Medium range order engineering in amorphous silicon thin films for solid phase crystallization

Abstract

In recent years, it has been recognized that medium range ordering (MRO) in amorphous silicon (a-Si:H) plays a role in controlling its solid phase crystallization (SPC) behavior. Information on the MRO can be obtained from the width of the first X-ray diffraction (XRD) peak of a-Si:H centered around 2θ = 27.5°. The broader the full width half maximum (FWHM) of the first XRD peak, the less ordered the a-Si:H material in the medium range length scale (up to 5 nm). In this work, it was found that the FWHM of the first XRD peak changes with the pressure used during the deposition of a-Si:H. A threshold SPC behavior was observed as a function of the a-Si:H deposition pressure and a good correlation between the SPC behavior and the a-Si:H XRD peak width was found. Results in this study indicate that higher MRO in a-Si:H led to faster SPC rates and smaller grain sizes, suggesting the presence of relatively active and high density of nucleation sites. High angle annular dark field scanning transmission electron microscopy and ultraviolet reflectance indicate that films with higher MRO yielded polycrystalline silicon (poly-Si) grains which were more defective and non-columnar in morphology. Results suggest that a-Si:H material with lower MRO were preferred as a precursor for SPC, which forms a better quality poly-Si thin film material. It was proposed that ion bombardment seems to play a role in altering the a-Si:H properties.