Progress and perspectives on polycrystalline germanium thin films: Advances in solid-phase crystallization
Kaoru Toko, Kenta Moto, Toshifumi Imajo, Takamitsu Ishiyama, Koki Nozawa, S. Maeda, Kota Igura, Takashi Suemasu
Abstract
Polycrystalline germanium (Ge) thin films have reemerged as promising materials for next-generation electronic and optoelectronic devices because of their superior electrical and optical properties. However, challenges such as high defect densities, small grain sizes, and unstable n-type conduction have limited their practical application. In this paper, we review the progress in the solid-phase crystallization of Ge thin films on insulating substrates. We first discuss conventional processes with excessive nucleation that lead to poor crystallinity and electrical properties. We then introduce advanced solid-phase crystallization strategies, highlighting the critical role of controlling the amorphous deposition temperature to control atomic density, increase grain size, and reduce acceptor defects. Additionally, the incorporation of tin (Sn) has been shown to passivate grain boundaries, while the modulation of film thickness and the introduction of interfacial layers have demonstrated the ability to mitigate carrier scattering at the substrate interface. This review also addresses impurity doping techniques for precise Fermi level control, strain engineering effects on grain boundary barrier energies, and metal-induced lateral crystallization for grain alignment. These innovations have culminated in achieving record-high carrier mobilities of 690 cm2 V−1 s−1 for p-type and 450 cm2 V−1 s−1 for n-type films, approaching single-crystal performance. The insights provided herein will pave the way for the development of high-performance Ge-based thin-film transistors and flexible electronic devices via low-temperature processing.