Charge-transfer excitons in atomically thin GaN quantum wells*

We investigate the properties of spatially indirect excitons (IXs) confined in pairs of atomically thin GaN wells, separated by polar AlN barriers. Atomically thin GaN is a promising material for realizing strongly bound excitons because of its extreme quantum confinement effect. Also, the spontaneous polarization fields in nitride heterostructures allow IXs to form in atomically thin GaN quantum wells even without external electric fields. We show that the overlap of the electron and hole wavefunctions, the degree of electron-hole interaction, and the character (IX or DX) of the lowest-energy exciton can be controlled by changing the thickness and the resulting electrical polarization of the separating AlN barrier. We demonstrate that room-temperature stable IXs, with radiative decay rates several orders of magnitude lower than DXs, can be realized in these atomically thin polar nitride heterostructures for potential excitonic applications at room temperature based on a commercial semiconductor platform.