Abstract
Polycrystalline Silicon (poly-Si) and polycrystalline silicon-germanium (poly-SiGe) thin films are of interest as the basic channel materials in high performance thin film transistors for optoelectronic devices. Metal-induced crystallization of amorphous thin films is a useful method to fabricate the high performance polycrystalline thin films. In this dissertation research, the crystallization behaviors of a-Si and a-SiGe enhanced by the presence of metallic species or electric current are investigated. Ultrafast directional crystallization that combined the electric current stressing with metal-induced crystallization has been achieved for BF2+-implanted amorphous Si (a-Si) at room temperature. Polycrystalline Si was observed to grow from anode towards cathode and the channels of a-Si strips with a length of 140 □m and a width of 10 □m can be fully crystallized with a stressing time less than 0.2 s. The directional growth of crystalline Si nanowires, 50 nm in width and as long as 3 □m in length, with an extraordinarily high aspect ratio of 60, indicates a strong electric- field-induced effect on the growth. The uneven thermal distribution in the a-Si channels also caused the variation in grain size between the central and edge regions along A-A’ direction. On the other hand, the effects of electric field and doping species on directional crystallization of a-Si channels under high-density current stressing have been also investigated. The a-Si channels were implanted by 30 keV BF2+ or As+ to a dose of 3 x 1015 ions/cm2. A preferential growth of poly-Si from anode toward cathode was found on BF2+, As+ and un-implanted a-Si samples. The results indicate that directional growth of poly-Si is caused by the strong electric field effect on positively charged Ni ions under high-density current stressing. In a word, the growth method provides a promising scheme to solve the problems caused by high-temperature and long-term annealing treatment for the applications of optoelectronic devices. Metal-induced crystallization of amorphous Si1-xGex (x = 0.2 and 0.3) thin films on SiO2 by rapid thermal annealing at 300-600 oC has been investigated. At low annealing temperature, Ni reacted with a-Si1-xGex films to form Ni germanosilicides including Ni, Si, and Ge atoms. The crystallization temperature of a-Si0.7Ge0.3 and a-Si0.8Ge0.2 was lowered from 500 to 400 oC and 600 to 500 oC, respectively, with capping Ni. The individual grains of Ni germanosilicide were observed to form at the poly-SiGe/SiO2 interface in the annealed Ni/a-Si0.7Ge0.3 and Ni/a-Si0.8Ge0.2 samples after annealing at 500 oC. The formation of individual structure containing a small amount of Ge at the bottom of polycrystalline Si1-xGex films is attributed to the preferential reactions of Ni with Si to Ge. In-situ observation of the morphological changes of the metals (including the silicide-forming metal and eutectic-forming metal) induced crystallization of a-Si0.8Ge0.2 films have been carried out. The crystallization of a-Si0.8Ge0.2 thin films induced by Ni and Au was observed by the in-situ transmission electron microscope. For the Ni-induced crystallization, NiSi2 precipitate was first formed then migrated along a straight line in a-Si0.8Ge0.2. The crystallization of a-Si0.8Ge0.2 was achieved along the path of NiSi2 migration. The needle-like crystal regions were obtained after Ni-induced crystallization of a-Si0.8Ge0.2. The change of the growth direction of the needle-like crystallites was also observed. The direction is parallel but opposite to the original growth direction. It appeared that the NiSi2 precipitates migrated along the interface of the crystallized and amorphous regions. The results were different from the branching needle-like structure for Ni-induced crystallization of a-Si and may be caused by the presence of Ge atoms. On the other hand, for the Au-induced crystallization, Au clusters was first melted above the eutectic temperature of the Au and a-Si0.8Ge0.2. Then, the Au alloys migrated on the a-Si0.8Ge0.2 to initiate the crystallization and the fully crystallized a-Si0.8Ge0.2 films were observed. The crystallization of the 150 nm-thick a-Si0.8Ge0.2 films can be enhanced by Ni and Au metals below 500 oC.