Metal-contact-induced crystallization of semiconductors

The crystallization of amorphous semiconductors in amorphous Si (a-Si)/Al, a-Si/Ag, and amorphous Ge (a-Ge)/Ag layered thin films has been investigated by cross-section in situ transmission electron microscopy and differential scanning calorimetry. In all cases, the amorphous semiconductors were found to crystallize at substantially lower temperatures than those without the metal contact. During the reaction, the metal phase always exists between the amorphous and crystalline semiconductor phases, migrating into the amorphous matrix, leaving the crystalline phase behind. This observation strongly indicates that the semiconductor atoms diffuse through the metal grain from amorphous to crystalline phases, which was confirmed by in situ high resolution recordings. We suggest that this mechanism provides the fastest reaction path for the system to reduce its excess free energy, as the direct rearrangement of the amorphous tetrahedral networks cannot occur at such low temperatures.