Characterization of Monolayer-Level Composition and Optical Gap Profiles in Amorphous Silicon-Carbon Alloy Bandgap-Modulated Structures
Proceedings Paper (Web of Science)
ABSTRACTOver the past few years we have applied real time spectroscopie ellipsometry (RTSE) to characterize the structural, compositional, and optical gap profiles in continuously-graded amorphous silicon-carbon alloy films (a-Si1-xCx:H). Most recently, we have extended the RTSE methods to their monolayer sensitivity and resolution limits. In this study, continuous triangular variations in the carbon content × (0.02≤x≤0.24) within ∼25 to 130 Å thick graded layers were introduced at the i/p interfaces of the n-i-p solar cell structures using continuous variations in the flow ratio z=[CH4]/{[SiH4]+[CH4]} during rf plasma-enhanced chemical vapor deposition (PECVD). A virtual interface approximation has been applied to interpret the RTSE data collected during the growth of the graded interface layers. This analysis yields C-content depth-profiles with monolayer-level resolution and a compositional uncertainty of ±0.004. Even compositional gradients in which x changes by >0.2 within a few monolayers’ thickness are readily characterized. Lastly, a continuous increase in open circuit voltage with increasing graded interface layer thickness, saturating at ΔVoc=0.1 V after 100 Å, is observed in the n-i-p solar cells with graded layers. These results demonstrate the importance of the RTSE analysis in assessing bandgap engineered device designs.