Structural and optical properties of a-Si:H/μc-Si:H:B junctions in the a-Si:H-based N-I-P solar cell configuration

We have extended previous real time spectroscopic ellipsometry (RTSE) capabilities in order to investigate the effects of H-2-plasma treatment of I-type hydrogenated amorphous silicon (a-Si:H) on the deposition of the overlying p-type microcrystalline silicon (mu c-Si:H:B) in the formation of an n-i-p solar cell structure. In this study, we compare in detail the nucleation and growth of p-layers by plasma-enhanced chemical vapor deposition (PECVD) from SiH4 highly diluted in H-2 on the surfaces of untreated and H-2-plasma treated a-Si:H i-layers. We find that for intended single-phase mu c-Si:H:B p-layer PECVD under optimum conditions on an untreated i-layer surface, a wide gap (similar to 2.0 eV Tauc gap) amorphous layer nucleates and grows in the first similar to 150 Angstrom. This layer develops uniformly to a bulk thickness of similar to 150 Angstrom, but gradually acquires a crystalline structure for thicknesses greater than the desired p-layer thickness (200 Angstrom). In contrast, for p-layer PECVD under identical conditions on the H-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-2-plasma treated i-layer. Specifically, an absorption onset near similar to 2.5 eV is observed for a 48 Angstrom fully-coalesced player, as measured by RTSE at 200 degrees C. For this mu c-Si:H:B p-layer, the optical gap decreases by similar to 0.15 eV with increasing thickness from 50 to 200 Angstrom. This effect is attributed to a reduction in the quantum confinement energy with an increase in the average crystallite size in the film.