Cation sublattice stacking faults in Cu-rich chalcopyrite CuInSe 2
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Cation sublattice stacking faults in Cu-rich chalcopyrite CuInSe2 Olof Hellman and Shun-ichiro Tanaka Tanaka SolidJunction Project/ERATO, Research Development Corporation of Japan, 1-1-1 Fukuura, Kanazawa-ku, Yokohama 236, Japan
Shigeru Niki and Paul Fons Optoelectronics Division, ElecTroTechnical Laboratory, 1-1-4 Umezono Tsukuba, Ibaraki 305, Japan (Received 25 September 1995; accepted 14 November 1995)
Using transmission electron microscopy, we have found stacking faults on the cation sublattice in the chalcopyrite structure of CuInSe2 . These films are grown by molecular beam epitaxy under Cu-rich conditions. These stacking faults are found to extend large distances in the plane of the film, and are not found to be present in samples not grown in Cu-rich conditions. We suggest that this defect is triggered by a Cu-induced transformation of the surface structure of the growing film. I. INTRODUCTION
II. EXPERIMENTAL
CuInSe2 is of interest for photovoltaics due to its large absorption coefficient for solar radiation.1 A variety of crystal structures have been observed for CuInSe2 films grown by molecular beam deposition/epitaxy, including the stable chalcopyrite structure,2–4 the cation disordered sphalerite structure (CuIn)Se,4 and a tetragonal structure in which the cations are ordered in a CuPt-type structure.4 Additionally, compounds of the stoichiometry CuIn3 Se5 5–9 and CuIn2 Se3.5 10 have been grown. Many other growth techniques have been used to make CuInSe2 in both thin film and bulk form. Kiely et al. give a fairly complete survey of the references for a number of these techniques.11 The various structures and defects in CuInSe2 have been shown to affect its optical properties drastically.12 This would suggest that careful study of these defects and their correlation with optical properties should be interesting. One feature of CuInSe2 is that wide deviations from stoichiometry are allowed while maintaining a crystal structure relatively free of extended defects. The ordering of point defects is shown to provide a surprisingly smooth transition between CuInSe2 and CuIn3 Se5 .7–9 The nature of defects associated with Cu enrichment has not been investigated, although a method for systematic investigation of these defects by diffuse electron beam scattering has been proposed.11 Another special feature of CuInSe2 solar cells is hardening; solar conversion efficiency has been shown to increase with use, which is quite the opposite of most high conversion efficiency materials. This effect is almost certainly due to a complex defect structure in the thin film material, and is perhaps also correlated with the stability of off-stoichiometric structures. For these reasons we would like to understand more intimately the nature of defects in CuInSe2 materials grown through the range of In-rich and Cu-rich environments.
The films discussed in this paper were grown by conventional MBE on GaAs substrates at a temperature of about 450 ±C. (001) GaAs substrates were clean
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