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Yiwei Lu*, Sangbo Kim*, J. S. Burnham*, Ing-Shin Chen*, Yeeheng Lee*, Y.E. Strausser**, C.R. Wronski*, and R.W. Collins* * Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802. "**DigitalInstruments, Santa Barbara, CA 93 103. ABSTRACT

We have applied real time spectroscopic ellipsometry (RTSE) to monitor the successive growth of p-type a-Sil-xCx:H and i-type a-Si:H on specular Sn02:F (i.e., the superstrate solar cell configuration) in a single-chamber deposition system. Both the microstructural evolution, which includes the surface roughness and bulk layer thicknesses versus time and bulk layer void volume fraction, as well as the optical properties, which include the dielectric function (1.5-4.0 eV) and optical gap of the individual layers, were determined from RTSE data collected during growth. The accuracy of our approach is demonstrated by correlating structural parameters obtained both by RTSE and atomic force microscopy. Based on prior information deduced by RTSE, the TCO/p/i structure was fabricated with optimized procedures that have sought to minimize TCO/p and p/i interfacial problems. These studies illustrate that RTSE can be a valuable tool for identifying problems in the fabrication of a-Si:H solar cells and ultimately improving cell performance. INTRODUCTION

In the preparation of hydrogenated amorphous silicon-based solar cells by plasma-enhanced chemical vapor deposition (PECVD), the properties of very thin doped layers (-200 A used for devices) may differ substantially from those of thick layers (-0.8 ýtrn used for electrical property characterization).l, 2 These differences arise since processes such as nucleation cause the microstructure to evolve with thickness. In addition, the layers of a device may interact during deposition or in subsequent processing. For example, interactions between a transparent conducting oxide (TCO) substrate layer and the hydrides in the plasma used to deposit the overlying layer can modify the TCO properties. 3 ,4 Such problems yield device performance characteristics that depend critically on processing procedure details. Thus, in order to expand current knowledge of the relationships between the preparation and characteristics of devices, monolayer-sensitive real time probes capable of characterizing the individual layers and interfaces in devices are needed. In an effort to address problems such as these, real time spectroscopic ellipsometry (RTSE) has been developed. The applications of RTSE discussed here emphasize the development of processing procedures and the ability to characterize thin layers in situ with precision and accuracy. Using RTSE, we have obtained a detailed fingerprint of a TCO/p/i structure prepared using procedures that have sought to minimize TCO/p and p/i interfacial problems. EXPERIMENTAL DETAILS

The p-i structures studied here were prepared by parallel-plate PECVD in a high vacuum single-chamber system. 5 The substrates for devices and for RTSE monitoring were textured Sn02:F-coated glass and specul