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Characterization of a hollow cathode styled plasma reactor for photovoltaic applications

Heated pocket deposition sources are often used for the rapid manufacture of thin film CdS/CdTe photovoltaic devices, however, new process methods are needed to further improve the quality of the films, increase cell efficiency, and reduce production costs. A plasma-enhanced, close-spacing sublimation (PECSS) technique utilizing a hollow cathode style discharge in a heated pocket has recently been developed as a candidate process method. It has been successfully used to eliminate pin holes, to dope CdS with oxygen, and dope CdTe absorption layers; all of which have resulted in higher device efficiencies. This work presents a characterization of plasma properties inside the hollow cathode style reactor including: plasma density, electron temperature, and plasma potential. Additionally, the uniformity of the ion current flux to the substrate is presented for nitrogen/oxygen and argon feed gases by means of in situ surface probes fabricated by segmenting a transparent conductive oxide film on a glass substrate. A three-dimensional model was developed and used to calculate plasma production and transport processes, and to gain an understanding of the relative roles of energetic primaries and bulk cold electrons on spatial ionization rates that develop within the reactor plasma as a function of gas pressure and geometry. Comparisons between the model and experimental measurements are presented and good agreement has been observed when the appropriate spatially varying ionization rates are estimated.