Real time spectroscopic ellipsometry study for the design and optimization of hydrogenated amorphous silicon-based solar cells

Real time spectroscopic ellipsometry (RTSE) has been applied to characterize the preparation and processing of a-Si:H-based materials and solar cell structures. Post-deposition data analysis yields the evolution of bulk, surface roughness, and interface layer thicknesses with {dollar}sim{dollar}0.2 A sensitivity. In addition, the dielectric functions and optical gaps of the separate layers are also determined in the analysis. With the real time measurement approach, the layer properties are determined in the actual device configuration, rather than being inferred indirectly from studies of thick film counterparts.; In this thesis, RTSE has been applied for the first time to investigate the growth of microcrystalline silicon n- and p-layers ({dollar}mu{dollar}c-Si:H:(P,B)) incorporated into amorphous silicon (a-Si:H) p-i-n and n-i-p solar cells, respectively. In particular, RTSE measurements have been performed to identify the optimal conditions for the nucleation and growth of 100 A microcrystalline silicon ({dollar}mu{dollar}c-Si:H) p-layers by rf plasma enhanced chemical vapor deposition (PECVD) on amorphous silicon (a-Si:H) i-layers in the n-i-p solar cell configuration. From RTSE spectra ({dollar}rm1.5le hnule14.5{dollar} eV) collected every {dollar}sim{dollar}4-15 s during growth, we have extracted the time evolution of the {dollar}mu{dollar}c-Si:H p-layer thickness and microstructure, along with the modification that the near-surface i-layer properties undergo in the formation of the i/p interface. Analysis of the RTSE data provide the bulk p-layer dielectric function (2.5-4.5 eV), whose amplitude and shape yield insights into the structural quality and crystallinity of the p-layer. Among the deposition parameters varied include the underlying i-layer surface treatment, the p-layer plasma power flux, and the p-layer dopant source gas (e.g., {dollar}rm Bsb2Hsb6, B(CHsb3)sb3, BFsb3rbrack{dollar} and its flow ratio. We find significant differences attributed to the differing effects on silicon crystallite growth of doping gas generated H, CH{dollar}sb3,{dollar} and F radicals in the plasma. Based on these RTSE studies, insights have been obtained into the design and optimization of solar cells based on a knowledge and understanding of the growth processes, rather than on trial-and-error parameter variations.