Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In₂Se₃, β-In₂Se₃, γ-In₂Se₃, etc.). In this work, the 2D α-In₂Se₃ film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In₂Se₃ surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In₂Se₃ phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In₂Se₃ to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).

In this work, we demonstrated the dependence of crystallographic characteristics and electronic structures on indium amounts in the indium–zinc–selenium alloy fabricated by vapour-phase deposition. By taking advantage of molecular beam epitaxy and a synchrotron radiation apparatus, it is shown for the first time that the significant difference in vapour pressure between zinc and indium induced zinc deficiency in the solid state and resulted in a transition of five distinct phases in the whole range of the alloy from zinc selenide to indium selenide. The binary ZnSe with indium dopants, the ZnSe-rich alloy with a minority of ZnIn₂Se₄, the disordered structure owing to the comparable mixture of ZnSe and ZnIn₂Se₄, the ZnIn₂Se₄-rich alloy with a minority of ZnSe, and the binary In₂Se₃ phase are separated by indium incorporation ratios of 0.01, 0.02, 0.18, and 0.69, respectively. Moreover, a pronounced carrier localisation effect was observed in the ZnSe-rich and the disordered mixture. The widely distributed mid-gap states were attributed to a high density of zinc vacancies in the ZnSe-based lattice. In brief, these fundamental properties show the feasibility of Se-based II–III₂–VI₄ alloys for optoelectronics.