Isao Kikuma

Indirect optical absorption of single crystalline β-FeSi2

We investigated optical absorption spectra near the fundamental absorption edge of β-FeSi2 single crystals by transmission measurements. The phonon structure corresponding to the emission and absorption component was clearly observed in the low-temperature absorption spectra. Assuming exciton state in the indirect allowed transition, we determined a phonon energy of 0.031±0.004eV. A value of 0.814eV was obtained for the exciton transition energy at 4K.

β-FeSi2 Single Crystals Grown from Solution

We have grown high-quality β-FeSi2 bulk single crystals by a temperature gradient solution growth method using Ga solvent. Polyhedral shaped bulk crystals with clear facet planes were obtained below the growth temperature of 900°C. Laue observation confirmed that crystals are high-quality single crystals without twins. Full-width at half maxim of the X-ray rocking curve at β-FeSi2 (800) reflection was 53 arcsec. The conduction was p-type and the resistivity was 0.03–0.04 Ωcm at room temperature.

Direct observation of the 3C-2H transformation in ZnSe by high-temperature x-ray diffraction

Abstract The crystal structures of ZnSe at high temperatures up to 1440°C are directly observed by a high-temperature oscillation technique. ZnSe samples cut from melt-grown crystals with a zincblende structure are sealed in fused quartz ampoules to avoid sublimation. The ampoule is fixed in a graphite heater. An X-ray diffraction study of the structural transformation in ZnSe is performed through the graphite heater. The X-ray patterns by the 12° oscillation method with Cu K α show that the cubic zincblende (3C) structure of ZnSe transforms to the hexagonal wurtzite (2H) structure above a reported transition temperature of 1425°C and that the 2H to 3C transformation in ZnSe occurs by cooling.

Solution Growth of Single-Phase β-FeSi 2 Bulk Crystals

We have succeeded in growing single-phase β-FeSi2 crystals by a temperature gradient solution growth method. FeSi2 solute and Ga solvent were placed in a quartz ampule, which was evacuated under a high vacuum (<2×10-6 Torr) and sealed with a quartz rod. The ampule was heated for 24–144 h in a gradient temperature profile. The grown crystals had multiple facet planes and the color was metallic silver white. X-ray diffraction showed that the grown crystals were single-phase β-FeSi2. The conduction of the β-FeSi2 grown crystals was p-type and the resistivity was between 0.05 and 0.2 Ωcm.

Solution Growth and Optical Characterization of β-FeSi2 Bulk Crystals

We have succeeded in growing large-sized β-FeSi2 crystal (10 mm in diameter and 0.3–0.5 mm in thickness) by a temperature gradient solution growth method using a carbon plate. The grown crystals had large-sized grain boundaries with an area of about (1–2)×(1–4) mm2. Optical absorption measurements were performed at room temperature on single crystalline specimens prepared from the grown crystals. The absorption spectra suggest that β-FeSi2 has an indirect band-gap structure with a gap energy of approximately 0.75 eV.

Formation of defects in zinc selenide crystals grown from the melt under argon pressure

The formation of macroscopic defects in ZnSe crystals is described. Crystals were grown by the Bridgman technique at the lowering rate of a crucible of 0.8–10.1 mm/h under an argon pressure of 100 kg/cm2. The formation of the assembly of rod-like low angle grain boundaries was dependent upon the angle θ between the growth axis and the (111) twin plane normal. For 75° < θ < 90° the rod-like low angle grain boundaries occurred and for θ < 75° crystals free of the boundaries grew at the lowering rates used in our experiments. The formation of voids was dependent on the lowering rate of the crucible. Many crystals free of these defects were obtained in the growth range 0.8–2.5 mm/h.

Melt growth of ZnSe crystals under argon pressure

Abstract The growth conditions of ZnSe crystals from the melt under an argon pressure of 100 kg/cm2 by the Bridgman technique are described. The temperature distributions in a high pressure furnace and the lowering rate of a graphite crucible were varied. The composition of the quenched melts was determined by chemical analysis of Zn. The excess Se in the melt is one of the important factors which limit the growth rate of ZnSe single crystals.

Observation of Etch Pits of β-FeSi2 Single Crystals

We have observed etch pits of p-type β-FeSi2 single crystals grown by the temperature gradient solution growth method. Characteristic etch pits which depend on the surface orientation of the crystals were observed on the surface etched by diluted hydrofluoric acid and HF:HNO3:H2O=1:2:2–8 solutions. These etchants are suitable for the observation of etch pits and surface orientation of β-FeSi2 single crystals.

Growth of ZnSe crystals from the melt under Zn partial pressure

Abstract This paper describes melt growth of ZnSe crystals by a modified Bridgman method under argon pressure. Stoichiometric deviations of the ZnSe melt are controlled by Zn partial pressure. A crucible and a Zn reservoir are placed in a pyrolitic boron nitride (pBN) vessel, which is covered with a deep, closed pBN cap to minimize loss of Zn. A definite Zn partial pressure over the melt during growth is maintained by controlling the temperature of the Zn reservoir. The melt composition is analyzed as a function of the temperature of the Zn reservoir. The stoichiometric melt is obtained under Zn partial pressure. Low-resistivity n-type crystals of 0.2-0.9 Ω cm are grown from Zn-rich melts in a one-step process.

Crystal growth of β-FeSi2 by temperature gradient solution growth method using Zn solvent

Single crystalline β-FeSi 2 with a low carrier density has been grown by a temperature gradient solution growth method using Zn solvent. The crystals were polyhedral with clear growth facets. Hall measurements revealed that the crystals showed p-type conductivity. The hole concentration and Hall mobility at room temperature were about 4 x 10 17 cm -3 and about 19 cm 2 /V s, respectively.