Vertical transport in GaAs/Ga1-xAlxAs superlattices observed by photoluminescence

Photoluminescence experiments on GaAs/Ga_1-xAl_xAs small period superlattices in which enlarged wells have been purposely introduced reveal a transfer of photoexcited carriers from the superlattice to the enlarged well localized levels. The transfer efficiency characterized by the relative intensities of luminescence peaks increases when the superlattice period decreases. Within a simple model, ionized impurity scattering and well size fluctuations account for this carrier transfer.     

Hopping mobility in semiconductor superlattices

We present results on the electrical transport perpendicular to interfaces in GaAs/Al_xGa_1?xAs superlattices. We have measured the current-voltage characteristics on a series of superlattices. This has been simulated numerically, the superlattice being replaced by an effective medium. Using this model we obtain the values of the effective mobility as a function of the superlattice period. Our data are in good agreement with a theory of phonon-assisted hopping transport between localized states, rather than the theory of phonon-limited band transport of Bloch waves.     

Suppression of the singularly localized states in dimer quasiperiodic Fibonacci superlattices

Using the transfer-matrix technique, we have numerically investigated the effect of introducing the dimer on the nature of the states across Dimer Fibonacci semiconductor superlattices on the miniband structure of the GaAs/Al_xGa_1?xAs superlattices. By the introduction of the dimer model, the transmission spectra reveal the appearance of a miniband structure with a concomitant disappearance of the singularly localized states. This behavior is due to the interaction between the states of the dimer wells inside the potential and, therefore, the system is seen by the particle as two overlapped ordered structures.     

Shallow impurity centers in semiconductor quantum well structures

Interest in the study of the behavior of shallow impurity centers in superlattices and quantum well structures is fairly recent. This paper reviews briefly both the theoretical and experimental work done in this field in the last few years. Several recent calculations of the energy levels of hydrogenic impurity states in quantum well structures, such as Ga_1?xAl_xAsGaAsGa_1?xAl_xAs, are reviewed. The behavior of these levels as a function of the quantum well size is discussed. Recent experimental data concerning the variations of the binding energies of shallow donors and acceptors as a function of the GaAs quantum well size are reviewed. A comparison between these experimental measurements and the results of recent calculations is presented.     

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.     

Raman spectroscopy of acoustic phonons in periodic and Fibonacci superlattices

Results of Raman scattering experiments on (a) periodic superlattices made up of GaAs/In_xGa_1−xAs layers with high indium concentrations, (b) GaAs/Ga_1−xAl_xAs Fibonacci superlattices, are presented. We discuss the observed peak positions and intensities using the continuum theory of acoustic wave propagation in layered media and the photo-elastic coupling model.     

1MeV Si+衬底加温注入Al0.3Ga0.7As/GaAs超晶格和GaAs的晶格损伤研究

用卢瑟福背散射／沟道技术研究了１ＭｅＶＳｉ⁺在衬底加温和室温下以不同剂量注入Ａｌ_0.3Ｇ_0.7Ａｓ／ＧａＡｓ超晶格和ＧａＡｓ后的晶格损伤。在衬底加温下，观察到Ａｌ_0.3Ｇａ_0.7Ａｓ／ＧａＡｓ超晶格和ＧａＡｓ都存在一个动态退火速率与缺陷产生速率相平衡的剂量范围，以及两种速率失去平衡的临界剂量。超晶格比ＧａＡｓ更难以损伤，并且它的两种速率失去平衡的临界剂量也大于ＧａＡｓ中的相应临界剂量，用热尖峰与碰撞模型解释了晶格损伤积累与注入剂量和衬底温度的关系。用ＣＮＤＯ／２量子化学方法计算了ＧａＡｓ和Ａｌ_xＧａ_1-xＡｓ中化学键的相对强度，并根据计算结果解释了注入过程中Ａｌ_0.3Ｇａ_0.7Ａｓ／ＧａＡｓ超晶格和ＧａＡｓ中晶格损伤程度的差别。 关键词：     

Binding energies of wannier excitons in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of Wannier excitons in a quantum well structure consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. Due to reduction in symmetry along the axis of growth of these quantum well structures and the presence of band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed leading to two exciton systems, namely, the heavy hole exciton and the light hole exciton. The variations of the binding energies of these two excitons as a function of the size of the GaAs quantum wells for various values of the heights of the potential barrier are calculated and their behavior is discussed.     

Absorption measurements at high pressure (0–10kbar) on strained superlattices

Absorption data on strained GaAs_1?xP_x-GaAs superlattices (SL, 128-period, barrier size L_B≈75 Å, quantum-well size L_z≈75 Å, alloy composition x≈0.25) are presented in the range 0–10 kbars. The absorption curves obtained show no exciton show no exciton peaks such as seen in lattice- matched Al_xGa_1?xAs-GaAs SL's, and the pressure coefficient decreases from 11.5 meV/kbar to ≈ 10.5 meV/kbar in the wells and ≈6.5 meV/kbar at energies approaching and above the barrier energies. This behavior is attributed to the fluctuations in strain caused by the alloy disorder, and clustering, of the barriers.     

Impurity absorption of light involving resonant states of shallow donors in quantum wells

The energy spectrum of localized and resonant states of shallow donors in heterostructures GaAs/Al_xGa_1?xAs with quantum wells is calculated. The widths of the resonant states belonging to the second size quantization subband are determined. It is shown that the width of a resonance level is mainly determined by the interaction with optical phonons. The spectrum of impurity absorption of light due to electron transitions from the ground state of the donor to the resonant states belonging to the second size quantization subband is calculated.     

Electron–hole superlattices in GaAs/Al x Ga1− x As multiple quantum wells

A GaAs/Al _x Ga_{1? x} As semiconductor structure is proposed, which is predicted to superconduct at T _c?≈?2?K. Formation of an alternating sequence of electron- and hole-populated quantum wells (an electron–hole superlattice) in a modulation-doped GaAs/Al _x Ga_{1? x} As superlattice is considered. This superlattice may be analogous to the layered electronic structure of high-T _c superconductors. In the structures of interest, the mean spacing between nearest electron (or hole) wells is the same as the mean distance between the electrons (or holes) in any given well. This geometrical relationship mimics a prominent property of optimally doped high-T _c superconductors. Band bending by built-in electric fields from ionized donors and acceptors induces electron and heavy-hole bound states in alternate GaAs quantum wells. A proposed superlattice structure meeting this criterion for superconductivity is studied by self-consistent numerical simulation.     

Effect of interface roughness on the density of states of finite barrier height quantum wells

We calculate the density of states of a 2D electron gas in finite barrier height quantum wells with the explicit inclusion of the interface roughness effect. By using Feynman path-integral method, the analytic expression is derived. The results show that the 2D density of states is dependent on the RMS of the fluctuation potential. The interface roughness causes localized states below the subband edge. We also apply the theory to model the finite barrier height quantum wells in Al_xGa_1?xAs/GaAs.     

Microscopic approach to the quantum size effect in superlattices

A new method for the microscopic calculation of electronic properties of superlattices is introduced and used to study the effects of carrier confinement in GaAs-Ga_1?xAl_xAs heterostructures. Our approach allows to evaluate the superlattice band structure and layer densities of states for arbitrary thickness of the GaAs and of the Ga_1?xAl_xAs slabs. We discuss the quantum size effect, the subband dispersion, the effective electron masses and the layer density of states near the conduction band edge for varying periodicity length and varying barrier thickness. Our results confirm simple potential well calculations and agree very good with experimental data.     

Phonon freedom and confinement in GaAsAlxGa1−xAs superlattices

Phonon modes in GaAsAl_xGa_1?xAs superlattices simplify when the phonon wavevector q is perpendicular to the plane of the layers. We have studied such modes using a Raman back-scattering technique on SL's grown by MBE. The results are consistent with simple ideas of LA phonon freedom and LO phonon confinement suggested by one-dimensional lattice dynamical calculations. The longitudinal acoustic (LA) modes show zone folding due to mini-zone formation. Their frequencies occur in doublets linearly dependent on q and show little mini-gap formation. This is consistent with a picture of approximately free plane wave propagating through the interfaces with Raman coupling due to SL layering of the photoelastic coefficient. By contrast, Raman data on LO modes in small period GaAsAlAs SL's suggest that these modes are standing waves strongly confined in either GaAs or AlAs.     

Single quantum well transistor with modulation doped AlGaAs/GaAs/AlGaAs structures

Self-consistent calculations have been performed to obtain the wave functions and energy subbands of the two-dimensional electrons confined in a single quantum well of a Al_xGa_1?xAs/GaAs/Al_xGa_1?xAs heterostructure. The wave functions of the two-dimensional electron gas are found to be easily controlled by an external gate voltage applied between the AlGaAs-barriers, indicating a capability of fabricating a novel quantum well device, a modulation-doped single quantum well transistor.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Well defined superlattice structures made by phase-locked epitary using RHEED intensity oscillations

Long continuing intensity oscillations of the RHEED pattern in the [100] azimuth on a (001) oriented substrate were observed during MBE growth of GaAs and Al_xGa_1−xAs. Using these oscillations, growth rates of GaAs and Al_xGa_1−xAs, and the Al mole fraction x of the Al_xGa_1−xAs were accurately monitored during the growth. The phase of the RHEED oscillations was analyzed by computer and molecular beam shutters were operated at a particular phase of the oscillations. This computer controlled phase-locked epitaxy (PLE) was used to grow precisely defined (GaAs)₂(AlAs)₂ bi-layer superlattices. Raman scattering spectra of the bi-layer superlattice showed split lines characteristic of superlattices. From TEM observation of a GaAs-AlAs multi-layered structure, it was verified that one cycle of oscillations corresponds to one monolayer growth of GaAs and AlAs. This PLE has a great advantage over the conventional MBE growth method for the precise control of very thin films and superlattice structures because it is invulnerable to fluctuations of molecular beam flux intensity.     

Effect of interdiffusion on the band structure and absorption coefficient of a one-dimensional semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential, band structure and absorption coefficient of electromagnetic radiation of a one-dimensional superlattice, composed of GaAs/Ga_1?x Al _x As quantum wells with the initially rectangular potential profile, is studied within the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the widening of the energy minibands and to the blueshift of the absorption spectrum observed in experiments.     

Refractive index of Ga1−xAlxAs

A semi-empirical method for calculating the room temperature refractive index of Ga_1?xAl_xAs at energies below the direct band edge is presented. This quantity is important in the design of GaAs heterostructure lasers as well as other wave-guiding devices using these materials. The calculated values compare favorably with recent data. The method is shown to be useful for the Ga_1?xAs_xP system as well.     

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).