Hirokazu Tatsuoka

Strain relaxation of CdTe(100) layers grown by hot‐wall epitaxy on GaAs(100) substrates

The strain relaxation of CdTe(100) layers grown on GaAs(100) substrates by hot‐wall epitaxy was investigated by measurement of optical properties, x‐ray analysis, and transmission electron microscopy. It is considered from transmission electron microscopy observation that relaxation of most of the strain due to lattice mismatch occurred at the interface. However, a small amount of strain, of the order of 10−3, remained in layers thicker than 0.7 μm, and it was relaxed as the layer thickness increased. The residual strain of 4×10−4, which exists in layers thicker than 10 μm, was due to the difference between the thermal expansion coefficients of the layer and the substrate. Moreover, for layers thicker than 17 μm, split ground (n=1) and first excited (n=2) free‐exciton states due to internal strain have for the first time been observed by photoluminescence and reflectance spectroscopy. The results show that CdTe layers with excellent crystallinity and homogeneity in strain are obtained.

Growth of CdTe on GaAs by hot‐wall epitaxy and its stress relaxation

CdTe(100) layers of the thickness range from 0.7 to 15 μm were grown on GaAs(100) substrates by hot‐wall epitaxy. The crystallinity of the layer was examined by reflection high‐energy electron diffraction, x‐ray rocking curve, and photoluminescence. The lattice relaxation were investigated by x‐ray analysis and optical reflectance spectra. CdTe layers with thickness up to 15 μm were under compressive biaxial stress. In addition to the split exciton lines (n=1), the emission from excited states (n=2) was observed.

Growth of Ca2Si layers on Mg2Si/Si(111) substrates

Single phase Ca 2 Si layers are successfully grown on Mg 2 Si/Si substrates for the first time. These Mg 2 Si and Ca 2 Si layers are formed by heat treatment of Si and Mg 2 Si/Si substrates in Mg and Ca vapor, respectively. The replacement of Ca atoms with Mg in Mg 2 Si leads to the formation of single phase Ca 2 Si layers. It is confirmed that the formation of other silicide phases is suppressed, when the layers are grown under optimum growth time. The structural property of the resultant layers is examined by X-ray diffraction technique, scanning electron microscopy and transmission electron microscopy.

Preparation and aging of sputtered tungstic oxide films

Conditions for the preparation of electrochromic tungstic oxide films with high efficiency for coloration have been investigated. Tungstic oxide films were deposited on glass substrates by rf sputtering in Ar‐O2 mixture from a compressed powder WO3 target. As‐deposited films require an ‘‘aging process,’’ in which the density of charges extracted in bleaching is smaller than that injected in coloring. It is considered that a part of the protons introduced during the aging combine with bonds of unstable oxygen contained in the as‐deposited films. After the aging period, sputtered films have reversible coloration and bleaching cycles. Maximum electrochromic coloration efficiencies are 0.09 and 0.04 cm2/mC at wavelengths of 800 and 500 nm, respectively. They are obtained when the sputtering atmosphere is 0.04–0.05‐Torr total pressure and 5% oxygen content. At any rate, substrate temperature should be kept at 100 °C or less.

Formation of CaMgSi at Ca2Si/Mg2Si interface

Abstract The structural property and interfacial morphology have been investigated for the Ca 2 Si/Mg 2 Si interface. The Ca 2 Si layers were grown on Mg 2 Si layers by heat treatment of the Mg 2 Si/Si substrates in Ca vapor. It is found that the CaMgSi phase is formed at the Ca 2 Si/Mg 2 Si interface. The structural property of the resultant silicides was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical composition of the layers was examined by energy dispersion spectroscopy (EDS). In addition, the chemical analysis by X-ray photoelectron spectroscopy (XPS) revealed the Mg segregation on the silicide surface. The growth mechanism of Ca 2 Si on Mg 2 Si/Si is also discussed.

CdTe (111)B growth on oriented and misoriented GaAs (100) grown by hot‐wall epitaxy

CdTe (111)B layers were grown on oriented and misoriented GaAs (100) substrates by hot‐wall epitaxy. The crystallinity of the layers was examined by x‐ray diffraction. The strain relaxation was investigated by x‐ray diffraction and optical reflectance spectra. (1) For the layers on oriented substrates, it is found that the strain of the layer is relaxed as the layer thickness increases, but additional strain, which is probably due to the formation of twinned domains, remains and is hardly relaxed. (2) Layers on misoriented substrates are twin free with good crystalline quality. Strain is relaxed as the layer thickness increases. The residual strain of layers thicker than 10 μm is for the most part due to the difference of thermal expansion coefficients between CdTe and GaAs.

Simple Fabrication of Mg2Si Thermoelectric Generator

Mg2Si polycrystals have been grown by heat treatment of bulk Si under Mg vapor. The structural property of the Mg 2Si has been investigated. The Mg 2Si shows n-type conduction. The impurity doping to obtain p-Mg 2Si was also demonstrated. In addition, simple fabrication procedure of Mg2Si thermoelectric generators has been developed.

Growth of Epitaxial ZnO Thin Film by Oxidation of Epitaxial ZnS Film on Si(111) Substrate

The growth of an epitaxial ZnO thin film on a Si substrate by oxidation of an epitaxial ZnS film is a novel method and we are reporting it for the first time. The merits of using the Si substrate are to make the driving voltage of light-emitting diodes (LEDs) lower and the cost of the LEDs less expensive than that using a sapphire substrate. In this study, the epitaxial ZnO thin film could be successfully grown on the Si substrate. The epitaxial films showed a strong near-ultraviolet emission with a peak at around 3.32 eV at room temperature under 325 nm excitation.

Growth of β-FeSi2 and FeSi layers by reactive deposition using Sb-related intermetallic compounds

Abstract β-FeSi 2 and FeSi layers were grown on Si substrates by deposition of Sb-related intermetallic compounds and simultaneous reaction with Si substrates. Higher quality epitaxialβ-FeSi 2 layers with smooth interfaces were obtained on the Si(111) substrates at the substrate temperatures ranging from 650 to 700°C in comparison to the layers grown by conventional reactive deposition epitaxy (RDE). Moreover, the growth mechanism of FeSi and Sb-related intermetallic compounds have been discussed.

High-Quality Epitaxial MnSi(111) Layers Grown in the Presence of an Sb Flux.

MnSi epitaxial layers have been grown on (111) and (001)-oriented Si substrates by Mn deposition and reaction with Si in the presence of an Sb flux. Characterization using transmission electron microscopy (TEM) confirmed the formation of high-quality epitaxial layers with smooth interfaces between the MnSi and the Si(111) substrate, when grown under optimal conditions, without the deposition of elemental Sb or Sb-based compounds. MnSi layers are found to be rotated 30° with respect to the Si(111) substrate to reduce the lattice mismatch. Evidence only for the presence of MnSi was found and there was no evidence of any other Mn–Si phases. The additional formation of MnSb is found to depend on the rate of formation of MnSi, which is primarily governed by the Mn flux rate and the growth temperature. By way of comparison, polycrystalline mixed phase Mn-silicide layers were formed by direct deposition of Mn and reaction with Si(111) at elevated temperatures in the absence of an Sb flux.