Influence of grain boundaries on the properties of polycrystalline germanium

High-speed thin film transistors based on plastic substrates are indispensable to realize next-generation flexible devices. Here, we synthesized a polycrystalline Ge layer, which had the highest quality ever, on GeO2-coated substrates using advanced solid-phase crystallization at 375 °C. X-ray diffraction and Raman spectroscopy revealed that Ge on plastic had a compressive strain, while conventional Ge with a glass substrate had a tensile strain. This behavior was explained quantitatively from the difference in the thermal expansion coefficients between Ge and the substrate. Electron backscatter diffraction analyses showed that the Ge had large grains up to 10 μm, while many intragranular grain boundaries were present. The potential barrier height of the grain boundary was lower for the plastic sample than that for the glass sample, which was discussed in terms of the strain direction. These features resulted in a hole mobility (500 cm2/V s) exceeding that of a single-crystal Si wafer. The findings and knowledge will contribute to the development of polycrystalline engineering and lead to advanced flexible electronics.