Masahiro Nagano

Resonant Raman Scattering in ZnS

Three Raman lines were observed in addition to excitonic emission lines in cubic ZnS using a He-Cd laser 325 nm line. The lines were assigned to the Raman lines of the laser line scattered by one or two phonons. The energies of the LO phonon and the TO phonon were estimated to be 44 meV and 35 meV from the energy of the Raman lines. These values agreed well with those measured by right-angle Raman scattering with an Ar laser 514.5 nm line. With the rise in temperature from 4.2 K, the intensity of the Raman lines declined until 80 K and was then enhanced to about 130 K. The temperatures where the intensity was enhanced corresponded to those where the energy of the free exciton level approached the energies of the Raman lines. The temperature dependence of the Raman intensity was interpreted in terms of resonance between the incident or scattered light with the temperature-dependent free exciton level.

Plan-View and Cross-Sectional Photoluminescence Imaging Analyses of Threading Dislocations in 4H-SiC Epilayers

We performed a plan-view and cross-sectional photoluminescence (PL) imaging and a spectral analysis of threading dislocations in 4H-SiC epilayers in the near-infrared region. The bright PL spots of threading screw dislocations (TSDs) and threading edge dislocations (TEDs) observed in the plan-view PL imaging are compared with the grazing incidence synchrotron X-ray topography contrast, and precise discrimination of threading dislocations using the PL technique and the direct acquisition of Burgers vector directions of TEDs are demonstrated. The inclination angles of TSDs and TEDs across a thick epilayer are revealed by the cross-sectional PL imaging, and the variations in the plan-view PL appearances of the threading dislocations are confirmed to originate from the line directions of such dislocations.

Three-dimensional imaging and tilt-angle analysis of dislocations in 4H-SiC by two-photon-excited band-edge photoluminescence

We demonstrate the three-dimensional imaging of threading screw dislocations (TSDs), threading edge dislocations (TEDs), and basal plane dislocations (BPDs) in 4H-SiC using two-photon-excited band-edge photoluminescence. Three-dimensional images of TSDs, TEDs, and BPDs are obtained successfully as dark contrasts on a bright background of band-edge emission. Dislocation images extending ~200 µm from the surface are demonstrated. The tilt angles of TSDs and TEDs in 4H-SiC epilayers are also measured, and the mechanisms governing the line directions of TEDs and TSDs are discussed.

Spectroscopic and laser oscillation characteristics of highly Cr/sup 4+/-doped forsterite

We investigated the absorption, fluorescence and lasing characteristics of chromium-doped forsterite with high Cr/sup 4+/-ion concentration. This crystal has a large absorption coefficient of 3.2 cm/sup -1/ at the fundamental wavelength (1064 nm) of Nd: YAG laser which is used as the pump laser. Using this crystal of which size was only 5/spl times/5/spl times/5 mm/sup 3/, laser oscillation was achieved by pumping with 1064-nm pulses from a Q-switched Nd:YAG laser. The output energy of over 1.2 mJ and 25% slope efficiency, which is the highest value achieved in the case of pulse operation to our knowledge, were obtained. In this case, 70% of the pump energy was absorbed in the crystal during the first pass through at the maximum output energy.

Low-Damage Sputtering of GaAs and GaP Using Size-Selected Ar Cluster Ion Beams

We have investigated the irradiation damage depths produced by an Ar gas cluster ion beam (GCIB) and an Ar monomer ion beam (MIB) on GaAs and GaP, in a comparison of the cathode luminescence (CL) spectra of ion-irradiated and nonirradiated areas. The depths of irradiation damage in both substrates were estimated from the relationship between the CL intensity and the electron beam acceleration voltage.

Atomic Layer Epitaxy of AlAs Using Dimethylethylamine Alane

AlAs was grown by the atomic layer epitaxy (ALE) technique using a novel precursor, dimethylethylamine alane (DMEAA), as the aluminium source, for the first time. One-monolayer saturated growth was observed in the temperature range from 360°C to 440°C. The epilayers exhibited very smooth surface morphology. The lowest carrier concentration in the ALE layer was 3×1018 cm-3.

Laser beam forming by fabricated aspherical mirror

A Fresnel error-reduction algorithm is compared with a stationary phase method as the method for designing the mirror surface profile of a laser beam forming system. We reshaped a Gaussian laser beam into a more uniform-intensity circular beam using a fabricated aspherical mirror.

Indirect-to-Direct Transition of (AlAs)n/(AlP)n Strained Short-Period Superlattices

We carried out first-principles calculation of (AlAs)n/(AlP)n superlattices (SLs) (n=1–5) using the full-potential linearized augmented plane-wave (FLAPW) method. The SLs are assumed to grow epitaxially on the (001) GaAs and GaP substrates. Indirect-to-direct transition was observed for n2 on the (001) GaAs substrate while all SLs grown on the (001) GaP substrate were indirect gap semiconductors. Optical properties were evaluated by the square of vertical transition matrix elements. The matrix elements of (AlAs)n/(AlP)n SLs grown on (001) GaAs exhibited oscillating behavior. The matrix elements for odd n are two orders larger than those for even.

GAS-SOURCE MIGRATION-ENHANCED EPITAXIAL GROWTH OF ALAS/ALP SUPERLATTICES

Gas-source migration-enhanced epitaxial growth of AlAs/AlP short-period superlattices on (001) GaAs substrates at low growth temperature was studied by X-ray diffraction. The intensities of the peaks, which are related to the superlattices in X-ray diffraction patterns, increased with decreasing growth temperatures. The intensities of the peaks were influenced by the group V supply. According to the X-ray diffraction patterns of (AlAs)n/(AlP)n (n=1, 2) short-period superlattices grown at 400°C, the average heterointerface roughness of the (AlAs)1/(AlP)1 superlattice is estimated to be 0.17 A and our growth technique shows good potential for obtaining As- and P-containing compounds.

Photoluminescence in Sb-Doped ZnS

This paper describes the photoluminescence properties of cubic ZnS grown from three kinds of solution: Sb0.4S0.6, Sb and Zn0.6Sb0.4. The ZnS crystals grown from these three solutions showed n-type conduction. The concentration of Sb in the ZnS crystal grown from the Sb0.4S0.6 solution was 0.01 at.%. The emission band at 3.794 eV, which was first observed on the ZnS crystal from the Sb0.4S0.6 and Sb solutions, was due to the annihilation of excitons bound to neutral donors with a binding energy of 7 meV. It is considered that the Sb atom acts as the donor related to the 3.794 eV band. It seems that the Sb donor is also related to the band at 3.770 eV due to the transitions from the donor level to the valuence band.