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Figure 1. Schematic illustration of a structure of /,c-Si:H. Region A is a crystalline grain cluster and region B is a disordered zone which surrounds region A.

Figure 1. Schematic illustration of a structure of /,c-Si:H. Region A is a crystalline grain cluster and region B is a disordered zone which surrounds region A.

temperature is taken to be around 200 °C. Higher plasma excitation frequency up to 100 MHz leads to a simultaneous increase of the deposition rate (commercially very important and is ~0.1 nm/s) [15, 16], the crystalline volume fraction (~80%), and the average grain size (~30 nm) [15]. It is known that atomic hydrogen (H) plays an important role for nucleation of crystallization, although the reason for this is not clear.

2.2. Hot-Wire Assisted CVD

Hot filament, such as tungsten (W) or tantalum (Ta) filament, assists catalytic decomposition of a silane and hydrogen gas mixture in this technique [17-19]. A wide range of deposition parameters is adjustable for the deposition. The most important parameter for this technique to produce nc-Si:H is the silane to hydrogen gas ratio (less than ~3% in gas ratio). A filament diameter, for example, is 250 /m and substrates are placed in the cm range from the filament whose temperature is kept around 1600 °C. Substrate temperature is around 200 °C. Much higher deposition rate in this technique than PECVD has been reported and it approaches ~3 nm/s depending on deposition conditions. Although overall qualitative electronic properties of HWCVD nc-Si:H are similar to those of PECVD nc-Si:H, the mobility-lifetime product, for example, is smaller in HWCVD samples than PECVD ones. This can be attributed to larger number of defects in HWCVD nc-Si:H films.

2.3. Pulsed Laser Crystallization

Crystallization of amorphous silicon films occurs at a liquidsolid phase induced by pulsed excimer laser heating (see, e.g., [20]). To reach the melting threshold, the laser energy, 160 mJ/cm2 for example, is required for the pulse width around 30 ns. This required energy is very small, since the region heated by irradiation is limited to near surface. A rapid crystallization with low processing temperature is achieved by this method and the grain size around 100 nm is obtained. Uniformity and reproducibility in crystallization may still be a problem.

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