Structural peculiarities of CCSVT-grown CuGaSe2 thin films (original) (raw)
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Surface and bulk properties of CuGaSe2 thin films
Journal of Physics and Chemistry of Solids, 2003
Using complementary techniques, namely X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS), we present a comparative study of the bulk and surface composition in device grade CuGaSe 2 (CGSe) thin films. The films were deposited in two stages by an open-tube chemical vapor deposition (CVD) process. The first stage leads to a nearly stoichiometric polycrystalline CGSe film of approximately 1.5 mm thickness. During the second stage the film is annealed in a Ga-and Se-rich atmosphere. While the XRF-data show a nearly stoichiometric integrated film composition, the surface composition, as determined by XPS analysis, is Cu-poor, pointing towards a highly non-stoichiometric surface layer. In addition, sodium was found at the film surfaces. The data are discussed in the framework of an ordered defect compound formation and the formation of a (Cu,Na) -Ga-Se compound at the surface of the CuGaSe 2 films. Complementary ultraviolet photoelectron-and inverse photoelectron spectroscopy investigations of the film surface derive a widening of the surface energy band gap up to 2.2 eV in comparison with a bulk energy band gap around 1.65 eV (obtained by optical transmission analysis). The observed data are consistent with our model of a two layer film structure containing a defect-rich near-surface region and a defect-poor bulk. q
CuGaSe2–CuGa3Se5 phase transition in CCSVT-grown thin films
Thin Solid Films, 2006
The system CuGaSe 2-CuGa 3 Se 5 in thin films has been investigated. Layer synthesis was carried out by chemical close-spaced vapour transport (CCSVT) using Cu precursors on Mo/soda-lime glass substrates. The extension of deposition times in a two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 1 to 3, allowing the study of the phase transition mentioned above. Films showing chalcopyrite (1:1:2), OVC (1:3:5) and twophase were grown. X-ray emission spectroscopy and x-ray diffraction (XRD) techniques have been combined for a compositional and structural study of this material system probing both bulk and near surface properties of the films. This analysis was also extended to the rearsurface investigation of selected two-phase thin films and complemented with surface sensitive photoelectron spectroscopy (PES). From these results a growth model is presented for CuGa 3 Se 5 formation in gallium-rich, CCSVT-grown CuGa x Se y-films.
Thin Solid Films, 2012
CuGaSe 2 (CGS) thin films were grown on uncoated and Mo-coated soda lime glass by Pulsed Electron Deposition (PED) technique at substrate temperatures comprised between 25°C and 475°C. X-ray diffraction analysis reveals that CGS samples exhibit a noteworthy crystal quality even at low growth temperature, T g = 100°C, whereas the out-of-plane preferential orientation of CGS chalcopyrite phase switches from b 220 > to b112 > by increasing the substrate temperature. Annealing treatments seem to enhance the crystallinity of the film and to release the residual strain energy. Visible/near-infrared absorbance spectra show a monotonic decrease of CGS optical bandgap (from 1.75 to 1.65 eV) by enhancing the substrate temperature. Yet the morphology of CGS films strongly depends on T g , which promotes the formation of larger columnar grains perpendicular to the growth plane. Grain dimensions of~2 μm are achieved when CGS films are grown at high temperature (> 400°C) on Mo-coated glass. The results indicate that PED is a promising growth technique for achieving good-quality CGS that can be useful as absorber layers in thin film solar cells.
Solar Energy Materials and Solar Cells, 1992
The growth conditions, the composition and the structural, optical and electrical properties of thin films of CuGaSe 2 and CuGaTe 2 have been studied using "flash" and "slow" evaporation in vacuum. Single phase films, when analyzing the absorption coefficient, present several energy gaps. For CuGaSe> they are 1.59, 1.66, 2.03 and 2.11 eV, for CuGaTe 2 1.23 and 1.89 eV. Both the CuGaSe~ and CuGaTe 2 evaporated films are p-type; the resistivities, carrier densities and mobilities are appropriate for thin film solar cells.
Growth and physical properties of CuGaSe2 thin films by r.f. sputtering
Journal of Materials Science Letters, 1990
CuGaSe 2 (CGS) is a I-III-VI 2 chalcopyrite semicon ductor with electro-optical properties very suitable for photovoltaic applications. Its gap value (1.68 e V) [1], l very high absorption coefficient (o: ~ 10 5 cm -) and easily controllable electrical resistivity in a wide range (0.1 to 10 5 Q cm) [2] make CGS an ideal high band gap partner for CuInSe 2 in tandem structures, for which a theoretical maximum achievable efficiency as high as 33% has been predicted .
Effects of Ge-Implantation on the Photoluminescence of CuGaSe2 Thin Films
MRS Proceedings, 2005
Photoluminescence (PL) spectra of as grown Ga-rich, and Ga-rich plus Ge-doped and annealed CCSVT (Chemical Close-Spaced Vapor Transport) — CuGaSe2 thin films have been investigated. Visible (514.5 nm) and Ultra- Violet (351.1 nm) excitation energies of the laser have been used in order to determine intrinsic and extrinsic defects created due to the implantation as well as separating near surface from bulk recombination.Both visible and UV-PL spectra of the undoped films show the well known luminescence of Ga-rich CuGaSe2, which can be described by the widely accepted model of fluctuating potentials. Unlike the visible- and UV-PL emissions of Ge- implanted and annealed films differ strongly. Obviously, Ge-implantation in combination with the thermal treatment results in an extrinsic doping of the material producing so far unknown states in the CuGaSe2 band gap. Comparing the visible- and the UV-PL spectra we found an accumulation of these extrinsic doping levels in the near-surface-r...
Solar Energy Materials and Solar Cells, 2011
CuGaSe 2 chalcopyrite thin films Chemical vapor transport Structure and phase transition Oxidation Doping Local and electronic band structure a b s t r a c t Device-grade ternary Cu-Ga-Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10 Â 10 cm 2 . A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe 2 -related defect compound structures (DC) with stoichiometries of CuGa 3 Se 5 and CuGa 5 Se 8 . In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe 2 -related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa 3 Se 5 formation in gallium-rich CCSVT-grown CuGa x Se y films. The chemical and electronic surface and interface structure of CuGaSe 2 thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced ''cliff-like'' conduction band offset at the respective interface.
Effects of chemical and electrochemical etching on polycrystalline thin films of CuGaSe2
Journal of Electronic Materials, 1989
Electrochemical and, especially, chemical oxidative etching drastically improves the photoresponse of liquid electrolyte/CuGaSe2-0n-Mo junctions. This is expressed in decreased effective doping levels and increased effective minority carrier diffusion lengths. It is accounted for by removal of highly defective surface layers, which also leads to an increase in the barrier height, as judged from a positive shift of the flat band potential (on the electrochemical scale). The etching effects are seen clearly in Zn/CuGaSe2 devices, by electron beam-induced current. This last method also reveals a supra-grain structure, which is tentatively explained by thermal stress-induced strain at the Mo-CuGaSe2 interface.