Dielectric Function and Defect Structure of CdTe Implanted by 350-keV Bi Ions
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Dielectric Function and Defect Structure of CdTe Implanted by 350-keV Bi Ions Peter Petrik1, Miklós Fried1, Zsolt Zolnai1, Nguyen Q. Khánh1, Jian Li2, Robert W. Collins2, and Tivadar Lohner1 1 Research Institute for Technical Physics and Materials Science, H-1525 Budapest, POB 49, Hungary 2 Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606 USA ABSTRACT In this work we have developed optical models for the ellipsometric characterization of Bi-implanted CdTe. We have characterized the amount and nature of disorder using Rutherford Backscattering Spectrometry combined with channeling (RBS/C). Samples with a systematically varying degree of disorder were prepared using ion implantation of Bi into single-crystalline CdTe at an energy of 350 keV with increasing doses from 3.75×1013 cm-2 to 6×1014 cm-2. The motivation for use of the high atomic mass Bi ions was that previous studies using lighter ions revealed damage at a low level, even for doses several times higher than the amorphization threshold estimated by simulation [P. Petrik et al., phys. stat. sol. (c) 5, 1358 (2008)]. In contrast, Bi ions create sufficient disorder for investigation of the changes in dielectric function critical point (CP) features in a wider variety of structures from single-crystalline to the disordered state. The CP features can be described by numerous methods starting from the standard CP model, through the parameterization of Adachi [Adachi et al., J. Appl. Phys. 74, 3435 (1993)], and finally to the generalized CP models. The standard CP model has been demonstrated to be a reliable approach for polycrystalline CdTe characterization used in photovoltaic applications [Li et al., phys. stat. sol. (a) 205, 901 (2008)]. INTRODUCTION This work is a continuation of previous studies on the ion implantation of singlecrystalline and polycrystalline CdTe [1]. Our aim is to study the optical properties and the defect structure of disordered CdTe prepared in a controlled way. Ion implantation provides a well established method to introduce defects reproducibly and systematically. Using ion implantation disordered regions of controlled extent can be created, which may be analogous -- in terms of their abilities to scatter optically excited carriers -- to the grain boundary regions in deposited thin film polycrystalline CdTe of different grain sizes. This study supports a better understanding of the optical properties of the thin film CdTe materials used for solar applications [2] (For example, using a low-temperature sputter-deposition technology for CdS/CdTe, a solar cell efficiency of 14 % has been demonstrated [3].) EXPERIMENT c-CdTe (Nippon Mining & Metals Co., (111)B) samples have been implanted using 175 kV Bi ions at fluences ranging from 3.75×1013 cm-2 to 6×1014 cm-2. The high-mass Bi ions were chosen because the damage created in previous studies using Xe implantation was too low 2+
[1]. Using double-charged Bi ions at 175 kV, each Bi ion will have an energy of 350 keV. Simulation with the SRIM (Stoppin
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