Charge control of defect colour centres in hexagonal boron nitride

Abstract

Colour centres in hexagonal boron nitride (hBN) have gained significant interest as they are active at room temperature and embedded in a two-dimensional material. The latter leads to minimal total internal reflection and coupling to photonic devices with nanometre proximity. Colour centres in hBN are particularly interesting as they have applications in quantum information, communications, metrology and LED technology. Current research has largely focused on photoluminescence from these defects. However, the integration of solid-state emitters into electronics necessitates the defects to be electrically driven. It is therefore essential to understand charge control of these defects. Recently, it was discovered that the charge state of defects in hBN can be controlled by interfacing it with graphene. It was observed that charge transfer between optically active defects in hBN and graphene led to the quenching of photoluminescence from these emitters. Therefore, interfacing hBN with graphene offers a pathway to inject carriers into hBN for the electrical control of defects. This is especially useful as graphene, also being a two-dimensional material, is a suitable gate to apply vertical electric fields across emitters in hBN. In this thesis, the study of charge transfer between hBN and graphene has been ex plored both from a theoretical and experimental standpoint. The theoretical approach involves using density functional theory to perform electronic structure calculations of de fects in hBN and subsequently deriving defect properties, such as band structure, defect geometry, and formation energies. The results of these calculations allowed the determina tion of the degree of charge transfer between defects in hBN and graphene. Furthermore, periodic boundary conditions and the choice of methodology for the assignment of charge to different layers had a significant impact on the derived value of charge transfer. A methodology to determine charge transfer that was robust against these effects was de veloped. The impact of the thickness of encapsulating hBN and graphene layers on charge transfer and the thermodynamics of the defect was also explored. The energy path for the Charge control of defect colour centres in hexagonal boron nitride reorientation of the defect between two equilibrium structures was studied to determine metastable intermediate states. It was found that the energy ordering of different phases of a defect was sensitive to the encapsulation of the host layer by graphene and hBN. As such, encapsulating layers can impact the spectroscopic properties of the defect. Experimentally, site control of emitters in hBN was studied. This involved the growth of an aluminium oxide (Al2O3) spacer between graphene and hBN to mitigate the quench ing of emitters. The Al2O3 was then patterned using electron beam lithography and etched to create an array of pillars. hBN was then deposited on top hosting emitters, with the aim of emitters being quenched in regions where there was direct contact with graphene and active in regions where an intermediate Al2O3 layer was present. Emitters were found predominantly on the edges of the pillars and quenching occurred in the regions where the pillars were absent. Hence spatial contol over colour centres in hBN was achieved.