Large-area optoelectronic-grade InSe thin films via controlled phase evolution

Indium monoselenide (InSe) is an emerging two-dimensional semiconductor with superlative electrical and optical properties whose full potential for high-performance electronics and optoelectronics has been limited by the lack of reliable large-area thin-film synthesis methods. The difficulty in InSe synthesis lies in the complexity of the indium-selenium phase diagram and inadequate understanding of how this complexity is manifested in the growth of thin films. Herein, we present a systematic method for synthesizing InSe thin films by pulsed laser deposition followed by vacuum thermal annealing. The controlled phase evolution of the annealed InSe thin films is elucidated using a comprehensive set of in situ and ex situ characterization techniques. The annealing temperature is identified as the key parameter in controlling phase evolution with pure thin films of InSe developed within a window of 325 °C to 425 °C. To exert finer stoichiometric control over the as-deposited InSe thin film, a co-deposition scheme utilizing InSe and In2Se3 pulsed laser deposition targets is employed to mitigate the effects of mass loss during annealing, ultimately resulting in the synthesis of centimeter-scale, thickness-tunable e-InSe thin films with high crystallinity. The optimized InSe thin films possess a strong optoelectronic response, exhibited by phototransistors with high responsivities up to 103 A/W. Additionally, enhancement-mode InSe field-effect transistors are fabricated over large areas with device yields exceeding 90% and high on/off current modulation greater than 104, realizing a degree of electronic uniformity previously unattained in InSe thin-film synthesis.