PACS 68.55. Jk, 78.30.Hv, 78.55.Hx This paper reviews the structural properties of ternary chalcopyrite absorbers for solar-cell device applications studied by Raman Spectroscopy. Raman mode assignment in CuGaSe 2 is reviewed according to the results of previous and present Raman and IR spectroscopic studies. The compositional dependence of Raman modes, in particular, the composition dependent frequency shift of the A 1 -mode, is suggested for the analysis of the chalcopyrite film structure. Order and disorder effects are discussed with reference to the excitation of phonon modes assigned to Cu -Au ordered phases and to defect related phases, respectively. Phase separation effects associated with the formation of secondary microcrystalline Cu x Se-or Cu x S-phases and the observation of Raman scattering from these phases are also discussed.
The structural properties of copper selenide (CuxSe), formed as a secondary phase on the surface of CuGaSe2 films grown under Cu-rich conditions on GaAs (100) substrates, were studied by micro-Raman spectroscopy. Raman bands at 45 and 263cm−1, observed on crystallites dispersed on the CuGaSe2 film, are in agreement with the Raman modes of CuSe and Cu2Se. Polarization- and angular-dependent micro-Raman measurements reveal that the CuxSe-crystallites are grown with a preferential orientation on the CuGaSe2 surface.
Silicon nanopillars were fabricated by using deep UV lithography, highly anisotropic silicon reactive ion etching based on fluorine chemistry, and high-temperature thermal oxidation for further thinning. Pillars with a diameter below 10 nm and a height in the 0.4–0.6 μm range were obtained while lying on a very smooth bottom silicon surface. An isolating transparent polymer was then used to fill in the etched area containing the pillars and, therefore, planarize and isolate the pillars. Oxygen plasma was used in order to remove the resistance from the top of the pillars. They were then contacted by a thin contact layer (gold or indium tin oxide), evaporated on the top of them. An Ohmic contact was also formed on the back side of the wafer. The obtained device showed rectifying behavior and forward voltages exceeding 10–12 V electroluminescence was observed, visible with the naked eye.
Raman scattering and photoluminescence (PL) emission of CuxGaySe2 thin films, grown by metalorganic chemical vapor deposition (MOCVD) on GaAs (100) and by physical vapor deposition (PVD) on Glass/Mo substrates, were studied at room and low temperatures as a function of composition. Line width changes of Raman bands in the temperature range 20–300 K indicate the formation of a more disordered CuxGaySe2 phase with increasing Ga content. It is most likely that Raman bands observed at 193 and 199 cm−1 in the Ga-rich samples at low temperatures are associated with defect-related interface modes. The intensity increase of these bands and the photoluminescence intensity enhancement may be correlated to an increase in defect concentration in the Ga-rich phase. The radiative recombination in defect-rich Ga-rich samples with a higher degree of disorder apparently supports the PL emission. The observed changes are more pronounced in MOCVD than in PVD grown films.
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