| Amorphous semiconductors, such as amorphous Si (a-Si) or amorphous Ge (a-Ge), must have thermal stability in order to be useful for applications, including solar cells and thin-film transistors. Crystallization of amorphous structures can have a catastrophic effect upon the performance of these devices.;In this respect, the crystallization of a-Si or a-Ge when they are in contact with metals presents a challenging problem. For example, while a-Si normally crystallizes at about 600-700$spcirc$C, it crystallizes at about 180$spcirc$C when it is in contact with Al. This catalytic crystallization of amorphous semiconductors is sometimes called Metal-Contact Induced Crystallization (MCIC). The mechanism of MCIC is, however, not clearly understood.;We studied MCIC in the a-Si/Al, a-Si/Ag, and a-Ge/Ag layered thin films using in-situ cross-section transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Cross-section samples were heated inside the TEM and the reaction was followed in real time.;During the low-temperature crystallization, our in situ TEM observation showed that the metal phase is always present between the amorphous matrix and the growing crystalline semiconductor phase, and that the metal phase was observed to "migrate" toward the amorphous matrix, as the crystalline phase grows from behind. Furthermore, the in situ high-resolution TEM observations showed that the lattice points of the metal grains are stationary during the migration. These observations suggest that the growth of the semiconductor phase is brought about by the diffusion of the semiconductor elements through the metal, while the migration of the metal phase is caused by the self-diffusion of metal elements towards the amorphous matrix.;We, then, extended our study on MCIC to the crystallization of amorphous carbon. It is shown that the presence of cobalt can catalyze the crystallization of sputter-deposited amorphous carbon at 500-600$spcirc$C.;It is suggested that the mechanism, where a metal phase acts as a transport medium for semiconductor atoms, provides the shortest reaction path for the amorphous semiconductor to reduce its excess free energy. |
