Nozomu Tsuboi

Resonant Raman scattering and free-exciton emission in CuGaS2 crystals

Origins of lines appearing in the exciton region spectra of CuGaS2 crystals at low temperatures under 4579, 4765 or 4880 A excitation of an Ar+ laser are discussed in terms of emissions from upper- and lower-branch excitonic polaritons (UBPs and LBPs) and multi-phonon Raman processes. The line at 2.500 eV is an emission from LBPs. The line at 2.504 eV seen in our samples under 4765 A excitation is shown to be interpretable as a three-phonon Raman process in resonance with UBP states, while the line reported at the same energy by Shirakata is likely to be an emission from UBPs.

Growth and characterization of twin-free ZnSe single crystals by the vertical Bridgman method

The crystallographic characteristics of twin-reduced ZnSe crystals grown by a high-pressure melting method and the approach to stoichiometric growth are given. The crystals were grown by the seeded vertical Bridgman method and gradient freezing with in-situ composition control by a defined Zn source temperature. One inch ZnSe single crystals having twin-reduced parts and an average etch pit density of 2.0 × 105cm−2 were successfully grown by the seeded Bridgman method. Multi-twin patterns, typically for melt-grown boules, were not detected from the observation by optical microscopy with crossed polaroids and by transmission X-ray topography. The crystallinity of the as-grown crystal was characterized by X-ray rocking curve analysis using a double crystal arrangement. A full width at half maximum of 24 arcsec was detected. The influence of the Zn source on the crystal composition was studied by chemical analysis and photoluminescence.

Absorption Edge Studies of CuGaS2 Single Crystal

The transmission and reflection spectra have been measured in CuGaS2 single crystals near the absorption edge in the temperature range 77–293 K. The absorption spectra for E⊥ Z calculated by assuming that the absorption band of the A-exciton is superposed on the absorption tail expressed as Urbachs rule fits the experimental spectra fairly well in that temperature range. Thus, the temperature dependence of the A-exciton energy (E01) becomes clear. The reflectivity maximum coincides with E01 only at low temperatures (< 100 K). Other parameters such as the damping constant, steepness factor and oscillator strength for the Γ5 exciton are also discussed.

Growth of CuInS2 and CuIn5S8 on Si(001) by the Multisource Evaporation Method

Thin films of CuInS2, CuIn5S8 and a mixture of these were epitaxially grown on Si(001) wafers at 400°C using a multisource evaporation method in which the In source temperature was varied. X-ray diffraction and reflection high energy electron diffraction observation showed that the CuInS2 film did not crystallize in the chalcopyrite structure, but in the Cu–Au structure with the c-axis normal to the substrate coexisting with the sphalerite structure. The CuIn5S8 film crystallized in the relaxed spinel structure with fairly good orientation. No evidence of CuIn3S5 was found from the X-ray diffraction patterns.

Improvement in Layer Quality of CuGaS2 Grown by Vapor Phase Epitaxy with Metal Chlorides and H2S Sources

In vapor phase epitaxial growth of CuGaS2 on GaP substrates under simultaneous feeding of H2S and metal chlorides, two-step growth was found to be useful for obtaining thick layers with single orientation of c-axis of the chalcopyrite normal to the substrate. Under alternate feeding of H2S and mixed gas of the chlorides, saturation behavior of growth rate was observed for the feeding amount of copper chloride and that of gallium chloride, and also for the supplying period of H2S gas. These results strongly suggest atomic layer epitaxy of CuGaS2. The layers grown under alternate feeding as well as layers by the two-step process under simultaneous feeding showed strong free exciton emission at low temperatures, indicating high quality of these layers.

Absorption and Photoluminescence Spectra of Heavily Zn-Doped CuGaS2 Crystals

In heavily Zn-doped (up to 5%) CuGaS2 crystals, reductions of excitonic absorption and above-band-gap absorption were observed for optical transitions of the polarization parallel to the optic axis, probably related to a change in the exciton oscillator strength ratio of the transitions of polarizations parallel and perpendicular to the optic axis. An impurity absorption whose concentration dependence is consistent with electron transitions from ZnGa acceptors to the conduction band is reported for the first time. An emission appearing at 2.1~2.3 eV is considered to be due to the electron transitions from a donor level of sulfur vacancies or their complexes with ZnGa to the ZnGa acceptor level, involving the effect of broadening of the latter level. The existence of isoelectronic-trap ZnCu-ZnGa pairs is suspected from time-resolved emission spectra.

Vapor-Phase Atomic Layer Epitaxy of CuGaS2 at Atmospheric Pressure Using Metal Chlorides and H2S

In the alternate feeding of metal chlorides (CuCl, GaCl3) and H2S sources, the growth rate saturation of CuGaS2 on GaP was observed for substrate temperatures between 530° C and 570° C, and also for some range of H2S flow rate. Together with the growth rate saturation behavior for metal chlorides reported in our previous paper, these results can be considered to indicate achievement of atomic layer epitaxy (ALE) of CuGaS2. The growth orientation change leading to c-axis growth was observed for a large H2S flow rate. Some possibility of stoichiometry control by ALE was suggested from photoluminescence spectra.

Raman Scattering and Two-Phonon Infrared Transmission Spectra of Cu(AlxGa1-x)S2 Crystals.

First- and second-order Raman scattering and two-phonon infrared (IR) transmission measurement results for Cu(Alx Ga1-x )S2 single crystals have been examined as a function of x. It is apparent that the highest frequency B2 and E modes at the zone center exhibit two-mode(persistence)-type behavior. The appearance of the two-mode-type behavior in a ternary alloy system is shown to be explainable by theoretical consideration based on a modified model of the type judging criterion. In addition, some of the two-phonon IR absorption bands and second-order Raman peaks are found to be due to phonons located at zone-boundary points. Compositional dependence of the A1(W1) mode Raman line shape and the appearance of a cross-coupled phonon between the two modes indicate that the clustering effect is essentially suppressed in this alloy system.

Resonant Raman Scattering and Excitonic Polariton States in CuGaS2

Scattering processes and the relevant intermediate state for intense Raman lines appearing in the exciton region spectrum of CuGaS2 under 4765 A excitation of an Ar+ laser are discussed from measured intensity changes of the lines with temperature as well as photon energy of excitation. Experimental results are consistent with the previous interpretation that the line at 2.504 eV corresponds to a three-phonon Raman scattering in resonance with the upper-branch excitonic polariton (UBP) state. The line at 2.509 eV is thought to correspond to a two-phonon Raman scattering in resonance with the UBP state.

Lattice Strain in AgGaS2 Epitaxial Layers Grown on GaAs (100) by Multisource Evaporation

Lattice strain in AgGaS2 epitaxial layers of different thicknesses, which are grown on GaAs (100) by the multisource evaporation method, is described. Biaxial compressive mismatch strain in thin layers relaxes with increasing layer thickness, and biaxial tensile thermal strain becomes dominant in thick layers. The thin AgGaS2 epitaxial layers are completely oriented toward the c-axis, but the thick layers contain 180°-rotated { 112 } twin crystals. The generation of twin crystals is considered to cause partial relaxation of the thermal strain. The observed variation in the A1-mode frequency with thickness is fully explained by considering the layer thickness dependence of the lattice strain. Full width at half-maximum of the A1-mode Raman scattering line is affected by the spatial inhomogeneity of the strain magnitude in the depth direction. Raman scattering measurement of the A1 mode can be used as a sensitive technique to evaluate the lattice strain in AgGaS2.