Hybridization three subbands at Dirac point in special designed strained HgTe thin films with structural inversion asymmetry

Abstract

We study specially designed strained thin HgTe layers with structural inversion asymmetry (SIA) which allow us to distinguish the topological surface states (TSS) typical for a two dimensional (2D) quantum well system in a subband state. To obtain such a dispersion relation on the basis of the eight-band kp model, the theoretical investigation calculations of thin (below 25 nm wide) HgTe strained films with SIA are investigated. The numerical band-gap engineering and dispersion relation allow us to obtain a new class of materials that are characterized by a Dirac-like dispersion and hybridization of the three different charges describing two TSS and one quantum well subband at Γ-point (zero gap). This opens up many possibilities from the applications point of view. An external electric field removed this degeneration and opened a band gap between Γ _6 , ${{\Gamma}}_{8}^{\mathrm{l}\mathrm{h}}$ (lh—light hole) and ${{\Gamma}}_{8}^{\mathrm{h}\mathrm{h}}$ (hh—heavy hole) subbands characteristic for TSS and the subband characteristic for the 2D quantum well state, respectively. The width of the band gap as a function of the external electric field is also considered. Due to consideration being given to the possible applications, analysis of the dispersion relation and Landau levels (LL) energy shape with SIA is also investigated. The possibility of tuning a band gap is promising from the point of view of, for example, THz detectors and emitters. What is very important, the proposed structures allow the avoidance of the coexistence of TSS with bulk states, as very often occurs in so-called 3D strained HgTe-like materials. Analysis of the wave function as a function of the width of the investigated structure as well as the external electric field is also presented. Due to the strong correlation between both states (2D and TSS), and their very well known properties, we expect that such HgTe films can be used as optical active layers in the THz region.

Keywords

• topological insulator
• structural inversion asymmetry
• kp model