Investigation of Hydrogen Dependent Long-Time Thermal Characteristics of PECV-Deposited Intrinsic Amorphous Layers of Different Morphologies

Abstract

Hydrogenated intrinsic amorphous silicon ((i) a-Si:H) layers deposited on n-type crystalline silicon (c-Si) by plasma enhanced chemical vapour deposition (PECVD) are investigated during long-time thermal treatment (100 h at 200°C) with regard to the depth profile of hydrogen in the a-Si layer and its diffusion into the c-Si bulk. The morphology of the (i) a-Si:H is manipulated by the PECVD process parameters. A columnar and a non-columnar growth can be distinguished. Microscopic investigations are carried out by scanning electron microscopy (SEM). Minority carrier lifetime (eff) measurements permit an evaluation of the surface passivation and thus the saturation of defects like dangling bonds at the (i) a-Si:H/c-Si interface. A non-columnar structure leads to a high stability of the passivation during thermal treatment of up to 100 h. In contrast a columnar structure of the amorphous silicon layer results in a better but less stable passivation of the c-Si wafer surface. Microvoids in the columnar layer are the reason for this behavior. Fourier transform infrared spectroscopy (FTIR) measurements confirm the formation of microvoids, i. e. a high concentration of Si-H2 bonds. Investigating the changes in hydrogen depth profile by nuclear resonant reaction analysis (NRRA) reveals a higher loss in hydrogen concentration during thermal treatment of the (i) a-Si:H layers with columnar morphology. The hydrogen concentration profiles as measured by NRRA illustrate the dependency of passivation quality with time on the specific morphology of different amorphous layers.