Microstructural and Microchemical Characterisation of CuInSe 2 Ingots Grown by the Vertical Bridgman Technique
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MICROSTRUCTURAL AND MICROCHEMICAL CHARACTERISATION OF CuInSe 2 INGOTS GROWN BY THE VERTICAL BRIDGMAN TECHNIQUE. C.A.MULLAN, S.M.CASEY, C.JONES, C.J.KIELY, M.IMANIEH* and R.D.TOMLINSON*. Department of Materials Science and Engineering, The University of Liverpool, England. *Department of Electronic and Electrical Engineering, The University of Salford, England.
ABSTRACT The microstructure and microchemistry of a number of melt textured CuInSe 2 ingots prepared from a variety of starting charge compositions have been investigated using a combination of TEM, EPMA and Auger spectroscopy. If a stoichiometric starting mixture is used, In-rich and Cu-rich precipitates are found at the first and last to freeze zones of the ingot respectively, accompanied by an intervening mid-portion of inclusion free (although slightly off-stoichiometry) CuInSe 2.We have found that in order to achieve an optimum matrix composition, a Cu-rich charge is required; however, this leads to a fine distribution of copper selenide particles throughout the ingot. If the starting charge is In-rich, then the boule contains In-rich precipitates and the matrix composition is In-rich and Cu-deficient. These observations are correlated with the observed electrical characteristics of the boules.
INTRODUCTION CuInSe 2 is a direct band gap ternary (I-III-VI2 ) semiconductor which can be cost effectively used in polycrystalline thin film form as a high efficiency and very stable absorber layer in photovoltaic devices. CuInSe 2 has a tetragonal chalcopyrite type (142d) crystal structure which can be derived from the sphalerite structure by substituting Cu and In atoms onto the sphalerite cation sublattice in an ordered fashion [2]. The electrical characteristics of CuInSe 2 are controlled by the complex interplay of the various intrinsic point defect populations which can exist in the crystal [1]. Formation energies for the various types of point defect have been estimated by adapting models for binary semiconductors to CuInSe 2[3].The Incu (donor) and Culn (acceptor) antisite defects have formation energies of 1.4eV and 1.5eV respectively. Other plausable point defects in CuInSe 2 are the cation acceptor vacancies (Vin (2.8eV) and Vcu (2.6eV)), as well as the anion vacancy Vse(2.4eV) which is donor type in nature. All other possible antisite defects and interstitial type defects in CuInSe 2 have formation energies in excess of 5eV which makes them unlikely to form. An understanding of the complex electrical and optical properties exhibited by CuInSe2 is only likely to emerge from fundamental studies of single crystal material where the compound stoichiometry can be closely controlled and grain boundary effects can be eliminated. Recently, it has been demonstrated by Tomlinson [4] that directional solidification techniques can be used to grow polycrystalline ingots of CuInSe 2 which contain crystals of centimetre dimensions. He also noted that for a nominally stoichiometric starting mixture, the resultant textured ingot frequently underwent a conversion fro
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