Ga 0.95In 0.05As 0.98N 0.02 layers, respectively. When the same value has been assumed for annealed layers, an increase in the energy of the resonant nitrogen level E N has been obtained. Two possible mechanisms leading to the increase in this energy are discussed in this work.