Influence Of Metal Underlayers On Low-temperature Deposition Of Poly-silicon Thin Films By Hot-wire CVD

High quality poly-Si thin films were deposited on silver- and copper-coated glass substrates by hot-wire chemical vapor deposition (HWCVD) at low temperature. The influence of the metal underlayers on the crystallographic growth orientation, the crystalline fraction and the grain sizes of poly-Si films deposited at the different distance between the filament and the substrate (5-10mm), the filament temperature range of 1800～1300℃and corresponding the substrate temperature range of 320～200℃were characterized by means of X-ray diffraction (XRD), Raman spectrum, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Information about Cu cross-section distribution of the sample was given by Electro Probe Microanalysis(EPMA). It is speculated that the presence of Cu in the amorphous silicon layer which consequently results in the formation of poly-Si indicates a diffusion of Cu atoms through a-Si.Experimental results showed that when the Ag underlayer thickness was 700nm, polycrystalline silicon films with micrometre-size grains and high crystalline fraction were obtained at the filament temperature range of 1800～1700℃(corresponding substrate temperature range of 295～270℃) for the d_f-s=5mm or at the filament temperature range of 1700～1600℃(corresponding substrate temperature range of 265～240℃) for the d_f-s=1.5 mm. The crystalline fraction of the poly-Si films increased to a maximum value and then rapidly reduced with decreasing filament temperature.Especially,the crystalline fraction deposited at the filament temperature of 1700℃(corresponding substrate temperature range of 270℃) for the d_f-s=5mm was 98.7%. The crystallographic growth orientation of poly-Si films deposited on silver-coated glass substrates did not vary with various parameters: intensity peak ratios of three strong diffraction peaks at (111), (220), and (311) planes for all samples are similar to those of Si powder, suggesting that there is no preferential orientation of crystallites. When the Ag underlayer thickness was decreased to 30nm, the crystallographic growth orientation had not any changes for the d_f-s of 7.5mm. The crystalline fraction increased to a maximum value and then rapidly reduced and the grain sizes continuously decreased with decreasing filament temperature at the filament temperature range of 1700～1300℃(corresponding substrate temperature range of 265～180℃). Polycrystalline silicon films composed of large and uniform columnar grains with a moderate lateral grain size of～1μm, vertical grain size over 15μm have been fabricated on copper-coated glass substrates at the d_f-s of 5mm. Influence of Cu underlayer on grain growth process appeared to be more visible at the filament temperature range of 1800～1700℃,which is consistent with the results obtained from the films deposited on silver-coated glass substrates under the same conditions. Owing to the presence of Cu underlayer, the crystalline fraction of poly-Si films was increased (the crystalline fraction of poly-Si films deposited on Cu-coated glass substrate was over 90% at the d_f-s of 5mm or 8mm) and the crystallographic growth orientation was changed in certain experimental conditions. Moreover, the grain size and the crystalline fraction of poly-Si films notablely decreased with the increasing distance between the filament and the substrate. Larger d_f-s and lower filament temperature have passitive effect on the nucleation and growth of poly-Si films. Cu atoms in the underlayer had diffused through a-Si layer which resulted in the formation of a silicon-copper solid solution.