A New Field-Aided Germanium-Induced Lateral Crystallization of Silicon
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A New Field-Aided Germanium-Induced Lateral Crystallization of Silicon Kianoush Naeli, Shamsoddin Mohajerzadeh, Ali Khakifirooz*, Saber Haji, Ebrahim A. Soleimani Electrical and Computer Eng. Dept., University of Tehran, Tehran 14395-515, IRAN. * Currently at Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. [email protected]. ABSTRACT The effect of an electric field on germanium-seeded lateral crystallization of a-Si is studied for the first time and compared to this effect in Ni-induced lateral growth. While the crystallization rate is lower when Ge is used as the nucleation seed and annealing should be done at higher temperatures, filed-aided crystallization shows a similar behavior to that observed for Ni-induced crystallization. Optical microscopy results indicate that grain growth starting from the negative electrode occurs in Si films at annealing temperatures higher than 480ºC, while the applied electric field ranges form 200 to 1400V/cm. SEM was also used to confirm the crystallinity of the films. INTRODUCTION Metal-induced lateral crystallization seems a promising technology for low-temperature fabrication of thin-film transistors needed for large-area electronics. Since the fabrication of the first thin-film transistor based on this effect [1,2], much attention has been paid to it as a new method for realization of TFT's on glass substrates. Although high-performance TFT's with carrier mobilities exceeding 100cm2/V.s and low leakage currents have been reported in the last few years [1-5], the presence of metal contamination in the channel area is still a major concern. In fact, this seems to be the main barrier in commercialization of the MILC technique. Several studies thus have been carried out to examine metal concentration in the channel. While SIMS and EDX data show that the nickel concentration in the MILC region is about 0.2 atomic percent, metal contamination reaches 4% at the crystallization front [6]. Unfortunately, in conventional MILC and starting from both source and drain, these regions meet at the middle of the channel. A unidirectional scheme has been also proposed in which crystallization starts only from the drain region, pushing this highly contaminated frontier to the source area [7]. It is well known that in nickel-induced lateral crystallization, NiSi2 precipitates, which are formed at temperatures as low as 350ºC, act as a nucleation seed, promoting the crystallization in the silicon film. In fact, any crystalline material with a reasonable lattice constant can be used as the seed. So, polycrystalline germanium seems a suitable candidate for lateral crystallization of silicon starting from S/D area in a similar manner [8,9]. Being a semiconductor, Ge contamination in the channel is hoped to have a less severe effect on the performance of the transistor. On the other hand, applying an external electric field during MILC has been reported to enhance the growth rate significantly [10-14]. In addition, this tech
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