Laser nucleated grain growth in hydrogenated amorphous silicon films

Abstract

Large grained film crystal silicon has been pursued for decades in hopes of replacing expensive wafer devices with lower cost film products with comparable electronic properties. The focus of these pursuits has been on directly producing thicker (>1um) films with bulk crystallization treatments. The recent development of a low cost, thin film compatible epi-thickening process of thin film template layers has opened up new possibilities for utilizing different methods of producing film crystal silicon. The focus of this research was an understanding of the nucleation and growth process sufficient to develop a "thin"(<1um) seed layer on an inexpensive (glass) substrate that replicates a large grained c-Si surface, thus presenting a template surface for the subsequent epitaxial growth of a "thick"(~10um) c-Si film. By combining laser nucleation and solid phase crystallization of a-Si:H films with different film hydrogen content (CH), the nucleation and crystallization processes were explored. The laser fluence necessary for nucleation through direct crystallization were explored for 100nm a-Si:H films with different film H content (CH). The affect of film H content and sub-crystallization threshold laser fluence were investigated by X-ray diffraction (XRD) measurements during in situ thermal annealing at 600°C of films with different CH treated with an array of fluences. The laser-treated films showed a reduced incubation time (t0) for crystallization compared to as-grown films, with the largest incubation time reduction exhibited for samples with higher film H and higher laser fluences. The grain sizes of HWCVD a-Si:H films which had been exposed to sub-threshold laser prior to thermal annealing were measured with electron backscattering diffraction (EBSD) and compared to unexposed films. For large area laser processed and thermally annealed films which exhibited a reduced to, the EBSD grain sizes were unchanged as compared to those for the same film which were annealed directly from the as grown state. The trends in EBSD grain size with in situ XRD crystallization time tc are consistent with theoretical predictions obtained from the classical model of nucleation and grain growth. An optical method was developed as a new and simple method to investigate crystallite nucleation and growth in stepwise, thermally annealed PECVD and HWCVD a-Si:H films. By confining film thicknesses to the range 500-4000Å, optical microscopy in the reflection mode was used to readily detect crystallites in the thermally annealed a-Si:H lattice. Measurements of the crystallite density versus annealing time for identically prepared films of different thicknesses demonstrated crystallite nucleation rates smaller for thinner films, suggesting homogeneous nucleation, in agreement with previous results. The effect of film stress on crystallite nucleation was investigated in 0.11µm thick, thermally annealed hydrogenated amorphous silicon films. The nucleation rate was significantly suppressed around scratches, cleaved film edges, and laser ablated areas, extending laterally as much as 100-150um from these regions where the film connectivity was disrupted. u-Raman measurements of the transverse optical mode of Si demonstrated an accompanying reduction in tensile stress in the regions where nucleation was suppressed. The first measurements of nucleation rate in stress and in stress relieved areas in the same film are presented.