GaAs Multiple Quantum Wells Research Articles

Interplay of GaAsP barrier and strain compensation in InGaAs quantum well at near-critical thickness

Abstract The effect of GaAs1−yPy tensile-strained barriers on suppressing the partial strain relaxation of InGaAs/GaAs multiple quantum wells (MQWs) is investigated when the thickness of a heavily strained InxGa1−xAs QW is near-critical thickness. The strain relaxations of In0.4Ga0.6As MQWs with and without strain-compensating GaAs1−yPy barriers are characterized using X-ray diffraction reciprocal space mapping (RSM) and micro-photoluminescence (µ-PL) mapping. A significant amount of strain relaxation (~1.53%) is measured when the thickness of each In0.4Ga0.6As QW within a 4-period MQW becomes 9.5 nm in the absence of strain-compensating layers. By adding two ~5 nm GaAs0.67P0.33 tensile-strained barriers sandwiching each QW, the strain relaxation in the In0.4Ga0.6As/GaAs0.67P0.33/GaAs MQWs is reduced to ~0.3% together with decreased surface roughness. Our study shows that tensile barriers with proper elastic energy densities are essential to achieve efficient strain compensation in a heavily-strained InGaAs MQW structure, which provides an important insight into the understanding of how to better achieve the benefits of strain compensation in III-V based QWs and superlattices.

InGaAs quantum well based dual-wavelength external cavity surface emitting laser for wideband tunable mid-infrared difference frequency generation

A two-chip vertical external cavity surface emitting laser with dual- wavelength output is presented. The active region of gain chip 1, designed for 960 nm wavelength, is composed of 6 repeats of strain uncompensated In0.185Ga0.815As/ GaAs multiple quantum wells, and the active region of gain chip 2, targeted for 1100 nm emission, is consisted of 16 repeats of strain compensated In0.26Ga0.74As/ GaAs0.94P0.06 multiple quantum wells. Two 1 mm thick uncoated fused silica birefringent filters are employed as the tuning elements, and the shorter wavelength can be tuned between 949 and 957 nm, while the longer wavelength can be changed from 1071 to 1106 nm by rotating the filters. The total intracavity power under dual-wavelength operation exceeds 36 W when the pump power is 10.5 W. For the use of intracavity different frequency generation, the wavelength tunability of down-converted mid-infrared radiation can cover a wavelength range from 6.7 to 9.0 µm.

GaAsBi/GaAs MQWs grown by MBE using a two-substrate-temperature technique

Eleven periods of GaAsBi/GaAs multiple quantum wells (MQWs) on (100) GaAs substrates were grown by molecular beam epitaxy (MBE) using the two-substrate-temperatures (TST) technique. In the TST technique, the substrate temperature was set at TGaAsBi = 350 °C for GaAsBi layers and TGaAs = 550 °C for GaAs layers for the growth of MQWs. The structural and optical characterization of GaAsBi/GaAs MQWs using TST technique was carried out for growth optimization by changing As4/Ga beam equivalent pressure (BEP) ratio. Atomic force microscope observation shows high As4/Ga BEP ratio supply can prevents roughening of the GaAsBi layer surface of MQWs. High resolution x-ray diffraction analysis results reveal Bi incorporation enhancement with increasing As4/Ga BEP ratio. The reciprocal space mapping shows highly strained MQWs layers can be grown without relaxation with respect to the GaAs substrate at optimized growth conditions. At room temperature 1.22 μm photoluminescence emission was obtained from GaAs0.962Bi0.038/GaAs MQWs grown using TST technique.

Prolonged spin relaxation time in Zn-doped GaAs/GaAsP strain-compensated superlattice

A GaAs/GaAsP strain-compensated superlattice (SL) is a highly promising spin-polarized electron source. To realize higher quantum efficiency, it is necessary to consider spin relaxation mechanisms. We have investigated the electron spin relaxation time in a Zn-doped GaAs/GaAsP strain-compensated SL by time-resolved spin-dependent pump and probe reflection measurements. The long spin relaxation time of 104 ps was observed at room temperature (RT), which is about three times longer than that of conventional undoped GaAs multiple quantum wells. Even when the excitation power increases from 30 to 110 mW, the change in the spin relaxation time at RT was small. This relationship implies that the intensity of the electron beam can be increased without affecting the spin relaxation time. These results indicate that a Zn-doped GaAs/GaAsP strain-compensated SL has the great advantage for use as a spin-polarized electron source.

High nitrogen composition–induced low interfacial roughness of GaAs0.978N0.022/GaAs multiple quantum wells grown through solid-source molecular beam epitaxy

GaAs1−xNx/GaAs multiple quantum wells (MQWs) were grown on GaAs(001) substrates through solid-source molecular beam epitaxy under various nitrogen background pressures (NBPs), and the crystal quality at the interface of GaAs1−xNx and GaAs was investigated. X-ray diffraction and electron microscopy confirmed the low interface roughness of MQWs grown at a NBP of 5×10−6Torr. Surface morphology measurements revealed a smooth surface without whisker-like defect structures. The fabricated MQWs exhibited high photoluminescence intensity because of the reduction in surface recombination with high nitrogen incorporation. Raman spectroscopy confirmed the presence of N-like local vibrational mode, and this was attributed to the presence of phase separation in GaAsN alloys. Rapid thermal annealing improved photoluminescence intensity by 100-fold and substantially reduced full width at half maximum because of MQW homogenization. These results evidence the favorable crystal interface of GaAs0.978N0.022 alloys. Hence, GaAs0.978N0.022/GaAs MQWs grown under high pressure might be useful in fabricating optoelectronic devices.

GaAsBi/GaAs MQWs MBE growth on (411) GaAs substrate

Eleven periods of GaAsBi/GaAs multiple quantum wells (MQWs) grown on (411)A and (411)B GaAs substrates at TGaAsBi= 350 °C and TGaAs= 550 °C by molecular beam epitaxy (MBE) and their structural and optical properties were investigated. The high resolution X-ray diffraction (HRXRD) analysis shows increment in Bi composition with the increasing As4 beam equivalent pressure (BEP). The reciprocal space mapping (RSM) reveals highly strained GaAsBi/GaAs MQWs layers can be grown without relaxation with respect to the (411) GaAs substrate on both A and B sides at an optimized growth condition of Bi and As4 BEP; while (411)A sample with higher bismuth content (5.5 %) shows lattice dislocation of MQWs due to lateral strain. The temperature-dependent photoluminescence (PL) shows 1.26 μm emission at room temperature from both the (411)A and (411)B samples for GaAs0.97Bi0.03/GaAs MQWs. It is perceived that GaAsBi/GaAs MQWS can be successfully grown on both sides of (411) GaAs substrate.

Room temperature spin injection into (110) GaAs quantum wells using Fe/x-AlOx contacts in the regime of current density comparable to laser oscillation

We investigate the electrical spin injection into (110) GaAs single quantum wells (SQWs) and multiple quantum wells (MQWs) using light-emitting diodes (LEDs) having Fe/crystalline-AlOx (x-AlOx) tunnel barrier contacts. A degree of circular polarization (Pc) of 5.0% is obtained for the SQW LED at 4 K with the current density of 1 kA/cm2 which is comparable to that for the laser oscillation in vertical-cavity surface-emitting lasers (VCSELs). On the basis of electron spin relaxation time and carrier lifetime in the (110) GaAs SQW measured by time-dependent photoluminescence and the value of Pc = 5.0%, the degree of spin polarization of initially injected electrons (P0) in the SQW is estimated to be 6.6% at 4 K. By using the MQW LED having a much stronger electroluminescence, a Pc value of 2.6% is obtained at room temperature (RT) with the current density of 1.5 kA/cm2. The temperature and current density dependences of Pc are found to be weak in both the SQW and MQW LEDs. The estimated P0 of 9.3% at RT suggests that the Fe/x-AlOx contacts can be used for the RT electrical spin injection for spin-controlled VCSELs.

AlAsxSb1-x단계 성분 변화 완충층을 이용한 Si (100) 기판 상 Al0.3Ga0.7As/GaAs 다중 양자 우물 형성

실리콘(Silicon, Si) 기판과 <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs 다중 양자 우물(multiple quantum wells, MQWs) 간의 격자 부정합 해소를 위해 <TEX>$AlAs_xSb_{1-x}$</TEX> 층이 단계 성분 변화 완충층(step-graded buffer, SGB)으로 이용되었다. <TEX>$AlAs_xSb_{1-x}$</TEX> 층 상에 형성된 GaAs 층의 RMS 표면 거칠기(root-mean-square surface roughness)는 <TEX>$10{\times}10{\mu}m$</TEX> 원자 힘 현미경(atomic force microscope, AFM) 이미지 상에서 약 1.7 nm로 측정되었다. <TEX>$AlAs_xSb_{1-x}$</TEX>/Si 기판 상에 AlAs/GaAs 단주기 초격자(short period superlattice, SPS)를 이용한 <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQWs이 형성되었다. <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQW 구조는 약 10 켈빈(Kalvin, K)에서 813 nm 부근의 매우 약한 포토루미네선스(photoluminescence, PL) 피크를 보였고, <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQW 구조의 RMS 표면 거칠기는 약 42.9 nm로 측정되었다. 전자 투과 현미경(transmission electron microscope, TEM) 단면 이미지 상에서 AlAs/GaAs SPS 로부터 <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQWs까지 격자 결함들(defects)이 관찰되었고, 이는 격자 결함들이 <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQW 구조의 표면 거칠기와 광 특성에 영향을 주었음을 보여준다. The <TEX>$AlAs_xSb_{1-x}$</TEX> step-graded buffer (SGB) layer was grown on the Silicon (Si) substrate to overcome lattice mismatch between Si substrate and <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs multiple quantum wells (MQWs). The value of root-mean-square (RMS) surface roughness for 5 nm-thick GaAs grown on <TEX>$AlAs_xSb_{1-x}$</TEX> step-graded buffer layer was ~1.7 nm. <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQWs with AlAs/GaAs short period superlattice (SPS) were formed on the <TEX>$AlAs_xSb_{1-x}$</TEX>/Si substrate. Photoluminescence (PL) peak at 10 K for the <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQW structure showed relatively low intensity at ~813 nm. The RMS surface roughness of the <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQW structure was ~42.9 nm. The crystal defects were observed on the cross-sectional transmission electron microscope (TEM) images of the <TEX>$Al_{0.3}Ga_{0.7}As$</TEX>/GaAs MQW structure. The decrease of PL intensity and increase of RMS surface roughness would be due to the formation of the crystal defects.

Top-Hat HELLISH-VCSOA for optical amplification and wavelength conversion for 0.85 to 1.3μm operation

The Top-Hat hot electron light emission and lasing in semiconductor heterostructure (HELLISH)-vertical cavity semiconductor optical amplifier (VCSOA) is a modified version of a HELLISH-VCSOA device. It has a shorter p-channel and longer n-channel. The device studied in this work consists of a simple GaAs p-i-n junction, containing 11 Ga0.35In0.65 N0.02As0.08/GaAs multiple quantum wells in its intrinsic region; the active region is enclosed between six pairs of GaAs/AlAs top distributed Bragg reflector (DBR) mirrors and 20.5 pairs of AlAs/GaAs bottom DBR mirrors. The operation of the device is based on longitudinal current transport parallel to the layers of the GaAs p-n junction. The device is characterised through I-V-L and by spectral photoluminescence, electroluminescence and electro-photoluminescence measurements. An amplification of about 25 dB is observed at applied voltages of around V = 88 V.

Observation of Intrinsic Inverse Spin Hall Effect

We report observation of intrinsic inverse spin Hall effect in undoped GaAs multiple quantum wells with a sample temperature of 10 K. A transient ballistic pure spin current is injected by a pair of laser pulses through quantum interference. By time resolving the dynamics of the pure spin current, the momentum relaxation time is deduced, which sets the lower limit of the scattering time between electrons and holes. The transverse charge current generated by the pure spin current via the inverse spin Hall effect is simultaneously resolved. We find that the charge current is generated well before the first electron-hole scattering event. Generation of the transverse current in the scattering-free ballistic transport regime provides unambiguous evidence for the intrinsic inverse spin Hall effect.

An Electrooptic Multiple-Quantum-Well Device for Image Processing

We investigated the use of an analog self-electrooptic effect device based on GaAs multiple quantum wells as an image processor chip to early vision processes. We focused our attention at the edge detection process using layouts of photodetectors to extract Laplacians of images. Laplacians of linear, constant, step, and Gaussian intensity profiles agree with the expected values. These results show that the device works properly as an edge detector.

Coherent control of dephasing process and many-body interactions among excitons in GaAs multiple quantum wells

We show the coherent control of exciton–exciton scattering and dephasing process in GaAs multiple quantum wells by using four-wave mixing measurements. The heavy hole (HH)–light hole (LH) quantum beat has been clearly observed. The quantum beat is suppressed when the delay time of the phase-locked pulses coincides with half the period of the oscillation. The phenomenological dephasing time hardly depend on the suppression or amplification of the quantum beat. However the magnitude of the diffracted signal intensity in the 2 k 2 - k 1 direction increases (decreases) when the quantum beat is suppressed (amplified). In other words, the dephasing process is suppressed (accelerated) depending on the strength of the HH–LH scattering. We show that the suppression and acceleration of the dephasing process is due to a fifth-order nonlinear effect.

Transitions of epitaxially lifted-off bulk GaAs and GaAs/AlGaAs quantum well under thermal-induced compressive and tensile strain

Low temperature photoluminescence and reflectance measurements on epitaxially lifted-off (ELO) bulk GaAs and GaAs/AlGaAs multiple quantum wells (MQWs) bonded to Si and MgO substrates are reported. Photoluminescence measurements indicate no strain at room temperature for the ELO bulk GaAs film but show biaxial strain at 10 K. Si-bonded films undergo tensile strain, while films with MgO host substrates experience compressive strain. Reflectance measurements at 10 K show that light hole band is closer to the conduction band for the tensile strained film. In GaAs MQW ELO films, the separation of the heavy hole and light hole band is reduced in tensile strained films by 4.7 meV, corresponding to a strain ε=−0.7±0.05×10−3 and stress X=0.9±0.05 kbar (90±5 MPa). For compressively strained films, this separation is enhanced by 3.9 meV, equivalent to a strain ε=0.6±0.05×10−3 and X=0.8±0.05×10−3 kbar (80±5 MPa). The findings demonstrate that ELO is an effective technique to introduce tensile and compressive strain in GaAs heterostructures and is appropriate for strain-related spectroscopy.

Superfluorescence from Magnetically Formed Quantum Dots: the Excitation Pulse-Width Dependence

We investigated the laser pulse-width dependence of dense plasmas confined within the magnetic length of <TEX>$In_{0.2}Ga_{0.8}As$</TEX>/GaAs multiple quantum wells under high magnetic fields up to 31 T. To fully fill the Landau levels of effectively zero-dimensional system, we used intense femtosecond (fs) laser pulses to create carrier densities near <TEX>$10^{13}/cm^2$</TEX>. The observed photoluminescence showed a characteristic of superfluorescence, above critical magnetic field when being excited by pulses shorter than coherence buildup time.

Modulation optical spectroscopy of excitons in structures with GaAs multiple quantum wells separated by tunneling-nontransparent barriers

Contactless optical electroreflectance measurements at different temperatures are used to study exciton states in a structure involving a periodic system of 36 GaAs quantum wells separated by tunneling-nontransparent AlGaAs barriers with thickness 104 nm. In the structure, the width of 32 of the quantum wells is 15 nm, while the width of the remaining four quantum wells, numbered 5, 14, 23, and 32, is 20 nm. The periodicity of the structure corresponds to the Bragg interference condition at the excitonic frequency in quantum wells at the angle of incidence of light ∼43°. From the quantitative analysis of the shape of the contactless electroreflectance line, the parameters of the exciton ground states and excited states are determined for both types of quantum wells. It is established that, for the system of four 20-nm-wide quantum wells separated by a distance of 830 nm, the size-quantization energy in the ground state is 8.4 ± 0.1 meV, and the parameter of broadening of the excitonic peak is 1.8 ± 0.1 meV at 17 K and increases with temperature up to 2.0 ± 0.1 meV at 80 K. For the system of 32 wells with the width 15 nm, the quantum confinement energy in the ground state is 14.9 ± 0.1 meV, and the parameter of broadening of the excitonic peak is 2.2 ± 0.1 and 2.6 ± 0.1 meV at 17 and 80 K, respectively. The possible causes of radiative and nonradiative broadening of exciton states in the systems are discussed.

Coherent control of electron–phonon scattering and the dephasing process in gallium arsenide multiple quantum wells

Exciton-folded acoustic phonon scattering has been observed in three types of GaAs multiple quantum wells (MQWs) by using four-wave-mixing spectroscopy. The energies of the phonons correspond exactly to those of the folded acoustic phonons calculated by the Rytov model in each MQW. We show the coherent control of the exciton–phonon scattering by the phase-locked pulses irradiation. The phenomenological dephasing time in the long-time region does not depend on the scattering. However, the magnitude of the diffracted signal intensity increases (decreases) when the exciton–phonon scattering is suppressed (amplified) at time less than the correlation, or memory time.

Nitrogen-enhanced indium segregation in (Ga,In)(N,As)/GaAs multiple quantum wells grown by molecular-beam epitaxy

Transmission electron microscopy (TEM) is used to determine the composition of quaternary (Ga,In)(N,As) quantum wells (QWs). Through a combined analysis of the chemically sensitive (002) dark-field (DF) images and the lattice-resolving high-resolution TEM images, the local distributions of nitrogen and indium in the growth direction are determined. In particular, we are able to directly detect the existence of indium segregation in (Ga,In)(N,As) QWs. A comparison with the indium distribution profile in the nitrogen-free (In,Ga)As QWs, grown under similar conditions, revealed that incorporating N into the alloy enhanced indium segregation.

Characterization of band gap in GaAsSb/GaAs heterojunction and band alignment in GaAsSb/GaAs multiple quantum wells

Photoreflectance (PR) and photoluminescence (PL) spectra are used to investigate the band alignment of GaAsSb/GaAs multiple quantum wells (MQWs). PR and PL spectra are measured for strained and unstrained GaAs 1− x Sb x /GaAs heterojunctions and coherently strained MQWs grown on a GaAs substrate by molecular beam epitaxy. The band gaps of the unstrained GaAs 1− x Sb x obtained from PL agree well with E g( x) = 1.43 − 1.9 x + 1.2 x 2. For the strained heterojunctions, the strain relaxation factor and the Sb mole fraction determined from PR measurements correspond to the results from X-ray diffraction. In the MQWs, the thickness of the GaAsSb layer is less than its critical thickness so the GaAsSb layer is coherently strained and the band gaps of the GaAs 1− x Sb x layers are estimated under this condition. In this study, the indirect transition from the electron levels in the GaAs layer to the hole levels in the GaAsSb layer is smaller than the band gap of the GaAsSb layer in the MQWs indicating that the band alignment of coherently strained GaAs 1− x Sb x /GaAs MQWs must be type-II.

Interdiffusion in narrow InGaAsN∕GaAs quantum wells

Interdiffusion in In0.32Ga0.68As0.984N0.016∕GaAs multiple quantum wells with well widths of 2 and 4nm, respectively, was investigated both experimentally and theoretically. Maximum blueshifts of 206 and 264meV in the photoluminescence spectra were observed. Secondary ion mass spectrometry showed that both In–Ga and N–As interdiffusions played key roles for the large blueshifts. The significant In–Ga interdiffusion occurred at 650°C while the N diffusion occurred at a temperature above 700°C. The theoretical results are in good agreement with the experimental observations.

Efficient generation of coherent acoustic phonons in (111) InGaAs∕GaAs multiple quantum wells through piezoelectric effects

The authors demonstrate the generation of coherent acoustic phonons in (111)-strained InGaAs∕GaAs multiple quantum wells through piezoelectric coupling by an optical pump-probe technique. Dependence of the induced strain amplitude on the photocarrier density reveals that coherent acoustic phonons can be efficiently excited by the piezoelectric effect with an unscreened built-in electric field. Under high fluence excitation, the generation of coherent acoustic phonons saturates. The experimental results are supported by a macroscopic theoretical model. No backward Raman signal and no zone-folded phonon modes were observed in this low impedance-mismatch and finite-period heterostructure.