Large grained single-crystalline-like germanium thin film on flexible Ni–W tape

Abstract

Roll-to-roll processing of single-crystalline semiconductor thin films on low-cost flexible substrates is of high importance for flexible electronics and photovoltaic applications. In this paper we demonstrate roll-to-roll (R2R) heteroepitaxial deposition of single-crystalline-like Ge thin film on flexible cube-textured Ni–W metal substrates using an intermediate buffer layer of CeO₂. Strongly biaxially-textured Ge thin film with large grain sizes in the range of 30–60 μm was obtained. The Ge film exhibited (004) out-of-plane orientation and (111) in-plane orientation spread of 6.6°. Transmission Electron Microscopy (TEM) diffraction patterns and Electron Backscattered Diffraction (EBSD) mapping confirmed the single-crystalline-like nature and highly-oriented grain structure with low angle grain boundaries. Raman measurement showed the presence of only crystalline Ge phase with TO peak width of 4.3 cm⁻¹, close to that of single-crystal Ge wafer (3.8 cm⁻¹), confirming the high crystalline quality of the film. The Ge film was p-type and exhibited high carrier mobility of ∼690 cm² V⁻¹ s⁻¹. This alternative inexpensive, flexible and lightweight single-crystalline Ge thin film template, functionally nearly equivalent to single crystal Ge, may be a potential candidate for cost-effective R2R manufacturing of optoelectronic devices.