Hisao Uchiki

Photoluminescence properties of ZnO epilayers grown on CaF2(111) by plasma assisted molecular beam epitaxy

Photoluminescence (PL) properties of ZnO films grown on CaF2(111) with a low-temperature buffer layer by plasma assisted molecular beam epitaxy are investigated, in which lattice misfit tensile strain is expected to be compensated by the thermal compressive strain. The low-temperature buffer layer is further introduced to accommodate lattice strain leading to the growth of almost strain-free and high quality ZnO films. PL spectra of ZnO layers measured at 10 K are dominated by neutral-donor bound exciton emission at 3.366 eV (I4) with a linewidth of 12 meV. Commonly observed deep level emission at around 2.3 eV is negligibly small in intensity. Free exciton emission develops as temperature is raised and eventually dominates at temperatures higher than 70 K. Detailed study on temperature-dependent PL spectra indicates that the energy position of the free exciton emission is located at the same energy as bulk materials suggesting the growth of strain-free ZnO layers. Consequently, stimulated emission due to...

Optical properties and photo-induced memory effect related with structural phase transition in TlGaS2

Abstract Anomalies of the temperature coefficient of exciton absorption peak shift of TlGaS 2 were observed around 180–190 and 240–250 K. Some Raman lines were found to split at temperatures around 230–260 K. At low temperatures the light irradiation with the photon energy above the band gap energy caused a memory effect of photoluminescence quenching. This effect was observed for the emission appearing at 609 nm. Restoration of this photoluminescence quenching occurred with temperature range of 180–270 K. Complete recovery was attained at temperature above 270 K. The correspondences among the temperature ranges of the exciton absorption peak shift anomalies, the Raman line splitting, and the disappearance of the memory effect are considered to show that in TlGaS 2 successive phase transitions occur around 180–190 and 230–260 K.

Photoluminescence studies of p- and n-type ZnS layers grown by vapor phase epitaxy

Abstract In order to clarify the donor and acceptor levels in p- and n-type ZnS layers grown on GaAs substrates by vapor phase epitaxy, delayed photoluminescence excitation spectra were measured at some lapse of time after excitation. This method enabled us to observe a direct resonant excitation of the selective donor-acceptor pair transition having a certain Coulomb energy. Through comparison of the resonant and non-resonant portions of the excitation spectrum, the p-type layer grown under the simultaneous presence of NH 3 and excess Zn atmospheres was found to have a shallow acceptor level of either ≈60 meV or ≈100 meV ionization energy. A description is also given on the structures of the excitation spectra related to bound excitons and on decays of bound excitons.

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.

Donor-acceptor pair recombination luminescence from monoclinic Cu2SnS3 thin film

The defect levels in Cu2SnS3 (CTS) were investigated using photoluminescence (PL) spectroscopy. A CTS thin film was prepared on a soda-lime glass/molybdenum substrate by thermal co-evaporation and sulfurization. The crystal structure was determined to be monoclinic, and the compositional ratios of Cu/Sn and S/Metal were determined to be 1.8 and 1.2, respectively. The photon energy of the PL spectra observed from the CTS thin film was lower than that previously reported. All fitted PL peaks were associated with defect related luminescence. The PL peaks observed at 0.843 and 0.867 eV were assigned to donor-acceptor pair recombination luminescence, the thermal activation energies of which were determined to be 22.9 and 24.8 meV, respectively.

Cu2ZnSnS4 thin film deposited by sputtering with Cu2ZnSnS4 compound target

Cu2ZnSnS4 (CZTS) thin films were prepared by a single sputtering process with a CZTS compound target or a co-sputtering process with Cu, ZnS, and SnS targets followed by annealing in a H2S-containing atmosphere at several temperatures. Between the CZTS thin films prepared by both single and co-sputtering processes annealed at 500 °C, there are no major differences in the X-ray diffraction patterns, optical properties, and chemical composition. However, there is a difference in the scanning electron microscopy images. Although the CZTS thin films prepared by the co-sputtering process have some voids, the CZTS thin films prepared by the single sputtering process have no voids. The solar cells prepared with the CZTS thin films deposited by the single sputtering process followed by annealing at 500 °C show a higher efficiency of 4.40% than those prepared with the CZTS thin films prepared by the co-sputtering process (2.06%).

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.

Decay Characteristics of Photoluminescence Lines in CuGaS 2 Crystals

Time-resolved photoluminescence (PL) measurements have been carried out for CuGaS2 single crystals with a slightly Cu-rich composition grown by an iodine transport method. PL lines at 2.460 eV, 2.457 eV, 2.417 eV, 2.404 eV, 2.397 eV and 2.295 eV exhibited biexponential decay. The relationship between the decay time of the fast component (τf) and the localization energy (Eloc) can be given by τf∝Eloc1.25. On the other hand, the decay times of the slow components (τs) have been classified into two groups having different trends τs∝Eloc2.7 and Eloc0.5. The difference between the two groups is discussed in terms of native defects and the iodine impurity included in the luminescence centers.

Fabrication of Three-Dimensional-Structure Solar Cell with Cu2ZnSnS4

We fabricated three-dimensional (3D)-structure solar cells with Cu2ZnSnS4 (CZTS). A CdS buffer layer was deposited around nc-TiO2 by the chemical bath deposition (CBD) method. The CdS deposition time was varied from 5 to 30 min at 65 °C. CZTS absorber was deposited by spray pyrolysis deposition (SPD) onto the CdS buffer layer. Cu-, Zn-, Sn-, and S-containing solutions were used in SPD. The metal sources of copper(II) acetate monohydrate, zinc(II) acetate dehydrate, and tin(II) chloride dehydrate, and pure sulfur powder were desolved in N,N-dimethylformamide (DMF) as a solvent and mono-ethanolamine as a stabilizer. The solution-sprayed 3D-structure substrate was annealed in a nitrogen atmosphere at 250 °C for 30 min. A 3D-structure CZTS solar cell presented the best conversion efficiency η of 0.51% with a CdS buffer layer deposition time of 20 min. The dependence of each solar cell characteristic on CdS buffer layer deposition time was measured.

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.