Masakazu Yagi

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.

Various Shapes of ZnO and CdO Nanostructures Grown by Atmospheric-Pressure Chemical Vapor Deposition

Various shapes of ZnO and CdO nanostructures were successfully grown on a- and c-plane sapphire substrates coated with Au nanocolloidal solution by atmospheric-pressure chemical vapor deposition methods under a simultaneous source supply of metal powder (Zn or Cd) and H2O. The ZnO and CdO nanorods (NRs) grown at higher substrate temperatures (TSs) exhibited tapered shapes, resulting from the competition between the axial growth due to the vapor–liquid–solid (VLS) mechanism and the radial growth due to the vapor–solid (VS) mechanism. The alternate source supply of Zn and H2O was found to be effective for suppressing the tapering of ZnO NRs. The appearance of the Y- and T-shaped nanotrees of CdO may be due to the splitting and migration of catalytic particles during the growth process. These results suggest that both the source supply sequence and the substrate temperature are important factors for the shape design of oxide nanostructures.

Deep Region Emissions of CuGaS2 Crystals

The broad emission bands appearing at 6700 A, 7500 A and 8600 A were discussed in terms of preparation processes of epitaxial films and bulk crystals. For the emission at 7500 A showing spectral shifts during decay, characteristic to donor-acceptor-pair type recombinations, possible relation is pointed out to sulfur vacancies and interstitial halogen (Cl or I) atoms in films or bulk crystals.

Characterization of Cu(In,Ga)Se2 thin films and solar cells by photoacoustic spectroscopy

Cu(In,Ga)Se2 (CIGS) thin films and solar cells were studied by photoacoustic spectroscopy (PAS) which is non-contact and non-destructive characterization of optical absorption properties. As preliminary experiment, the band gap of CIGS bulk alloy crystals were characterized by PAS, photoluminescence, and electroreflectance measurements, and the utilization of PAS was examined as the characterization of CIGS alloys. Photoacoustic (PA) spectra of CIGS thin films grown on a Mo/soda-lime glass (SLG) substrate showed the existence of the deep absorption band. The deep absorption band was observed only in PA spectra of CIGS/Mo/SLG films grown by three-stage method. The microphone-based PAS method detected the signal from the deep interface in CIGS/Mo/SLG structure sensitively. On the other hand, the PA spectrum of CdS/CIGS/Mo/SLG obtained by the transparent piezoelectric photo-thermal (Tr-PPT) method showed the clear absorption edge. Therefore, microphone-based PAS and Tr-PPT measurements can give complementary absorption properties of CIGS films.

Comparative study on structural and optical properties of ZnO films grown by metalorganic molecular beam deposition and metalorganic chemical vapor deposition

Polycrystalline zinc oxide films were grown on glass substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) and metal organic molecular beam deposition (MOMBD) using diethlyzinc and water as precursors. With increasing substrate temperature (TS), preferential growth direction of the LP-MOCVD films changed from c-axis direction to a-axis direction, while that of the MOMBD films changed from random direction to c-axis direction. Photoluminescence (PL) spectra of the LP-MOCVD films exhibited an orange band emission related to interstitial oxygen atoms, while those of the MOMBD films exhibited a green band emission related to oxygen vacancies. This PL result suggests that the LP-MOCVD films are in oxygen excess condition, but the MOMBD films are in oxygen deficient condition. Photoacoustic measurements revealed that the energy level of the nonradiative center formed in the LP-MOCVD films is deeper than that in the MOMBD films.