Quantum Light Generation from Bound Excitons in ZnSe

Abstract

Quantum light sources and spin-based qubits are essential building blocks for on-chip scalable quantum computation and information processing. To achieve scalability, information-storing qubits should exhibit long coherence times. These qubits should also be efficiently interfaced with information-carrying single photons. Semiconductors are not only able to host such qubits and single photon sources but also, they offer a platform to interface them with the help of photonic structures. Hence, optically active solid-state qubits such as quantum dots, crystal defects and color centers have been extensively studied to date in various semiconductors. However, we still lack a suitable platform to satisfy all the requirements needed to realize a scalable quantum technology.

Impurities in epitaxially grown ZnSe are particularly promising single photon sources and qubit candidates due to the direct bandgap of the material and potential for isotopic purification to achieve nuclear spin-zero background. These impurities possess impurity bound electrons that can serve as spin-qubit. They also form impurity-bound excitons that can generate single photons. Various impurities have been studied in ZnSe, but only F impurities have been isolated as single emitters to date. Despite the great potential suggested by previous results, there are many impurities waiting to be explored for their quantum capabilities.

In this thesis, we study isolated Cl impurities in ZnSe for their photon emission and spin properties. We utilize a ZnMgSe/ZnSe/ZnMgSe quantum well to increase the binding energies bound excitons and to better separate donor bound exciton emission from the free excitons. In the PL spectrum, we observe narrow emission lines around 440 nm, which are originated from the single bound excitons. We calculate the average binding energy as 15 meV (at least 2 times higher than bulk values) and inhomogeneous broadening as 6 meV. We confirm the single photon emission by observing clear photon antibunching in the second order autocorrelation measurements. The time-resolved photoluminescence measurements show short radiative lifetimes of 192 ps. Our results demonstrate first time isolation of donor impurities in an unstructured ZnSe and provide complete characterization of radiative properties single Cl bound excitons.

The bound electron of a donor impurity atom can serve as a spin qubit. We verify that the presence of ground state electron of the Cl donor complex by observing two electron satellite emission. We also characterize the Zeeman splitting of the exciton transitions by performing polarization-resolved magnetic spectroscopy on the single emitters.

We also discover the presence of single biexcitons bound to Cl impurities. We demonstrate a radiative cascade from the decay of bound biexcitons. The emission exhibits both single photon statistics and clear temporal correlations revealing the time–ordering of the cascade. Finally, we discuss the design of nanophotonic cavities in the ZnSe platform. We develop a nanofabrication recipe to create suspended photonic crystal cavities. Then, we optically characterize the fabricated cavities.

The results presented in this thesis provide the first complete study of single Cl impurities in ZnSe. Based on the results discussed, single Cl impurities in ZnSe manifest themselves as promising quantum light sources and appealing solid-state qubit candidates.