REAL-TIME SPECTROELLIPSOMETRY CHARACTERIZATION OF OPTICAL GAP PROFILES IN COMPOSITIONALLY-GRADED SEMICONDUCTOR STRUCTURES - APPLICATIONS TOBANDGAP ENGINEERING IN AMORPHOUS SILICON-CARBON ALLOY SOLAR-CELLS

We have applied a real time spectroellipsometry data analysis procedur e developed previously [S, Kim and R. W. Collins, Appl. Phys. Lett. 67 , 3010 (1995)] to characterize depth profiles in the optical gap for c ompositionally-graded semiconductor alloy thin films prepared by plasm a enhanced chemical vapor deposition. The analysis procedure employs a two-layer (four-medium) optical model consisting of the ambient, a th in surface roughness layer and outer-layer (5-15 Angstrom) whose prope rties ars to be determined, and a pseudo-substrate that contains the p ast history of the graded-layer deposition. The ellipsometric spectra (2.3-4.0 eV) are analyzed to provide, not only the depth-profile of th e optical gap and alloy composition for the graded layer, but also the instantaneous deposition rate and the surface roughness layer thickne ss versus time or accumulated layer thickness. To apply the previous a nalysis approach, it was necessary to (i) parameterize the dielectric function of the alloys as a continuous function of composition over th e desired alloy range and (ii) express the optical gap as an accurate function of alloy composition. As an example, we have applied the exte nded analysis to obtain the depth-profile of the optical gap and alloy composition with <15 Angstrom resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1-xCx:H) film prepared by continuously vary ing 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 pli interface of widegap a-Si1-xCx:H p-i-n s olar cells, and improvements in open-circuit voltage have been observe d. (C) 1996 American Institute of Physics.