Development of Deposition Phase Diagrams for Thin Film Si:H and Si 1-x Ge x :H Using Real Time Spectroscopic Ellipsometr
- PDF / 337,263 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 8 Downloads / 196 Views
A16.3.1
Development of Deposition Phase Diagrams for Thin Film Si:H and Si1−xGex:H Using Real Time Spectroscopic Ellipsometry N. J. Podraza,1 G. M. Ferreira,2 C. R. Wronski,2 and R. W. Collins1 1 Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606; 2 Department of Electrical Engineering and Center for Thin Film Devices, The Pennsylvania State University, University Park, PA 16802 ABSTRACT The growth of hydrogenated silicon (Si:H) and silicon-germanium alloys (Si1-xGex:H) by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) substrates has been studied by real time spectroscopic ellipsometry (RTSE). The motivation is to develop deposition phase diagrams that can provide greater insight into the optimization of amorphous Si1-xGex:H (a-Si1-xGex:H) for multijunction photovoltaics. In initial studies, the phase diagram for bottom cell a-Si1-xGex:H (Eg ~ 1.4 eV) is found to exhibit fundamental similarities to that for Si:H when both materials are prepared under standard PECVD conditions that optimize pure a-Si:H. These similarities suggest directions for optimizing a-Si1-xGex:H by identifying conditions under which a smooth, stable surface is obtained to the largest possible bulk layer thickness. In phase diagram development for PECVD Si1-xGex:H on c-Si, it has been found that the highest surface stability and smoothest surfaces are obtained using cathodic deposition (self bias: ~ −20 V) with a H2-dilution level just below that of the amorphous-to-(mixed-phase microcrystalline) [a→(a+µc)] transition for a thick layer. Due to the promising nature of these results, full phase diagrams are compared for cathodic and anodic Si1-xGex:H as well as for cathodic and anodic Si:H, all on c-Si substrates. The cathodic phase diagram for Si1-xGex:H reveals a narrow range of significant improvement in surface structural evolution near the a→(a+µc) transition, and for a-Si:H reveals an extension of the ultrasmooth protocrystalline regime to a much wider range of thickness. INTRODUCTION AND SUMMARY State-of-the-art solar cells based on amorphous silicon (a-Si:H) employ a triple-junction n-i-p design [1,2]. In this design, a deposition phase diagram approach has been developed for optimizing the top-most a-Si:H i-layer. The operative principle of this approach is to deposit the i-layer under conditions that yield a smooth, stable surface to the largest possible bulk layer thickness db, as determined in real time spectroscopic ellipsometry (RTSE) studies using crystalline silicon (c-Si) substrates [3]. In fact, under optimum a-Si:H deposition conditions, no more than a monolayer of roughening is observed throughout the growth of a 4000 Å thick a-Si:H i-layer. As a function of the H2-dilution ratio R=[H2]/[SiH4], the smoothest, highest stability surfaces inevitably occur at the highest possible R, but without crossing the amorphousto-(mixed-phase microcrystalline) transition [a→(a+µc)] throughout the desired thickness. Similar observations of an optimum electronic quality at maximal R had
Data Loading...
