H NMR Study of Hydrogenated Nanocrystalline Silicon Thin Films

Abstract

Hydrogenated nano-crystalline silicon (nc-Si:H) is a promising material for multi-junction solar cells. We investigated the local hydrogen environments in nano-crystalline silicon thin films by nuclear-magnetic-resonance (NMR). At room temperature, ¹H NMR spectra have broader components than those observed in standard device grade hydrogenated amorphous silicon (a-Si:H). As the temperature decreases, the ¹H NMR exhibits a broadening of the line shape attributed to hydrogen atoms at the interfaces between the amorphous silicon (a-Si) and the crystalline silicon (c-Si) regions. These results suggest that the local hydrogen structure in nc-Si:H is very different from that in a-Si:H. In particular, the hydrogen clusters contributing to broadened spectra may exist on the surfaces of the a-Si/c-Si interfaces which do not exist in the more dense matrix of a-Si:H and may contribute to certain unique optoelectronic properties of these nc-Si:H thin films. The dependence of the spin-lattice relaxation time (T₁) on temperature, however, is very similar to that in a-Si:H, which indicates the spin-lattice relaxation mechanism, i.e via spin diffusion through molecular hydrogen, is common to both systems.