Directional Growth of Si Nanowires on Insulating Films by Electric-Field-Assisted Metal-Induced Lateral Crystallization
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Directional Growth of Si Nanowires on Insulating Films by Electric-Field-Assisted Metal-Induced Lateral Crystallization Hiroshi Kanno, Atsushi Kenjo, Taizoh Sadoh, and Masanobu Miyao Department of Electronics, Kyushu University 6-10-1 Hakozaki, Fukuoka 812-8581, Japan ABSTRACT Metal-induced lateral crystallization of amorphous Si has been investigated under a wide range of electric fields (0-4000 V/cm). In the low field region (2000 V/cm), directional growth aligned to the electric field was observed. This new findings will be a powerful tool to achieve new poly-Si with highly controlled structures. INTRODUCTION The low-temperature formation of high quality polycrystalline Si (poly-Si) films on insulating films has been expected to enable system-in-displays and three-dimensional ultra large scale integrated circuits (ULSI). To prevent the diffusion of dopant atoms and the softening of glass substrates, the processing temperature for crystallization of amorphous Si (a-Si) should be lower than 550oC. To achieve this, various recrystallization processes of a-Si on SiO2 have been widely investigated. However, only poly-Si with small grains (2000 V/cm). EXPERIMENT In the experiment, a-Si films (50 nm thick) were deposited on quartz substrates using a molecular beam epitaxy system (base pressure: 5 x 10-11 Torr). Here a-Si films were deposited using an electron-beam gun at a rate of 0.1 nm/s with keeping the substrates at a room temperature. Subsequently, Ni films (15 nm thick) were deposited on the a-Si and then patterned into electrodes by using the photolithography technique. The spacing between the anode and the cathode was varied between 40 and 6000 µm. Finally, the samples were annealed at 525 and 550oC in an evacuated quartz tube. A DC bias was applied to the Ni electrodes using a DC power supply during annealing. Such experimental procedures are schematically shown in Fig. 1. The electric field strength E was estimated as E = V/d, where V is the applied voltage and d is the spacing between electrodes. The lateral growth lengths from Ni-pattern were evaluated by using Nomarski optical microscopy and scanning electron microscopy (SEM). RESULTS AND DISCUSSION Figures 2(a) and 2(b) show Nomarski optical micrographs of grown regions after annealing (525oC, 25 h) under the electric fields of 1.7 and 100 V/cm, respectively. Dark regions in the photographs show the anode and the cathode (Ni-patterns). Bright regions near the electrodes are the crystallized regions (poly-Si regions). It is clear that poly-Si regions at the cathode side were wider than those of the anode side, which agrees with the previous studies [12,13]. Jang et al. reported that Ni atoms in NiSi2 are negatively charged [14]. Recently, Yoon et al. calculated the averaged charge of a Ni atom in Si as – 0.33 e, where e is an electron charge [15]. These results suggest that migration of negatively charged Ni atoms is enhanced by an electric field, which results in the enhanced lateral crystallization at the cathode side. DC power supply (V)
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