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Keywords:

  • nanocrystalline silicon films;
  • hydrogenation;
  • phase transformations;
  • amorphous silicon solar cells

Abstract

We present a study on the morphological evolution of hydrogenated amorphous silicon layers obtained by plasma enhanced chemical vapor deposition at different H dilutions in the regime close to the formation of the nanocrystalline (nc-Si) phase. The role of hydrogen in the transition from the amorphous to the crystalline phase is investigated by accurate structural and chemical characterisation, from the early stages of nucleation, where the nuclei present a size slightly larger than the critical nucleus, i.e. about 0.8 nm in radius, up to the formation of crystalline grains larger than 30 nm in radius. Hydrogen is found to have a crucial role in the transition from a-Si:H to nc-Si:H, since it forms an intermediate bond-centred Si–H–Si configuration, and when the H moves away from the bond-centred location, the strained Si–Si bonds either break or relax, undergoing local structural rearrangements closer to those of c-Si. During this phase transition a part of H bonds at grain boundaries. A correlation between the structural characteristics of the crystalline phase and the bonding mechanism of Si with H through multiple hydrides, such as Si-H2 and Si-H3 is found. Particularly the SiH3 are found to be directly correlated to the shape and the size of the nanocrystallites present in the films (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)