Influence of the Permeable Membrane

The permeable membrane (barrier layer) plays a crucial role for the ALILE process. Usually, the barrier layer is formed by exposure of the Al-coated glass substrate to air. The thickness of this barrier layer (A1 oxide), which is on a nanometer scale, can be influenced by the variation of the exposure time. With increasing exposure time, the thickness of the barrier layer increases. The thicker the barrier layer, the lower is the nucleation density and the longer is the process time necessary to form a continuous poly-Si film [19]. By X-ray photoelectron spectroscopy (XPS) measurements it was shown that the barrier layer stays in place during the whole ALILE process. 

In order to demonstrate the importance of the barrier layer for the layer exchange process, an initial glass/Al/a-Si stack without a barrier layer was prepared [50]. To prevent the formation of a barrier layer the a-Si was deposited directly onto the A1 (without vacuum break). The SEM images in Fig. 12.11 make the influence of the barrier layer clear. Two samples are shown - one with barrier layer (by exposure to air for 2 h) (left hand side) and one without barrier layer (right hand side). Both samples were annealed for 45 min at 480° C. After the annealing step, the A1 was etched off chemically. To show the cross section as well as the surface, the samples were tilted by 30° for the SEM measurements. The sample with barrier layer (left) shows a continuous poly-Si film with Si islands on top (similar to what is shown in Fig. 12.5), whereas the sample without barrier layer (right) does not show a continuous poly-Si film but a porous film structure. The former interface between the A1 layer and the a-Si layer is not visible. In some parts, the 

In addition to the experiments with very thin barrier layers, rather thick barrier layers have also been investigated [51]. The thick barrier layers were formed by thermal oxidation of the Al surface (e.g., for 2h at 560°C). With such a thick barrier layer, an estimated average grain size of above 200 pm was reached. But at the corresponding Ta of 450°C, it took days to form a continuous poly-Si film. The influence of a barrier layer formed by thermal oxidation on the preferential orientation of the poly-Si surface was already described (see Fig. 12.10). 

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