Takeo Takizawa

Single-crystal growth and optical properties of undoped and Ce3+ doped CaGa2S4

To improve the crystallinity of C a Ga 2 S 4 single crystals, we have grown the crystals under various conditions by the horizontal Bridgman method using a travelling temperature gradient. If we lower the growth speed to < 0.5 cm/h while keeping the growth temperature close to but a little higher than the melting point, cracks and voids in the crystals are substantially reduced. A carbon boat is adopted in order to avoid the reaction of an ampoule with the material solution, which is found effective to improve the quality of a grown crystal. However, an absorption band with a peak at 3.2 eV newly emerges to turn a grown crystal yellowish. Absorption spectra of Ce 3+ -doped crystals are also measured. As a result, the energy separation between the 5d sublevels of Ce 3+ ion, i.e., T 2g and E g is first estimated as 0.8 eV.

Phase diagram of the CuIn2Se system for CuInSe2 crystal growth by controlling Se contents

Abstract The detailed phase diagram of a CuIn2Se system is indispensable to control Se contents in growing CuInSe 2 bulk single crystals by the selenization method. The phase diagram is constructed by the differential thermal analysis (DTA) and powder X-ray diffraction measurements. The solidifying point in the composition ranging from CuInSe 2 to CuInSe 3 decreases by more than 100°C with increasing the Se content, and the samples do not contain heterogeneous phases. In addition, the Se vapor pressure dependence of the solidifying point of CuInSe 2 is investigated by the DTA measurement, using a electric furnace with two temperature zones. The solidifying point decreases by about 130° with increasing the Se vapor pressure up to 760 Torr. It is shown that the high-quality bulk single crystals with controlled Se content can be grown by controlling the Se vapor pressure.

Thermal analysis of chemical reaction process forming CuInSe2 crystal

In order to establish the preparation method of high-quality CuInSe2 single crystals, we have investigated the chemical reaction process of CuInSe2 by differential thermal analysis and by powder X-ray diffraction measurements. It is confirmed that CuInSe2 compound undergoes a phase transition from chalcopyrite to sphalerite structure at 815°C and melts at 996°C. In the chemical reaction process of a Cu+In+2Se mixture, an explosive exothermic reaction occurs at 250°C, which is ascribed to the formation of intermediate products of the In-Se system. In the chemical reaction process of a CuIn+2Se mixture, an explosive selenization reaction forming CuInSe2 compound occurs at 575°C. It is concluded that the selenization of CuIn alloy at temperatures higher than 600°C suppresses the formation of lattice defects and heterogeneous products.

Single crystal growth of CuInSe2 by selenization horizontal Bridgman method

Abstract Bulk single crystals of CuInSe 2 are grown by the selenization horizontal Bridgman method where the temperature profile is moved electrically with controlling Se vapor pressure. The p-type single crystals with large Hall coefficients and mobilities are prepared at Se vapor pressures of 10 and 25 Torr. Overshoots are observed in the Hall coefficient versus temperature curves for most of the p-type crystals. The peak temperature at the overshoot depends on the donor density, which is controlled by Se vapor pressure ranging from 5 to 25 Torr. Acceptors of the p-type crystals have two kinds of activation energies of ∼ 20 meV for Cu vacancy or copper in the In site, and ∼ 60 meV for the In vacancy. The number ratio of those densities is determined by the variation of the Hall coefficient versus temperature curves.

Defect Properties of CuInS2 Single Crystals Grown by Horizontal Bridgman Method with Controlling S Vapor Pressure

We have analyzed the lattice defects of CuInS2 bulk single crystals prepared by the horizontal Bridgman method with controlling S vapor pressure. The grown crystals have p-type conduction and electrical resistivities of more than 103 Ωcm at room temperature. From measurements of the Hall effect, photoluminescence, optical absorption and photoconductivity, the activation energies of acceptors in CuInS2 crystals are shown to be of 85 meV, 115 meV and 360 meV, which are respectively ascribed to Cu-vacancies, In-vacancies and extrinsic impurities; whereas donors of 35 meV activation energy are ascribed to S-vacancies.

DEFECT PROPERTIES OF CUINSE2 SINGLE CRYSTALS PREPARED BY SELENIZATION HORIZONTAL BRIDGMAN METHOD

CuInSe2 bulk single crystals have been grown using the selenization horizontal Bridgman method. On the basis of the temperature variation of Hall coefficient curves, we have classified the p-type CuInSe2 into the deep (D), shallow (S) or a combination of both (B) types, and identified that the activation energies of acceptors are 50–60 meV for In vacancies and 10–20 meV for Cu vacancies and Cus in the In site. The donors, whose densities decrease with annealing in Se atmosphere, are ascribed to Se vacancies having an activation energy of 10 meV. The optical band gaps of the D- and B-type samples are estimated to be 1.04 eV at 0 K, but that of the S-type sample is 1.00 eV. A photoluminescence peak is observed at 0.97–0.98 eV for the D- and B-type samples, and often other emissions at ~1.00 and 1.04 eV, while for the S-type sample, the peaks are observed at 0.93 and/or 0.95–0.96 eV. In conclusion, the activation energies of donors are ~30 meV for Ins in the Cu site and 5–10 meV for Se vacancies, and another level is expected to be ~60 meV.

Impurity band in p-type CuInSe2

The results of transport measurements on p-type CuInSe 2 single crystals in the temperature range of 20 to 300 K are presented and explained assuming the existence of an impurity band. The values of the activation energies of the acceptors, their concentrations, the concentration of the compensating donors, as well as the impurity band width were calculated.

Observation of Flux Movement in Quench-and-Melt-Grown YBCO Generated by a Pulsed Magnetic Field

The dynamics of the quantized flux in a ring-shaped YBa2Cu3O7 sample has been studied through the observation of the magnetic flux motion generated by a pulsed magnetic field. The flux speed is obtained as a function of the rise rate of the external field and as a function of temperature. It is shown that the flux speed is not intrinsic as determined by the creep theory but is determined by the external field and the Jc value. To interpret the slow rise of the observed signal, a distribution of different kinds of pinning centers in a sample is suggested.

Compensated Electrical Properties of CuInSe2 Single Crystals

Electrical properties of stoichiometry-controlled single crystals of CuInSe2 are investigated by the Hall effect measurement over the temperature range of 20 to 300 K. Overshoot characteristics have been observed in the temperature dependence of the Hall coefficient for all p-type crystals. The compensating donor density is shown to be a dominant factor for the Hall overshoot rather than the acceptor density. If we assume a relation that µn/µp=Const.T-η with η around 1.5, the Hall overshoot is well explained.

P-type sp3-bonded BN/n-type Si heterodiode solar cell fabricated by laser–plasma synchronous CVD method

A heterojunction of p-type sp3-bonded boron nitride (BN) and n-type Si fabricated by laser–plasma synchronous chemical vapour deposition (CVD) showed excellent rectifying properties and proved to work as a solar cell with photovoltaic conversion efficiency of 1.76%. The BN film was deposited on an n-type Si (1 0 0) substrate by plasma CVD from B2H6 + NH3 + Ar while doping of Si into the BN film was induced by the simultaneous irradiation of an intense excimer laser with a pulse power of 490 mJ cm−2, at a wavelength of 193 nm and at a repetition rate of 20 Hz. The source of dopant Si was supposed to be the Si substrate ablated at the initial stage of the film growth. The laser enhanced the doping (and/or diffusion) of Si into BN as well as the growth of sp3-bonded BN simultaneously in this method. P-type conduction of BN films was determined by the hot (thermoelectric) probe method. The BN/Si heterodiode with an essentially transparent p-type BN as a front layer is supposed to efficiently absorb light reaching the active region so as to potentially result in high efficiency.