Nanocalorimetric high-temperature characterization of ultrathin films of a-Ge

The crystallization temperature of ultrathin films of a-Ge sandwiched between SiO2 layers increases with decreasing thickness and also with the heating rate. Based on this premise we used ultrafast heating to probe the amorphous-to-liquid transition in ultrathin films of amorphous Ge. Upon cooling, the melt solidifies into nanocrystalline Ge. We show that sensitive nanocalorimetric measurements can provide kinetic and thermodynamic information involved during the amorphous-to-liquid- and nanocrystalline-to-liquid-phase transitions in the Ge system. A 3-nm a-Ge ultrathin layer is condensed from an e-beam evaporator under high vacuum conditions onto the 180 nm thick free-standing SiNx membranes of the calorimeters that form the calorimetric cells. The a-Ge film is sandwiched between a-SiO2 layers to minimize heterogeneous nucleation at the interfaces. Nanocalorimetric measurements are carried out in-situ inside the e-beam setup. The apparent heat capacity (Cp) of the a-Ge layers is extracted measuring the power released to the sample at fast heating rates (104-105 K/s), in the temperature range from 350 to 1200 K. © 2006.