Thermal conductivity of amorphous Si and Ge thin films
A Si and Ge films were deposited by magnetron sputtering with a laboratory-built combinatorial sputtering system at temperature of 25, 300, 500, and 600°C. The Si and Ge thin films prepared at each deposition temperature were 15, 100, and 250 nm thick. We characterized the crystalline states and microstructures of the thin films by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The XRD patterns and HRTEM images showed no evidence of crystallinity. We measured the thermal conductivity of the a-Si and a-Ge films by a frequency domain thermoreflectance method. For both types of films regardless of substrate temperatures, thermal conductivity increased with the film thickness. Furthermore, in a-Si and a-Ge thin films, thermal conductivity was dependent on deposition temperature. For the 250-nm-thick a-Si thin films, thermal conductivity increased sharply as deposition temperature was increased from 300 to 600°C. However, such deposition temperature dependence was not observed in the 100-nm-thick a-Si thin films. The a-Ge thin films of 100 and 250 nm in thickness exhibit thermal conductivity dependent on deposition temperature. As deposition temperature was increased, thermal conductivity increased more strongly in the a-Ge thin films of 250 nm in thickness than those of 100 nm in thickness. We concluded the deposition temperature and film thickness dependent thermal conductivity as a result of the increase of long mean free path (MFP) phonon with increased deposition temperature, and cut off of the phonon MFP by film thickness.