Real Time Analysis of Magnetron-Sputtered Thin-Film CdTe by Multichannel Spectroscopic Ellipsometry
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Real Time Analysis of Magnetron-Sputtered Thin-Film CdTe by Multichannel Spectroscopic Ellipsometry Jian Li,1 Jie Chen,1 J. A. Zapien,2 N. J. Podraza,1 Chi Chen,1 J. Drayton,1 A. Vasko,1 A. Gupta,1 S. L. Wang,1 R. W. Collins,1 and A. D. Compaan1 1 Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606 2 The City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR ABSTRACT Real time spectroscopic ellipsometry (RTSE) based on rotating-compensator modulation and multichannel detection has been implemented to characterize polycrystalline thin film CdTe deposition for photovoltaic applications. RTSE is capable of providing routine deposition information on substrate temperature T and deposition rate. It is also capable of providing detailed information on the thickness evolution of microstructure and optical properties. In this study, we highlight the differences in nucleation that occur under different CdTe deposition conditions on smooth crystalline Si wafer substrates. Differing behavior in the initial stages of deposition has been observed, ranging from layer-by-layer growth to nucleation and coalescence of 45 Å thick clusters. We also consider the thickness and substrate dependence of the microstructure, comparing depositions on smooth Si wafer and rough thin film Mo substrates. INTRODUCTION Real time spectroscopic ellipsometry (RTSE) has been developed as an effective tool for the analysis and optimization of thin film materials and photovoltaic (PV) device structures. Extensive RTSE studies have been performed on hydrogenated amorphous and microcrystalline Si:H (a-Si:H and µc-Si:H) materials and structures that have successfully guided optimization procedures; however, few such studies have been reported on polycrystalline II-VI's or chalcopyrites [1-3]. Thus the full range of RTSE capabilities, including, for example, determination of (bulk layer)/(surface roughness layer) thickness evolution and bulk layer structural and compositional profiling, have yet to be exploited in these thin film PV technologies. In this study, rotating-compensator based RTSE has been implemented to analyze CdTe sputter deposition for thin film PV applications. EXPERIMENTAL DETAILS The design of the rotating-compensator multichannel ellipsometer used in this study is analogous to that first developed to study Si:H-based materials and solar cells [4]. This instrument provides the capability of collecting complete spectra (0.75 to 6.5 eV) in the ellipsometric angles (ψ, ∆) as an average over a minimum of two optical cycles in a time of (30.7 Hz)−1 = 32 ms. Here, the spectra were collected in times from 2 to 3 s, as averages over ~60 to 90 optical cycle pairs, respectively. During the acquisition time for one set of (ψ, ∆) spectra, a CdTe thickness of < 1 monolayer (2.6 Å) accumulates at the maximum bulk layer deposition rate used in this study (~1.3 Å/s). Analyses of all spectra apply an algorithm combining numerical inversion and least-squares regression [1]. The angle of incidence for RTSE w
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