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

ABSTRACTWe have extended previous real time spectroscopie ellipsometry (RTSE) capabilities in order to investigate the effects of H2-plasma treatment of i-type hydrogenated amorphous silicon (a-Si:H) on the deposition of the overlying p-type microcrystalline silicon (μc-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 H2 on the surfaces of untreated and H2-plasma treated a-Si:H i-layers. We find that for intended single-phase μc-Si:H:B p-layer PECVD under optimum conditions on an untreated i-layer surface, a wide gap (∼2.0 eV Taue gap) amorphous layer nucleates and grows in the first ∼150 Å. This layer develops uniformly to a bulk thickness of ∼150 Å, but gradually acquires a crystalline structure for thicknesses greater than the desired p-layer thickness (200 Å). In contrast, for p-layer PECVD under identical conditions on the H2-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 H2-plasma treated i-layer. Specifically, an absorption onset near ∼2.5 eV is observed for a 48 Å fully-coalesced p-layer, as measured by RTSE at 200°C. For this μc-Si:H:B p-layer, the optical gap decreases by ∼0.15 eV with increasing thickness from 50 to 200 Å. This effect is attributed to a reduction in the quantum confinement energy with an increase in the average crystallite size in the film.