Search

Abstract: The structural properties of Ge doped polycrystalline CuGaSe2 films with potential applications in solar cell device technology have been studied by Raman spectroscopy at 300 K and by Photoluminescence (PL) spectroscopy at 300 K and 2 K. The films were intentionally doped with max. 200 keV Ge ions using one- and three-stage ion implantation processes. The Raman spectra of as grown films are dominated by the A1-mode (breathing mode) of the CuGaSe2 absorber at 184 cm−1. It was found, that in doped films the Raman mode intensities are reduced and the B2-modes (TO at 249 cm−1 and LO at 273 cm−1) take over. This implies an increase in structural disorder induced, probably, by bond reorientation effects that favor excitation of asymmetric lattice vibrations (B2) instead of the symmetric ones (A1). Moreover, it was found, that the Raman bands of doped films exhibit asymmetric broadening representative of a Fano line-shape. Changes were more pronounced in films doped at one-stage. The PL-emission of films subjected to one-stage process was enhanced, which supports an increase in structural disorder particularly for these films. On the contrary, for films doped in three-stages, the PL bands are less intensive and the Raman bands are less broadened. [Copyright &y& Elsevier]

Abstract: The structural and optical properties of CuIn1−xGaxSe2 epitaxial layers were studied in dependence of composition by Photoreflectance (PR) spectroscopy at room and low temperatures and by Raman spectroscopy at room temperature. For compositions with [Ga]/([In]+[Ga]) varying between 0.08 and 0.82, the band gap, determined by fitting the PR-spectra with a third derivative functional form, shows a quadratic dependence on composition. The Raman spectra, recorded under excitation with a vertically polarized Kr+-laser beam operated at the 647.1 nm line in the region 50–700 cm−1, consist mainly of peaks assigned to the A1- and B2-phonon modes. The A1-mode dominates the spectra and shifts linearly, with the increase of the [Ga]/([In]+[Ga]) content, from 172 cm−1 (A1-mode frequency of CuInSe2) to 181 cm−1 (A1-mode frequency of CuGaSe2). Combining the Raman and PR-data, an analytical expression has been derived which correlates the band gap energy with the A1 Raman mode frequency for a given composition of the quaternary compound. [Copyright &y& Elsevier]

The optical properties of carbon materials formed by pyrolysis of novolac-resin/biomass composites were studied by Raman, photoluminescence (PL) and (modulated) photoreflectance (PR) spectroscopy in dependence of the pyrolysis temperature and the heating rate. In the Raman spectra, the frequency, width and intensity ratio of the disordered graphitic D-band to the graphitic G-band, ID/IG, indicate an increase in ordered sp2-bonds in composites pyrolyzed at 1000 °C in comparison with those at lower pyrolysis temperatures. Moreover, the intense PL, observed at 2.2 eV for pyrolysis temperatures in the range 400–600 °C, is significantly decreased at temperatures ≥800 °C, which implies that the materials pyrolyzed at higher temperatures have less defects. According to the results of Raman-studies and complementary FTIR-studies, the material pyrolyzed at 1000 °C exhibits spectral characteristics similar to those of activated charcoal. A weak PR-band observed at 2 eV in the spectra of activated charcoal gives an indication of the (optical) band gap of charcoal and related composite materials. Materials pyrolyzed at high temperatures (≥1000 °C) are expected to be suitable for electrochemical applications. [Copyright &y& Elsevier]