Photoreflectance of GaAs doping superlattices

We have performed room-temperature photoreflectance measurements on two GaAs doping superlattices having considerably different built-in potentials (1.2 eV and 85 meV). The first sample exhibits Franz-Keldysh oscillations, the period of the oscillations corresponding to the . A second dc pump beam has been used to change the electron-hole concentration and hence the built-in field. The spectrum of the second sample displays a number of features corresponding to quantized electron and hole states. There is qualitative agreement between experiment and theoretical calculation based on a two-band tight-binding model. In both samples the dependence of the amplitude of the photoreflectance signal on pump chopping frequency yields the minority carrier lifetime.     

Computerized setup for double-monochromator photoreflectance spectroscopy

An experimental setup for studying semiconductor structures by photoreflectance spectroscopy is designed. The double-monochromator-based optical scheme of the setup makes it possible to depress uncontrolled heating of the sample and diminishes a bending of the energy bands due to charge carrier photogeneration. Accordingly, the photoreflectance spectra are detected with a minimal influence of the modulating and probe radiations on the sample. With this setup, the room-temperature photoreflectance spectra from GaAs/GaAsP superlattices are taken and the interband transition energies, as well as a potential step in the conduction band of these superlattices, are measured.     

Photoconductivity and photoreflectance studies of quantum-wells and superlattices

We have performed room-temperature in-plane photoconductivity and photoreflectance measurements on various types of GaAs/AlAs quantum-wells and superlattices. The photoconductivity spectra of samples with even a few (1 to 7) quantum-wells show sharp excitonic structures superimposed on the bulk GaAs signal, and provide a useful alternative to absorption spectra. The photoreflectance spectra are demonstrated experimentally to follow the third derivative of the photoconductivity spectra, as previously reported for bulk materials, even though the mechanism of photoreflectance must be different in the present two-dimensional systems.     

Ⅱ—Ⅵ族稀磁半导体Cd1—xMnxTe／Cd1—yMnyTe超晶格的光调制反 …

本文报道用分子束外延技术生长的Ｃｄ１－ｘＭｎｘＴｅ／Ｃｄ１－ｙＭｎｙＴｅ超晶格样品在８０Ｋ下的光调制反射谱实验结果，观测到１１Ｈ，２２Ｈ，３３Ｈ和１１Ｌ等激子跃迁结构。计及晶格失配导致的应力效应，对子能级结构进行了理论计算。     

GaAs掺杂超晶格的光反射谱

不同调制光强下测量了MBE GaAs掺杂超晶格的室温光反射谱。较低光强下观察到一系列对应于禁戒跃迁(△n≠0)的精细结构,因此否定了它是三阶微商谱的可能性。明确提出了掺杂超晶格的光反射为一阶微商谱,并讨论了其产生机制。理论计算结果能较好地解释实验现象。 关键词：     

Role of beryllium doping in strain changes in II-type InAs/GaSb superlattice investigated by high resolution X-ray diffraction method

We have studied the influence of the beryllium doping on strain in the II-type InAs/GaSb superlattices (SLs) by means of high-resolution X-ray diffraction (HRXRD). Three analyzed superlattices were grown by molecular beam epitaxy. One of the examined superlattices was undoped. Two others structures, called doped SLs, composed of two superlattices: Be-doped and undoped which were grown one on the top of each other. The doping concentration was determined by secondary ion mass spectroscopy. The doping level was 1×10¹⁷ cm^?3 and 2×10¹⁹ cm^?3. For doped superlattices, the HRXRD measurements showed splitting of satellite (ST) peaks. Furthermore, the separation of ST peaks increase with doping level. In contrast, for undoped superlattice, the splitting of the ST peaks was not observed. Sometimes the separation of ST peaks can be caused by change of thickness period of superlattice or partial relaxation of the structure. However, we claim that in our experiment the splitting is caused by another mechanism: The presence of Be atoms in SL causes the change of average lattice constant of the superlattice. The influence of Be dopant on lattice parameter of superlattice was confirmed by theoretical simulations. Furthermore, the change of the lattice constant (Δa/a) of the GaSb:Be buffer was examined. The reduction of lattice parameter of GaSb was noticed. It was caused by the presence of Be doping and unintentionally incorporated As-atoms in the GaSb layer. It is very important to know that even very small Be concentration (1×10¹⁷ cm^?3) causes the change of average lattice parameter of SL.     

MOVPE Growth and optical characterization of ZnSe/ZnS strained layer superlattices

We have grown ZnSe/ZnS superlattices using low pressure MOVPE. The superlattices were grown onto a relaxed ZnSe buffer and were constituted of 45 periods. The well and barrier thicknesses were chosen from the calculation of critical thicknesses for coherent and free standing situations. Following this analysis we have grown samples with individual ZnSe, ZnS thicknesses of 3 to 9 monolayers.The optical properties of the samples were studied using photoluminescence, photoreflectance and reflectivity experiments. The results of the photoluminescence and photoreflectance experiments are consistent with a free standing model of the strain state, and a typical sketch of potential profile, describing the band structure of such superlattices is proposed. The photoluminescence linewidth is analyzed in terms of interface roughness. The asymmetry of the photoluminescence peaks is modelled using a density of state with a low energy exponential tail, due to the competition between radiative recombination and non-radiative recombination mechanisms. In some samples of lower crystalline quality, bound excitons were observed in the near band edge photoluminescence, which we attribute to impurity interdiffusion, which is favoured by crystalline defects.     

Thermal conductivity of symmetrically strained Si/Ge superlattices

This paper reports temperature-dependent thermal conductivity measurements in the cross-plane direction of symmetrically strained Si/Ge superlattices, and the effect of doping, period thickness and dislocations on the thermal conductivity reduction of Si/Ge superlattices. The Si/Ge superlattices are grown by molecular beam epitaxy on silicon and silicon-on-insulator substrates with a graded buffer layer. A differential 3 ω method is used to measure the thermal conductivity of the buffer and the superlattices between 80 and 300 K. The thermal conductivity measurement is carried out in conjunction with X-ray and TEM sample characterization. The measured thermal conductivity values of the superlattices are lower than those of their equivalent composition bulk alloys.     

Anomalous mobility enhancement in Si doping superlattices

Enhanced Hall mobility has been measured in Si doping superlattices with pipi or nini doping profiles grown by molecular beam epitaxy. Although the carrier concentration is large, in the range of 10¹⁸cm⁻³, the mobility of holes in the pipi doping superlattice has the temperature dependence such as that expressed in the T^−2.2 law which is characteristic of high purity bulk crystals. The hole mobility is about twice that of high purity crystals, rising up to 40,000 cm²/V.s at about 30 K. In the case that p-type impurity concentration is comparable to n-type impurity concentration, no mobility enhancement is observed and the mobility is anomalously depressed. The mechanisms for drastical change in the mobility of doping superlattices have not yet been clarified.     

Determination of energy difference and width of minibands in GaAs/AlGaAs superlattices by using Fourier transform photoreflectance and photoluminescence

In this work, Fourier transform photoreflectance (in a form of fast differential reflectance spectroscopy) has been used to study the interband optical transitions in molecular beam epitaxially grown GaAs/AlGaAs superlattices. The dependence of the measured features on the growth parameters (QW and barrier widths) has been studied. The minibands widths and energy differences between them have been obtained and matched to these coming from effective mass calculations. In addition, it has been shown that Fourier transform photoluminescence measurement might be used in the far infrared region (up to ∼15 μm) to the direct detection of the energies of intraband transitions between the electron minibands (subbands) in the superlattice and QW system.     

Experimental evidence for electron channeling in Fe /Aau (100) superlattices.

We present transport and structural data from epitaxial (100) and (111) Au/Fe superlattices grown by molecular beam epitaxy. From their analysis, we conclude that an electron channeling mechanism, due to strong specular reflection of the minority spin carrier at the Au/Fe interfaces, is responsible for the high conductivity in the (100) superlattices.     

a-Si:H/a-SiNx:H超晶格的掺杂效应

采用带有可转动掩板的沉积系统,合成出一类新的a-Si:H/掺杂a-SiN_x:H超晶格。样品中各子层厚度及a-SiN_x:H子层中N/Si比固定,仅改变掺杂浓度。结果发现:此类超晶格中的费密能级可以通过a-SiN_x:H层中的掺杂来控制,即a-Si:H/a-SiN_x:H超晶格可以从n型转变为p型,依赖于a-SiN_x:H子层中B的掺杂比。然而,a-SiN_x:H子层中P的掺杂对a-Si:H/a-SiN_x:H超晶格传输特性影响并不大。 关键词：     

LWIR detector arrays based on nipi superlattices

We propose that nipi superlattice structures in InSb or InAs can be grown with modern techniques to achieve tunable and stable LWIR detectors with high performance. We examine key device and material considerations for the application of such nipi superlattices to LWIR detectors. It is shown that practical absorption coefficients (≈ 100 cm⁻¹) can be achieved with high doping concentrations (≈ 10¹⁹ cm⁻³) achievable in these materials. In particular, recent delta doping techniques being developed in molecular beam epitaxy offer promise of higher doping concentrations, improved uniformity, and greater flexibility in tailoring the structures for optimum detector performance.     

The effects of photoexcitation on excitons in semiconductor doping superlattices

Calculations of the properties of excitons in doping superlattices have been made as a function of doping density using a variational approach. Interesting new features are obtained when the exciton energy becomes comparable to the superlattice potential energy. These results are compared to recent experimental data on GaAs doping superlattices.     

Determination of photoexcited carrier concentration and mobility in GaAs doping superlattices by hall effect measurements

The photo-Hall effect in a new type of periodicp-n doping multilayer structures (superlattices) of GaAs grown by molecular beam epitaxy has been investigated. In these space charge systems electrons and holes are separated in real space. As a consequence, large deviations from thermal equilibrium become quasi-stable. Carrier generation by optical absorption occurs in these doping superlattices even at photon energies far below the gap of the homogeneous semiconductor material. The photoexcitation results in a strong enhancement of the conductivityparallel to the layers and in a substantial photovoltaic response. An increase in carrierconcentration as well as an increase in carriermobility both contribute to the observed enhancement of the conductivity under excitation. The absolute values of changes in free-carrier concentration are very large due to the manyfold active layers of the structure. The measured free-carrier mobilities depend on the population of the multilayer system. A reduction in mobility as compared to bulk material is found to be more pronounced in weakly populated systems. This finding is caused by the larger weight of the boundary regions of the total active layers where the free-carrier density is lower than the density of ionized impurities resulting in an enhanced impurity scattering.     

Laser-modulated reflectance of SbSI near Tc

Reflectance modulation spectra of the semiconducting ferroelectric compound SbSI, at wavelengths near the fundamental edge and at temperatures around the phase-transition are reported. Modulation was obtained by the periodical injection of free carriers produced by a laser beam chopped at 10.18 Hz. In order to explain the mechanism of photoreflectance in SbSI, a comparison of the experimental data has been made both with electroreflectance spectra and eith the theoretical curve expected under the assumption of a periodical change of free carriers density.     

Photoreflectance study of folded above-barrier states in (InAs)1/(GaAs)m strained-layer superlattices

We have performed a photoreflectance study of the above-barrier states in (InAs)₁/(GaAs)_m strained-layer superlattices (m=10 and 30 monolayers) grown on a (001) GaAs substrate by molecular-beam epitaxy. We have clearly observed the optical transitions associated with the above-barrier states at the Γ and π (mini-Brillouin-zone edge) points. The layer-thickness dependence of the transition energies is explained by the zone-folding effect on the above-barrier states based on a simple Kronig-Penney analysis taking account of the lattice-misfit strain effects.     

Impurities in type-II-staggered InAs/AℓSb superlattices

The physics of impurities in Type-II staggered superlattices is reviewed, with emphasis on changes of doping character, such as shallow to deep and deep to false-valence transitions, that can result from altering layer thicknesses. It is shown how some impurities can change their doping characters from "deep acceptors" (semi-insulating) to shallow donors (n-type) as functions of layer thicknesses in superlattices such as InAs/AℓSb. A particularly important case is an Aℓ_Sb antisite defect in an AℓSb layer of an InAs/AℓSb superlattice. This defect is a deep trap or "deep acceptor" in AℓSb, and a remote donor in some superlattices, depending on the layer thicknesses. It can, by itself, control the electron gas density in a superlattice structure, and so can play an important role in the development of high-speed field-effect transistors.     

Improvement of doping efficiency in Mg-Al0.14Ga0.86N/GaN superlattices with AlN interlayer by suppressing donor-like defects

We investigate the mechanism for the improvement of p-type doping efficiency in Mg-Al0.14Ga0.86N/GaN superlattices(SLs).It is shown that the hole concentration of SLs increases by nearly an order of magnitude,from 1.1×1017 to 9.3×1017cm-3,when an AlN interlayer is inserted to modulate the strains.Schro¨dinger-Poisson self-consistent calculations suggest that such an increase could be attributed to the reduction of donor-like defects caused by the strain modulation induced by the AlN interlayer.Additionally,the donor-acceptor pair emission exhibits a remarkable decrease in intensity of the cathodoluminescence spectrum for SLs with an AlN interlayer.This supports the theoretical calculations and indicates that the strain modulation of SLs could be beneficial to the donor-like defect suppression as well as the p-type doping efficiency improvement.     

Photoreflectance and photoluminescence study of short period (GaAs)n/(AlAs)n superlattices with n = 1 – 15

Photoreflectance and photoluminescence measurements were performed in (GaAs)_n/(AlAs)_n (n = 1 – 15) short-period superlattices in the temperature range from 25 K to 275 K. Weak signals of photoreflectance associated with the critical point of the pseudodirect transition, weakly allowed direct transition arising from the zone-folding effect, have been observed as well as main signals associated with the direct allowed transitions. This assignment is supported by the measurement and analysis of the temperature dependence of the photoluminescence intensity.