Abstract Body: In this work, we investigate the properties of spatially indirect excitons (IXs) confined in pairs of atomically think 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 performed first-principles calculations based on density functional theory and many-body perturbation theory to investigate the properties of IXs and DXs in pairs of atomically thin GaN quantum wells separated by polar AlN layers with varying thickness. Our calculation shows 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. For realization of IXs in atomically-thin GaN quantum wells, molecular beam epitaxial growth was performed on an n-type Si substrate using a Veeco GEN II system with radio frequency plasma-assisted nitrogen source. The epitaxy includes ~100 nm long GaN nanowires which serves as a template for the subsequent epitaxy of ~150 nm long AlN nanowires. Then, monolayer GaN quantum well/AlN barrier/monolayer GaN quantum well heterostructure was grown on top of AlN nanowires. The thickness of AlN barrier was varied by changing the growth duration. Figure 1 shows the photoluminescence spectra of two samples with 1 ML and 7 MLs thick AlN barriers incorporated, respectively. The emission peak around 5.93 eV can be attributed to the excitonic emission from AlN. DX emission dominates in the spectrum of sample with 7 MLs thick AlN barrier. In comparison, there’s a clear shoulder emerging on the lower energy side observed in the sample with 1 ML thick AlN barrier, which can be assigned to indirect exciton (IX) emission. The work is supported by the University of Michigan College of Engineering Blue Sky Research Program. W.L. was partially supported by the Kwanjeong Educational Foundation Scholarship. Computational resources were provided by the DOE NERSC facility.

Deep Ultraviolet Luminescence and Charge-Transfer Excitons in Atomically Thin GaN Quantum Wells

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Deep Ultraviolet Luminescence and Charge-Transfer Excitons in Atomically Thin GaN Quantum Wells

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