An ellipsometry study of a hydrogenated amorphous silicon based n-i structure

Abstract

Spectroscopic ellipsometry measurements have been used as a probe to provide information on the structure of a hydrogenated amorphous silicon based n‐i two‐layer system. The n‐i structure is prepared under the same conditions as an a‐Si:H p‐i‐n cell with a thin (∼150 Å) n layer deposited in a glow discharge from a mixture of SiF₄, SiH₄, H₂, and PH₃ on top of intrinsic a‐Si:H. An effective medium approximation and a linear regression analysis have been used to determine the bulk microstructure of the thick (0.3 μm) n layer on quartz as a mixture of volume fractions of a‐Si (0.74±0.01), c‐Si (0.09±0.02), and void (0.17±0.03). When applied to the thin n layer in the solar‐cell configuration, however, the corresponding analysis shows evidence of a 100‐Å overlayer of very low density a‐Si, not observed on the thicker n‐type material, which dominates the dielectric properties of the thin layer. These results are suggestive of nucleation and growth surface microstructure on the 150‐Å n layer and lead to the conclusion that, in the fabrication of high‐performance a‐Si:H based devices using thin layers, such structure must be minimized.