AlGaAs Layers Research Articles

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.

Multiexciton complex from extrinsic centers in AlGaAs epilayers on Ge and Si substrates

The multiexciton properties of extrinsic centers from AlGaAs layers on Ge and Si substrates are addressed. The two photon cascade is found both in steady state and in time resolved experiments. Polarization analysis of the photoluminescence provides clearcut attribution to neutral biexciton complexes. Our findings demonstrate the prospect of exploiting extrinsic centers for generating entangled photon pairs on a Si based device.

Tuning Characteristics of Monolithic MEMS VCSELs With Oxide Anti-Reflection Layer

We demonstrate a micromachined tunable vertical-cavity surface-emitting laser with a monolithically formed anti-reflection (AR) layer for highly efficient electro-thermal wavelength tuning. The AR layer is formed by the lateral oxidation of an aluminum-rich AlGaAs layer and no extra process is needed. We fabricated a micromachined GaAs vertical-cavity surface-emitting laser and measured its wavelength tuning characteristic. The measurement result shows that the oxide layer functions as an AR layer for linear and efficient tuning characteristics. A large negative temperature dependence of wavelength from -0.24 to -2.0 nm/K was demonstrated using a thermally actuated cantilever for efficient electro-thermal wavelength tuning. An electro-thermal tuning efficiency of -2.3 to -4.8 nm/mW was obtained.

Asymmetric design of photonic crystal surface-emitting lasers with low-threshold characteristics

We present AlGaAs-InGaAs multiquantum wells photonic crystal surface-emitting lasers by using the transfer matrix method and coupled wave method to achieve a low-threshold operation. The extremely low-threshold gain is achieved by adopting an asymmetric cladding layer design to enhance both of the vertical optical confinement factors for the quantum wells and photonic crystal (PC). By modifying the composition of the AlGaAs layer to raise the refractive index in the p-type cladding, optical field distribution will obviously be shifted to the p side. Hence, it results in a significant coupling enhancement between the optical mode profile and the PC layer. The optimized value of the vertically optical confinement factor of the PC layer is 13.94%, and the corresponding threshold gain can be as low as 19.45 cm(-1).

Terahertz-Induced Optical Emission of Photoexcited Undoped GaAs Quantum Wells

Intense terahertz (THz) pulse induces photoluminescence (PL) flash from undoped high-quality GaAs/AlGaAs quantum wells under continuous wave laser excitation. The number of excitons increases 10,000-fold from that of the steady state under only laser excitation. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes impurity states during the 1 ps period of the THz pulse and release carriers from a giant reservoir containing impurity states in the AlGaAs layers.

Tunnel Junction with Autodoped AlGaAs on InP

We describe the low resistance of tunnel junction structures with a p+-AlGaAs layer grown on an InP substrate using the autodoping technique. An Al0.4Ga0.6As layer showed a hole concentration of 2.4 × 1020 cm−3 without additional material sources. We demonstrated that the proposed tunnel junction structure with a p+-Al0.4Ga0.6As/In0.52Al0.48As multiple-quantum-well layer on an InP substrate exhibited a low resistance of 2.5 × 10−5 Ω cm2, as estimated from reverse current–voltage characteristics, and a tunnel peak current density of 170 A/cm2.

Characterization of semiconductor devices and wafer materials via sub-nanosecond time-correlated single-photon counting

Time-correlated single-photon counting (TCSPC) of semiconductor photoluminescence is presented as a versatile technique for addressing diverse aspects of charge carrier dynamics on a pico- to microsecond timescale. Particularly, advantages of expanding this time-domain technique with spectral and spatial resolution are demonstrated. By differentiating the spectral channels within the photoluminescence signal, dynamics of the charge carriers can be correlated to particular materials and substructures for analyzing their functions in complex, multicomponent systems. Diffusion and transport phenomena become directly accessible, and localized variations of charge carrier lifetimes can be associated with a particular morphology when the measurements are carried out in the context of microscopic imaging. These general capabilities are demonstrated specifi cally on a GaAsP quantum well embedded both in an AlGaAs layer structure and in a multi-layer CdTe-CdS heterojunction system.

Quantitative analysis of the quantum dot superlattice by high-resolution x-ray diffraction

A new high-resolution x-ray diffraction approach for quantitative analysis of superlattice structures (SLs) with self-assembled quantum dots (QDs) was developed. For numerical simulations of the 2D angular distribution of diffracted x-ray radiation, both the coherent and diffuse scattering components have been calculated. Direct comparison of simulated patterns and experimental results revealed good agreement of the calculated intensity distribution with experimental reciprocal space maps for the superlattice GaAs(001)-AlGaAs-{InAs QDs-GaAs}SL with 20 periods of quantum dots. The simulation procedure allows one to obtain data about the shape, average size, elastic strains around the QDs, average density of the QDs, the presence of short- or long-range order in the arrangement of QDs in the semiconducting matrix, the vertical and lateral correlation lengths of the ensemble of quantum dots, and the parameters of the intermediate GaAs and AlGaAs layers.

A GaAs/Air Multilayer Cavity for a Planar-Type Nonlinear Optical Device

GaAs/air multilayer cavity structure is expected as a novel planar-type nonliner optical device because of the strong enhancement of optical electric fileds in the cavity. The optical Kerr signal was estimated to be two order of magnitude larger than that from GaAs/AlAs multilayer cavity with the same Q-value by numerical simulation. We fabricated a GaAs/air cavity structure with 2-period distributed Bragg reflector (DBR) layers on each side of the λ-cavity layer by wet etching of sacrificial AlGaAs layer. Measured reflection spectra showed the expected structure was successfully fabricated in some parts.

Carrier localization and in-situ annealing effect on quaternary Ga1−xInxAsySb1−y/GaAs quantum wells grown by Sb pre-deposition

Using temperature-dependent photoluminescence spectroscopy, we have investigated and compared intrinsic InGaAs, intrinsic GaInAsSb, and p-i-n junction GaInAsSb quantum wells (QWs) embedded in GaAs barriers. Strong carrier localization inside the intrinsic GaInAsSb/GaAs QW has been observed together with its decrease inside the p-i-n sample. This is attributed to the effect of an in-situ annealing during the top p-doped AlGaAs layer growth at an elevated temperature of 580 °C, leading to Sb-atom diffusion and even atomic redistribution. High-resolution X-ray diffraction measurements and the decrease of both maximum localization energy and full delocalization temperature in the p-i-n QW sample further corroborated this conclusion.

MBE grown GaAsBi/GaAs double quantum well separate confinement heterostructures

GaAsBi/GaAs double quantum wells and double quantum well separate confinement heterostructures are grown at low temperatures using molecular beam epitaxy. Methods of achieving identical quantum wells in double quantum well structures without growth interruption are proposed and implemented. Cross-sectional transmission electron microscopy and room temperature photoluminescence measurements indicate that the samples have excellent structural and optical properties. The high optical quality of the samples is attributed to the surfactant effect of Bi throughout the low temperature growth of GaAs and AlGaAs layers. The proposed approach can be extended to grow laser diode structures with multiple quantum well separate confinement heterostructures containing more identical quantum wells.

Photoluminescence lifetimes in GaAs/Al<sub>0.3</sub>Ga<sub>0.7</sub>As structures designed for microwave and terahertz detectors

This paper presents the photoluminescence spectra and light emission lifetimes in GaAs/Al0.3Ga0.7As structures designed for microwave and terahertz detectors. The photoluminescence and light emission lifetimes were investigated both before and after etching of the cap-layers, and possible mechanisms of carrier recombination are discussed. The characteristic time of the free exciton emission corresponds to 0.5 ns at a temperature of 3.6 K. The recombination lifetime of the free electron acceptor was measured to be 22–25 ns in GaAs structures and 4–15 ns in AlGaAs structures. The emission lifetime of deep Si level having 170 meV activation energy in AlGaAs layer was found to be equal to 40 ns. The excitonic photoluminescence maxima of n+/n-GaAs and n+/n-Al0.3Ga0.7As homojunction structures in comparison to n-GaAs and n-Al0.3Ga0.7As were shifted by approximately 200 ps. This suggested that the drift of free carriers is important in the formation of free excitons in n-GaAs and n-Al0.3Ga0.7As layers.

Photonic crystal nanocavities in GaAs/AlGaAs with oxidised bottom cladding

We present a solution to the difficult task of removing an oxide-based hard mask from a photonic crystal fabricated in the GaAs/AlGaAs system. We use a polymer backfill technique to seal the AlGaAs layer, thereby making it inaccessible to the wet-etch solution. This allows us to use a GaAs active layer for the photonic crystal placed on an oxidised AlGaAs layer which provides mechanical and thermal support. Using this technique, we fabricated GaAs-based photonic crystal cavities and measured respectable quality factors (Q≈2200) despite the intrinsic asymmetry of the system. The technique presents a viable method for producing electrically injected photonic crystal cavities for operation on a mechanically stable and thermally conducting buffer layer.

2D irregular structure in the LED surface patterned by NSOM lithography

This contribution presents near-field scanning optical microscope (NSOM) in illumination mode as effective tool for semiconductor device surface patterning. Though the effect of periodic structure application on the light emitting diode (LED) surface is well known, for the first time, the effect of irregular structure is investigated. The non-contact mode of the NSOM lithography was used to pattern two-dimensional irregular structure on the emitting surface of GaAs/AlGaAs-based LED. The structure was patterned in the upper confinement AlGaAs layer and the enhancement of radiation 1.2–1.5 times from the patterned air holes in comparison with the surrounding surface was proven, which is similar to the enhancement of light extraction efficiency published for periodic structure application.

Single photon emission from impurity centers in AlGaAs epilayers on Ge and Si substrates

We show that the epitaxial growth of thin layers of AlGaAs on Ge and Si substrates allows to obtain single photon sources by exploiting the sparse and unintentional contamination with acceptors of the AlGaAs. Very bright and sharp single photoluminescence lines are observed in confocal microscopy. These lines behave very much as single excitons in quantum dots, but their implementation is by far much easier, since it does not require 3D nucleation. The photon antibunching is demonstrated by time resolved Hanbury Brown and Twiss measurements.

Current–Voltage Characteristics of GaAs/AlGaAs Coupled Multiple Quantum Well Solar Cells

We have studied how current–voltage (I–V) characteristics of GaAs/AlGaAs multiple quantum well (MQW) solar cells under illumination depend on the wavelength of incoming photons at 6 K. Coupled MQW structures used in this study consist of 4-nm-thick GaAs and 3-nm-thick AlGaAs layers. It is found that I–V curves under illumination measured at 6 K exhibit distinct shapes that depend systematically on the energy Eph of incoming photons; when Eph is smaller than the bandgap Eg of AlGaAs, the photocurrent shows a single-step decrease with voltage, while for Eg < Eph, it decreases in two or three steps. These behaviors are explained in terms of bias-voltage dependent changes in various dynamic processes of photogenerated carriers.

Current–Voltage Characteristics of GaAs/AlGaAs Coupled Multiple Quantum Well Solar Cells

We have studied how current–voltage (I–V) characteristics of GaAs/AlGaAs multiple quantum well (MQW) solar cells under illumination depend on the wavelength of incoming photons at 6 K. Coupled MQW structures used in this study consist of 4-nm-thick GaAs and 3-nm-thick AlGaAs layers. It is found that I–V curves under illumination measured at 6 K exhibit distinct shapes that depend systematically on the energy Eph of incoming photons; when Eph is smaller than the bandgap Eg of AlGaAs, the photocurrent shows a single-step decrease with voltage, while for Eg < Eph, it decreases in two or three steps. These behaviors are explained in terms of bias-voltage dependent changes in various dynamic processes of photogenerated carriers.

850-nm diode lasers based on AlGaAsP/GaAs heterostructures

Laser heterostructures with waveguides and emitter layers made of AlGaAs and AlGaAsP alloys are grown by hydride metal-organic vapor-phase epitaxy on a GaAs substrate and studied. It was shown that the heterostructure containing AlGaAsP layers has a large curvature radius in comparison with the structure consisting of AlGaAs layers. Ridge waveguide lasers with an aperture of 100 μm are fabricated based on the AlGaAsP/GaAs laser heterostructure and studied. The internal optical loss of the lasers is 0.75 cm−1; the characteristic parameter T0 = 140 K in the temperature range of 20–70°C. The maximum optical output power per mirror reached 4.1 W; cw lasing was retained at a heat sink temperature of 120°C.

Effects of Plasma Processes on the Characteristics of Optical Device Structures Based on GaAs

We investigate the effects of inductively coupled O2 plasma ashing, capacitively coupled C3F8 plasma etching, and capacitively coupled C3F8/O2 plasma etching on the optical and electrical characteristics of GaAs-related optical device structures. The O2 plasma ashing produces non radiative recombination centers close to the sample surface. The sample with a cap AlGaAs layer prevents the degradation of photoluminescence (PL) intensity, probably inhibiting the penetration of damaging ions into the buried quantum well. The C3F8 plasma etching also degrades the PL intensity; however, under moderate conditions, the effect of non radiative surface recombination can be suppressed by the formation of a surface oxide layer. On the other hand, the formation of oxide increases the specific contact resistance (ρc) of the electrode. The etching with C3F8/O2 mixed plasma also deteriorates the optical characteristics presumably owing to the synthesized impact of the constituent gases, while it has limited effect on ρc.

Single Photon Generation from Nitrogen Atomic-Layer Doped Gallium Arsenide

We have studied the properties of photoluminescence (PL) from individual isoelectronic traps formed by nitrogen-nitrogen (NN) pairs in nitrogen atomic-layer doped (ALD) GaAs. Micro-PL measurements were performed to investigate the properties of single photons generated from individual isoelectronic traps. Twin PL peaks were observed from individual isoelectronic traps in nitrogen ALD GaAs (001). The PL transitions at longer and shorter wavelength sides were linearly polarized in the [110] and [1-10] directions, respectively. The peak splitting and polarization properties can be explained by some in-plane anisotropy most likely due to strain in host crystal. From individual isoelectronic traps in nitrogen ALD GaAs (111), a single PL peak with random polarization was observed, showing that the growth on (111) surface is an effective way to obtain unpolarized single photons. As for nitrogen ALD GaAs (110), different polarization properties were obtained depending on the atomic configuration of NN pairs. In addition, we have used AlGaAs layers to diminish the in-plane anisotropy and could successfully obtained single emission lines with unpolarized character. Introducing AlGaAs layers was also useful for improving the luminescence efficiency.