Toshiya Yokogawa

ZnSe‐ZnS strained‐layer superlattice grown by low pressure metalorganic vapor phase epitaxy using methylalkyls

Good quality ZnSe‐ZnS strained‐layer superlattice (SLS) has been successfully fabricated for the first time by low pressure metalorganic vapor phase epitaxy (MOVPE) using methylalkyls (dimethylzinc, dimethylselenide, and dimethylsulfide). The satellite peak observed by x‐ray diffraction measurements and the periodic behavior of the atomic profiles by secondary ion mass spectrometry measurements confirm the formation of the SLS structure. From the photoluminescence measurements, the quantum size effect has been evidenced by the relationship between the ZnSe well‐layer thickness and the peak energy shift of the light emission. Our results show that low pressure MOVPE using VI group alkyls can be quite useful for the growth of ZnSe‐ZnS SLS.

Evidence of lattice tilt and slip in m-plane InGaN/GaN heterostructure

Using high-resolution microbeam x-ray diffraction and cross-section transmission electron microscopy, we investigated in-plane anisotropy resulting from epilayer lattice tilts in heteroepitaxial InGaN on a m-plane GaN substrate. The in-plane structure consists of two lattice tilts along the [112¯0] direction corresponding to (101¯0) and (01¯10) slip planes inclined at roughly 60° from the m-plane. Based on the Peierls-Nabarro model, we explain this structure by proposing a slip system via the {101¯0} prism plane with 〈112¯0〉-type slip directions.

Low-loss short-wavelength optical waveguides using ZnSe-ZnS strained-layer superlattices

Low‐loss short‐wavelength optical ridge waveguides using ZnSe‐ZnS strained‐layer superlattices (SLS’s) were successfully fabricated on GaAs substrates by low‐pressure metalorganic vapor phase epitaxy. A propagation loss α as low as 0.71 cm−1 was obtained for the SLS waveguide composed of 80 periods of ZnSe(50 A)‐ZnS(50 A) at the 0.633‐μm wavelength TE fundamental mode. Therefore, this waveguide may be suited for optoelectronic integrated circuits composed of II‐VI and III‐V compound semiconductor devices. Furthermore, these waveguides exhibited a large difference in the propagation loss between TE and TM polarizations which may be related to the birefringence for TE and TM polarizations due to the slight anisotropy of the refractive index in the ZnSe‐ZnS superlattice structure. This birefringence effect will be very useful for a polarizing optical device.

High quality ZnSe films grown by low pressure metalorganic vapor phase epitaxy using methylalkyls

High quality ZnSe layers have been successfully grown by low pressure metalorganic vapor phase epitaxy (MOVPE) using methylalkyls, dimethylzinc, and dimethylselenide. Streaks and Kikuchi lines were observed by reflective high‐energy electron diffraction measurements. Also the sharp and strong excitonic emission line due to free exciton and no emissions related to deep levels or residual impurities were observed by photoluminescence measurements. These results show that the ZnSe layers are of good crystalline quality. Furthermore, good selective growth of the ZnSe films on structured substrates was easily achieved by using this low pressure MOVPE technique.

Temperature dependence of localized exciton transitions in AlGaN ternary alloy epitaxial layers

The optical properties of Ga-rich AlxGa1−xN (x=0.019, 0.038, 0.057, 0.077, and 0.092) ternary alloy epitaxial layers have been studied by means of temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. The luminescence intensity of excitons in five epitaxial layers indicated a thermal quenching process with two activation energies. The two quenching activation energies were attributed to the delocalization of excitons and thermal dissociation of excitons. Anomalous temperature dependence of the PL peak energy was also observed in the epitaxial layers, which enabled the evaluation of the localization energy of the excitons. The localization energy increased as the 1.7th power of the PL linewidth, which reflected a broadening of the density of localized exciton states. In addition, the luminescence decay of the localized excitons for the five epitaxial layers became longer with decreasing emission energy. These observations suggest that the decay of excitons is caused not only by ra...

Electronic Properties of Nitrogen Delta-Doped Silicon Carbide Layers

Nitrogen delta-doped silicon carbide (SiC) layers were grown by a new pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1.×10 18 cm −3 ) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.

Growth of ZnSe/ZnS strained‐layer superlattices on Si substrates

The high‐quality ZnSe‐ZnS strained‐layer superlattices (SLSs), as well as the ZnS layers, were successfully grown for the first time on Si substrates. Photoluminescence (PL) spectra of ZnS layers grown on Si showed an intense excitonic‐emission line. In transmission electron microscopy analyses, no misfit dislocations and no moire fringes were observed on the ZnS layer with a thickness of less than about 500 A. We have also characterized ZnSe‐ZnS SLSs grown on Si substrates. By the PL measurements, an intense excitonic‐emission line, and no emissions due to deep levels, were observed. As the ZnSe well‐layer thickness decreased, the peak of the line largely shifted towards the higher‐energy side. This behavior may be related to the quantum size effect. In the temperature dependence of PL intensity, there appeared the thermal quenching process, which may be related to the thermal release of excitons in quantum wells. As the thickness of the ZnSe well‐layer decreased, the activation energy abruptly increased...

Dependence of elastic strain on thickness for ZnSe films grown on lattice‐mismatched materials

We present a new approach for calculation of the dependence of elastic strain on layer thickness concerning ZnSe films grown on GaAs and on ZnS. The basic concept involved in our model is that the interfacial misfit dislocations are generated only when the areal strain energy density exceeds an energy barrier. It is found that the elastic strain calculated by this model is quite in agreement with experimental results for the ZnSe/GaAs system with the energy barrier of 79 erg/cm2. To compare the theory with experiments, the difference of thermal expansion coefficient between the epitaxial layer and the substrate must be taken into consideration. Applications of the model to other systems are also discussed.

Role of the buffer layer thickness on the formation of basal plane stacking faults in a-plane GaN epitaxy on r-sapphire

The thickness of low temperature AlGaN buffer layers grown on r-sapphire substrates has been found to directly affect the crystalline structure of the buffer layer as well as the structural and optical properties of subsequently grown a-plane GaN films. A buffer layer with a thickness of 30nm results in randomly distributed fine domains without extended defects. Increasing the thickness to 90nm leads to a uniform and largely coalesced crystalline structure, with well-defined stacking faults. GaN films grown on the thinner buffer layer contain a lower density of larger stacking faults, and exhibit brighter stacking-fault luminescence as compared to films grown on thicker buffer layers. Our studies indicate that the optimum buffer layer thickness for growth of a-plane GaN is about 30nm.

Nanoindentation study on insight of plasticity related to dislocation density and crystal orientation in GaN

Yield shear stress dependence on dislocation density and crystal orientation was studied in GaN by nanoindentation examination. The yield shear stress decreased with increasing dislocation density, and it decreased with decreasing nanoindentation strain-rate. It reached and coincided at 11.5 GPa for both quasi-static deformed c-plane and m-plane GaN. Taking into account theoretical Peierls–Nabarro stress and yield stress for each slip system, these phenomena were concluded to be an evidence of heterogeneous mechanism associated plastic deformation in GaN crystal. Transmission electron microscopy and molecular dynamics simulation also supported the mechanism with obtained r-plane dislocation line.