III-Nitride Based Visible Single-Photon Sources
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A quantum dot (QD) is a nanometer-scale inclusion of a smaller bandgap semiconductor material inside a larger bandgap one, where electrons and holes occupy only a given set of discrete energy levels. Thus, a QD can act like an atomic two-level system, where the relaxation from an excited state to ground state releases a single photon. Single photons are essential for quantum cryptography, where information is encoded in the quantum state of a single photon such as its polarization. Single photon sources also have applications in quantum computation, and metrology.
III-nitride quantum dots, with their wide bandgap and large exciton binding energy, have been used as single-photon sources up to room temperature but only through optical excitation. In this work, two different quantum dot architectures have been investigated: the InGaN/GaN disk-in-nanowire and InGaN/GaN self-organized QD to demonstrate a practical single-photon source, one that emits single photons at room temperature upon electrical excitation.
We have investigated the optical and structural properties of the InGaN/GaN disk-in-nanowire heterostructures, grown on silicon by molecular beam epitaxy (MBE). A single-dot-in-nanowire light emitting diode (LED) was fabricated on Si substrates. The single nanowire LED exhibits single-photon emission upon both optical and electrical excitation. By engineering the confinement in the quantum dot and improving the electrical performance of the diode, single-photon emission, with a g(2)(0) = 0.3 was observed from a single InGaN quantum dot emitting in the green spectral range at 125 K. Additionally, a self-organized quantum dot-based single photon diode with g(2)(0) = 0.29 at room temperature was demonstrated. The g(2)(0) value in both cases indicates an almost three-fold decrease in probability of multi-photon emission events compared to a classical emitter. On-demand single-photon emission at an excitation repetition rate of 200 MHz was achieved, by electrical pumping of the quantum dot.