Haruhiko Udono

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.

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.

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.

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.

Crystal growth and characterization of Mg2Si for IR-detectors and thermoelectric applications

We have investigated the melt growth of Mg2Si crystal and its electrical and optical properties. Progress in Mg source purity and stoichiometric control during the growth enabled the development of a high purity Mg2Si crystal with low carrier density and a high stable Mg2Si with good doping controllability. The Mg2Si crystal grown by the pressure controlled Bridgman method using 5N purity or 6N purity of Mg source and purified PG crucible showed low electron density (~1015 cm−3) and high electron mobility (485 cm2 V−1 s−1 at 300 K and 21900 cm2 V−1 s−1 at 40 K). Silver doping in the high purity crystals performed the low-hole density of p-type Mg2Si (~3 × 1016 cm−3). Ionization energy of residual Al donor in the high purity crystal and Ag acceptor in the Ag doped crystals was determined as 8–9 meV and 26 meV, respectively. Indirect band gap energy Eg of approximately 0.61 eV at 300 K and 0.69 eV at 4 K were estimated by the optical transmission measurements on the high purity crystals. It is also found that the Sb-doped melt grown crystal had good power factor around room temperature (26 µW cm−1 K−2 at 270 K).

Convenient Melt-Growth Method for Thermoelectric Mg2Si

We have succeeded in growing single-crystalline-like n-type Mg2Si bulk crystals by a convenient melt-growth method that requires no vacuum or inert gas. The Sb-doped, n-type Mg2Si crystals had a density equivalent to the theoretical ideal of 1.99 g cm3 to 2.00 g cm−3 and well-developed crystalline grains. Powder x-ray diffraction measurements and scanning electron microscopy observations confirmed the single-phase Mg2Si nature of the grown crystals, with no MgO or unreacted Si and Mg observed. The crystals had high Hall mobility and power factor compared with Sb-doped sintered Mg2Si crystals. The achieved ZT values were 0.10 at 300 K and 0.36 at 600 K for 0.317 at.%Sb-doped Mg2Si.

Structural study of FeSi2 under pressure

The structural properties of β- and α-FeSi2 were examined by high-pressure x-ray diffraction at room temperature. In β-FeSi2, the lattice compression in the longest a axis is larger than in the b and c axes. The expected pressure-induced quenching by the Jahn-Teller effect on γ-FeSi2 was not achieved in the pressure range investigated. The bulk modulus B0 of β-FeSi2 and its derivative B0′ were determined to be 243.5GPa and 3.2GPa, respectively. A high-pressure phase, tentatively assigned to be tetragonal, appeared at about 20GPa. For α-FeSi2, a higher temperature phase of β-FeSi2, the bulk modulus B0 and its derivative B0′ were determined to be 183GPa and 6.2GPa, respectively. The experimental values of B0 are compared with the values estimated by various first-principles calculations, where the characteristic tendency is shown.

Growth and characterization of Br-doped ZnSe single crystals grown by a vertical sublimation method

Bromine doping of ZnSe single crystals grown by using a vertical sublimation method was studied. ZnBr2 was used as a dopant at concentrations of 0.003 to 0.1 mg/cm 3 in the ampoules. When the Br-doping was 0.01 mg/cm 3 or less, the average growth rate of the crystals was about 34 mg/h cm 2 ; the same as that without doping. When the Br-doping exceeded 0.01 mg/cm 3 , the average growth rate decreased with increasing dopant concentration. The undoped ZnSe crystals were yellow, whereas the Br-doped ZnSe crystals were orange. The high quality of the grown crystals was confirmed by X-ray diffraction (XRD), which showed that the full width at half maximum (FWHM) of the rocking curve ranged from 6.7 to 8.9 arcs. Bromine concentration of the grown crystals ranged from 4 10 17 to 1.5 10 19 cm 3 (measured by secondary ion mass spectrometry, SIMS), depending on the dopant concentration. All of the as-grown crystals showed high resistivity. After growth, all of the grown crystals were annealed in a Zn atmosphere at 11008C for 100 h to activate the bromine as an n-type donor. When the dopant concentration was increased, the carrier concentration of Br-doped ZnSe increased from 1.4 to 4.1 10 17 cm 3 and the mobility decreased from 366 to 146 cm 2 / V s (determined by Hall effect measurements). Our results show that high quality, low resistivity ZnSe single crystals can be grown by this vertical sublimation method. # 2001 Elsevier Science B.V. All rights reserved.