AVS 56th International Symposium & Exhibition
    Applied Surface Science Tuesday Sessions
       Session AS+EM+MS+TF-TuM

Paper AS+EM+MS+TF-TuM11
Analysis of CdTe and CdS Thin Films and Photovoltaic Device Structures by Spectroscopic Ellipsometry

Tuesday, November 10, 2009, 11:20 am, Room C2

Session: Spectroscopic Ellipsometry III
Presenter: M.N. Sestak, University of Toledo
Authors: M.N. Sestak, University of Toledo
J. Li, University of Toledo
J. Chen, University of Toledo
C. Thornberry, University of Toledo
D. Attygalle, University of Toledo
R.W. Collins, University of Toledo
Correspondent: Click to Email
The techniques of in-situ real-time and ex-situ spectroscopic ellipsometry (SE) have been applied for the analysis of polycrystalline II-VI thin films and device structures fabricated by magnetron sputtering onto various substrates for photovoltaics (PV) applications. The CdS/CdTe heterojunction PV technology has led to efficiencies as high as 14% (for an all-sputtered device), and to the lowest module manufacturing costs in the PV industry (< $1/W). In our SE studies, depositions of individual CdTe, CdS, and CdTe1-x Sx films on smooth crystalline silicon substrates have provided information on the nature of thin film nucleation and coalescence, as well as on the evolution of the dielectric function with thickness and quantum size effects in very thin films (< 30 nm). Films remain sufficiently smooth to a thickness of ~ 50 nm under a wide range of deposition conditions so that accurate dielectric functions could be determined as a function of temperature upon suspending the deposition at this thickness and cooling the film to room temperature. Such dielectric functions are deemed accurate because they are measured in situ under high vacuum, thus avoiding surface oxidation and contamination. In addition, surface roughness corrections are made based on roughness determinations obtained from the full real time SE data set. Variations with deposition conditions in the critical point parameters of the resulting room temperature CdTe and CdS dielectric functions have provided information on film stress, defects, and grain size. Upon reheating the film to the deposition temperature and resuming the deposition for the fabrication of the thick (~ 1 micron) films used in PV devices, surface roughness evolution and void volume fraction depth profiles have been extracted. These provide key insights into the optimization of a subsequent CdTe processing step – an anneal in CdCl2 vapor which promotes grain growth and strain relaxation in the active CdTe layer for high efficiency PV devices. Ex situ ellipsometry techniques have also been developed that involve smoothening the thick CdTe film with a succession of Br+CH3OH treatments that enables step-by-step etching and time reversed spectroscopic ellipsometry on CdCl2 treated device structures. Finally, through-the-glass SE has been developed for multilayer analysis of completed PV devices with the potential for scale up to full 2' x 4' modules in a mapping mode using the reference dielectric function database developed by real time SE.