(a) 100 µm - Helmholtz-Zentrum Berlin

26 State of the art
Figure 2.13: ALILE growth model suggested by O. Nast [53]. Step 1: Si dissolution and
transport into the Al layer. Step 2: Si diffusion in the Al layer. Step 3: Si nucleation.
Step 4: growth of Si grain.
This model was extended by P. Widenborg et al. [55]. They considered also the
process in the top layer (Fig. 2.14). While process steps 1, 2 and 3 correspond to
the steps suggested by O. Nast, here step 4 describes the diffusion of Al into the
top layer. Al is displaced by Si grains and forced to diffuse into the a-Si top layer
(step 4a). The a-Si crystallizes in presence of the Al and small Si crystallites form
in the Al top layer. The Al in this mixed phase dissolves further a-Si and grows
(step 4b) competing with the Si growth in the bottom layer in this model. While
the poly-Si in the bottom layer grows supplied by step 5, silicon is also crystallized
in the top layer. Small nanocrystalline Si (nc-Si) crystallites will agglomerate at the
more compact central Si islands, even when the actual layer exchange is completed
(step 6)
The formation of a stable c-Si phase within the Al in the top layer can be thought
of as formation of idealized ’double cylinder’ shaped Al/c-Si-clusters in top view.
A sketch of such a ’double cylinder’ embedded in the a-Si matrix is shown in
Fig. 2.15(a). Fig. 2.15(b) shows several of these ’double cylinders’ corresponding
to the optical micrograph in Fig. 2.15(c) [56] obtained at the top layer.
The driving force of the entire process was assumed by O. Nast to be the chemical
potential difference between the amorphous and crystalline phase (Fig. 2.16). It
is proposed that the Al layer is in equilibrium with both the amorphous and the
crystalline silicon phase at the interfaces. Then the difference of the chemical
potentials leads to silicon diffusion within the aluminum from the a-Si/Al interface
to the Al/c-Si interface.