Tunable quasiparticle band gap in few-layer GaSe/graphene van der Waals heterostructures

Aziza, Zeineb Ben; Pierucci, Debora; Henck, Hugo; Silly, Mathieu G.; David, Christophe; Yoon, Mina; Sirotti, Fausto; Xiao, Kai; Eddrief, M.; Girard, Jean-Christophe; Ouerghi, Abdelkarim

doi:10.1103/physrevb.96.035407

Cited by 112 publications

(113 citation statements)

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“…We have further investigated the electronic properties of nominal bilayer GaSe films by photoemission electron momentum microscopy (k-PEEM) and confirmed the Mexican-hat shaped dispersion in the upper valence band. 20 These results demonstrate that MBEgrown GaSe films can be an ideal platform for investigation of new physics in atomically thin monochalcogenides.…”

Section: Introductionmentioning

confidence: 88%

“…Therefore, we speculate that our MBE-grown film contains a lower concentration of vacancies or impurities associated with p-type doping than the bulk counterparts or n-type GaSe grown on epitaxial graphene. 20 In order to have a finer look at the upper valence band features of bilayer GaSe, we performed k-PEEM measurements at a higher analyzer energy resolution of~100 meV and focused on the topmost upper valence band (Γ 1 + ) near the Γ point. 12 The acquired band structures and the second-derivative band structures are shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…8 The film morphology and uniformity are also difficult to control. Spiral and island growth modes were commonly reported for epitaxial GaSe films grown by MBE, 15,[18][19][20] making it more difficult to control the film thickness, especially in the atomically thin limit. While oscillations in reflection high energy electron diffraction (RHEED) patterns during the growth of GaSe on mica were used to determine the thickness, indicating that layer-by-layer growth could be achieved, 21 the morphology evolution, particularly in atomically thin layers was not addressed at all.…”

Section: Introductionmentioning

confidence: 99%

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Large-grain MBE-grown GaSe on GaAs with a Mexican hat-like valence band dispersion

et al. 2018

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Atomically thin GaSe has been predicted to have a non-parabolic, Mexican hat-like valence band structure due to the shift of the valence band maximum (VBM) near the Γ point which is expected to give rise to novel, unique properties such as tunable magnetism, high effective mass suppressing direct tunneling in scaled transistors, and an improved thermoelectric figure of merit. However, the synthesis of atomically thin GaSe remains challenging. Here, we report on the growth of atomically thin GaSe by molecular beam epitaxy (MBE) and demonstrate the high quality of the resulting van der Waals epitaxial films. The full valence band structure of nominal bilayer GaSe is revealed by photoemission electron momentum microscopy (k-PEEM), confirming the presence of a distorted valence band near the Γ point. Our results open the way to demonstrating interesting new physical phenomena based on MBE-grown GaSe films and atomically thin monochalcogenides in general.

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Section: Introductionmentioning

confidence: 88%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Large-grain MBE-grown GaSe on GaAs with a Mexican hat-like valence band dispersion

et al. 2018

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“…For this reason, previous ARPES experiments on monolayer metal monochalcogenides have used materials grown by molecular beam epitaxy. [18][19][20][21] Most transport and optical investigations instead use mechanically exfoliated flakes which, though higher-quality, are typically only a few µm across. We have recently demonstrated that sub-micrometer spatially resolved ARPES (µARPES) enables high resolution measurements from mechanically exfoliated flakes.…”

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confidence: 99%

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

et al. 2019

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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-

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“…GaSe is a layered metal monochalcogenide crystal with two planes of Ga atoms sandwiched by two planes of Se atoms in each primitive layer. Unlike the well‐studied layered transition metal dichalcogenides, the thinning of GaSe from bulk to few‐layers will drive a direct‐to‐indirect bandgap transition, and the bandgap could be tuned reliably by controlling the layer numbers and doping levels in GaSe's van der Waals heterostructures . As a well‐known nonlinear crystal over the spectral range from visible to terahertz, GaSe's monolayer has a second‐order nonlinear coefficient about one order of magnitude larger than that of monolayer MoS 2 .…”

Section: Introductionmentioning

confidence: 99%

Multiple Optical Frequency Conversions in Few‐Layer GaSe Assisted by a Photonic Crystal Cavity

Fang

Yuan

Fang

et al. 2018

Advanced Optical Materials

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those with third-order nonlinearity. [2,3] Recently, 2D materials with a thickness of few atomic layers have been revealed as a suitable candidate for OFCs, presenting a variety of advantages over bulk materials: 1) The low dimensionality of 2D materials gives rise to reduced dielectric screening of Coulomb interactions between charge carriers, allowing their strong light-matter interactions and exotic excitonic effects. [4,5] The second-order nonlinear susceptibility χ (2) could be extraordinarily high. A value of χ (2) ≈ 1000 pm V −1 is estimated in a monolayer WSe 2 , which is about three orders of magnitude higher than those in conventional bulk materials. [6,7] 2) Strong exciton charging effect in 2D materials allows for control over the oscillator strengths at the exciton and trion resonances, promising an electrical modulation of the exciton-enhanced second harmonic generation (SHG) as well as the corresponding χ (2) by one order of magnitude. [8] 3) χ (2) is a third-rank tensor and therefore must vanish if the material has an inversion symmetry. When the layered materials are thinned down to 2D materials, the crystal structures are determined by the layer number strongly, leading to variations of the inversion symmetry. It is reported few-layer MoS 2 or boron nitride (BN) with odd (even) numbers of layers present appreciable (negligible) SHGs, showing an obvious layer-dependent oscillation. [9,10] In addition, 2D materials' crystal structures could also be governed by their layer stacking orders or spiral angles, [11][12][13][14] which provides more degrees of freedom to engineer their second-order nonlinearity. 4) And besides, 2D materials' intriguing electrical properties and ability of chip-integration make them have benefits in constructing optoelectronic devices, such as high-performance photodetectors, modulators, and light-emitting diodes. [15][16][17][18][19][20][21] It would facilitate device applications of 2D materials' second-order nonlinearities.While materials with χ (2) could support a variety of OFCs, 2D materials' prominent second-order nonlinearities have only been illustrated by the realizations of SHGs. There are challenges to achieve other OFCs, such as sum-frequency generation (SFG), difference-frequency generation (DFG), and spontaneous downconversion (SPDC). First, 2D materials are so thin that a normal incident light could not couple with them efficiently. Hence, in the reported SHGs of 2D materials, a pump of pulsed laser was essential to provide an extremely high peak power. This requirement makes it difficult to realize While 2D materials have intriguing second-order nonlinearities with ultrahigh coefficient and electrical tunability, their atomic layer thicknesses hinder explorations of other optical frequency conversions (OFCs) than second harmonic generation (SHG) due to inefficient light-coupling and unachievable phase-matching. By resonantly pumping a photonic crystal cavity integrated with a few-layer GaSe, it is reported that it is possible to realize multiple second-ord...

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Tunable quasiparticle band gap in few-layer GaSe/graphene van der Waals heterostructures

Cited by 112 publications

References 49 publications

Large-grain MBE-grown GaSe on GaAs with a Mexican hat-like valence band dispersion

Large-grain MBE-grown GaSe on GaAs with a Mexican hat-like valence band dispersion

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

Multiple Optical Frequency Conversions in Few‐Layer GaSe Assisted by a Photonic Crystal Cavity

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