Reversible Thermally Driven Phase Change of Layered In2Se3 for Integrated Photonics

Polymorphic phases in 2D In2Se3: fundamental properties, phase transition modulation methodologies and advanced applications

Two-dimensional (2D) In2Se3, which is a multifunctional semiconductor, exhibits multiple crystallographic phases, each of which possesses distinct electronic, optical, and thermal properties. This inherent phase variability makes it a promising candidate for a wide range of applications, including memory devices, photovoltaics, and photodetectors. This review comprehensively explores the latest progress of various polymorphic phases of 2D In2Se3, emphasizing their unique properties, characterization methods, phase modulation strategies, and practical applications. Commencing with a rigorous examination of the structural attributes inherent in its various phases, we introduce sophisticated techniques for its characterization. Subsequently, modulation strategies, encompassing variations in temperature, application of electric fields, induced stress, and alterations in pressure, are explored, each exerting an influence on the phase transitions in 2D In2Se3. Finally, we highlight recent advancements and applications resulting from these phase transitions, including homoepitaxial heterophase structures, optical modulators, and phase change memory (PCM). By synthesizing insights into phase properties, modulation strategies, and potential applications, this review endeavours to provide a comprehensive understanding of the significance and prospects of In2Se3 in the semiconductor field.

High-Performance Photoresponse and Nonvolatile Photomemory Effect in a Partially Gated MoS2/α-In2Se3 Heterojunction Photodetector

2D ferroelectric materials exhibit exceptional properties such as atomically thin layers, strong ferroelectric polarization, high carrier mobility, and suitable band gaps, making them highly promising for electronics, optoelectronics, ferroelectronics, and nonvolatile memory applications. In this study, we present a partially gated MoS2/α-In2Se3 heterojunction photodetector that allows the manipulation of ferroelectric polarization in α-In2Se3 by both gate and drain voltages. The device shows excellent photoresponse performance and a nonvolatile photomemory effect. It achieves a maximum photoresponsivity of ∼698 A/W at 405 nm, ∼1210 A/W at 473 nm, ∼1076 A/W at 515 nm, and ∼1306 A/W at 638 nm. By enhancing the ferroelectric polarization of α-In2Se3, the photoresponsivity, external quantum efficiency, and detectivity are greatly improved. The photomemory effect has an extremely long retention time exceeding 4000 s with an expected information retention rate of over 95% after 10 years. The device performance is a result of the combined effects of vertical-electric-field-controlled ferroelectric polarization, horizontal-electric-field-controlled ferroelectric polarization, and light-induced depolarization. The proposed partially gated MoS2/α-In2Se3 heterostructure provides a new perspective for designing 2D ferroelectric devices with significant potential for optoelectronics and nonvolatile memory applications.

Femtosecond Laser Manipulation of Multistage Phase Switching in Two-Dimensional In2Se3 Visualized via an In Situ Transmission Electron Microscope

Phase transitions critically determine material properties for applications, making them central to material science. The two-dimensional (2D) van der Waals material In2Se3 has been extensively studied as a model system for multiphase switching due to its intricate phase transition behaviors and outstanding ferroelectric properties for device applications. However, the lack of an efficient method for precise phase control and the poorly defined conditions for multiphase transitions have severely hindered its practical use. Here, we report that the femtosecond (fs) laser can serve as a potent tool for fast and precisely manipulating multiphase transitions in In2Se3 thin flakes. Using a transmission electron microscope capable of in situ fs laser irradiation, we realize controllable fast phase switching between four phases of 2D In2Se3 by controlling the laser fluence, including the transition from the ferroelectric α phase to the antiferroelectric β' or paraelectric β phase, reversible switching between antiferroelectric β' and paraelectric β phases at room temperature, as well as reversible transformation between the ferroelectric α' phase and antiferroelectric β' or paraelectric β phase at liquid nitrogen temperature. Notably, these multiphase transitions are accompanied by rapid formation and annihilation of domain structures and superlattices, resulting in fast changes in electric conductivity. Our first-principles calculations verify the multiphase transition pathways and reveal that the conductivity change stems from electronic band structure variation among the different phases. This work systematically investigates the phase transition behaviors in In2Se3 through spatially and temporally resolved characterization methods, providing foundational insights into memory device optimization.

Phase Tailoring of In2Se3 Toward van der Waals Vertical Heterostructures via Selenization of γ-InSe Semiconductor

The polymorphic nature of In2Se3 leads to excellent phase-dependent physical properties including ferroelectricity, photoelectricity, and especially the intriguing phase change ability, making the precise phase modulation of In2Se3 of fundamental importance but very challenging. Here, the growth of In2Se3 with desired-phase is realized by temperature-controlled selenization of van der Waals (vdW) layered bulk γ-InSe. Detailed results of Raman spectroscopy, scanning electron microscopy (SEM), and state-of-the-art spherical aberration-corrected transmission electron microscopy (Cs-TEM) clearly and consistently show that β-In2Se3, 3R α-In2Se3, and 2H α-In2Se3 can be perfectly obtained at ≈270, ≈300, and ≈600 °C, respectively. Further comprehensive atomic imaging analyses confirm that the seeding material, InSe, plays a critical role in the low-temperature epitaxial growth of vdW-layered In2Se3, and, more interestingly, β-In2Se3 acts as an intermediate phase between 3R and 2H α-In2Se3 transitions. This investigation not only provides a simple yet versatile strategy for the phase modulation of In2Se3, but also sheds light on the temperature-dependent phase evolution of In2Se3.

Melting-free integrated photonic memory with layered polymorphs

Chalcogenide-based nonvolatile phase change materials (PCMs) have a long history of usage, from bulk disk memory to all-optic neuromorphic computing circuits. Being able to perform uniform phase transitions over a subwavelength scale makes PCMs particularly suitable for photonic applications. For switching between nonvolatile states, the conventional chalcogenide phase change materials are brought to a melting temperature to break the covalent bonds. The cooling rate determines the final state. Reversible polymorphic layered materials provide an alternative atomic transition mechanism for low-energy electronic (small domain size) and photonic nonvolatile memories (which require a large effective tuning area). The small energy barrier of breaking van der Waals force facilitates low energy, fast-reset, and melting-free phase transitions, which reduces the chance of element segregation-associated device failure. The search for such material families starts with polymorphic In2Se3, which has two layered structures that are topologically similar and stable at room temperature. In this perspective, we first review the history of different memory schemes, compare the thermal dynamics of phase transitions in amorphous-crystalline and In2Se3, detail the device implementations for all-optical memory, and discuss the challenges and opportunities associated with polymorphic memory.

Photodetectors integrating waveguides and semiconductor materials

Photodetectors integrating substrates and semiconductor materials are increasingly attractive for applications in optical communication, optical sensing, optical computing, and military owing to the unique optoelectronic properties of semiconductor materials. However, it is still a challenge to realize high-performance photodetectors by only integrating substrates and semiconductor materials because of the limitation of incident light in contact with sensitive materials. In recent years, waveguides such as silicon (Si) and silicon nitride (Si3N4) have attracted extensive attention owing to their unique optical properties. Waveguides can be easily hetero-integrated with semiconductor materials, thus providing a promising approach for realizing high-performance photodetectors. Herein, we review recent advances in photodetectors integrating waveguides in two parts. The first involves the waveguide types and semiconductor materials commonly used to fabricate photodetectors, including Si, Si3N4, gallium nitride, organic waveguides, graphene, and MoTe2. The second involves the photodetectors of different wavelengths that integrate waveguides, ranging from ultraviolet to infrared. These hybrid photodetectors integrating waveguides and semiconductor materials provide an alternative way to realize multifunctional and high-performance photonic integrated chips and circuits.