Intrinsic hydrogenated amorphous silicon (a-Si:H) films can yield in outstanding electronic surface passivation of crystalline silicon (c-Si) wafers as utilized in the HIT (heterojunction with intrinsic thin layer) solar cells. We have studied the correlation between the passivation quality and the interface nature between thin amorphous layers and an underlying c-Si substrate for understanding the passivation mechanism. We found that a thin (~5nm) intrinsic layer is inhomogeneous along the growth direction with the presence of a hydrogen rich layer at the interface and that completely amorphous films result in better passivation quality and device performance than an epitaxial layer. Post annealing improves carrier lifetime for the amorphous layer, whereas the annealing is detrimental for the epitaxial layer. We have also found that the passivation quality of intrinsic a-Si:H(i) film deteriorates severely by the presence of a boron-doped a-Si:H(p+) overlayer due to Si-H rupture in the a- Si:H(i) film. Finally, for a passivation layer in the hetero-junction structure, a-Si1-xOx will be demonstrated in comparison with a-Si:H.