Strain-Induced Bandgap Enhancement of InSe Ultrathin Films with Self-Formed Two-Dimensional Electron Gas

Abstract

Atomically thin indium selenide (InSe) is a representative two-dimensional (2D) family that have recently attracted extensive interest for their intriguing emerging physics and potential optoelectronic applications with high-performance. Here, by utilizing molecular beam epitaxy and scanning tunneling microscopy, we report a controlled synthesis of InSe thin films down to the monolayer limit and characterization of their electronic properties at atomic scale. Highly versatile growth conditions are developed to fabricate well crystalline InSe films, with a reversible and controllable phase transformation between InSe and In 2 Se 3 . The band gap size of InSe films, as enhanced by quantum confinement, increases with decreasing film thickness. Near various categories of lattice imperfections, the band gap becomes significantly enlarged, resulting in a type-I band alignments for lateral heterojunctions. Such band gap enhancement, as unveiled from our first-principles calculations, is ascribed to the local compressive strain imposed by the lattice imperfections. Moreover, InSe films host highly conductive 2D electron gas, manifesting prominent quasiparticle scattering signatures. The 2D electron gas is self-formed via substrate doping of electrons, which shift the Fermi level above the confinement-quantized conduction band. Our study identifies InSe ultrathin film as an appealing system for both fundamental research and potential applications in nanoelectrics and optoelectronics.

Nearly Ideal Two-Dimensional Electron Gas Hosted by Multiple Quantized Kronig–Penney States Observed in Few-Layer InSe

“…Figure b displays an STM image of the as-grown monolayer InSe on a highly oriented pyrolytic graphite (HOPG) substrate (Figure S1). The inset of Figure b shows an atomic-resolution STM image revealing the closed packed arrangement of the top-layer Se atoms, with a lattice constant of 4.0 ± 0.1 Å similar to that reported in previous theoretical and experimental works. − The STS is an effective method to measure the DOS in real space. In the STS experiments, we can directly obtain the high-resolution d I /d V signal by using the lock-in amplifier, which should reflect the local DOS of the sample.…”

“…These results indicate that the effective mass of the carrier can be controlled by varying the number of InSe layers, which may further control carrier mobility in the 2DEG system and lead to device applications. Moreover, since the quantum confinement effect in the InSe layer is very strong, ,− the induced high-quality 2DEG on the surface in combination with high carrier mobility provides an effective quantum Hall platform. ,− …”

Properties, Synthesis, and Device Applications of 2D Layered InSe

“…For 2D InSe, different single-crystalline substrates were used in the MBE method, such as GaAs(100), and graphene. [142,[144][145][146] For example, Poh et al developed an MBE method to synthesize 2D InSe on a graphene substrate. [144] In this work, In 2 Se 3 and Se powers were used as the precursors, and the monolayer graphene on SiO 2 /Si is used as the substrate.…”

Growth of Nanometer-Thick γ-InSe on Si(111) 7 × 7 by Molecular Beam Epitaxy for Field-Effect Transistors and Optoelectronic Devices

“…The scalability down to a single layer accompanied by the BEOL-friendly synthesis parameters makes InSe a promising candidate as a channel material in high-performance, ultrathin body field-effect transistors (FET). , To date, exfoliation from a single crystal is the most commonly used approach to study the electronic properties of InSe, ,,,,− while large-scale synthesis with precisely controlled thickness is still required for realistic industry purposes. Thin film synthesis techniques like pulsed laser deposition, , physical vapor deposition, , metal–organic chemical vapor deposition, , and molecular beam epitaxy (MBE) have successfully been demonstrated for InSe thin film growth. − MBE has advantages over other methods in growing high-quality crystalline InSe films, as it excels in precise control of atomic flux, growth temperature, and deposition rate. Despite the strength of precise control of growth parameters, growing a phase pure InSe film generally requires an extensive growth window mapping process.…”