Tadashi Koyama

Nonlinear optical property of CdTe microcrystallites doped glasses fabricated by laser evaporation method

Samples of CdTe microcrystallites doped glasses were fabricated by a high energy pulsed laser evaporation method. In order to fabricate a CdTe doped glass, synthesis of CdTe microcrystallites and formation of SiO2 films were carried out alternately on a fused silica glass substrate. The absorption edge of the CdTe doped glasses shifted to a higher energy region than that of the bulk CdTe due to the quantum size effect as the particle size of CdTe microcrystallites decreased. The third‐order nonlinear susceptibility of χ(3) was estimated to be 4×10−7 esu at 580 nm using the method of degenerate four wave mixing.

Growth of high-quality ZnSe by MOVPE on (100) ZnSe substrate

Single crystalline layers of undoped ZnSe have been on (100) ZnSe substrates by atmospheric pressure MOVPE using dimethylzinc (DMZ) and hydrogen selenide (H2Se). The layers are typically 6 μm thick and the surface morphologies, measured by Nomarski phase contrast interference microscopy, appear superior to those of ZnSe layers grown on GaAs subrates. The photoluminescence (PL) spectra at 4.2 K exhibit strong band edge PL peak dominated by donor bound exciton (DBE) and free exciton (FE) emissions. The full width at half maximum (FWHM) of DBE is 0.8 meV and sharper than that of layers grown on GaAs substrates. However, the intensity of the self-activated (SA) PL of the homoepilayer measured at 77 K is several times more intense than that of heteroepilayers. From the investigations on the gas source molar ratio, it is inferred that the reason for this is that Zn vacancies diffuse from the substrate into the layer even at a growth temperature as low as 250°C. The characterization of the crystallographic properties by double X-ray diffraction rocking curves shows that the crystalline quality of the homoepilayers is drastically improved by heat treatment (300–650°C) of the ZnSe substrate under hydrogen atmosphere in the reactor before growth. The FWHM of the (400) diffraction of the homoepitaxial layer grown under the optimum conditions is 77 arc sec and this value shows that the ZnSe layer has the highest crystalline quality which has ever been obtained by any growth technique.

Fabrication of microcrystallites of II–IV compound semiconductors by laser ablation method

Abstract Microcrystallites of CdTe and CdS were obtained by pulsed laser ablation in argon gas. Average particle size depended on laser power, and on gas pressure during ablation. Particle diameter of CdTe could be controlled from 4 to 10 nm and their particles dispersed in methanol showed a quantum size effect on measurements of absorption property. Photoluminescence spectra of microcrystallites of CdS had a strong band edge emission related to impurities or defects. We confirmed that the laser ablation method to fabricate microcrystallites of II-VI compound semiconductors was useful.

Growth of ZnSe single crystals by iodine transport

Abstract Large ZnSe single crystals (14x14x20 mm3) have successfully been grown from the vapor phase by iodine transport. Substrates with large area (10x10 mm2) for epitaxial growth were prepared by cutting the crystal. It is important to control the temperature fluctuation and seed direction for rapid growth. As an etchant for the measurement of dislocation density, the NaOH solution on (111)B is better than the Br-CH3OH solution on (111)A. The etch-pit density is (2–7)x104/cm2. The iodine concentration in the crystal is about 200 ppm. The resistivity of the ZnSe crystal annealed in molten Zn is 0.03 Ω cm.

Direct formation of a surface-relief grating on glass by ultraviolet visible laser irradiation.

Surface-relief gratings were directly fabricated onto a glass surface by UV-visible laser irradiation. The glass surface was pretreated by molten salt, including Ag ions. Periodic intensity modulation of the laser light was conducted with a phase mask or by an interference technique. A pattern generated by intensity modulation was precisely transcribed onto the glass surface and a surface-relief grating was formed. The period and depth of the grating were 0.5 to 10 microm and less than 0.8 microm, respectively. The cross-sectional profile of the grating was sinusoidal or triangular, with very smooth surface morphology.

Epitaxial Growth of High Quality ZnSe on GaAs Substrate by Atmospheric Pressure MOVPE Using Dimethylzinc and Hydrogen Selenide

Single crystalline layers of undoped ZnSe have been grown on GaAs substrates by atmospheric pressure MOVPE using dimethylzinc (DMZ) and hydrogen selenide (H2Se). Premature reactions typically encountered with this source combination can be eliminated completely, even at atmospheric pressure, by controlling the flow velocity of each source gas and the source gas mole ratio. For the first time, an excellent mirror surface morphology, as revealed by Nomarski interference microscopy, was obtained even for 8.2 µm thick epilayers grown at 300°C.

Ultrafast nonlinear optical effect in CdTe-doped glasses fabricated by the laser evaporation method

Abstract The laser evaporation method is an attractive technology to synthesize composite materials. We tried to apply the laser evaporation method on fabrication of CdTe-doped glasses for the first time. CdTe microcrystallites embedded in SiO2 films were confirmed by transmission electron microscope images. The third-order nonlinear susceptibility χ(3) and the decay time τ of CdTe-doped glasses were measured by three-beam forward type degenerate four-wave mixing. The maximum value of χ(3) was estimated 4.2 x 10-7 esu when the absorption coefficient was 6000 cm-1. From the limitation of pulse duration of 5 ps, τ was not exactly measured but imagined to be shorter than 10 ps, which was much faster than the other semiconductor-doped glasses ever reported. The figure of merit defined as χ(3)/ατ is probably larger than 7.

Fabrication of Micro-Optic Elements by the Sol-Gel Method

Micro-lens arrays composed of organic-inorganic materials were molded on glass substrates by the sol-gel method. Each lens had the same focal length and accurate pitch. There was no change in appearance or focal length after heat treatment at 350°C. The arrays had a refractive index of 1.53 (@1550 nm) and a transmittance of 98%/mm (@1550 nm). The film material did not change in appearance during evaluation for acid resistance.

High‐quality epitaxial growth of ZnSe on (100) ZnSe by atmospheric pressure metalorganic vapor phase epitaxy

Single crystalline layers of undoped ZnSe have been grown on ZnSe substrates at 250 °C by atmospheric pressure metal organic vapor‐phase epitaxy using dimethylzinc and hydrogen selenide (H2 Se). The substrates are ZnSe wafers with (100) orientation cut from bulk crystals grown by iodine vapor transport. A homoepitaxial layer (homoepilayer) of ZnSe with excellent crystallographic properties, as revealed by double crystal x‐ray diffraction, was obtained. The full width at half maximum (FWHM) of the (400) diffraction pattern was 38 arcsec, which was superior to the crystallographic property (the FWHM was 250 arcsec) of the heteroepitaxial layer (heteroepilayer; a GaAs substrate was used). However, the intensity of deep emissions near 560 nm (PL at 77 K) is several times stronger than that of the heteroepilayer. The origin of deep emissions is not supposed to be due to iodine or copper diffused from the ZnSe substrate but to complexes between Zn vacancies and impurities in the layer.

Crystallographic properties of ZnSe grown by sublimation method

Abstract The crystallographic properties of ZnSe grown by the sublimation method were investigated. The (111) B face of the seed crystals was suitable for crystal growth. The crystal morphology was the hexagonal prism and they were constructed by {110} faces only. These properties were compared with the crystal grown by the iodine transport method. The seed direction and crystal morphology were different between the two methods. Particularly, the twin density of the crystal by the sublimation method was 5–6 mm -1 , and was higher than the crystal by the iodine transport method. This high value relates to the hexagonal prism morphology and the change of the surface energy when the twin was generated.