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

Abstract

Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of dipolar excitons. We demonstrate the inplane confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface. This result is a prerequisite for the electrical control of the exciton densities and fluxes, as well for studies of the complex phase diagram of these dipolar bosons at low temperature. Keywords exciton fluid, electrostatic traps, cooling, gallium nitride Dipolar excitons, Coulomb-bound but spatially separated electron-hole pairs, have a long life-time and a built-in dipole moment that offer an opportunity for the cooling and electrical 1 arXiv:1902.02974v1 [physics.app-ph] 8 Feb 2019 control of exciton fluids. 1-7 Various intriguing quantum phenomena including Bose-Einsteinlike condensation, darkening and superfluidity of excitons have been recently reported. 8-15Albeit demonstrated at very low temperatures, those phenomena are promising for better understanding of new states of matter, but also for potential applications in excitonic devices with novel functionalities. 7The recent emergence of high quality wide-bandgap semiconductor quantum wells (QWs) and two-dimensional Van der Waals heterostructures, hosting dipolar excitons with large exciton binding energies and built-in electric fields, has given a new impetus to this research. [16][17][18][19][20] Room temperature exciton transport in GaN/(AlGa)N QWs has been demonstrated, 17 as well as its electrical control in MoS 2 −WSe 2 heterostructures. 20 The latter is

Complexity of the dipolar exciton Mott transition in GaN/(AlGa)N nanostructures

“…In GaN/(Al,Ga)N structures studied in this paper and up to 9 T, we seem to deal with the opposite limit: χ 1. Numerical modeling of the IX transport within a drift-diffusion model (see Appendixes) [32,47] allows us to estimate that the enhancement of the exciton mass M(B)/M does not exceed a few percent at B < 9 T, consistent with estimations that can be done in the χ < 1 regime [63,64]:…”

“…Such decrease of the emission energy and intensity indicates that the exciton density decreases. The relation between PL energy and IX density is particularly strong in wide QWs due to interactions between IXs that have large dipole moments, and the resulting screening of the electric field along the growth direction: the highest density under the pumping spot corresponds to the maximum screening (lowest electric field) and thus to the highest 045308-3 emission intensity and energy [32,47,50,51]. The interactioninduced increase of the IX energy reaches 40 meV (10 meV) at the spot center and at highest (lowest) power densities that we could explore in our experiments.…”

“…The interactioninduced increase of the IX energy reaches 40 meV (10 meV) at the spot center and at highest (lowest) power densities that we could explore in our experiments. This shift can be used to estimate the IX density as n = E int /φ 0 , where φ 0 = 1.5 × 10 −13 eV cm −2 is the coefficient extracted from the selfconsistent solution of the Schrödinger and Poisson equations (plate capacitorlike model) [32,47,50,51]. This estimate needs to be improved by taking into account the exciton binding energy (which decreases when the IX density increases) and excitonic correlations, which become significant at low temperatures and high densities.…”

“…We have recently demonstrated the electrostatic trapping and control of the cold the two ways to study IX emission: IX in a trap (upper sketch)pointlike excitation followed by spatially separated PL analysis in the trap (used in Ref. [32]) and free space (lower sketch)-broad excitation followed by detection in the center of the excitation spot (used in this paper).…”

“…IX density in such structures [32]. IXs at a quasithermal equilibrium state could be studied in spatial regions situated tens of micrometers away from the laser excitation spot.…”