Fabrication of Solar-Cell-Quality Cadmium Sulfide Layers Via Laser-Assisted Chemical Vapor Deposition (LCVD)
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Fabrication of Solar-Cell-Quality Cadmium Sulfide Layers Via Laser-Assisted Chemical Vapor Deposition (LCVD) Johnnie L. Hixson,1 Christopher S. Cassidy,1 Robert L. Stewart,1 Robert M. Taylor,1 Lowell R. Matthews,1 Kenneth H. Church,1 Robert Mamazza,2 and Christos S. Ferekides2 1 CMS Technetronics, Inc., 5202-2 North Richmond Hill Road, Stillwater, OK 74075, U.S.A. 2 Department of Electrical Engineering, University of South Florida, Tampa, FL 33626, U.S.A. ABSTRACT The laser-assisted chemical vapor deposition (LCVD) technique has proved to be an effective means of depositing cadmium sulfide layers in a well-controlled manner over select small areas. Observations of favorable conditions for deposition and typical pitfalls in the process are discussed. Analysis of the LCVD CdS layers is presented in conjunction with comparisons to conventionally grown CdS layers. The stoichiometry, thickness, and grain structure of the LCVD CdS layers have been suitably optimized for use in solar-cell applications. INTRODUCTION Cadmium sulfide–cadmium telluride (CdS–CdTe) solar cells are fabricated by the methods of chemical vapor deposition (CVD) [1–2] and close-spaced sublimation (CSS) [3], both generally considered well-defined processes. Other deposition methods include chemical bath deposition [4–6] and electrodeposition from nonaqueous [7–8] and aqueous solution [9–11]. These techniques of deposition are useful when the area upon which the solar cell is to be deposited is a nominally simple geometry encompassing the entire surface of the target substrate. Deposition of solar-cell materials in complex geometries, over smaller areas, poses a significantly more difficult challenge. The technique of laser-assisted chemical vapor deposition (LCVD) is one means of overcoming the challenge of complex deposition. In this work, CdS films have been deposited on fluoride-doped tin(IV) oxide (SnO2:F)–glass substrates by LCVD using the reaction between dimethylcadmium (DMCd) and various organosulfur precursors. EXPERIMENTAL DETAILS A computer-controlled flow system, shown in schematic in Figure 1, was constructed to facilitate the deposition of solar cells by the LCVD technique. The computer interface allows for graphical user input and has full functional control of the system. The system functions under computer control include the precursor flow rates rII (DMCd) and rVI (the sulfur precursor), gas mixture ratios, chamber pressure pc, substrate temperature Ts, and the substrate translation speed vs. The value of Ts at the laser spot was maintained by allowing the computer to control the laser power with feedback from an optical pyrometer. Dimethylcadmium (DMCd) was chosen as the organometallic source for cadmium deposition. Four organic sulfur precursors, diisopropyl sulfide (DIPS), tert-butyl methyl sulfide (TBMS), ethyl mercaptan or ethanethiol (EtSH), and tert-butyl mercaptan or 2-methyl-2propanethiol (TBSH) were tested for suitability in producing CdS films. H8.4.1
Figure 1: Schematic of the CMS Flow System These precursors were selec
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