Phase control can prepare large-area two-dimensional In2Se3 single crystal film

On December 12, 2022, the team of Prof. Zhao Jiong and Prof. Yang Ming from The Hong Kong Polytechnic University and Prof. Shuhui Li from the City University of Hong Kong published an article entitled “Phase-controllable large-area two-dimensional In2Se3 and ferroelectric heterophase junction” in the journal Nature Nanotechnology Research results.

Through short-range chemical vapor deposition and phase transition principle, the research group realized the preparation of in2Se3 single crystal films with centimeter size β and β′ and α three phases, and the field-effect transistor devices based on the three phases showed excellent mobility and storage performance.

The corresponding authors of the paper are Zhao Jiong, Yang Ming, Li Shuhui; The first authors are Han Wei, Zheng Xiaodong, and Yang Ke.

Two-dimensional ferroelectric semiconductors (2D-FES) with spontaneous polarization and atomic thickness are considered to be one of the best material systems for future “storage-computing” integrated brain-like devices and non-von Neumann computing architectures. In the reported 2D-FES, α-In2Se3 has a suitable band gap (1.39 eV), high electron mobility, and in-plane out-of-plane coupling ferroelectricity as thin to monolayer. In addition, In2Se3 is a multiphase material, in which β-In2Se3 has paraelectric (PE), β′-In2Se3 has antiferroelectric (AFE), and α-In2Se3 has ferroelectricity (FE), which provides convenience for realizing ferroelectric heterojunction devices. However, the uncontrollable phase of In2Se3 and the micron-scale crystal size seriously restrict its research on basic physical properties and its application in large-scale integrated devices. How to achieve large-area and phase-controlled growth of In2Se3 films are two major problems in the field of two-dimensional materials today.

Recently, the team of Professor Zhao Jiong and Professor Yang Ming of the Hong Kong Polytechnic University and Professor Li Shuhui of the City University of Hong Kong reported a strategy for controlling two-dimensional In2Se3 single crystal films with centimeter size based on chemical vapor deposition (CVD) and phase transition principle (Figure 1). First, they prepared a centimeter-sized two-dimensional β-In2Se3 single crystal film by CVD. Then, inspired by the results of β-InSe phase change to β′-In2Se3 in in situ EM experiments, a centimeter-sized β′-In2Se3 film was obtained by adding β-InSe to the precursor (Figure 2). The seed effect of InSe and Se vacancies are the two principles that achieve β→β′. In in situ Raman and in situ transmission electron microscopy experiments, they found that β′-In2Se3 was transformed into α-In2Se3 by stress release, achieving a phase transition from β′ to α (Figure 3, Figure 4, Sci. Adv. article). Inspired by this mechanism, they transferred the β′-In2Se3 film to an uneven substrate and successfully realized the large-area preparation of α-In2Se3 film. Finally, an in-plane β′-α heterogeneous junction was prepared by transferring the β′-In2Se3 film to the gold particle array.

Figure 1: Phase controlled synthesis of large-area two-dimensional In2Se3 films.

Figure 2: Phase control mechanism of a two-dimensional In2Se3 film.

Figure 3: Phase transition relationship of two-dimensional In2Se3 films derived from in situ TEM.

Figure 4: Phase transition from β′-In2Se3 to α-In2Se3 by in situ TEM.

The prepared two-dimensional In2Se3 film has good electron mobility and storage performance. Among them, FET devices prepared from β′ and α-In2Se3 films have large memory window, long durability and good cycle stability. Thanks to ferroelectric coupling, β′-α heterojunctions exhibit a larger hysteresis window than the α phase and better non-volatile memory performance (Figure 5).

Figure 5: Conceptual diagram of a device with a 2D α-β′ in-plane out-of-phase junction.

In this work, In2Se3 single crystal films with three phases, paraelectric, ferroelectric and antiferroelectric, with centimeter-sized scale, were successfully prepared by revealing the phase transition mechanism of β→β′ and β′→α based on CVD method. At the same time, it also realizes the excellent performance of single-phase FET devices and out-of-phase junction FET devices, which lays a solid foundation for the realization of storage-computing integrated ferroelectric devices in the future. (Source: Science Network)

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