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The control of the crystal phase in self-catalyzed nanowires (NWs) is one of the central remaining open challenges in the research field of III/V semiconductor NWs. While several groups analyzed and revealed the growth dynamics, no experimental growth scheme has been verified yet, which reproducibly ensures the phase purity of binary self-catalyzed grown NWs. Here, we demonstrate the advanced control of self-catalyzed molecular beam epitaxy of GaAs NWs with up to a grade of 100% wurtzite (WZ) phase purity. The evolution of the most important properties during the growth, namely, the contact angle of the Ga droplet, the NW length, and the diameter is analyzed by scanning electron microscopy and transmission electron microscopy. Based on these results, we developed a comprehensive NW growth model for calculating the time-dependent evolution of the Ga droplet contact angle. Using this model, the Ga flux was dynamically modified during the growth to control and stabilize the contact angle in a certain range favoring the growth of phase-pure GaAs NWs. Although focusing on the self-catalyzed growth of WZ GaAs NWs, our model is also applicable to achieve phase-pure zinc blende (ZB) NWs and can be easily generalized to other III/V compounds. The self-catalyzed growth of such NWs may pave the way for substantial improvement of GaAs NW laser devices, the controlled growth of WZ/ZB quantum disks, and novel heterostructured core/multishell NW systems with a pristine crystalline order.

Defects in wide-bandgap semiconductors are promising qubit candidates for quantum communication and computation. Epitaxially grown II-VI semiconductors are particularly promising host materials due to their direct bandgap and potential for isotopic purification to a spin-zero nuclear background. Here, we show a new type of single photon emitter with potential electron spin qubits based on Cl impurities in ZnSe. We utilize a quantum well to increase the binding energies of donor emission and confirm single photon emission with short radiative lifetimes of 192 ps. Furthermore, we verify that the ground state of the Cl donor complex contains a single electron by observing two-electron satellite emission, leaving the electron in higher orbital states. We also characterize the Zeeman splitting of the exciton transition by performing polarization-resolved magnetic spectroscopy on single emitters. Our results suggest single Cl impurities are suitable as single photon source with potential photonic interface.