ZnSe-ZnTe Strained-layer Superlattices Research Articles

The influence of tellurium interdiffusion on the optical properties of MOVPE-grown ZnSe-ZnTe superlattices

ABSTRACTWe have used low pressure MOVPE to grow a series of short-period ZnSe-ZnTe strained-layer superlattices onto either ZnSe or ZnTe buffer layers. Typical superlattice periods range from 2 to 6.5 nm with constant ZnSe thickness of 2 monolayers. X-Ray diffraction analyses have shown that the superlattices are free-standing if grown on a ZnSe buffer but lattice matched to ZnTe if grown on ZnTe buffer layers. The luminescence emitted by the free standing samples has been studied using both the photoluminescence technique at low temperature and cathodoluminescence at higher temperature, for improved pumping. Our samples exhibit a complicated temperature dependence similar to the behaviour of ZnSeTe alloys where exciton self-trapping to tellurium gives strong photoluminescence bands labelled SI and S2. A high temperature, high energy feature is observed and is associated with the fundamental superlattice transition. The energy position of this structure suggest a high value of the ZnSe/ZnTe valence band offset, in the range 1200-1400 meV.

Phonon Folding in ZnSe–ZnTe Strained Layer Superlattices

Folding of longitudinal acoustic (LA) mode phonons in ZnSe–ZnTe strained layer superlattices, grown on the (001) GaAs without buffer layers by molecular beam epitaxy (MBE), was measured as a function of period and thickness of the layers by Raman scattenng. The dispersion relation between observed LA frequencies and wave number of the superlattices was analyzed by fitting the sound velocity as a parameter in the Rytov model. Two analytical solutions were obtained from the model. It is supported by consideration of calculation and experimental results that the sound velocity of the ZnSe and ZnTe layers in the superlattices is smaller and larger than their bulk values respectively, and this implies elongation of the ZnSe layers and compression of the ZnTe layers.

The study of Raman spectra of II–VI wide gap ZnSeZnTe strained layer superlattices

Vacuum heat treatment is an increasingly popular thermal processing technique which is now widely established practice in the aerospace, automotive and machine tool industries.This paper is concerned with the development of a pressure quenching system for vacuum furnaces which provides both the optimum metallurgical properties and minimal distortion of components during hardening.First introduced by Consarc in the mid-1980s the Multiflow system utilises multiple gas flow patterns to achieve uniform quenching conditions and hence minimise the distortion of component during hardening. This is in contrast to vacuum furnaces employing a conventional, single gas flow pattern which results in a less uniform quench.Since 1989 the ongoing development programme at Consarc Engineering Ltd, has resulted in further improvements to the Multiflow system. The latest generation of furnaces incorporates an improved design of heat exchanger which has significantly increased the cooling rate of the furnace. The improved heat exchanger is the by-product of extensive in-house testing and state-of-the-art computer aided design techniques. The latest multiflow furnaces therefore offer the capability to harden awider range of materials while maintaining the uniformity of the quench and the dimensional stability of components.

Atomic layer epitaxial growth of ZnSe, ZnTe, and ZnSe-ZnTe strained-layer superlattices

ZnSe and ZnTe thin films and ZnSe-ZnTe strained-layer superlattices (SLS) have been successfully grown by atomic layer epitaxy (ALE) using molecular-beam epitaxy. The ALE growth of ZnSe and ZnTe was achieved at substrate temperatures between 250 and 350 °C and between 240 and 280 °C, respectively. The Zn, Se, and Te beam intensities necessary to achieve ALE growth were evaluated. Under optimum growth conditions, the surfaces of ZnSe and ZnTe thin films were very specular without any surface structure. Modulation-doped n-type SLSs have been prepared by ALE using Cl as a n-type dopant. The carrier concentration of superlattices grown by ALE (3.8×1016 cm−3) was three orders of magnitude higher than those of superlattices grown by conventional molecular-beam epitaxy.

Atomic layer epitaxy of ZnSe-ZnTe strained layer superlattices

ZnSe-ZnTe strained layer superlattices (SLSs) were grown by atomic layer epitaxy (ALE). The optical properties of the SLSs were evaluated by photoluminescence (PL) and compared with those grown by molecular beam epitaxy (MBE). The PL intensity of ALE-SLSs was several orders of magnitude greater than that of MBE-SLSs with an improvement in the full width at half maximum (FWHM). The structure of the SLS was evaluated from the high-angle X-ray diffraction pattern. Furthermore, the direct observation of SLS structure by transmission electron microscopy (TEM) showed clearly the layered structure with the designed numbers of ZnSe and ZnTe monolayers.

Atomic Layer Epitaxy of Wide Bandgap II-VI Compound Semiconductor Superlattices

ABSTRACTZnSe, ZnTe and ZnSe-ZnTe strained-layer superlattices (SLS's) have been successfully grown by atomic layer epitaxy (ALE) using molecular beam epitaxy (MBE-ALE). The ideal ALE growth, i.e., one monolayer per cycle of opening and closing the shutters of the constituent elements, was obtained for ZnSe in the substrate temperature range of 250-350° C. However, for ZnTe, precise control of the Te beam intensity is needed to obtain the ALE growth. Optical properties of the (ZnSe)l-(ZnTe) 1 SLS were evaluated by photoluminesence. ZnSe films were also grown by ALE using metalorganic molecular beam epitaxy (MOMBE-ALE). Diethylzinc (DEZn), diethylsulfur (DES) and diethylselenium (DESe) were used as source gases for Zn, S and Se, respectively. The ALE growth of ZnSe was achieved at substrate temperature between 250 and 300° C which is about 150° C lower than that for the conventional MOMBE.

Interdiffusion in ZnSe-ZnTe strained-layer superlattices

A detailed study has been made of interdiffusion in ZnSe-ZnTe strained-layer superlattices (SLSs) grown by molecular-beam epitaxy (MBE) at a growth temperature of 320 °C. In x-ray diffraction measurements, the satellite peak intensities relative to the zero-order peak intensity decreased with annealing time. The interdiffusion coefficient D was calculated assuming a linear diffusion model. The values of D=3.6×10−21 to 2.2×10−19 cm2/s at an annealing temperature of 500 °C were obtained for the ZnSe-ZnTe SLSs. In the high-resolution transmission electron microscopy (HRTEM) image of as-grown SLSs, the presence of fine superlattice structures was seen, but for interdiffused samples stripes due to the periodic superlattice structures were not visible and many dislocation lines were observed. These results suggest that the structure of SLSs is significantly modulated by thermal annealing at a temperature higher than the growth temperature.

Electrical properties of p-type and n-type ZnSeZnTe strained-layer

ZnSe and ZnTe semiconducting materials are highly promising candidates for the fabrication of short-wavelength light-emitting devices. We have grown ZnSeZnTe strained-layer superlattices (SLSs) on InP substrates by molecular beam epitaxy. In addition to undoped SLSs, two kinds of modulation doped SLS samples were prepared in this study, the first with Ga-doped ZnSe layers and undoped ZnTe layers, and the second with undoped ZnSe layers and Sb-doped ZnTe layers. Van der Pauw measurements of the SLS samples at room temperature showed that their electrical properties can be controlled by using the modulation doping technique. The undoped sample and the Ga-doped sample exhibited n-type conduction, whereas p-type conduction was observed for the Sb-doped sample. Interdiffusion profiles of dopants were measured by secondary ion mass spectroscopy, and significant Ga redistribution was observed. Finally, we have fabricated pn junctions from ZnSeZnTe SLSs, and measured their current-voltage characteristics.

ZnSe-ZnTe strained-layer superlattices: A novel material for the future optoelectronic devices

ZnSe-ZnTe strained superlines (SLSs) were grown by molecular beam epitaxy 9MBE). Th optical properties of the ZnSe-ZnTe SLS were evaluated by photoluminescence (PL). The PL peak position was shifted by tailoring the structure of the superlattice. The luminescence color in the visible changed from blue-green to red. In order to obtain both p- and n-type conduction in wide-bandgap II–VI compound semiconductors, we have prepared ZnSe-ZnTe superlines with a modulation doping technique. When Sb was selectively doped in ZnTe layers, all the samples exhibited p-type conductivity with hole concentrations of (0.5–1.0)X1014cm-3. On the other hand, Ga-doped SLSs were type with electron concentrations of (2–7)X1013cm-3. Furthermore, the growth of ZnSe-ZnS SLS was also demostrated by metalorganic molecular beam epitaxy (MOMBE). Blue luminescence related. to the quantized levels in the SLS was detected.

Realization of both p- and n-type conduction for ZnSe-ZnTe strained-layer superlattices

The conduction types of ZnSe-ZnTe strained-layer superlattices (SLS’s) have been controlled by using the modulation doping technique. Two kinds of modulation-doped SLS’s were prepared. One of them consisted of gallium (Ga) doped ZnSe layers and undoped ZnTe layers. The other consisted of undoped ZnSe layers and antimony (Sb) doped ZnTe layers. The conduction types of the samples modulation doped with Ga and Sb were shown to be n and p type, respectively whereas the undoped samples exhibited n-type conduction. The electrical properties of the undoped and modulation-doped samples were evaluated by the van der Pauw method. The carrier concentrations of all types of samples were about 5×1013/cm3 at room temperature. The temperature dependence of the electrical properties was measured for an undoped sample and a sample modulation doped with Sb. The carrier concentrations increased with temperature and reached about 1×1017/cm3 at 500 K.

Growth and characterization of ZnSe-ZnTe strained-layer superlattices

ZnSe-ZnTe strained-layer superlattices (SLSs) were grown on InP, GaAs and InAs substrates by molecular beam epitaxy (MBE). The samples were characterized by transmission electron microscopy (TEM) observation, and photoluminescence (PL) measurement techniques. Uniform and fine superlattice structures were confirmed by TEM observation. Moreover, a distinguishable change in the direction of (111) lattice-fringes from ZnSe layer to ZnTe layer was clearly observed in lattice image observation. The PL peak energy depended on the thickness of the alternating layers and the luminescence was related to the size quantization of the SLS structure. The luminescence peak was also affected by the substrate materials.

Lattice strain and lattice dynamics of ZnSe-ZnTe strained-layer superlattices

Lattice strain and lattice dynamics of ZnSe-ZnTe strained-layer superlattices (SLSs) were examined by means of transmission electron microscopy and Raman scattering. Superlattice structures were confirmed from both transmission electron diffraction patterns and bright field images of the samples. Lattice distortions which originated from misfit strains were observed by the lattice image mode. The crystallinity of the adjacent layers was degraded when the layer thickness became large and polycrystalline structures were observed when the layer thickness exceeded the critical layer thickness. Raman scattering was used to assess the magnitude of the strain in the SLSs. Optical phonon frequencies of the components in SLSs were different from those frequencies of the bulk materials. The frequency shifts were used to quantify the strains in the SLSs. The observed shifts corresponded to the calculated values.