Takeshi Miyauchi

Photoluminescence Properties of CuGaSe2 Grown by Iodine Vapour Transport

The photoluminescence spectra of CuGaSe2 are measured on melt-solidified crystals, iodine vapour transported as-grown crystals and annealed crystals in various conditions. From the change in the spectra among them and the influence of annealing on luminescence, it is concluded that the iodine impurity acts as a deep donor which has been unintentionally incorporated into crystals during growth, and that the selenium vacancy acts as a shallow donor. The latter defect can be filled back by annealing in selenium vapour at 600°C. All photoluminescence bands observed in the iodine vapour transported crystals are explained in trems of these two donors (0.38 eV and 80 meV) and one acceptor (40 meV), which is probably due to the copper vacancy.

Excitonic Structure of CuGaS2xSe2(1-x) and CuAlS2xSe2(1-x)

Phase-shift-difference (PSD) spectra upon reflection are measured at 110 K and 20 K on CuGaS2xSe2(1-x) and CuAlS2xSe2(1-x) alloy systems. The spectra reveal so-called A, B and C excitonic structures which are associated with the splitting of the uppermost valence band. In addition, the first excited state of the A-exciton is observed at 20 K in CuGaS2xSe2(1-x) system, and the binding energy of the exciton is estimated by using the hydrogen model. Analysis by Hopfields quasi-cubic model for the spin-orbit and crystal field splittings is found to be inapplicable to these compounds. The crystal field splitting is always larger than expected in these compounds. This effect as well as anomalously small spin-orbit splitting are explained by p-d hybridization of the valence band.

Synthesis and Some Properties of Solid Solutions in the GaP-ZnS and Gap-ZnSe Pseudobinary Systems

Solid solutions of heterovalent systems, GaP-ZnS and GaP-ZnSe are prepared by the solution growth method. The energy gaps determined by optical absorption vary anomalously with composition. Measurements of photoconductivity and photovoltaic effect support the validity of the energy gap values determined by the absorption. The resistivities of the solid solutions are extremely large except for GaP-rich alloys. Photoluminescent spectra are closely related to deep recombination centers. The attempt to obtain a material having a direct wide energy gap and controllable electrical properties is unsuccessful because of anomalous composition dependences of the energy gap and the resistivity in the solid solutions studied here.

Spin-dependent luminescence enhanced by interface stress between III-V alloy layers on excitation of circularly polarized light

The relation between the luminescence polarization for circularly polarized light and the band structure of zincblende semiconductor under two-dimensional stress is theoretically discussed. Two type of samples with an unstrained InP layer and a strained InP layer due to the lattice mismatch are prepared. The luminescence from the InP layer excited by circularly polarized light is analyzed using a Babinet-Soleil compensator, (or phase-modulator) and polarizer. The large luminescence polarization of circularly polarized light is observed in the strained sample. At temperatures below 60 K, the luminescence polarization exceeds 25% which is the maximum value predicted in the unstrained crystal. The experimental results show that the luminescence polarization is increased by the internal strain due to the lattice mismatch.

Optical Band-Pass Filter Using Accidental Isotropy and Optical Activity of AgGaSe2 (II)

This paper is a sequel to a previous paper (Jpn. J. Appl. Phys. 24 (1985) 463), which dealt with an optical band-pass filter which utilized the accidental degeneracy of the refractive indices and the optical activity of an AgGaSe2 crystal. It theoretically expressed a method for controlling the band width and, further, predicted that the central wavelength of the filter could be changed by using a mixed alloy of AgGaS2 and AgGaSe2. In this research, these two problems were confirmed experimentally.

A New Method for Measuring Optical Activity in Crystals and Its Application to Quartz

This report describes a theoretical study of a convenient method for measuring the optical activity in crystals in which birefringence and optical activity coexist. The method was applied to right-handed quartz. The gyrations in the [100] direction and at 45° from the optical axis were measured. The gyration tensor component g33 calculated from the gyrations measured in these two directions was found to be in good agreement with the value of g33 measured by other workers in the direction of the c-axis (the optical axis) by conventional methods.

Electrical Properties of Gallium Phosphide

Hall coefficient and conductivity of GaP crystals produced by the solution-growth method were measured at temperatures between 4.2°K and 300°K. Activation energies of donors in undoped, Si-doped and S-doped samples are almost the same indicating about 0.10 eV. In Zn-doped Gap, the level of Zn lies 0.031 eV above the valence band. Electron mobility in undoped samples and hole mobility in Zn-doped samples are 140–180 cm2/voltsec and about 50 cm2/voltsec respectively at room temperature. These samples exhibited impurity conduction at considerably high temperature, for example, at 50°K for undoped samples, and its characteristics are similar to those of germanium in the metallic and the transition region.

Vapour Growth of CuGaS2 and Its Optical Properties

Single crystals of CuGaS2 are prepared by the iodine vapour transport method using either solution grown crystals or two constituent binary compounds, Cu2S and Ga2S3 as the starting materials. In both cases, a slight difference in the growth condition leads to different colouration of the crystals yielded; yellow, red and black varieties. Polarization measurements of transmission, reflection and photoconduction on these different coloured crystals also show different characteristics of these crystals. From the results of both growth experiments and optical measurements, the differences in colouration and optical properties among these crystals are expected to be due to a slightly off-stoichiometric unbalance between Cu2S and Ga2S3, and the yellow crystal is considered to be the most stoichiometric one.

Linear Electro-Optic Effect of AgGaSe2

Single crystals of AgGaSe2 with sufficient transparency to measure the electro-optic effect have been prepared by the Bridgman technique. The linear electro-optic coefficients are measured by a phase modulated method which is devised to remove the inaccuracy induced by the existence of natural birefringence. The coefficients at constant stress, γ63 and γ41 are found to be 3.9×10-12 and 4.5×10-12m/V respectively at a wavelength of 1.15 µm. The influence of optical activity on the observation of the electro-optic effect is also discussed.

Luminescences from Solution-Grown AlxGa1-xP Alloys

Single crystals of AlxGa1-xP alloys with several compositions up to x=0.74 are prepared by the solution growth method. Photoluminescence peaks at 77 K lie at about 1.96 eV for undoped n-type alloys, and near 2.1 eV for Zn-doped p-type alloys. The peaks show almost no shift as x increases. Zn-doped p-type alloys also exhibit electroluminescence when carriers are injected into the crystals. The peak energy nearly follows the energy gap with increasing x, in contrast to the photoluminescences. The photoluminescent emissions are assigned to the Si–Si pair for the 1.96 eV peak, and to Si–Zn and Si–C pairs for the 2.1 eV peaks. Transitions of free electrons to neutral Zn acceptors might play a role in the electroluminescence. The conductivities of the alloys are of the same order as that of GaP.