Photoluminescence from Ion Implanted CdTe Crystals
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Photoluminescence from Ion Implanted CdTe Crystals Xiangxin Liu and A. D. Compaan, Department of Physics and Astronomy, The University of Toledo, Ohio 43606 ABSTRACT We have studied the photoluminescence of CdTe crystals doped with two elements, Cu and Cl, that are frequently used in CdTe-based solar cells. Ions were implanted into high-quality single crystals of CdTe at the Toledo Heavy Ion Accelerator Lab in our Department. We used a standard Monte Carlo simulation program to plan an implant dosage at three different energies. The lattice damage was removed by thermal annealing in an inert atmosphere using a proximity cap to avoid surface deterioration. The PL spectra at 40K were obtained at 488 nm or 752nm to match the absorption depth with the implant profile. Using implant densities typically of 1016, 1017, and 1018 /cm3, and laser excitation power densities ranging over several orders of magnitude, we have identified band-to-band transitions, free-to-bound transitions, bound-exciton lines, and donor-acceptor pair transitions related to these species. INTRODUCTION Photoluminescence (PL) is a convenient and powerful method to probe the material properties of an operating solar cell and to investigate the defect states in the active semiconductor material. However, the unambiguous identification of the transitions responsible for the light emission, particularly in polycrystalline thin films, has been elusive. Partly to address this difficulty of identification, we have made a series of measurements on single crystals which have received calibrated doses of known atoms from ion implantation. Doping with ion implantation is a convenient way to introduce specific dopants and to introduce a uniform doping level in a thin surface layer. Monte Carlo simulations enable us to produce uniform doping by selecting the ion doping energies and doses. However, the disadvantage of ion implantation is that large numbers of defects are created, typically 1000 displacements for each atom implanted. Therefore, we have annealed all samples before PL acquisition. High performance CdS/CdTe thin film solar cells are usually completed with a low resistance Cu back contact. The copper appears to be critical for achieving heavy p-type doping of the CdTe at the contact. It is also known that Cu doping can increase the open-circuit voltage. However, copper is also a fast diffuser,1 which can accumulate at the CdS/CdTe junction and is suspected of playing a role in cell performance deterioration under certain conditions. High-temperature CdCl2 treatment in the presence of oxygen is a critical step needed to improve the performance of CdTe thin-film cells, which can improve the cell efficiency a factor of two or more.2,3 However the process is not well understood and information on the role Cl as a function of density is particularly important. EXPERIMENTAL DETAILS Monte Carlo calculations using “SRIM” (The Stopping and Range of Ions in Matter4) were performed before ion implantation. Three different ion kinetic energies and doses w
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