Crystallization of amorphous silicon thin films induced by nanoparticle seeds

Crystallization of amorphous Si (a-Si) thin film has received extensive interest for their attractive applications into Si thin film transistors and Si based solar cells. Among various crystallization techniques, Solid phase crystallization (SPC) and Excimer laser crystallization (ELC) were investigated. Firstly, Solid phase crystallization (SPC) of amorphous silicon thin films deposited by the DC magnetron sputtering system with a modification in nucleation step was investigated at low temperature. The thin film consists of polycrystalline nanoparticles embedded in an amorphous matrix which can act as nuclei during crystallization, resulting in a lower thermal energy for the nucleation. The lowering energy barrier for nucleation would shorten the transition time from amorphous into polycrystalline silicon resulting from the reduction of incubation time and also lower the processing temperature spontaneously. In addition, a comprehensive study of the growth mechanism of the sputtered amorphous silicon thin films is presented during annealing. Samples were prepared with various substrate temperatures and RF power in order to optimize the crystallization of a-Si after the deposition. Also, the effects of annealing condition were examined. Low pressure N2 ambient during SPC promoted crystallization of a-Si thin films and the crystallinity. The low pressure annealing had a large impact on the crystallinity and growth behavior of subsequent films. In addition, the crystallinity, incubation time, the crystallized volume fraction and growth rate of the films annealed in a conventional furnace have been extensively studied by XRD and HRTEM. It was believed that crystalline Si nanoparticles would act as nuclei for growth of crystalline Si thin films, thus removing the high temperature requirement for nucleation, resulted in the improvement of the crystallization of a-Si.;Secondly, the controlled Super Lateral Growth (SLG) can be obtained by nanoparticle induced crystallization (NIC) technique during laser annealing, which led to enhance the random super lateral growth (SLG) of Si thin films for the excimer laser crystallization (ELC). The crystallinity and surface information of the films irradiated by excimer laser have been studied by Raman spectroscopy, FESEM and AFM. Also, Transmission electron microscopy (TEM) was employed in order to obtain structural information. Polycrystalline Si nanoparticles, which have higher melting point than those of amorphous phase, would survive at high energy density of laser. In general, super lateral growth (SLG) occurs at vary narrow laser energy density region. Thus, it tends to be sensitive to laser energy density, which means not easy to control because of the characteristics of the mechanism of the SLG. In this study, poly-Si nanoparticles would act as nucleation seeds for the growth of the films during the solidification. Those nanoparticle seeds provided more probability to survive at higher density of energy compared to that without nanoparticle seeds, resulted in the large grain size distribution and the controlled super lateral growth (SLG), relatively independent of laser energy density.