High-electron-mobility (370 cm²/Vs) polycrystalline Ge on an insulator formed by As-doped solid-phase crystallization

High-electron-mobility polycrystalline Ge (poly-Ge) thin films are difficult to form because of their poor crystallinity, defect-induced acceptors and low solid solubility of n-type dopants. Here, we found that As doping into amorphous Ge significantly influenced the subsequent solid-phase crystallization. Although excessive As doping degraded the crystallinity of the poly-Ge, the appropriate amount of As (~10²⁰ cm^-3) promoted lateral growth and increased the Ge grain size to approximately 20 μm at a growth temperature of 375 °C. Moreover, neutral As atoms in poly-Ge reduced the trap-state density and energy barrier height of the grain boundaries. These properties reduced grain boundary scattering and allowed for an electron mobility of 370 cm²/Vs at an electron concentration of 5 × 10¹⁸ cm^-3 after post annealing at 500 °C. The electron mobility further exceeds that of any other n-type poly-Ge layers and even that of single-crystal Si wafers with n ≥ 10¹⁸ cm^-3. The low-temperature synthesis of high-mobility Ge on insulators will provide a pathway for the monolithic integration of high-performance Ge-CMOS onto Si-LSIs and flat-panel displays.