Three-Dimensionally Orientation-Controlled Ge Rods on an Insulator Formed by Low-Temperature Ni-Induced Lateral Crystallization

The carrier mobility of polycrystalline Ge(Sn) thin films has been updated significantly in recent years owing to the modulation of amorphous precursors in solid-phase crystallization. However, uncontrolled grain boundaries and crystal orientations are major problems associated with thin-film transistor (TFT) applications. To overcome these problems, we investigated the effects of amorphous Ge(Sn) precursors on metal-induced lateral crystallization using a Ni catalyst. In situ optical microscopy and Raman spectroscopy revealed that the densification of amorphous Ge promotes lateral crystallization by increasing the frequency factor rather than by reducing the activation energy. Low-temperature annealing at 325 °C allowed for the synthesis of large Ge rods (10 μm length, 2 μm width), which were Ni-free according to an energy-dispersive X-ray spectrometer. Furthermore, electron beam backscattering diffraction and transmission electron microscopy analyses revealed that the Ge rods grew in the ⟨110⟩ direction from the Ni pattern and were oriented in the {110} plane in the normal direction, resulting in the {100} orientation in the longitudinal sides of the Ge rods. Thus, we have developed three-dimensionally orientation-controlled Ge rods on insulators at low temperatures. This achievement will pave the way for TFTs with a stable and high-performance operation, enabling the development of advanced flexible devices.