Electroluminescent device based on AlxGa1−xAs–GaAs quantum well nanostructures

AlGaAs–GaAs based quantum well nanopillar arrays are fabricated by using the UV lithography and the chlorine based reactive ion etching. The nanostructure is fabricated so as to get the confinement of carriers within the i-GaAs quantum well layer of 9 nm thick sandwiched between two barrier layers of Al_0.33Ga_0.67As of 11 nm thick in order to induce the possible light emission from the quantum well region. The size of pillars is obtained from SEM analysis. The number of pillars available within the 1 m² mesa size is found to be around 400 having the pillar size between 10 and 50 nm. Electroluminesence (EL) is detected from the nanopillars when applying a forward bias voltage of 1.3 V and the emitted light is observed at around 830 nm.     

Density of states in a cylindrical GaAs/AlxGa1−xAs quantum well wire under tilted laser field

Within the effective-mass approximation the subband electronic levels and density of states in a semiconductor quantum well wire under tilted laser field are investigated. The energies and wave functions are obtained using a finite element method, which accurately takes into account the laser-dressed confinement potential. The density of states obtained in a Green's function formalism is uniformly blueshifted under the laser's axial field whereas the transverse component induces an additional non-uniform increase of the subband levels. Our results confirm that the tilted laser field destroys the cylindrical symmetry of the quantum confinement potential and breaks down the electronic states' degeneracy. Axial and transversal effects of the non-resonant laser field on the density of states compete, bringing the attention to a supplementary degree of freedom for controlling the optoelectronic properties: the angle between the polarization direction of the laser and the quantum well wire axis.     

Fractional-dimensional approach for biexcitons in GaAs/AlxGa1−xAs quantum wells

The energy of a biexciton in a GaAs/Al_xGa_1?xAs quantum well structure with finite barriers is investigated by using the geometrical model of two-dimensional biexcitons proposed by Singh et al. [J. Singh, D. Birkedal, V.G. Layssenko, J.M. Hvam, Phys. Rev. B 53 (1996) 15909; I.-K. Oh, J. Singh, Phys. Rev. B 60 (1999) 2528]. A fractional-dimensional approach is used to obtain the binding energy of the biexciton in both square quantum wells and parabolic quantum wells. Theoretical results show that the binding energy of a biexciton in a finite quantum well exhibits a maximum with increasing well width. The ratio of the binding energy of a biexciton to that of an exciton in a quantum well structure is found to be sensitive to the electron-to-hole mass ratio and larger than that in the three-dimensional system. The results agree fairly well with previous experimental results. The results of our approach are also compared with those of earlier theories.     

Doping profile effects on modulation-doped single nonabrupt GaAs/AlxGa1 − xAs quantum wells

We study inhomogeneous doping effects on the confinement properties of modulation-doped single nonabrupt GaAs/Al_xGa_1 − xAs quantum wells. We describe the inhomogeneous doping using error function profiles, and we solve self-consistently the coupled Schrödinger (with a position dependent kinetic energy operator) and Poisson equations to obtain the electron energy levels. When the nonabrupt interfaces (spacer layer) are 10Å(100Å) wide and the presence of Si-dopant density in a 100 Å GaAs well region is only 10% of the Si-dopant density in the Al_0.3Ga_0.7As barriers, the lowest intersubband transition energy increases 37 meV in comparison with that calculated within the homogeneous doping-abrupt interface picture.     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Temperature dependence of excitonic transitions in AlxGa1 − xAs/GaAs quantum wells

The temperature dependence of excitonic transitions in double quantum well heterostructures in the temperature range of 2–300 K were investigated. A crossing between excitonic transition experimental curves as a function of temperature in quantum wells of the same thickness and different barrier height is observed. The influence of the barrier height on the temperature dependence of excitonic states in the quantum wells is analyzed.     

Index of refraction of GaAsAlxGa1−xAs superlattices and multiple quantum wells

Recent research of superlattices and multiple quantum wells has generated considerable interest in the optical waveguiding properties of these structures for optoelectronic applications. As a result we present a theoretical study of the index of refraction of superlattices and determine its variation as a function of frequency and the superlattice parameters, i.e., layer width and AlAs composition. Γ-region exciton and valence-band mixing effects are included in the model. It is found that these two effects have an important influence on the value of the index of refraction and that superstructure effects rapidly decrease for energies greater than the superlattice potential barriers. Because of the quasi-two-dimensional character of the Γ-region excitons, our results indicate that the superlattice index of refraction can vary by ∼ 2% at the quantized, bound-exciton, transition energies. Overall, the theoretical results are in good agreement with the experimental data.     

Fundamental studies and device application of δ-doping in GaAs Layers and in AlxGa1−xAs/GaAs heterostructures

In addition to the realization of atomically abrupt interfaces in III–V semiconductors by molecular beam epitaxy, the confinement of donor and acceptor impurities to an atomic plane normal to the crystal growth direction, called-doping, is important for the fabrication of artifically layered semiconductor structures. The implementation of-function-like doping profiles by using Si donors and Be acceptors generates V-shaped potential wells in GaAs and Al_xGa_1–xAs with a quasi-two-dimensional (2D) electron (or hole) gas. In this review we define three areas of fundamental and device aspects associated with-doping. (i) The prototype structure of-doping formed by a single atomic plane of Si donors in GaAs allows to study the 2D electron gas by magnetotransport and tunneling experiments, to study the metal-insulator transition, and to study central-cell and multivalley effects. In addition, non-alloyed ohmic contacts to GaAs and GaAs field-effect transistors (-FETs) with a buried 2D channel of high carrier density can be fabricated from-doped material. (ii) GaAs sawtooth doping superlattices, consisting of a periodic sequence of alternating n- and p-type-doping layers equally spaced by undoped regions, emit light of high intensity at wavelengths of 0.9 < <1.2 [m], which is attractive for application in photonic devices. The observed carrier transport normal to the layers due to tunneling indicates the feasibility of this superlattice as effective-mass filter. (iii) The confinement of donors (or acceptors) to an atomic (001) plane in selectively doped Al_xGa_1–xAs/GaAs heterostructures leads to very high mobilities, to high 2D carrier densities, and to a reduction of the undesired persistent photo-conductivity. These-doped heterostructures are thus important for application in transistors with improved current driving capabilities.Extended version of a paper presented at the18th Int. Symp. GaAs Related Compounds (Heraklion, Crete, 1987)     

Nonlinear properties in InxGa1 − xAs/GaAs multiple quantum well laser structures

In this paper we investigated nonlinear properties and lasing in In_xGa_1 − xAs/GaAs multiple quantum wells grown by metal-organic chemical vapour deposition. A systematic study, performed by high excitation photoluminescence measurements as a function of excitation intensity, allowed us to identify the minimum well width for observing stimulated emission from well states. We also determined the threshold for stimulated emission for well and barrier lasing. Radiative recombination energies are identified by using theoretical data obtained in the effective mass approximation, including boundary conditions and strain.     

Magnetoreflectance and luminescense study of beryllium acceptors in selectively doped GaAs/AlxGa1−xAs multiple quantum wells

We present a magneto-optical study of p-type GaAs/Al_xGa_1−xAs quantum wells doped with Be acceptors over the central one-third of the GaAs layers. Using magneto-reflectance and magneto-luminescence spectroscopies, we have investigated (a) interband Landau transitions and (b) transitions from the conduction band Landau levels to the Be acceptors. Binding energies of the acceptors were determined and the dependence of the impurity ground state energy on magnetic field has been studied.     

Accumulation layer and interface effects in doped nonabrupt GaAs/AlxGa1 − xAs single quantum wells

The electron energy levels in doped nonabrupt GaAs/Al_xGa_1 − xAs single quantum wells 100 Å wide are calculated. Interface widths varying from zero to four GaAs unit cells are taken into account, as well as band bendings of 0–90 meV. It is shown that interface effects on the energy levels are important and sensitive to the level of doping. When interfaces of only two GaAs unit cells and a band bending of 40 meV are considered, the ground-state (first excited state) energy level shifts toward energies as high as 4 meV (20 meV).     

The influence of growth patterns on the transmission properties of nonabrupt GaAs/AlxGa1−xAs heterojunctions

The influence of growth patterns in the transmission properties of nonabrupt GaAs/Al_xGa_1−xAs heterojunctions is investigated. Five interfacial growth patterns, representative of interfacial alloy variations generated by different growth techniques, are used. It is shown that carrier transmission depends on the type of the aluminum molar fraction variation through the interface. A study of the role of the interface width in carrier transmission is done for each compositional growth profile.     

Effect of SiO2–metal–SiO2 plasmonic structures on InGaAs/GaAs quantum well intermixing

Dielectric–metal–dielectric sandwich structures have been fabricated on top of an InGaAs/GaAs single quantum well (QW) structure to enhance atomic interdiffusion across the QW interfaces at elevated temperature during rapid thermal annealing using a halogen lamp as the heating source. The QW intermixing enhancement is realized during rapid thermal annealing. By placing a properly designed SiO₂–Ag–SiO₂ structure on top of the QW sample, a blueshift in photoluminescence emission from 920 to 882 nm was observed, larger than that obtained in a SiO₂-capped QW annealed at the same condition. Finite-difference time-domain simulation and optical reflectance measurements showed that the enhanced QW intermixing is due to the plasmonic resonance-enhanced light absorption and suppressed light reflection from the SiO₂–Ag–SiO₂ structure.     

Structural fluctuations and randomness in GaAsAlxGa1−xAs superlattices

We discuss the use of X-ray and Raman scattering to probe structural disorder in aperiodic GaAsAl_xGa_1−xAs superlattices, including random and quasiperiodic examples. Evidence is found for the presence of monolayer-thick steps at the interfaces. The X-ray data appear far more sensitive to this type of disorder than the acoustic phonon spectra obtained by Raman scattering.     

AlxGa1—xAs／GaAs异质结材料的XPS研究

用Ｘ射线光电子能谱（ＸＰＳ）结合氩离子溅射深度剖析对一系列不同ｘ值的ＡｌｘＧａ１－ｘＡｓ／ＧａＡｓ异质结材料中各主元素的分布及化学状态和相对含量的变化进行了分析，发现Ａｌ向表面偏析的现象及Ａｓ和Ａｌ的择优溅射问题，并对此进行了讨论。同时用ＸＰＳ法进行了Ａｌ的定量分析，并与光致发光法测得的Ｘ值进行了对比，发现二者有非常好的线性关系。     

Band structure of a cylindrical GaAs/AlxGa1 − xAs superwire

We investigate the existence of a band structure in GaAs/Al_xGa_1 − xsuperlattices with cylindrical symmetry, namely GaAs/Al_xGa_1 − xAs cylindrical superwires. These systems consists of a large number of concentric GaAs and Al_xGa_1 − xAs alternate cylindrical shells around a central GaAs cylindrical wire. Despite the radial configuration (that breaks the translational symmetry) and the electron confinement in the central three-dimensional well, a band structure can emerge depending on the number and thickness of the cylindrical shells.     

Effects of incident-light-intensity-dependent band gap narrowing on barrier heights of p-doped AlxGa1−xAs/GaAs heterojunction devices

Band gaps of semiconductor materials are reduced due to band gap narrowing (BGN). Photoluminescence measurements on GaAs and AlGaAs thin films revealed a dependency of incident light intensity, and temperature in BGN in addition to the doping density. As a result, the valence band offset of p-doped GaAs/AlGaAs heterojunctions were reduced under illumination and high temperatures. We present evidence of incident-light-intensity causing barrier reduction at temperature >50 K causing zero valence band offsets in low-barrier heterostructures such as p-GaAs/Al_0.01Ga_0.99As, in addition to the dark-current increase by thermal excitations, causing the device failure at high temperatures.     

Photoluminescence and dark current of p-doped InGaAs/AlxGa1−xAs strained multiple quantum wells

We report the temperature dependence of the photoluminescence spectra and current-voltage (I_d-V) characteristics of p-doped In_0.15Ga_0.85As/AlGaAs quantum well infrared photodetectors (QWIPs) with different barrier heights grown by molecular beam epitaxy (MBE). The dark current at low temperatures is found to be about three orders of magnitude lower than that reported for the n- and p-doped QWIPs made of other material systems. The PL spectra show two emission peaks which correspond to an intersubband absorption and are tunable by changing the mole fraction of Al. The low energy emission peak of the In_0.15Ga_0.85As/Al_0.45Ga_0.55As QWIP is found to be much lower in intensity than that of the high one, due possibly to excess Al which may result in defects or imperfection at/or near well-barrier interfaces.