MICROSTRUCTURAL EVOLUTION OF A-SI-H PREPARED USING HYDROGEN DILUTION OF SILANE STUDIED BY REAL-TIME SPECTROELLIPSOMETRY

We have applied real time spectroellipsometry to measure the nucleatio n and growth of hydrogenated amorphous silicon (a-Si:H) films prepared by plasma-enhanced chemical vapor deposition from H-2-diluted SiH4 on crystalline Si (c-Si) substrates at 200 degrees C. For a H-2-dilution ratio R = [H-2]/[SiH4] of 10, optimum microstructural evolution is ob served during the growth of 0.3 mu m a-Si:H film, namely, smoothening during coalescence followed by long-term surface stability. At lower a nd higher R values, surface roughening in the thick film regime (d > 2 0 nm) is larger, particularly for R greater than or equal to 20 due to crystallite development. Although dilution levels of 20 less than or equal to R less than or equal to 30 lead to microcrystallinity in thic k films (0.3 mu m), the structural evolution and optical properties in thin films (< 20 nm) are characteristic of high quality a-Si:H. Thus, real time spectroellipsometry suggests that in the preparation of i-l ayers for solar cells, R similar to 10 may be optimum for the bulk i-l ayer whereas a 10 to 20 nm layer with much larger R may be beneficial at the p/i-interface. These suggestions have been verified in p-i-n ce lls deposited on specular SnO2-coated glass substrates. (C) 1998 Elsev ier Science B.V. All rights reserved.