ABSTRACT
Single photon sources are a critical resources for quantum information science and technologies [1]. Although methods to generate on-demand single photons have been developed for many years on a wide variety of material platforms including atomic vapors, ion traps, crystals, and semiconductors, the quest for a practical single photon source is still ongoing [2]. The challenges include not only finding materials with appropriate properties to enable a precise control of the quantum properties of light, but developing a scalable manufacturing technology at both the device and system levels. In semiconductors, on-demand single photons can be generated from quantum dot (QD) structures with very high brightness and indistinguishability [3–5]. In QDs, the electrons and holes are prohibited from moving freely, resulting in quantized energy states, each of which can be occupied by up to two electrons or holes with opposite spins. If the Coulomb interaction between the electrons and holes are strong enough such that two electrons and two holes form stable exciton and biexciton states, the energy of the biexciton can be spectrally discernible from that of the exciton such that single photon emission from the exciton (or the biexciton) state can be observed and utilized. The stability of excitons is, therefore, important for semiconductor QDs to operate effectively and efficiently as single photon sources. The exciton stability is characterized by its binding energy, which depends on the effective masses of the electrons and holes, the dielectric constant of the QD materials, and the QD potential profile. In bulk III-nitride semiconductors, the exciton binding energy is 28 meV. In QDs, this value can be even larger due to quantum confinement [6]. In comparison, the exciton binding energy in GaAs or InP is only a few meV. Therefore, excitons are much more stable in III-nitride QDs at an elevated temperature. Observation of single photon emission in III-nitride QDs has been made at room temperature by several groups [7,8]. Theoretical prediction of exciton binding energy as large as 1.4 eV has been shown in InN nanowires [9]. The goal of this chapter is to discuss the potentials and challenges for III-nitride QDs as a material of choice for single photon sources for practical applications. The large band gap tunability of the InGaN alloy, between 0.36 and 1.9 µm [10], makes GaN-based single photon sources technologically relevant, 662both for free-space [11,12] and fiber-based [13,14] applications. We will specifically focus on the aspect of manufacturing scalability which is often overlooked but is critical from a technology point of view.
