Real-Time Spectroscopic Ellipsometry of Sputtered CdTe Thin Films: Effect of Ar Pressure on Structural Evolution and Pho
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Real-Time Spectroscopic Ellipsometry of Sputtered CdTe Thin Films: Effect of Ar Pressure on Structural Evolution and Photovoltaic Performance Michelle N. Sestak, Jian Li, Naba R. Paudel, Kristopher A. Wieland, Jie Chen, Courtney Thornberry, Robert W. Collins, and Alvin D. Compaan Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization (PVIC), University of Toledo, Toledo, OH 43606 ABSTRACT In this study, 1 µm thick polycrystalline CdTe films were deposited by magnetron sputtering using a variable argon pressure, 2.5 ≤ pAr ≤ 50 mTorr, and a fixed substrate temperature, Ts = 230○C. Real time spectroscopic ellipsometry (RTSE) was performed during each deposition in order to analyze the nucleation and coalescence. The evolution of the surface roughness layer thickness ds with bulk layer thickness db and the depth profile in the void volume fraction fv were also extracted from each RTSE data set. A linear correlation was found between the final ds value measured by RTSE at the end of deposition and the root-mean-square (rms) surface roughness measured ex situ by atomic force microscopy (AFM) after deposition. A monotonic decrease in RTSE-determined roughness thickness is observed with decreasing Ar pressure from 18 to 2.5 mTorr. The lowest pressure also leads to the greatest bulk layer structural uniformity; in this case, fv increases by 0.02 with increasing CdTe thickness to 1 µm. The photovoltaic performance of ~1.25 µm thick CdTe films prepared at the lowest pressure of pAr = 2.5 mTorr is compared with that of previously optimized solar cells with pAr = 10 mTorr. INTRODUCTION Sputtering is advantageous for CdTe thin film fabrication first because it provides the capability of control over the film microstructure [1], and second because it yields efficient solar cells in a low temperature process [2]. The power applied to the target provides a means for controlling the thickness rate, and hence the kinetics of the deposition, whereas the substrate temperature provides a means for controlling the surface diffusion length of the deposited species. Finally, the Ar pressure provides a means for controlling the momentum per incident species as well as the directionality of these species. By varying the sputtering parameters, films of different microstructures can be deposited, including films with different void volume fraction profiles, and films having a wide range of surface roughness thicknesses. In previous research, real time spectroscopic ellipsometry (RTSE) has been demonstrated for the characterization of a variety of thin films used in photovoltaics technology [3]. Structural information of interest has been obtained from RTSE, including the characteristics of thin film nucleation and coalescence, as well as the evolution of the bulk and surface roughness layer thicknesses db and ds, respectively, along with variously defined deposition rates. The dielectric function of the growing film, also obtained by RTSE, can provide additional information, for exa
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