Metalorganic Vapor-Phase Epitaxy of ZnTe and CdZnTe on GaAs

ZnTe and Cd_{1 – y} Zn _y Te epilayers were grown on GaAs(111)B and GaAs(100) substrates by metalorganic chemical vapor deposition. The structural perfection and surface morphology of the layers were investigated by double-crystal x-ray diffraction and scanning electron microscopy. The effect of deposition conditions on the growth rate, zinc content (y), surface morphology, and structural perfection of the epilayers was studied. The results suggest that the optimal structures for depositing HgCdTe(111)B and HgCdTe(100) are CdTe(111)B/GaAs(111)B and CdTe(100)/ZnTe(100)/GaAs(100), respectively.     

Effect of thermal annealing on the structural and optical properties of CdTe (111)/GaAs (100) heterostructures

Reflection high-energy electron diffraction (RHEED), double-crystal X-ray rocking curve (DCRC), and photoluminescence (PL) measurements were performed to investigate the effect of thermal annealing on the structural and the optical properties of CdTe (111) epilayers grown on GaAs (100) substrates by molecular beam epitaxy (MBE) at low temperature. The results of the RHEED patterns showed that the oxidized layer on the GaAs substrate was removed in a Te atmosphere, and that the 20-Å CdTe layer was grown by three-dimensional process. When the rapid thermal annealing (RTA) was performed at 500°C for 14 s, the FWHM of the DCRC for the CdTe layer had the smallest value. After the RTA process, the luminescence intensity of the exciton remarkably increased, and the peak at 1.476 eV was dominant. As the RTA temperature increased, the luminescence intensity of the exciton peak related to neutral acceptors (A°, X) increased. The temperature dependence of the spectra showed that the (A°, X) peak originated from the recombination of the excitons bound in high-density defects. The excitation power intensity dependence of the PL spectra showed that the peaks of the transitions due to donor-acceptor pairs shifted to larger energies. These results indicate that the structural and the optical properties of the CdTe epilayers grown on GaAs (100) are improved by RTA, and that the RTA process is very useful for the growth of Hg_xCd_1−xTe on CdTe/GaAs heterostructures.     

CdTe介质膜与HgCdTe晶片间界面特性的研究

利用Ａｒ＾＋束溅射沉积技术在ＨｇＣｄＴｅ表面低温生长了ＣｄＴｅ介质薄膜。分别用ＣｄＴｅ介质膜和ＨｇＣｄＴｅ自身阳极氧化膜对ＨｇＣｄＴｅ表面钝化,利用光电导衰退信号波形的拟合,得到了不同表面钝化的ＨｇＣｄＴｅ非平衡载流子表面复合速度。结果表明,ＣｄＴｅ／ＨｇＣｄＴｅ界面质量已超过自身阳极氧化膜／ＨｇＣｄＴｅ界面质量。     

Transition from a disordered to a crystalline state in II–VI and III–V films

The initial stages of HgCdTe growth on Al₂O₃, GaAs, CdTe, and KCl substrates have been studied by electron diffraction. HgCdTe films were produced by pulsed laser deposition and isothermal vapor phase epitaxy. InGaAs films were grown by isothermal chloride epitaxy on GaAs substrates. In the initial stages of the growth process, we observed a transition from an amorphous to a textured polycrystalline phase and then to a mosaic single-crystal structure. We have calculated the critical size of crystalline grains below which amorphization occurs in II-VI and III-V compounds. The critical grain size agrees with the grain size of the disordered (amorphous) phase that forms in the initial stage of epitaxy. We have determined some characteristics of the heterostructures: critical film thickness below which pseudomorphic growth is possible without misfit dislocation generation, elastic stress in the epitaxial system, surface density of dangling bonds at dislocations, and the critical island radius above which no interfacial misfit dislocations are generated.     

Monolithic ZnTe-based pillar microcavities containing CdTe quantum dots

Micropillars of different diameters have been prepared by focused ion beam milling out of a planar ZnTe-based cavity. The monolithic epitaxial structure, deposited on a GaAs substrate, contains CdTe quantum dots embedded in a ZnTe λ-cavity delimited by two distributed Bragg reflectors (DBRs). The high refractive index material of the DBR structure is ZnTe, while for the low index material a short-period triple MgTe/ZnTe/MgSe superlattice is used. The CdTe quantum dots are formed by a novel Zn-induced formation process and are investigated by scanning transmission electron microscopy. Micro-photoluminescence measurements show discrete optical modes for the pillars, in good agreement with calculations based on a vectorial transfer matrix method. The measured quality factor reaches a value of 3100.     

High-throughput optical processors based on semiconductor nanostructures for incoherent-light optical analog computers with parallel image processing

Fast-response optical recording media based on semiconductor nanostructures (CdTe, GaAs) have been developed for image recording and processing at a speed of up to 10⁶ cps, which is 2–3 orders of magnitude higher than the speed of well-known media based on liquid crystals (MIS-LC). The new media are characterized by a photosensitivity of 10^?2 W/cm² and a spatial resolution of 5–10 lines/mm. Methods for the readout of images recorded in the nanostructures are developed and high-speed incoherent-light optical processors based on these structures are created. The possibility of using these processors for building optical analog computers and image correlators is demonstrated.     

Application of equations for incoherent internal reflections to HgCdTe/CdTe structures

The reflectance R and transmittance T of an epitaxial film of HgCdTe on CdTe were measured for a range of energies near the band edge. Equations for the incoherent addition of multiple internal reflections in multilayered media are derived and used to calculate the absorption coefficient and index of refraction from R and T. The exact calculation is compared with an approximation which simplifies the analysis of the data.     

Electrical and photoelectrical behaviour of CdTe structures

The electrical and photoelectrical behaviour of Au/n-CdTe junctions prepared on CdTe monocrystalline substrates and CdTe epitaxial layers grown on n⁺ GaAs substrates were studied. The electrical and photoresponse properties depended very strongly on the parameters of the compensated high-resistive layer at the CdTe surface formed by annealing during preparation.     

Effects of a ZnTe buffer layer on structural quality and morphology of CdTe epilayer grown on (001)GaAs by molecular beam epitaxy

The structural properties, crystalline quality and surface morphology of CdTe thin films without and with a ZnTe buffer layer grown on (001)GaAs by molecular beam epitaxy (MBE) have been studied. CdTe thin film directly prepared on GaAs substrate displays (111) orientation with an island growth mode, whereas the CdTe epilayers with a ZnTe buffer are (001)-oriented single-crystalline film with a two-dimensional (2D) growth mode. The morphology and surface root-mean-square (RMS) roughness of CdTe epilayers are also dramatically improved by using a ZnTe buffer. Furthermore, it is suggested that the high-temperature (HT) ZnTe buffer grown at 360 °C is more efficient for enhancing CdTe structural quality than the low-temperature (LT) one at 320 °C. The CdTe epilayer on the HT-ZnTe buffer shows a narrower full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (004) reflection and a smaller RMS roughness.     

Optical characterization of double layers containing epitaxial ZnSe and ZnTe films

In this paper results of the optical characterization of double layers consisting of ZnTe and ZnSe thin films prepared by molecular beam epitaxy onto GaAs single crystal substrates are presented. For this optical characterization the optical method based on combining variable angle spectroscopic ellipsometry and near-normal spectroscopic reflectometry is used in the multi-sample modification applied within the spectral region 230–850?nm. Using this method the spectral dependences of the optical constants of the upper ZnTe thin films are determined within the spectral region mentioned above. Spectral dependences of the optical constants of the lower ZnSe thin films were determined within the spectral region 450–850?nm. Boundary roughness of these double layers and overlayers is respected. RMS values of the heights of the irregularities of the boundaries and thicknesses and optical constants of the overlayers are determined by means of the combined optical method as well. The uppermost boundaries of the double layers are, moreover, analysed using atomic force microscopy because of verification of the RMS values of these boundaries obtained by the optical method. The spectral dependences of the optical constants of the upper ZnTe films and lower ZnSe films determined in this paper are compared with those presented for ZnTe single layers and ZnSe single layers in the literature.     

CdTe/CdMnTe多量子阱的生长和激子复合动力学性质的研究

用分子束外延在GaAs衬底上生长了CdTe/Cd0.8Mn0.2Te多量子 结构,利用X射线衍射（XRD）、低激发密度下的PL光谱和变密度激发的ps时间分辨光谱研究了CdTe/CdMnTe多量子阱的结构和激子复合特性。在变密度激发的ps时间分辨光谱中,发现不同激发密度下发光衰减时间不同,认为它的机理可能是无辐射复合引起的。     

HgCdTe探测器应力的多重晶X射线衍射分析

HgCdTe光伏探测器的钝化介质膜应力常常限制其低温性能,利用高分辨率多重晶X射线衍射仪中的三重晶衍射技术和倒易空间作图对钝化介质膜应力进行了表征,发现在较高溅射能量下沉积的钝化膜,由于应力的作用,HgCdTe晶片出现弯曲,并有大量镶嵌结构,而在较低的溅射能量下和热蒸发下沉积的钝化膜,晶面未出现明显弯曲,可获得较低应力的钝化介质层。     

Effect of pulsed laser irradiation on the photoconductivity of polycrystalline CdTe films

Polycrystalline CdTe films were prepared by molecular-beam deposition onto sapphire and glassceramic substrates at 670 K. The photoelectric behavior of the films (dependence of photoconductivity on incident light intensity and spectral response of photocurrent) was found to depend on grain size. The photoresponse of the fine-grained films in the intrinsic-absorption range and that of the coarse-grained films in the intrinsic- and impurity-absorption ranges increase as a result of above-gap irradiation with 20-ns laser pulses. The dependences of photoconductivity on incident intensity for the coarse-grained films are similar to those for CdTe single crystals. The grain boundaries in fine-grained CdTe films play an important part in the generation and recombination of charge carriers.     

Investigation of passivation processes for HgCdTe/CdS structure for infrared application

The effect of chemical and in-situ electrochemical etching on the morphology of the mercury cadmium telluride (HgCdTe) surface, prior to growth of the cadmium sulfide (CdS) films, has been examined. The CdS films have also been characterized using grazing angle X-ray diffraction and optical absorption spectroscopy. Preferential growth of (002) planes of CdS has been observed on the HgCdTe substrate. The effectiveness of the surface treatment and CdS growth methodology for passivation of the HgCdTe surface has been examined using capacitance–voltage studies of Au/HgCdTe/CdS/Au devices at 77 K. A negative fixed charge density  10¹⁵/m² was obtained.     

Optical properties of a strained GaAs/Si heterostructure after rapid thermal annealing

Photoluminescence (PL) and Raman spectroscopy measurements on GaAs/Si heterostructures grown by molecular beam epitaxy were performed in order to characterize the optical properties of the samples after thermal treatment. The crystalline quality of the annealed GaAs/Si was remarkably improved in comparison with that of the as-grown GaAs/Si. The results of PL spectra for rapidly thermally annealed GaAs/Si showed that the peaks at 1.501 and 1.485 eV, which are related to intrinsic bands, and the peak at 1.467 eV, which can be attributed to impurities, were well resolved. The magnitude of the deformation in the GaAs layer, as determined from the PL measurements, was approximately 2.1 × 10^7minus;3. The ratio of the peak intensity of the longitudinal optical phonon to that of the transverse optical phonon for rapidly thermally treated GaAs/Si increased dramatically in comparison with the corresponding rates for the as-grown GaAs/Si. These results indicate that the crystallinity of the GaAs epilayer is improved by thermal treatment.     

Activation energy and carrier dynamics of CdTe/ZnTe quantum dots on GaAs and Si substrates

We investigate the activation energy and carrier dynamics of CdTe/ZnTe quantum dots (QDs) grown on GaAs and Si substrates. The activation energy of the electrons confined in QDs on the Si substrate, as obtained from the temperature-dependent photoluminescence (PL) spectra, is lower than that of electrons confined in QDs on the GaAs substrate. Time-resolved PL measurements used to study the carrier dynamics show shorter exciton lifetimes for QDs on the Si substrate. This behavior is attributed to the fact that defects and dislocations in the QDs on the Si substrate provide nonradiative channels.     

A study of cadmium mercury telluride (CMT) thin film structures: some physical observations and characteristics

Polycrystalline thin film structures of the form Cd_1?x Hg _x Te (0 ≤ x ≤ 0.5) were obtained on ultrasonically cleaned glass substrates in an alkaline medium (pH≈ 10 ± 0.2) at 80^°C. The forerunner solutions were obtained from the equimolar solutions of CdCl₂, HgCl₂ and 0.33 M-refluxed Na₂TeSO₃. The preparation parameters such as temperature, pH, deposition time and speed of mechanical stirring were optimized. The as-prepared samples were tightly adherent to the substrate support, less smooth, diffusely reflecting and were analysed for composition. It appeared that incorporation of Hg²⁺ in the lattice of CdTe replaced divalent Cd²⁺ predominantly. However, a small amount of Te^2? replacement by Hg²⁺ ions has also been observed at higher levels of Hg in CdTe (> 0.1 mol. %). The samples were then characterized optically to determine the absorption coefficient, mode of optical transition and the optical bandgaps. The estimated band-gap decreased, typically from 1.51 to 0.83 eV as Hg content in CdTe was increased from 0 to 0.5. The dc electrical conductivity measurements were made in the temperature range from 300 to 550 K and it was observed that the conductivity increased rapidly up to x = 0.1 and thereafter remained more or less the same. The thermopower measurements showed n-type conduction of these films. Both carrier concentration (n) and mobility (μ) were computed from these studies and found to be dependent on the working temperature and the film composition. The surface morphologies of these as-grown layers were then observed via a scanning electron microscope. CdTe crystals are more or less crystalline spherical and hexagonal prismatic in shape whereas HgCdTe samples appear glassy and show hexagonal prismatic whiskers, some less frequent forms of needles.     

Optical gain of 1.3 /spl mu/m GaAsSbN/GaAs quantum well lasers

Optical gain of 1.3 mum GaAsSbN/GaAs quantum well (QW) structure is investigated using the multiband effective mass theory. The results are compared with those of 1.3 mum InGaNAs/GaAs and GaAsSb/GaAs QW structures. The optical gain of the GaAsSbN/GaAs QW structure is found to be similar to that of the InGaAsN/GaAs QW structure. In contrast, GaAsSbN/GaAs and InGaNAs/GaAs QW structures show significantly larger optical gain than the GaAsSb/GaAs QW structure. This is mainly attributed to the fact that the former has a larger optical matrix element than the latter. In addition, GaAsSbN/GaAs and InGaNAs/GaAs QW structures have much smaller threshold current density than the GaAsSb/GaAs QW structure. This is because the Auger recombination current density gives dominant contribution to the threshold current density and the former has smaller threshold carrier density than the latter. On the contrary, the threshold current density of the GaAsSbN/GaAs QW structure is shown to be similar to that of the InGaAsN/GaAs QW structure     

Strategies to increase CdTe solar-cell voltage

There is a significant difference in performance between today's highest efficiency of CdTe solar cells and single-crystal cells of comparable band gap. The largest contribution to this difference is the voltage, where the values for the best CdTe cells are about 230 mV below the best GaAs cells when an appropriate adjustment is made for band gap. CdTe voltage and fill-factor are currently compromised by low recombination lifetime (near 1 ns), low hole density (near 10¹⁴ cm^− 3), and in some cases an excessive back-contact barrier. Numerical simulation is used to evaluate how combinations of lifetime, carrier density, back electron reflection, and interfacial properties affect voltage and cell performance. Two different strategies for improving voltage and performance are explored.     

Optical evidence of a quantum well channel in low temperature molecular beam epitaxy grown Ga(AsBi)/GaAs nanostructure

A Ga(AsBi) quantum well (QW) with Bi content reaching 6% and well width of 11 nm embedded in GaAs is grown by molecular beam epitaxy at low temperature and studied by means of high-resolution x-ray diffraction, photoluminescence (PL), and time-resolved PL. It is shown that for this growth regime, the QW is coherently strained to the substrate with a low dislocation density. The low temperature PL demonstrates a comparatively narrow excitonic linewidth of ～ 40 meV. For high excitation density distinct QW excited states evolve in the emission spectra. The origins of peculiar PL dependences on temperature and excitation density are interpreted in terms of intra-well optical transitions.