Toshiaki Asahi

Growth and characterization of large diameter ZnTe single crystals

ZnTe crystals of 80 mm diameter were grown in melt by the vertical gradient freezing (VGF) method. B 2 O 3 was used to prevent evaporation of Zn and Te during the crystal growth. The mean dislocation density of the grown crystals was about 4000-7000 cm -2 . The lowest dislocation density was found to be less than 2000 cm -2 . Hole concentrations of more than 10 18 cm -3 were obtained using ZnP 2 as a dopant. When the indium concentration in ZnTe was 2.9 x 10 18 cm -3 , the resistivity was around 1 × 10 8 Ω cm. Thus, high quality large-diameter single crystals covering a wide range of resistivity were obtained.

Molecular Beam Epitaxy Growth of ZnTe Epilayers on c-Plane Sapphire

ZnTe epilayers were grown on transparent substrates by molecular beam epitaxy. The insertion of a low-temperature buffer layer was carried out, and the influence of the buffer layer thickness and its annealing on the crystallographic property were investigated. Pole figure imaging was used to study the domain distribution in the layer. It was shown that the (111) ZnTe epilayer with the decreased number of domains could be formed on c-sapphire when a 3.5-nm-thick annealed ZnTe buffer layer was inserted. It was shown that the XRD pole figure imaging was a useful means of analyzing domain distributions in the film.

Molecular beam epitaxy growth and pole figure analysis of ZnTe epilayer on m-plane sapphire

ZnTe epilayers were grown on transparent () oriented (m-plane) sapphire substrates by molecular beam epitaxy (MBE). The insertion of a low-temperature buffer layer was carried out, and the influence of the buffer layer annealing on crystallographic properties was investigated. Pole figure imaging was used to study the domain distribution in the layer. It was shown that strongest (211)- and (100)-oriented ZnTe epilayers were formed on m-sapphire when a ZnTe buffer layer annealed at 340 °C for 5-min was inserted. Also, it was confirmed that only (211) ZnTe epilayers were formed on the 2° tilted m-plane sapphire substrate. Thus, the single domain (211) ZnTe epilayer can be grown on the m-plane sapphire using MBE.

Random error estimation in refractive index measured with the terahertz time domain spectroscopy

This study proposes a practical method to estimate the random error in real part of refractive index measured with terahertz time domain spectroscopy (THz-TDS) for the single measurement of sample by using the phase spectra of the reference terahertz electric field and their standard deviation. The validity of this method is based on the experimental result that the phase correlates with its standard deviation and the signal to noise ratio is almost equal in the phase spectra of reference and sample signal. The random error estimated from the proposed method fitted well to the statistically computed standard deviation.

Fe Doping and Preparation of Semi-Insulating InP by Wafer Annealing under Fe Phosphide Vapor Pressure.

Semi-insulating (SI) InP has been industrially produced by doping Fe atoms as deep acceptors. Fe concentrations in InP are, however, largely varied from top to tail along the crystal growth axis due to impurity segregation. In the present work, we have examined the possibility of vapor-phase Fe doping for fabrication of 50- and 75-mm-diameter SI InP wafers with constant Fe concentrations using a wafer annealing procedure. A small amount of Fe was charged with red phosphorus in ampoules in which InP wafers were annealed. It was found that the vapor-phase doping is effective for Fe doping of InP. The present technology can be applied for the fabrication of low Fe-doped SI InP wafers with similar Fe concentrations of all wafers from one InP ingot.

Accurate Complex Refractive Index with Standard Deviation of ZnTe Measured by Terahertz Time Domain Spectroscopy

Accurate complex refractive index of ZnTe crystal in terahertz spectral region from 0.2 to 3.5 THz is obtained by measuring three samples with different thickness using transmission terahertz time-domain spectroscopy. Each sample is measured a number of times and the mean and standard deviation (SD) of the complex refractive index are calculated to quantify the random errors in the measurement. The ratios of SD to mean of the real and imaginary parts of the refractive index are approximately 1.0×10-4 and 1.2×10-2 at 1 THz respectively. The absorption coefficient of our ZnTe sample is found to be comparable with or smaller than the values reported in previous literatures.

(211)-oriented domain formation during growth of ZnTe on m-plane sapphire by MBE

ZnTe epilayers have been grown on 2°-tilted m-plane

Surface Texture and Crystallinity Variation of ZnTe Epilayers Grown on the Step-Terrace Structure of the Sapphire Substrate

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