Indirect to Direct Gap Crossover in Two-Dimensional InSe Revealed by Angle-Resolved Photoemission Spectroscopy

Abstract

Atomically thin films of III-VI post-transition metal chalcogenides (InSe and GaSe) form an interesting class of two-dimensional semiconductor that feature strong variations of their band gap as a function of the number of layers in the crystal 1-4 and, specifically for InSe, an earlier predicted crossover from a direct gap in the bulk 5,6 to a 1 arXiv:1901.06943v1 [cond-mat.mtrl-sci] 21 Jan 2019 weakly indirect band gap in monolayers and bilayers. 7-11 Here, we apply angle resolved photoemission spectroscopy with submicrometer spatial resolution (µARPES) to visualise the layer-dependent valence band structure of mechanically exfoliated crystals of InSe. We show that for 1 layer and 2 layer InSe the valence band maxima are away from the Γ-point, forming an indirect gap, with the conduction band edge known to be at the Γ-point. In contrast, for six or more layers the bandgap becomes direct, in good agreement with theoretical predictions. The high-quality monolayer and bilayer samples enables us to resolve, in the photoluminescence spectra, the band-edge exciton (A) from the exciton (B) involving holes in a pair of deeper valence bands, degenerate at Γ, with the splitting that agrees with both µARPES data and the results of DFT modelling. Due to the difference in symmetry between these two valence bands, light emitted by the A-exciton should be predominantly polarised perpendicular to the plane of the two-dimensional crystal, which we have verified for few-layer InSe crystals. KeywordsARPES, indium selenide, 2D materials, density functional theory, photoluminescence, spinorbit coupling Two-dimensional materials (2DM) and their van der Waals heterostructures, constructed by the mechanical assembly of individual 2D crystals, have a great potential for optoelectronic applications. 12 The fast growing family of 2DM 13 includes 2D insulators, 2D semiconductors with various band gaps, 2D metals and even 2D superconductors, with electronic and optical properties that often differ from their bulk allotropes. 14 In this family, post-transition metal monochalcogenides (PTMC), III-VI compounds such as GaSe and InSe, are emerging as important materials to study, due to their interesting layer-dependent optical properties and exceptionally high carrier mobility. [1][2][3][4]8,15,16 Both GaSe and InSe display a pronounced quantum confinement effect: an increase of the band gap upon decreasing the number of layers, L, which is stronger in InSe 3,11 than in GaSe 17 films as revealed recently by photolu-

Ultra-thin van der Waals crystals as semiconductor quantum wells

“…Indium selenide is a new material in the 2DS family. It has a layer-dependent bandgap spanning from 1.2 eV for bulk to almost 3.0 eV for a monolayer [15,16,6] and features a crossover [17,18,13] from weakly indirect (in mono-, bi-and tri-layer) to direct character (in crystals thicker than 4 layers). Its high crystal quality has recently been demonstrated, resulting in exceptionally high electron mobility [6] making it a promising avenue for developing atomically thin nanoelectronics [19].…”

“…While THz spectroscopy gives access to direct measurements of some intersubband transitions, it only works in a small energy window and requires a specific doping of the materials studied. At the same time, angleresolved photoemission spectroscopy (ARPES) only shows filled (valence) subbands and has diminished energy resolution when applied to micrometre-size crystals [13]. Here, we present a promising experimental approach that enables a comprehensive experimental study of the full subband structure of atomically thin 2DS, namely resonant tunneling spectroscopy combined with photoluminescence excitation (PLE) measurements.…”

Raman spectroscopy of GaSe and InSe post-transition metal chalcogenides layers

“…In addition to widely popular semiconducting transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, MoTe2, WS2 and WSe2, a new class of LMs, semiconducting post-transition metal chalcogenides (PTMCs) are increasingly studied. [2][3][4][5][6] These materials feature a large variation of the optical bandgap with number of layers, N: from 1.25 eV in bulk to 2.8 eV in monolayer (1L) InSe 3,4,6 and from 2.0 eV (bulk) to 2.4 eV (2L) in GaSe. 5 Furthermore, the band-gap evolves from being quasi-direct for 1L to direct in bulk.…”

“…in InSe mobilities correspondingly up to 10 3 cm 2 /(Vs) and 10 4 cm 2 /(Vs) at room and liquid-helium temperatures, 4 as well as one-dimensional quantization of electrons by electrostatic gating, 7 and unusual photoluminescence, polarized primarily out of the basal plane. 6 These materials can be combined into PTMC/PTMC or PTMC/TMD layered materials heterostructures (LMHs), with type-II band alignment and allowing direct optical transitions in reciprocal space, 8 and offering an even larger selection of emission energies, 5 with potential for novel optoelectronic applications in a broad spectral range from far infra-red to violet.…”

“…9 However the most commonly found polytypes are ε-GaSe with a unit cell containing 8 atoms and two layers thick, 5 and -InSe, with a unit cell extending over 3 layers, containing 12 atoms. 6 Raman spectroscopy is the prime non-destructive characterization tool for graphene and LMs. [10][11][12][13] In LMs there are 2 fundamentally different sets of Raman modes.…”