Structural and Optical Properties of a-Si:H/Mu zeta-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 H2-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 ofp-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 ..mu..c-Si:H:B p-layer PECVD under optimum conditions on an untreated i-layer surface, a wide gap (approx. 2.0 eV Tauc gap) amorphous layer nucleates and grows in the first approx. 150 ..ANG... Thislayer develops uniformly to a bulk thickness of approx. 150 ..ANG.., but gradually acquires a crystalline structure for thicknesses greater than the desired p-layer thickness (200 ..ANG..). 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 opticalproperties of the bulk p-layer that develops on the surface of the H2-plasma treated i-layer. Specifically, an absorption onset near approx.2.5 eV is observed for a 48 ..ANG.. fully-coalesced p-layer, as measured by RTSE at 200 degrees C. For this ..mu..c-Si:H:B p-layer, the optical gap decreases by approx. 0.15 eV with increasing thickness from 50 to 200 ..ANG... This effect is attributed to areduction in the quantum confinement energy with an increase in the average crystallite size in the film.