HgTe/CdTe superlattices grown by molecular beam epitaxy

In this paper we present results concerning the optical absorption in HgTe-CdTe superlattices. We confirm the narrowing of the superlattice band-gap (the increase of cut-off wavelength, λ_c) compared to the band gap of the equivalent Hg_1?xCd_xTe alloy. We show also, as predicted by the theory, an increase of the cut-off wavelength of the superlattice when the HgTe layer thickness increases. At 300K, the agreement between theory and experiment is fairly good if we consider the onset of the absorption. The λ_c tail shifting towards shorter wavelengths could be explained by the interdiffusion between HgTe and CdTe layers. At 30K, no important change in the I.R. absorption is noticed for all the superlattices.We present for the first time a superlattice exhibiting an absorption in the 8–12 μm window.We have carried out Hall measurements on several superlattices and present for the first time transport properties on these alternate microstructures. The most important features concern the unexpected and not yet understood very high hole mobilities at 10K.     

High resolution x-ray diffractometry of the structural characteristics of a semiconducting (InGa)As/GaAs superlattice

A statistical approach is used to construct a kinematic theory of x-ray diffraction on a semiconducting superlattice with a two layer period. This theory takes two types of structural deformations into account: crystal lattice defects caused by microdefects distributed chaotically over the thickness of the superlattice, and periodicity defects of an additional superlattice potential owing to random deviations in the thicknesses of the layers of its period from specified values. Numerical simulation is used to illustrate the effect of structural defects on the development of the diffraction reflection curve. The theory is used to analyze experimental x-ray diffraction spectra of semiconducting In_xGa_1−x As/GaAs superlattices. Zh. Tekh. Fiz. 69, 44–53 (February 1999)     

Tenseur de mobilite d'un super-reseau en presence de desordre d'interface

Mobility tensor, limited by island-like interface disorder, in type-I superlattices, is studied within a simple model (Kronig-Penney and plane waves). Computed results are presented, for GaAs-Ga_1?xAl_xAs superlattices, versus variations of temperature, superlattice period and defect parameters such as localisation, width and thickness. This interface scattering process has a significant influence on near equilibrium transport at low temperature.     

A Ground Miniband Width of a Superlattice with Rectangular Quantum Wells versus Constructive Parameters

A convenient formula for the ground miniband width as a function of constructive parameters of the superlattice is derived from the solution of the Kronig–Penney equation using the perturbation theory. The conditions of applicability of this formula are verified for the GaAs/Al_xGa_1–x As superlattices used as infrared photodetectors.     

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.     

Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices

Measurements of Raman scattering were performed on GaAs-In_xGa_1?xAs strained-layer superlattices, grown by molecular beam epitaxy, with lattice periods ranging from 30 ～ 250 Å and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-Al_xGa_1?xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk In_xGa_1?xAs alloys as the ratio of the one-layer thickness of In_xGa_1?xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer superlattice and the phonon frequency is determined by the averaged In concentration.     

Optimization of absorption in InAs/InxGa1-xSb superlattices for long-wavelength infrared detection

The linear absorption coefficient of InAs/In_xGa_1-xSb superlattices is optimized based on an 8×8 envelope-function approximation (EFA) model. The effect of layer widths, indium content, buffer choice, substrate orientation, interface type, layer growth order, piezoelectricity, and layer width variations on the cutoff wavelength and the linear absorption coefficient is investigated. We propose specific superlattice parameters that optimize absorption for superlattices grown on GaSb at three cutoff wavelengths.     

Far infrared measurements of bulk and surface phonons in GaAs/AlAs superlattices

We have used far-infrared oblique-incidence reflection spectroscopy to study bulk phonon polaritons, and attenuated total reflection (ATR) spectroscopy to study surface phonon polaritons, in long-period GaAs/Al_xGa_1–xAs and short-period GaAs/AlAs superlattices. Results on the former are in good agreement with an effective-medium bulk-slab model of the dielectric tensor of the superlattice; results on the latter are analysed in terms of a model that contains dielectric-tensor contributions from the confined optic phonons.     

硅基超晶格Si1-xSnx/Si的能带结构

由于Si基发光材料能与现有的Si微电子工艺兼容,其应用前景被广泛看好. 设计具有直接带隙的Si基材料,备受实验和理论研究者的关注. 本文根据芯态效应、电负性差效应和对称性效应设计了Si基超晶格Si_1－xSn_x/Si. 其中Si_0.875Sn_0.125/Si为直接带隙材料. 在密度泛函框架内,采用平面波赝势法计算表明,Si_0.875Sn_{0.125相似文献}   

窄带隙超晶格中载流子俄歇寿命和碰撞电离率的第一性原理研究

通过第一性原理的完整形式,基于全势能线性化增广平面波方法确定的精确能带结构和波函数,推算了技术上极为重要的窄带隙半导体超晶格中载流子俄歇复合时间.少数载流子的俄歇寿命由两种相关的方法来确定:1)由Fermi-金规则直接估算,2)联系俄歇复合和其相反过程碰撞电离,建立细致平衡公式,在一个统一的结构中进行间接估算.在n掺杂HgTe/CdTe和InAs/In_xGa_1－xSb超晶格中,由直接和间接的方法确定的寿命与一些实验结果相当一致.这说明该计算模式可以作为一种精确的手段用于窄带隙超晶格材料的性能优化. 关键词： 第一性原理 俄歇复合 碰撞电离 半导体超晶格     

Exciton oscillator strength in GaAs/AlAs superlattices near type I-type II transition

We present resonance reflectivity measurements performed on graded GaAs/AlAs superlattices in the transitional region from type I to type II. A theory of exciton states and exciton oscillator strength is developed making allowance for the mixing of Γ₁, X₁ and X₃ electron states in the superlattice. Experimental and theoretical results are compared taking into account effects of superlattice imperfections.     

Improved thermoelectric power factor in metal-based superlattices

In this paper we present a detailed theory of electron and thermoelectric transport perpendicular to heterostructure superlattices. This nonlinear transport regime above barriers is also called heterostructure thermionic emission. We show that metal-based superlattices with tall barriers can achieve a large effective thermoelectric figure of merit (ZT > 5 at room temperature). A key parameter to achieving high performance is the nonconservation of lateral momentum during the thermionic emission process. Conservation of lateral momentum is a consequence of translational symmetry in the plane of the superlattice. We also discuss the use of nonplanar barriers and embedded quantum dot structures to achieve high thermoelectric conversion efficiency.     

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.     

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.     

Morphology of GaAs1−xPx superlattices grown by MOCVD and chloride VPE

Distorted layer growth manifested by nonuniform etching may occur in GaAs_1−xP_x superlattices grown by MOCVD and VPE. The distortion was found to be more severe for MOCVD growth than for VPE. The distortion is decreased by decreasing the magnitude of the interlayer strain in the superlattice but does not depend upon the layer thickness. The rate of crystal growth, the temperature of growth and the strain rate in the layer supporting the superlattice also influence the distortion. Several possible causes of the effect are discussed, including nonuniform elastic stress and/or compositional nonuniformity.     

Intrinsic radiative recombination from quantum states in GaAs-AℓxGa1−xAs multi-quantum well structures

From absorption, emission, luminescence excitation and electron spin orientation studies of undoped GaAs-A?_xGa_1?xAs superlattices we demonstrate the $\text{intrinsic}$ nature of the radiative recombination process. This is in direct contrast to recombination observed in similar purity thick GaAs material. Moreover, our results do not support a recent suggestion that enhanced LO phonon-electron coupling should occur in such superlattice structures.     

Enhanced optical nonlinearities of superlattices within the Kronig-Penney model incorporating inherent bulk nonlinearities

Third order nonlinear optical susceptibilities χ⁽³⁾ of GaAs/Ga_1?xAlAs superlattices have been predicted which are two orders of magnitude larger than those of bulk GaAs. This enhancement is due to the band nonparabolicity arising from the additional periodicity of the superlattice. These predictions, based on a tight-binding model of the superlattice dispersion, are here extended to the more realistic Kronig-Penney (KP) model. Corrections to tight-binding are non-negligible; however, enhancements of χ⁽³⁾ are still large but reduced approximately 30%–50% over previous estimates. The KP model is also here applied to superlattices employing InSb as the quantum well material. Because of the smaller effective mass of InSb, and taking account of its bulk nonparabolicity, the minibands move to higher energy, enhancing the interwell overlap and increasing χ⁽³⁾ by about one order of magnitude over that of bulk InSb. The role of the barrier material in this case is important and is discussed. The interplay between the bulk nonparabolicity and that arising from the superlattice is also addressed.     

Two types of extended states in random dimer barrier superlattices

We study numerically the effects of short-range correlated disorder on the electronic and transport properties of intentionally disordered GaAs–Al_xGa_1−xAs superlattices. We consider layers having identical thickness where the Al concentration x takes at random two different values with the constraint that one of them appears only in pairs, i.e. the random dimer barrier. Various physical quantities such as the conductance, the universal fluctuation conductance, the localization length, the resistance and its probability distribution are statistically computed by means of the transfer matrix formalism to discriminate the nature of the electronic states. In spite of the presence of disorder, the system exhibits two kinds of sets of propagating states lying below the barrier due to the characteristic structure of the superlattice. The states close to the resonance can be viewed as consisting of weakly localized states with very large localization length. In the band tails, i.e. for vanishing conductance, the states are strongly localized. The nature of the transition between these two regimes is quantitatively investigated through relevant physical quantities.     

Effect of coherency strain on the deformation of In x Ga1− x As superlattices under nanoindentation and bending

It has been shown elsewhere that the room temperature yield pressure of In _x Ga_{1? x} As superlattices measured by nanoindentation, decreases from a high value as the volume averaged strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing strain modulation. The coherency strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The strain energy released by the loss of coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and depth of approximately 1.3?µm and 1.1?µm respectively. Under uniaxial compression and tension at 500°C the superlattices deform by dislocation flow along {111} planes. The most highly strained samples also partially relax through the formation of misfit dislocations.     

Magnetooptical investigation of PbTe/Pb1−xSnxTe superlattices

Intra- and interband magnetooptical experiments were carried out on PbTe/Pb_1?xSn_xTe superlattices (x=0.135) with layer thicknesses ranging from 13 – 225 nm. For the magnetic field parallel to the [111] growth direction, the Landau levels for the superlattice band structure are calculated within the framework of the envelope function approach for the [111] and the three obliquely oriented valleys. The experimental intra- and interband data agree qualitatively with the results of the model calculation and are consistent with the predictions of a LCAO calculation according to which the electrons or holes are confined in the Pb_1?xSn_xTe layers.