Real time analysis of amorphous and microcrystalline silicon film growth by multichannel ellipsometry

Abstract

Real time spectroscopic ellipsometry (SE) has been applied to obtain insights into the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) thin films by plasma-enhanced chemical vapor deposition as a function of the H2-dilution gas flow ratio R=[H2]/[SiH4], the accumulated film thickness db, and the substrate material. For depositions with 15≤R≤80 on clean amorphous semiconductor surfaces, for example, initial film growth occurs in a predominantly amorphous phase, as deduced from analyses of the real time SE data. However, after an accumulated thickness ranging from 3000 Å for R=15 to 30 Å for R=80, a roughening transition is observed in the SE analysis results as the Si film begins to develop a predominantly microcrystalline structure. We have identified this roughening transition as an amorphous-to-microcrystalline phase boundary in the deposition parameter space of db and R. The thickness at which this boundary occurs decreases continuously with increasing R, and the position of the boundary is strongly substrate dependent. Based on these real time SE studies and detailed device analyses, we have found that the highest performance p–i–n solar cells are obtained in i-layer deposition processes maintained at the highest possible R value versus thickness without crossing the deposition phase boundary into the microcrystalline regime.