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Metal induced crystallization
The metal induced crystallization of amorphous silicon is part of the seed layer concept. According to this concept, large grain polycrystalline silicon films are first generated through crystallization and then thickened by epitaxial silicon growth. The thickening of the polycrystalline seed layers avoids the challenge of low temperature deposition of crystalline thin films directly onto an amorphous surface.
The preparation of the silicon seed layers is usually conducted in two step
- First, an amorphous silicon film is deposited onto the substrates by means of chemical or physical vapor deposition
- Second, the amorphous silicon layer is transformed into a polycrystalline silicon layer.
Two low temperature crystallization techniques have previously been the focus of investigation:
- (SPC - solid phase crystallization)
- (LC - Laser crystallization).
A proposed alternative to these techniques is the aluminium-induced layer exchange (ALILE). ALILE takes advantage of the aluminium induced crystallization of amorphous silicon which occurs far below the eutectic temperature of the silicon/aluminium system at 577°C. This method is well suited for the low temperature approach to silicon thin film solar cell processing due to the following :
- Process temperatures below 550°C
- Glass substrates can be used
- Application of industry relevant technologies is possible (thermal evaporation, sputter deposition)
- Relatively short process duration (~30 min)
- Simplicity
The images display several steps of the ALILE process. The entire process leads to an exchange of layers during which a continuous polycrystalline silicon thin film is formed. At temperatures of about 500°C the entire process takes about 30min. The aluminium at the sample surface can be etched off selectively such that the poly-Si film can be used for further processing according to the seed layer concept.
Presently, metals other then aluminium are also investigated as candidates for metal induced crystallization techniques.
Figures: Cross sectional picture obtained from focused ion beam microscopy of four different a-Si/Al/Glas stacks. (a) Before the anneal, (b) after anneal (c) 10 min and (d) 60 min anneal at 500°C; the samples are tilted by 45º.
(Fig. a) Before the ALILE process is started, the sample consists of the glass substrate onto which an aluminium film is thermally evaporated before a thin amorphous silicon layer is sputtered on top.
(Fig. b) When the structure is annealed at a temperature betwen 350°C and 550°C, silicon atoms diffuse into the aluminium film. Therein, small silicon crystallites will eventually form at aluminium grain boundaries or the at the aluminium-silicon interface.
(Fig. c) The crystallites will grow as long as silicon atoms diffuse from the amorphous phase into the aluminium and eventually they become large grains which substitute the aluminium layer. The growth of the silicon grain is limited only by the glass substrate and the Al/a-Si interface.
(Fig. d) Once the interfaces are reached, the grains will grow laterally until the reach neighbouring grains.
