Real time spectroellipsometry characterization of optical gap profiles in compositionally-graded semiconductor structures: Applications to bandgap engineering in amorphous silicon-carbon alloy solar cells

Abstract

We have applied a real time spectroellipsometry data analysis procedure developed previously [S. Kim and R. W. Collins, Appl. Phys. Lett. 67, 3010 (1995)] to characterize depth profiles in the optical gap for compositionally-graded semiconductor alloy thin films prepared by plasma enhanced chemical vapor deposition. The analysis procedure employs a two-layer (four-medium) optical model consisting of the ambient, a thin surface roughness layer and outer-layer (5–15 Å) whose properties are to be determined, and a pseudo-substrate that contains the past history of the graded-layer deposition. The ellipsometric spectra (2.3–4.0 eV) are analyzed to provide, not only the depth-profile of the optical gap and alloy composition for the graded layer, but also the instantaneous deposition rate and the surface roughness layer thickness versus time or accumulated layer thickness. To apply the previous analysis approach, it was necessary to (i) parameterize the dielectric function of the alloys as a continuous function of composition over the desired alloy range and (ii) express the optical gap as an accurate function of alloy composition. As an example, we have applied the extended analysis to obtain the depth-profile of the optical gap and alloy composition with <15 Å resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1−xCx:H) film prepared by continuously varying the gas flow ratio z=[CH4]/{[CH4]+[SiH4]}. In order to demonstrate the technological importance of such structures, the graded layer has been incorporated at the p/i interface of widegap a-Si1−xCx:H p-i-n solar cells, and improvements in open-circuit voltage have been observed.