Thermal studies of individual Si/Ge heterojunctions — The influence of the alloy layer on the heterojunction

Phonon transport across an interface is of fundamental importance to applications ranging from electronic and optical devices to thermoelectric materials. The phonon scattering by an interface can dramatically suppress the thermal transport, which can benefit thermoelectric applications but create problems for the thermal management of electronic/optical devices. In this aspect, existing molecular dynamics simulations on phonon transport across various interfaces are often based on estimates of atomic structures and are seldom compared with measurements on real interfaces. In this work, planar Si/Ge heterojunctions formed by film-wafer bonding are measured for the interfacial thermal resistance (RK) that is further compared with predictions from existing simulations and analytical models. The twist angle between a 70-nm-thick Si film and a Ge wafer is varied to check the influence of the crystal misorientation. Detailed transmission electron microscopy studies are carried out to better understand the interfacial atomic structure. It is found that the alloyed interfacial layer with mixed Si and Ge atoms dominates the measured thermal resistance (RK). Some oxygen impurities may also help to increase RK due to the formation of glassy structures. Following this, RK reduction should be focused on how to minimize the interdiffusion of Si and Ge atoms during the formation of a Si/Ge heterojunction.