Thin GaAs Layer Research Articles

Design engineering of non-linear graded InAlAs metamorphic buffer layers for efficient reduction of misfit dislocation density

Metamorphic InxAl1-xAs (xmax > 0.75) buffer layer (MBL) grown on GaAs with an optimized non-linear graded composition profile along the growth direction is proposed for enhanced reduction of misfit dislocation (MD) density in highly mismatched III-V/GaAs metamorphic heterostructures. The equilibrium distributions of MD density throughout such InxAl1-xAs/GaAs MBLs are calculated. The influence of the initial composition (xmin) of MBL and the elastically strained thin GaAs layer embedded into the InxAl1-xAs MBL on the MD density distribution was studied. An optimum value of the inverse step (Δ), representing the difference between the top In content of the InxAl1-xAs MBL and that of a In0.75Al0.25As virtual substrate (VS) grown atop is determined. It was theoretically shown that Δ above 0.04 results in relaxation of the elastic stresses in VS via formation of the MDs, while the lower Δ values allow growing the VS completely free of MDs. Finally, the metamorphic In0.75Al0.25As/graded-InAlAs/GaAs heterostructures differing from each other by only the composition profile of the graded-InAlAs MBL were grown by MBE on GaAs(001) and studied by atomic force microscopy. The structure with proposed optimized non-linear graded (x = 0.05–0.77) MBL demonstrated the ∼ 1.5 times less MD density as compared to that with the convex-graded (x = 0.05–0.81) MBL, which was estimated to be at the level of ∼ 2·107 cm−2.

Effect of emitter-base spacer design on the performance of InP/GaAsSb/InP DHBTs grown by MOCVD

Effect of spacer layer design between InP emitter and GaAsSb base in InP/GaAsSb/InP double heterojunction bipolar transistors (DHBTs) grown by MOCVD was investigated. A very thin tensile-strained GaAs layer, or a thin GaInP layer, or combination of both was inserted between InP emitter and GaAsSb base to mitigate Sb segregation and/or eliminate electron pile-up between emitter and base. With a base sheet resistances of ∼1800 ohm sq−1 for all devices, DHBTs with a GaAs spacer, a GaInP spacer and the combination demonstrate a current gain of 24, 49 and 64, respectively. The conduct band discontinuity ΔEc at InP/GaAsSb interfaces and the current blocking effect are effectively eliminated by employing the combination of GaAs and GaInP layers.

Reduction of misfit dislocation density in metamorphic heterostructures by design optimization of the buffer layer with non-linear graded composition profile

Equilibrium distributions of misfit dislocation density along the growth direction of metamorphic buffer layers InxAl1-xAs/GaAs with maximum In content xmax≥0.77 and different non-linear graded composition profiles x propto z1/n are calculated. The effect of the initial In composition (n=2) of InxAl1-xAs buffer layer with convex-graded (xmin) composition profile on misfit dislocation density as well as amount of residual stresses at its top part is considered. Using computational approach, it was shown that a dislocation-free region is formed under thin tensile-strained GaAs layer (1-10 nm) inserted into InAlAs metamorphic buffer layer, which agrees with experimental data obtained early by transmission electron microscopy. Novel non-linear graded composition profile of metamorphic buffer layer has been proposed, which results in twice reduction of misfit dislocation density as compared to the convex-graded one. In addition, equilibrium distributions of misfit dislocation density in the HEMT heterostructures with two-dimensional electron channel InxAl1-xAs, which are based on In0.75Ga0.25As/In0.75Al0.25As metamorphic buffer layer of various designs, are calculated. The values of inverse steps (Delta), representing the difference between the maximum In content of InxAl1-xAs (xmax) and In content of In0.75Al0.25As virtual substrate, at which relaxation of the elastic strains in 2D channel In0.75Ga0.25As/In0.75Al0.25As doesn't occur, are calculated for metamorphic buffer layers InxAl1-xAs with convex-graded and optimized non-linear graded composition profiles. Keywords: metamorphic heterostructures, metamorphic buffer layer, misfit dislocations, elastic stresses, In(Ga,Al)As/GaAs.

Снижение плотности дислокаций в метаморфных гетероструктурах путем оптимизации конструкции буферного слоя с нелинейным профилем изменением состава

Equilibrium distributions of misfit dislocation density along the growth direction of metamorphic buffer layers InxAl1-xAs/GaAs with maximum In content xmax ≥ 0.77 and different non-linear graded composition profiles (x ∝ z1/n) are calculated. The effect of the initial In composition (xmin) of InxAl1-xAs buffer layer with convex-graded (n = 2) composition profile on misfit dislocation density as well as amount of residual stresses at its top part is considered. Using computational approach, it was shown that a dislocation-free region is formed under thin tensile-strained GaAs layer (1–10 nm) inserted into InAlAs metamorphic buffer layer, which agrees with experimental data obtained early by transmission electron microscopy. Novel non-linear graded composition profile of metamorphic buffer layer has been proposed, which results in twice reduction of misfit dislocation density as compared to the convex-graded one. In addition, equilibrium distributions of misfit dislocation density in the HEMT heterostructures with two-dimensional electron channel In0.75Ga0.25As/In0.75Al0.25As, which are based on InxAl1-xAs/GaAs metamorphic buffer layer of various designs, are calculated. The values of inverse steps (∆), representing the difference between the maximum In content of InxAl1-xAs (xmax) and In content of In0.75Al0.25As virtual substrate, at which relaxation of the elastic strains in 2D channel In0.75Ga0.25As/In0.75Al0.25As doesn’t occur, are calculated for metamorphic buffer layers InxAl1-xAs with convex-graded and optimized non-linear graded composition profiles.

Atomic-scale insights of indium segregation and its suppression by GaAs insertion layer in InGaAs/AlGaAs multiple quantum wells

The In segregation and its suppression in InGaAs/AlGaAs quantum well are investigated by using high-resolution x-ray diffraction (XRD) and photoluminescence (PL), combined with the state-of-the-art aberration corrected scanning transmission electron microscopy (Cs-STEM) techniques. To facility our study, we grow two multiple quantum wells (MQWs) samples, which are almost identical except that in sample B a thin GaAs layer is inserted in each of the InGaAs well and AlGaAs barrier layer comparing to pristine InGaAs/AlGaAs MQWs (sample A). Our study indeed shows the direct evidences that In segregation occurs in the InGaAs/AlGaAs interface, and the effect of the GaAs insertion layer on suppressing the segregation of In atoms is also demonstrated on the atomic-scale. Therefore, the atomic-scale insights are provided to understand the segregation behavior of In atoms and to unravel the underlying mechanism of the effect of GaAs insertion layer on the improvement of crystallinity, interface roughness, and further an enhanced optical performance of InGaAs/AlGaAs QWs.

Albedo‐Enabled Enhanced Energy Harvesting via GaAs Bifacial Thin‐Film Solar Cells

In addition to direct solar illumination to the photovoltaic cell, the albedo effect provides a unique opportunity to enhance energy harvesting. However, conventional solar cells are inefficient albedo energy harvesters because the rear side of the cell is usually blocked by a thick substrate or metal contact. In this study, structurally thin active layers of GaAs thin‐film solar cells covered with transparent Ag nanowires that enable bifacial solar cell operations are fabricated. The GaAs bifacial thin‐film solar cell is fabricated by bonding the active regions of the solar cell on colorless polyimide sheets with highly transparent epoxy adhesives. The fabricated bifacial solar cells without any anti‐reflection coatings demonstrated power conversion efficiencies of 8.54 and 6.78% at the front and rear sides, respectively, under AM 1.5 G irradiance conditions. The increase in energy harvesting by the GaAs bifacial thin‐film solar cells was estimated, which showed approximately a 72% increase in short‐circuit current density (JSC) when the solar cells operated on a high‐reflection‐coated ground. Positioning the solar cells on snow or sand (deserts and beaches) increasedJSCby ≈44 and 31%, respectively, compared to monofacial solar cells.

Amplification Lengths of Spectral Components of Intrinsic Stimulated Picosecond Emission. Dependence of the Characteristic Relaxation Time of These Components on Their Amplification Lengths. Relation Between Stimulated and Spontaneous Emission Spectra in GaAs

The high-power picosecond optical pumping of a thin GaAs layer generates intense picosecond stimulated emission. The emission spectrum is a light continuum. Based on the results of previous experimental work, the following is carried out. (a) The amplification lengths of the emission spectral components, i.e., the different distances over which different emission components are amplified in the active medium produced by pumping and (b) the dependences of the characteristic relaxation time of emission components on their gain lengths are estimated. It is shown that the spectrum of the product of the gain length and the gain linearly relates the spectra of the spontaneous and stimulated emission of GaAs. This relation is established upon gain saturation which is evidenced by a dip in the gain spectrum, “burned out” by intense emission.

Effect of Carrier Heating by the GaAs Picosecond Stimulated Emission on Duration of Emission

The real-time measured envelopes of high-power picosecond optical pumping and of the intrinsic stimulated picosecond emission of a thin GaAs layer are compared. The emission duration excess over the pump duration is explained on the basis of negative feedback between the emission intensity and carrier heating by this emission. The main parameter of this duration excess is the characteristic carrier cooling time slowed down due to carrier heating by emission. The effect of this time on renormalization of the band gap due to the Coulomb interaction of carriers is also noted. The part of the concept of the picosecond dynamics of the intense stimulated emission of GaAs, which has been obtained to date in our works with co-authors, is given in Conclusions.

Cathodoluminescence visualisation of local thickness variations of GaAs/AlGaAs quantum-well tubes on nanowires

We present spatially and spectrally resolved emission from nanowires with a thin radial layer of GaAs embedded in AlGaAs barriers, grown radially around taper-free GaAs cores. The GaAs layers are thin enough to show quantization, and are quantum wells. Due to their shape, they are referred to as quantum well tubes (QWTs). We have investigated three different nominal QWT thicknesses: 1.5, 2.0, and 6.0 nm. They all show average emission spectra from the QWT with an energy spread corresponding to a thickness variation of ±30%. We observe no thickness gradient along the length of the nanowires. Individual NWs show a number of peaks, corresponding to different QW thicknesses. Apart from the thinnest QWT, the integrated emission from the QWTs shows homogeneous emission intensity along the NW. The thinnest QWTs show patchy emission patterns due to the incomplete coverage of the QWT. We observe a few NWs with larger diameters. The QWTs in these NWs show spatially resolved variations across the NW. An increase in the local thickness of the QWT at the corners blocks the diffusion of carriers from facet to facet, thereby enabling us to visualise the thickness variations of the radial quantum wells.

Anticorrelation between the Intensity of Stimulated Picosecond Emission in GaAs and the Characteristic Time of Charge-Carrier Cooling

During the powerful picosecond optical pumping, intense stimulated picosecond emission arises in a thin GaAs layer. It is found that, first, the maximum emission intensity decreases as the diameter of the pump beam increases (while the pump energy density is fixed). Second, this dependence anticorrelates with the dependence of the characteristic relaxation time of the emission on the beam diameter. This time is related to the characteristic cooling time of charge carriers, which is slowed due to carrier heating by emission. This leads to the anticorrelation given in the title.

Антикорреляция интенсивности стимулированного пикосекундного излучения GaAs и характерного времени остывания носителей заряда

During the powerful picosecond optical pumping, intense stimulated picosecond emission arises in a thin GaAs layer. It was found that, firstly, the maximum emission intensity decreases with increasing diameter of the pump beam (the pump energy density is fixed). Secondly, this dependence is anticorrelated with the dependence on the diameter of the characteristic relaxation time of the emission. And this time, in turn, is associated with the characteristic cooling time of charge carriers, which is slowed down due to heating of the carriers by emission. As a result, the autocorrelation indicated in the title is revealed.

Microstructure of GaAs thin films grown on glass using Ge seed layers fabricated by aluminium induced crystallization

We perform the growth of GaAs epilayers by molecular beam epitaxy (MBE) on Ge pseudo-substrates obtained by the aluminium induced crystallisation (AIC) of thin amorphous Ge layers deposited on silica. Despite the apparent uniformity of the AIC-Ge layer, large domains (more than 50 µm wide) previously thought to be monocrystalline are found to actually consist in smaller grains (500 to 1000 nm wide), separated by low angle grain boundaries. These defects are transferred during epitaxy to the GaAs layer and degrade the quality of the III–V material. In our growth conditions, the MBE results in the selective deposition of thin GaAs layers on Ge with respect to the silica support, but selectivity is progressively lost with increasing layer thickness.

Spatial Optimization of Modulation Doping in P-I-P QDIPs: Towards Achieving Higher Operating Temperature

We report short wavelength infrared (SWIR) photodetectors using multi-stacked modulation doped Indium-Arsenide (InAs) self-assembled quantum dots (QDs). The detriments of direct doping on the energy levels of InAs QDs have been eliminated by choosing modulation doping. We have considered three P-I-P Indium Arsenide/Gallium Arsenide (InAs/GaAs) quantum dot infrared photodetector (QDIP) heterostructures in which a thin (3 nm) P-doped GaAs layer is introduced at a distance of 7, 12 and 17 nm away from each InAs dot layer respectively. The structures have been analyzed for their morphological and optical characteristics and an optimum separation between the active layer and modulation doped layer has been achieved. The fabricated single pixel detectors exhibit a broad spectral response from 1 to 3.5 μm in the SWIR region for all the samples. The infrared photocurrent survives up to a maximum temperature of 200K for the optimized device with a peak responsivity of 1.439 A/W at wavelength λ = 1.77 μm and temperature T = 200 K.

Integrated photonics for low transverse emittance, ultrafast negative electron affinity GaAs photoemitters

Photocathodes exhibiting simultaneous high quantum efficiency, low mean transverse energy (MTE), and fast temporal response are critical for next generation electron sources. Currently, caesiated negative electron affinity GaAs photocathodes have demonstrated good overall results [Bell and Spicer, Proc. IEEE 58, 1788 (1970); Pierce et al., Appl. Phys. Lett. 26, 670 (1975)]. However, due to the nature of the photoemission process and the details of the Cs surface structure, a tradeoff exists. A low mean transverse energy of ∼25 meV can be obtained by using photons with near bandgap energy, at the cost of an unacceptably high response time, or higher energy photons can be used with a mean transverse energy of ∼60 meV with acceptable response times of 2–5 ps [Karkare et al., J. Appl. Phys. 113, 104904 (2013); Honda et al., Jpn. J. Appl. Phys. 52, 086401 (2013); Pastuszka et al. Appl. Phys. Lett. 71, 2967 (1997)]. Here, it is shown through a calibrated simulation that a thin layer of caesiated GaAs on a waveguide can potentially exhibit photoemission with MTEs ∼30 meV, ultrafast response times of ∼0.2–1 ps, and quantum efficiency of 1%–10%, breaking the traditional tradeoffs associated with bulk negative electron affinity photoemitters.

Связь релаксации собственного стимулированного пикосекундного излучения GaAs с характерным временем остывания носителей заряда

During the powerful picosecond optical pumping of a thin (~ 1 µm) GaAs layer, a stimulated intense (up to 1 GW/cm^2) picosecond emission appeared. As was found, for a fixed density of the pump pulse energy, with an increase of its diameter the characteristic picosecond time τr of the emission and carrier density n relaxation increases. Due to interrelation of the density and the temperature of the carriers at high-intensity emission (in the saturation state of the emission amplification), time τr is associated with the characteristic temperature relaxation time τT of the photo-pumped carriers, which was determined earlier theoretically with the emission-caused carrier heating taken into account. The corresponding analytical expressions for τr as a functions of τT are consistent with the above experimental results.

Switching of Spectral Modes of Picosecond Stimulated Radiation of GaAs due to Stimulated Raman Scattering in the Presence of Interband Oscillations of Electrons in the Radiation Field

It was found that self-modulation of the spectrum of high-intensity stimulated radiation of a thin (about 1 μm) GaAs layer can be represented as switching of spectral modes of radiation caused by stimulated Raman scattering (SRS). It is shown that the SRS results from modulation of population of energy levels upon interband oscillations of electrons in the presence of the radiation field. It is shown that such oscillations become possible owing to the slowing down of elimination of deviations from the Fermi distribution due to energy transport of carriers. It is clarified that the oscillations are synchronized owing to the SRS.

Kinetically controlled dewetting of thin GaAs cap from an ErAs/GaAs nanoparticle composite layer

ErAs metal nanoparticles (NPs) embedded in GaAs have multiple applications in plasmonic, terahertz, and tunneling devices. Growing a high quality thin GaAs layer over the ErAs NP layer is vital to these applications. In this work, the authors study the surface stability of a thin GaAs cap (1–5 nm) annealed in a temperature range of 450–620 °C. The thin GaAs cap covered a single layer of ErAs NPs [0.5–1.33 monolayer (ML)] grown using molecular beam epitaxy on GaAs(001) substrates at 450–500 °C. For 1.33 ML ErAs coverage, although a 1 nm GaAs cap exhibited a root-mean-square surface roughness close to 0.3 nm, the authors expected that 1 nm GaAs was not thick enough to overgrow the NPs in a height of 3–4 nm; thus, a large number of pinholes should be left on the surface. By increasing the GaAs cap thickness to 3 nm, the authors were able to achieve atomically smooth surfaces with few remaining pinholes. At a lower coverage of ErAs, 0.5 ML, the authors were able to achieve atomically flat pinhole-free GaAs caps with a thickness of 3–5 nm. However, the key finding from this study is that distinct changes in the surface morphology occurred upon annealing depending on the film thickness and NP density. In the case of 1 nm GaAs caps, clumps were formed when annealed, whereas in the case of GaAs caps of 3–5 nm in thickness, the GaAs film uncharacteristically dewetted at the ErAs NP/GaAs composite interface. Thermodynamically, this dewetting is driven by the high interfacial energy resulting from the difference in the crystal structure between GaAs and ErAs (zinc-blende and rock salt); however, surface mobility plays an important kinetic role in this process. It has been demonstrated that the dewetting can be prevented by combining a higher As overpressure, a low growth/annealing temperature, lower surface coverage of ErAs NP, and thicker GaAs caps.

Enhanced efficiency of thin film GaAs solar cells with plasmonic metal nanoparticles

ABSTRACTIn the present work, a thin film solar cell structure consisting of thin GaAs layer, metallic nanoparticle array and aluminum back reflector is proposed and studied to calculate optical absorption and current density using finite difference time domain (FDTD) numerical simulation method. Simulation results show that in comparison to other metallic nanoparticles, aluminum nanoparticles have strong plasmonic scattering effect at optimized period of 100nm and radius 40nm to improve the efficiency of thin film GaAs solar cells through enhancement of current density integrated over solar spectrum by 31% compared to bare thin film GaAs solar cells.

On the high characteristic temperature of an InAs/GaAs/InGaAsP QD laser with an emission wavelength of ~1.5 μm on an InP substrate

We report on a study of lasers with an emission wavelength of about 1.5 μm and high temperature stability, synthesized on an InP (001) substrate. Self-organized InAs quantum dots capped with a thin GaAs layer are used as the active region of the laser. A quaternary InGaAsP solid solution with a band-gap width of 1.15 eV serves as the waveguide/matrix layer. A high characteristic temperature of the threshold current, T 0 = 205 K, is reached in the temperature range 20–50°C in ridge-waveguide laser diodes. A correlation between the values of T 0 and the band-gap width of the waveguide layers is found.

Below-band-gap absorption in undoped GaAs at elevated temperatures

This paper presents results of measurements of optical absorption in undoped epitaxial GaAs for photon energies below the band gap. Absorption spectra were determined from transmission spectra of a thin GaAs layer at several temperatures between 25 °C and 205 °C. We optimized our experiment to investigate the long-wavelength part of the spectrum, where the absorption is relatively low, but significant from the point of view of applications of GaAs in semiconductor lasers. Absorption of 100 cm−1 was observed over 30 nm below the band gap at high temperatures.