Two-step crystallization during the reverse aluminum-induced layer exchange process

In this work, the reverse aluminum-induced layer exchange (R-ALILE) process with an initial layer stack of substrate/amorphous Si/Si-oxide/Al was studied in detail. The influence of the annealing temperature on the sample properties was investigated by optical reflection/transmission measurements and Raman spectroscopy. In addition, focused ion beam measurements were conducted to elucidate the inner structure of the layers. Two steps during crystallization were observed: at first a substrate/Al-Si composite/closed poly-Si layer structure is formed with an activation energy EA poly-Si =1.1 eV, which can be transferred to the stable configuration of substrate/ Al+Si -islands (hillocks)/poly-Si by extended annealing or a high temperature step (EA hillocks =2.4 eV). Both processes are basically independent at low annealing temperatures due to the large difference in activation energy. The transformation of the Al-Si composite to the Al/Si-hillock structure involves the crystallization of a-Si regions and their subsequent coalescence, different to the feedback mechanism suggested for the normal ALILE process, where hillock and closed poly-Si growth are believed to influence each other. This insight into the process leads to the possibility to prepare poly-Si layers on pure Al back contacts by R-ALILE, possibly improving efficiencies of solar cells prepared by epitaxial overgrowth of poly-Si seed layers.