Real time spectroscopic ellipsometry of sputtered CdTe, CdS, and CdTe_1−xS_x thin films for photovoltaic applications

Abstract

Real time spectroscopic ellipsometry (RTSE) has been applied to study the deposition of polycrystalline CdTe, CdS, and CdTe1–x Sx thin films on crystalline silicon wafer substrates as well as the formation of CdS/CdTe and CdTe/CdS heterojunctions, all using a magnetron sputtering process. For CdTe and CdS, the key process variable is deposition temperature (T). The nucleation and growth behaviors of CdTe and CdS show strong variations with T and this influences the ultimate grain size of the polycrystalline films. For the co‐sputtered CdTe1–x Sx alloys, the key variables are the rf power levels applied to the CdTe and CdS targets and thus the resulting composition x. The dielectric functions of the alloys have been used as a database for real time investigation of inter‐diffusion at the CdS/CdTe and CdTe/CdS heterojunctions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopic Ellipsometry

“…The very broad dielectric function features for the thin film are due to its fine grained structure. Figure 6.6b compares the room temperature dielectric functions of polycrystalline CdS films 50 nm thick fabricated at substrate temperatures of 145 and 310 C [39]. From these results, one can conclude that the grain size of CdS increases with increasing substrate temperature.…”

“…In this case, e 0 is interpreted as the dielectric function of the near-surface region of the deposited film. Repeating this procedure as a function of time will result in a depth profile in the optical functions, which can be further analyzed to extract a depth profile of the structural phase (as in thin Si:H) or the chemical composition (as in CdTe 1Àx S x or CIGS solar cell materials) [38,39].…”

“…For thin film CdTe, RTSE studies currently explore the relationships between the nucleation and growth dynamics of asdeposited CdTe and CdS and the resulting solar cell performance [39]. In this case, the complexity of such relationships is enhanced as a result of the postdeposition CdCl 2 treatment.…”

“…These mechanisms determine the nano-and microstructure of the sputter deposited film [44,45]. Figure 6.6a shows the room temperature dielectric functions for bulk single crystal CdTe (c-CdTe) and an as-deposited polycrystalline CdTe film sputtered to 100 nm thickness at 188 C [39]. CdTe exhibits four CPs over the energy range of 0.75 < E < 6.5 eV.…”

Optical measurement techniques for in-line process control in CdS/CdTe solar cell manufacturing

“…A similar experiment was later repeated for sputtered ternary films [26]. This research used a real-time spectroscopic ellipsometer which was incorporated into the RF sputtering system.…”