Electrically active boron doping in the core of Si nanocrystals by planar inductively coupled plasma CVD

An improvement in the doping efficiency in p-type nc-Si:H, a two-phase structure consisting of Si-nanocrystallites embedded in an amorphous matrix, has been pursued via low-temperature, spontaneous, plasma processing of low-pressure and high-density SiH4 plasma with added B2H6, diluted in H-2, in inductively coupled plasma CVD. With increased dopant incorporation, the gradually reduced overall crystallinity systematically dominates via an ultrananocrystalline component in the thermodynamically stable < 220 > orientation. The microstructure factor decreases continuously, and the bonded-H content of the network increases arbitrarily, with a significant fraction being associated with Si in an Si-H-Si plateletlike configuration. Effective doping by B atoms, mostly in fourfold coordination with Si in its nanocrystalline core, contributes to the rapid increase in conductivity of the doped p-nc-Si:H network, up to a moderate B2H6 flow rate. However, gradually enhanced doping by threefold coordinated electrically inactive B-atoms, mostly at the nanocrystalline grain boundary at a higher B2H6 flow rate, induces gross structural deviation in the degree of nanocrystallinity and reduces dark conductivity substantially. Further, the logarithmic magnitude of the conductivity prefactor (sigma(0)) demonstrates the negative characteristic energy (E-MN) in the Meyer-Neldel (MN) relation. The high density of charge carriers in the heavily doped configuration of the nc-Si network forms a deep and continuous band tail near the valence-band edge and induces subsequent narrowing of the bandgap as well as a shift of the Fermi level into the valence band. Degenerate-semiconductor type behavior has been accomplished in a heavily doped p-nc-Si:H network, demonstrating reverse MN characteristics in electrical transport. Published under license by AIP Publishing.