Structural, optical and electronic properties of hydrogenated polymorphous silicon films deposited at 150°C

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to the formation of powder, a new type of material, called hydrogenated polymorphous silicon (pm-Si:H) is obtained. This material has increased transport properties with respect to device-grade hydrogenated amorphous silicon (a-Si:H). To understand the origin of such improved transport properties, we made electrical measurements from which we deduced that the density of states at the Fermi level N(E_F) and the carrier capture cross-section, σ_c, in pm-Si:H films are at least 10 times lower and 5 times lower, respectively, than in a-Si:H films. The crystallite sizes deduced from Raman spectra confirm high-resolution transmission electron microscopy measurements. The infrared stretching modes of pm-Si:H films have a band at ∼2035 cm^-1 which is attributed to hydrogen platelets. The smaller density of states at the Fermi level N(E_F) is explained in terms of improved amorphous matrix as confirmed by optical measurements. We suggest that the low capture cross-section, σ_c, observed in these films results from a preferential carrier recombination path at grain boundary dangling bonds as predicted by theoretical calculations.