A Study of Arsenic Dopant Concentration and Activity as a Function of Growth Conditions in Polycrystalline MOCVD-Grown C
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B1.5.1
A Study of Arsenic Dopant Concentration and Activity as a Function of Growth Conditions in Polycrystalline MOCVD-Grown CdTe Anne Stafford, Stuart J.C. Irvine, Ken Durose1 and Guillaume Zoppi1 Department of Chemistry, University of Wales, Bangor, Gwynedd, LL57 2AW, U.K. 1 Department of Physics, University of Durham, South Road, Durham, DH1 3LE, U.K.
ABSTRACT P-type CdTe can be produced via acceptor doping with As. However, as with other II/VI materials, the dopant behaviour is not simple, as there is the potential for compensating species to be formed from intrinsic defects and dopant-defect complexes. A further complication is introduced by the presence of grain boundaries in polycrystalline material. This study demonstrates that dopant concentration is a function of VI/II ratio in the growth ambient, and that resistivity is minimised for a dopant concentration of < 2 x 1018 at.cm-3. Grain size is also affected by the VI/II ratio, increasing slightly as the growth ambient becomes more Te-rich. INTRODUCTION Polycrystalline CdTe has considerable and well documented potential as a thin-film photovoltaic absorber material, due to the excellent band-gap/solar spectrum match and good absorption characteristics [1]. However, control of the electrical properties, such that the grains are p-type with passivated grain boundaries is crucial in obtaining high efficiency PV modules [2]. Conventionally, CdTe-based solar cells are produced by depositing the material via a number of different methods (Close Space Sublimation [3], electrodeposition [4], MOCVD [5]) and then performing a post-deposition p-type conversion anneal in the presence of CdCl2. The effects of this anneal are still not completely understood, but as well as converting n-type material to ptype, it is believed to enhance junction interdiffusion and near-interfacial grain sizes [6], resulting in large increases in photovoltaic efficiency. It is also possible to obtain p-type material by growing CdTe by MOCVD in a very Te-rich growth ambient, thus relying on intrinsic defects (Cd vacancies) to produce the required conductivity type [7]. However, because only fairly slight deviations from stoichiometry can occur, the carrier concentration remains relatively low. Another possible route is via acceptor doping, for example using Arsenic. It is well-known that doping in II/VI materials can be highly problematic due to the many potential compensation mechanisms involving both intrinsic defects and defect complexes [8]. Indeed, previous work by the authors in the area of As-doped CdTe has so far produced solar cells with efficiencies limited to only 1 or 2% without the use of a post-deposition annealing step [9]. The aim of this study is to gain a better understanding of the behaviour of the As dopant under various growth conditions, with a view to optimising the p-type conductivity and hence finding a route to improved photovoltaic performance.
B1.5.2
EXPERIMENTAL DETAILS CdTe single layers doped with As were grown by MOCVD onto insulating sapphire substrates
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