Transport of indirect excitons in ZnO quantum wells

Abstract

We report on spatially-and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.An indirect exciton (IX) in a semiconductor quantum well (QW) structure is composed of an electron and a hole confined to spatially separated QW layers. IXs were realized in wide single quantum wells (WSQW) [1][2][3][4] and in coupled quantum wells (CQW) [5][6][7][8] [2-4, 6, 7]. Their long lifetimes allow IXs to travel over large distances before recombination, providing the opportunity to study exciton transport by optical imaging [9][10][11][12][13][14][15] and explore excitonic circuit devices based on exciton transport, see [16] and references therein.Materials with a high IX binding energy allow extending the operation of the excitonic devices to high temperatures [17][18][19]. Furthermore, such materials can allow the realization of high-temperature coherent states of IXs [19]. These properties make materials with robust IXs particularly interesting. However, so far, studies of IX transport mainly concerned GaAs-based CQW. In this paper, we probe transport of IXs in ZnO/MgZnO WSQW structures. IXs in these structures are much more robust than in GaAs structures: their binding energy ∼ 30 meV [4] is considerably higher than that in GaAs/AlGaAs and GaAs/AlAs CQW (∼ 4 and ∼ 10 meV, respectively [7,20]). The binding energy of IXs is smaller than that of excitons in bulk ZnO (∼ 60 meV), however it is large enough to make the IXs stable at room temperature. At the same time, the measurements reported in this work show that transport lengths of IXs in WSQW ZnO structures reach ∼ 4 µm. In comparison, for excitons in bulk ZnO and direct excitons in ZnO structures, transport lengths are within ∼ 0.2 µm [21,22].In this work, we study polar and semipolar ZnO/MgZnO QW structures. The samples were grown by molecular beam epitaxy as in Refs. [4,23] Fig. 1a. The charges on the interfaces between ZnO and MgZnO result in a built-in electric field in the structure, which is stronger for polar samples [4,23]. The built-in electric field pulls the electron and the hole toward opposite borders of the QW, resulting in the spatial separation required for an IX.

Trapping Dipolar Exciton Fluids in GaN/(AlGa)N Nanostructures

“…This means that here the carrier densities are low enough, so that exciton radiative emission rate does not grow exponentially with density, as it is usually observed in polar heterostructures. 16,17,[25][26][27][28] In conclusion, we have shown that dipolar excitons can be efficiently trapped in the plane of GaN/(AlGa)N QWs. In previous works on GaN QWs hosting dipolar excitons, the mutual repulsion of excitons led to a fast radial expansion and dilution of the exciton gas, accompanied by dramatic variation of the exciton energy and lifetime.…”

Room-Temperature Transport of Indirect Excitons in(Al,Ga)N/GaNQuantum Wells

“…These properties make materials with robust IXs particularly interesting. Nevertheless, so far, even in these materials with robust excitons there were no evidence of room-temperature transport [17,18].…”

“…Moreover, they possess strong permanent dipole moments along the growth axis, which is essential for electrical control of excitonic fluxes, and for excitonic device operation [7][8][9][10][11]. IXs were most extensively studied in GaAs-based QWs [2,5,[12][13][14][15][16], but recently their realisation in wide bandgap semiconductors such as GaN [17], and ZnO [18], has been considered. GaN/(Al,Ga)N and ZnO/(Zn,Mg)O wide QWs grown along the (0001) crystal axis, naturally exhibit built-in electric fields up to MV/cm [19,20], so that IXs are naturally created in the absence of any external electric field.…”

Indirect excitons in van der Waals heterostructures at room temperature

“…IXs were explored in various III-V and II-VI semiconductor QW heterostructures based on GaAs 2,3,[8][9][10][11][12][13][14] , AlAs 15,16 , InGaAs 17 , GaN [18][19][20] , and ZnO 21,22 . Among these materials, IXs are more robust in the ZnO structures where their binding energy is about Van der Waals structures composed of atomically thin layers of TMD offer an opportunity to realize artificial materials with designable properties, forming a new platform for studying basic phenomena and developing optoelectronic devices 4 .…”