Field-induced dissociation of two-dimensional excitons in transition metal dichalcogenides

Abstract

Generation of photocurrents in semiconducting materials requires dissociation of excitons into free charge carriers. While thermal agitation is sufficient to induce dissociation in most bulk materials, an additional push is required to induce efficient dissociation of the strongly bound excitons in monolayer transition-metal dichalcogenides (TMDs). Recently, static in-plane electric fields have proven to be a promising candidate. In the present paper, we introduce a numerical procedure, based on exterior complex scaling, capable of computing field-induced exciton dissociation rates for a wider range of field strengths than previously reported in literature. We present both Stark shifts and dissociation rates for excitons in various TMDs calculated within the Mott-Wannier model. Here, we find that the field induced dissociation rate is strongly dependent on the dielectric screening environment. Furthermore, applying weak-field asymptotic theory (WFAT) to the Keldysh potential, we are able to derive an analytical expression for exciton dissociation rates in the weakfield region. arXiv:1905.02571v1 [cond-mat.mes-hall]

Interlayer excitons in van der Waals heterostructures: Binding energy, Stark shift, and field-induced dissociation

“…Turning to the dissociation rates, it is clear that the encapsulating media significantly alter how quickly interlayer excitons are dissociated. The same behaviour was observed for their monolayer counterparts 30 , and one therefore has a large degree of freedom in device design. For extremely weak fields, the dissociation rates are very low, but they grow rapidly with an increasing field strength.…”

“…For extremely weak fields, the dissociation rates are very low, but they grow rapidly with an increasing field strength. As an example, the dissociation rate of interlayer excitons in freely suspended MoS 2 /WS 2 is around Γ ≈ 1.7 × 10 4 s −1 already at E = 10 V/µm, and only Γ ≈ 5.3 × 10 −38 s −1 and Γ ≈ 2.7 × 10 −33 s −1 for monolayer MoS 2 and WS 2 , respectively 30 . It should be noted that dissociation rates of intralayer excitons are only important if they are comparable to the rate at which intralayer excitons tunnel over to interlayer excitons.…”

“…The partitioning of the radial domain is efficiently dealt with by resolving the radial part of the eigenstate in a finite element basis consisting of Legendre polynomials, and the angular part in a cosine basis. We have previously used the same numerical procedure to calculate dissociation rates for excitons in various monolayer TMDs 30 , and we refer the interested reader to that paper for the technical details of the method. The field induced dissociation rates and Stark shifts for interlayer excitons in the six van der Waals heterostructures are shown in Fig.…”

“…Without inducing dissociation in some way, excitons will typically recombine before they are dissociated. Applying an in-plane electric field to the excitons, however, enhances generation of photocurrents for two reasons: (i) the electric field counteracts recombination by pulling electrons and holes in opposite directions, and (ii) the electric field assists dissociation of excitons [27][28][29][30] .When two TMD monolayers are brought together with a type-II band alignment, the conduction band minimum and the valence band maximum reside in two different layers. Electrons and holes in the structure will therefore prefer to reside in separate layers, provided that the loss in exciton binding energy is smaller than the energy gained by band offsets.…”

Phonon-assisted exciton dissociation in transition metal dichalcogenides

“…To reach optimal operation of these devices, exciton dissociation mechanisms and their tunability need to be understood. Previous studies have investigated exciton dissociation via tunneling to the continuum in presence of strong electric fields [33,34], showing that field-induced dissociation dominates the response at large fields needed to dissociate excitons with large binding energies [35]. However, the dissociation of excitons via scattering with phonons, which is expected to play a major role at low electric fields, has been overlooked so far-despite its potential implications on the fundamental efficiency limits of TMD-based photodetectors and solar cells.…”