Thin Films of p-Type CdTe Grown with Ion-Beam-Assisted Doping
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THIN FILMS OF p-TYPE CdTe GROWN WITH ION-BEAM-ASSISTED DOPING Paul Sharps, Alan L. Fahrenbruch, Adolfo Lopez-Otero, and Richard H. Bube Dept. of Materials Science and Engineering, Stanford University, Stanford, CA 94305 The purpose of the present work is to investigate p-CdTe thin films grown by ionassisted doping (lAD). Controlled doping in homo-epitaxial films resulting in carder densities up to 2x1 017 cm-3 was obtained using P ions as the dopant. About 1.5% of the impinging P ions became electrically active in the films. Solar cells of n-CdS/pCdTe were prepared and used as a diagnostic tool in understanding the p-CdTe films. INTRODUCTION Many of the common dopants in semiconductors do not perform satisfactorily when used to dope films grown from the vapor phase. Control over doping has been difficult if not impossible due to low incorporation probabilities, surface segregation, and/or reevaporation. Many of the dopants evaporate in the form of molecular species (P4 , As4 , Sb 2, etc.) that require high dissociation energies to produce atomic species that can be readily incorporated into a growing film. However, by use of ionized and accelerated dopant beams, the dopant incorporation is increased, surface segregation is decreased, and reevaporation is reduced [1]. Dopant incorporation is increased due to trapping (low energy implantation) and to increased chemisorption of the ions on the growing surface. Surface segregation is decreased due to collisional mixing and increased diffusion through point defects created by ion bombardment [2]. Reevaporation is reduced due to increased chemisorption of the ions on the surface [2]. Some defects are annihilated at the surface, providing an annealing effect [2]. Sputtering of the growing film may also occur [2] (Fig. 1). Ion assisted doping has been used by several investigators to improve the dopant incorporation in epitaxially grown Si and GaAs [3-6]. Bajor and Greene have developed a model that describes the incorporation of an ionized dopant during film growth [7]. The motivation for this work is to improve the electrical properties of p-CdTe for use as a solar cell collector material. CdTe has an ideal bandgap (1.45 eV) for matching the solar spectrum. Although polycrystalline CdTe solar cells have demonstrated a high efficiency, e.g. [8], a major difficulty has been in obtaining a high enough p-type carrier density in order to lower high bulk and contact resistances. A second advantage in controlling the carrier density in p-CdTe is in the potential optimization of the open-circuit voltage, Voc [9]. Group V elements P, As, or Sb are readily incorporated into single crystals of CdTe 3 grown from the liquid [10], with the carder density reaching a maximum of =2x1017 cmusing P [11]. On the other hand, preparation of p-CdTe from the vapor by conventional film growth techniques has been difficult. In our lab we have used close spaced vapor transport (CSVT) and hot-wall vacuum evaporation (HWVE) with additional sources for the dopant in order to grow and simultaneously d
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