Ultrafast electron dynamics reveal the high potential of InSe for hot-carrier optoelectronics

Abstract

We monitor the dynamics of hot carriers in InSe by means of two photons photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to the electronic states degenerate with theM valley. Subsequently, the electronic cooling is dictated by Fröhlich coupling with phonons of small momentum transfer. Ab-initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with theM valley could travel through a multilayer flake of InSe with lateral size of 1 micrometer. The hot carriers pave a viable route to the realization of below-bandgap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides. PACS numbers:Van der Waals chalcogenides display a variety of different specifics that depend on their composition and number of layers. The weak mechanical binding of the atoms along the stacking direction facilitates the realization of heterostructures with different functionalities. Some recent achievements have been: the fine tuning of the band gap 1-3 , the control of valley polarization 4 and the realization of devices with high mobility 5-7 . In this context, InSe is one of the building blocks with the highest potentials.The bulk crystals of InSe can be thinned down to a few layers and encapsulated in hBN 7 . By these means, Bandurin and collaborators have fabricated transitors whose quality is high enough to observe Shubnikovde Haas oscillations and quantum Hall effect 7 . These results point out the two aspects making InSe particularly appealing. On one hand, the mobility of charge carriers rivals with the one measured in graphene 8 . On the other hand, the bulk band gap of 1.26 eV, is ideally suited for optoelectronic devices. Indeed, several groups have recently reported that InSe 10,11 and InSegraphene heterostructures 12 have excellent photoresponsivity in the visible spectral region. Such photodetectors could be patterned on a large scale over flexible supports 13 . Eventually, the application of larger bias can drive the photodetector in the avalanche regime 14 .The conception of devices based on InSe would greatly profit from a precise knowledge of the transient state following photoexcitation. Some pioneering experiments have investigated the coherent propagation and dephasing of the exciton-polariton 15,16 . These optical meth-ods revealed a beating polarization induced by resonant pulses, but provided no insights on the energy relaxation of hot carriers. Here, we address this issue by mapping the dynamics of excited electrons in reciprocal space 17,18 . Our two photon photoemission (2PPE) data show that photoexcitation above theM valley results in a high density of electrons with excess energy of ∼ = 0.6 eV and cooling time of 2 ps. First principle calculations of the electron-phonon coupling discriminate between the distinct scattering mechani...

Evidence of direct electronic band gap in two-dimensional van der Waals indium selenide crystals

“…This result suggests that InSe has a direct electronic bandgap at Γ point. In accord, pump-probe dynamics performed in this system have shown a dispersion of unoccupied states with CBM in the center of Brillouin zone31 .In summary, we have presented an exhaustive study of the electronic band structure of the InSe crystal combining spectroscopic tools and theoretical investigation (DFT). The joint analysis of the DOS using STS and of the band structure using 2PPE indicates that the InSe crystal presents a direct quasiparticle band gap of about 1.25 eV located at the  point of the BZ.…”

“…The three-dimentional ε-polytype of InSe (space group 3ℎ 1 in Schoenflies notation) has been build using the lattice parameters a=3.9553 Ǻ, dM-M= 2.741 Ǻ, dX-X= 5.298 Ǻ and c=16.64 Ǻ and AB stacking [8][9][10][11][12][13] . These values are very closed to experimental ones and no influence of such a small difference is expected.…”

“…Its crystal structure consists in covalently-bound "X-M-M-X" sheets, stacked vertically through van der Waals (vdW) interaction [7][8][9] The electronic band structure of many two-dimensional (2D) vdW materials changes as the thickness is reduced down to a few layers 10 . One well known example is the indirect to direct gap transition that occurs at monolayer thickness of Mo and W transition metal dichalcogenides (TMDs) 11 .…”

“…In bulk, these individual InSe layers are held together by vdW forces 29 . Here, high-quality InSe single crystals were synthesized by the flux method [30][31][32] . In order to study the structural and electronic properties of our sample, we carried out STM/STS measurements at low temperature of 4.2 K. An atomic lattice image, as shown in Figure 1 Figure 1(c).…”

Ultrafast dynamics of hot carriers in a quasi–two-dimensional electron gas on InSe

“…The choice of polar material is motivated by the fact that InSe is one of the best van der Waals structures for the fabrication of FET devices 15,16 . It has an electronic gap comparable to silicon 17 , small effective mass 18 , layered structure 19 and carrier mobility higher than transition metal dichalcogenides 16 . The electronic state and distribution function of hot electrons in the accumulation layer is directly monitored by time and angle resolved photoelectron spectroscopy (tr-ARPES).…”