Structural and optical properties of a-Si:H/{mu}c-Si:H:B junctions in the a-Si:H:-based n-i-p solar cell configuration

The authors have extended previous real time spectroscopic ellipsometry (RTSE) capabilities in order to investigate the effects of H{sub 2}-plasma treatment of i-type hydrogenated amorphous silicon (a-Si:H) on the deposition of the overlying p-type microcrystalline silicon ({micro}c-Si:H:B) in the formation of an n-i-p solar cell structure. In this study, the authors compare in detail the nucleation and growth of p-layers by plasma-enhanced chemical vapor deposition (PECVD) from siH{sub 4} highly diluted in H{sub 2} on the surfaces of untreated and H{sub 2}-plasma treated a-Si:H i-layers. They find that for intended single-phase {micro}c-Si:H:B p-layer PECVD under optimum conditions on an untreated i-layer surface, a wide gap ({approximately}2.0 eV Tauc gap) amorphous layer nucleates and grows in the first {approximately}150 {angstrom}. This layer develops uniformly to a bulk thickness of {approximately}150 {angstrom}, but gradually acquires a crystalline structure for thicknesses greater than the desired p-layer thickness (200 {angstrom}). In contrast, the p-layer PECVD under identical conditions on the H{sub 2}-plasma treated i-layer, high-density crystalline nuclei form immediately. This conclusion is drawn on the basis of the unique optical properties of the bulk p-layer that develops on the surface of the H{sub 2}-plasma treated i-layer. Specifically, an absorption onset near {approximately}2.5 eV ismore » observed for a 48 {angstrom} fully-coalesced p-layer, as measured by RTSE at 200 C. For this {micro}c-Si:H:B p-layer, the optical gap decreases by {approximately}0.15 eV with increasing thickness from 50 to 300 {angstrom}. This effect is attributed to a reduction in the quantum confinement energy with an increase in the average crystallite size in the film.« less