Hexagonal boron nitride: Investigations towards a single-photon source
Creators: name: Browning, Alexander Rowan
Publisher: name: University of Surrey
Year: 2025

description: This thesis aims to advance the application of hBN as a single-photon source by investigations of native
description: hexagonal boron nitride (hBN) single-photon emitters (SPE), unreported luminescent hBN-substrate
description: layers, and the positioning of emitters using an atomic force microscopy (AFM) nano-indentation
description: method.
description: Mechanically exfoliated mono-, bi-, and multi-layer hBN flakes are thermally annealed to optically
description: activate native defects in the hBN crystal lattice, which are known to produce SPE. Photoluminescence
description: (PL) mapping is used to survey flakes and locate emitters. Emitter zero phonon lines (ZPL) with
description: wavelengths between 560 – 620 nm are observed, accompanied by a characteristic phonon sideband
description: detuned by ~160 meV. Emission in mono- and bi-layer hBN was found to be highly unstable, and it
description: was not possible to revisit emitters to confirm SPE. SPE was confirmed from multilayer emitters using
description: Hanbury-Brown and Twiss (HBT) interferometry. Only one emitter initially passed the criteria for SPE
description: of g(2)(0) &lt; 0.5, but many displayed anticorrelation dips with 0.5 &lt; g(2)(0) &lt; 1, indicating the additional
description: presence of background photons or multiple emitters. All emitters displayed blinking or bleaching,
description: and this was leveraged to separate data into groups by emission intensity (‘bright’, ‘dim’ or ‘off’ states)
description: for analysis. This approach allowed careful accounting for background photons in some HBT data,
description: resulting in a background corrected g(2)(0) value of -0.009 ± 0.032 for one emitter. This represents one
description: of only two measurements of g(2)(0) = 0 within experimental error for hBN SPE at room temperature
description: currently. Further analysis using the intensity grouping method allowed identification of two distinct
description: energy level configurations for a rapid blinking defect, likely caused by charge state switching similar
description: to colour centres in diamond, supported by an associated single exponential state duration switching
description: time.
description: Annealing of hBN can produce broadband luminescence in addition to localised emission from SPEs.
description: Here, it was discovered a significant source of this luminescence is from previously unreported, ultrathin residue layers located between the hBN flake and substrate. Residues were isolated and identified
description: by transferring parent hBN flakes to a different substrate using a homebuilt all-dry transfer system
description: and performing PL and AFM imaging of the flake’s original location. Residues bore a striking
description: resemblance to monolayer hBN, with a height of just 0.3 nm and displaying the same luminescence
description: pattern as the parent flake. Furthermore, SPE like spectra are identified originating from residues at
description: 2
description: multiple locations. However, it is established, by correlation of the rate of occurrence of bubbles and
description: decline of residue height, the most likely origin of the residue layers is a thin uniform layer of
description: contamination trapped between hBN and substrate during fabrication. It is also found the key step to
description: activating the strong residue luminescence is high temperature annealing.
description: To create arrays of deterministically placed emitters, nano-indentation of polymer supported hBN was
description: performed using an AFM. A detailed account of the nano-indentation method is given for hBN of
description: different thicknesses, including the limitations found. For 2 nm and monolayer hBN, no indent related
description: emission was identified. Optical characterisation of emitters demonstrated poor photostability and
description: ZPLs spread over a wide wavelength range for 15 nm hBN. Thermal light contributions from the
description: underlying polymer due to high incident laser power impeded quantification of HBT data. 57% of
description: indent sites displayed signs of anticorrelation, with no optimisation of indent parameters,
description: demonstrating excellent potential for this method. The lowest anti-correlation dip measured was
description: g
description: (2)(0) = 0.87 ± 0.04, comparable to some other fabrication techniques. The results of this thesis
description: advance the knowledge of native hBN SPE, will assist in background correction and elimination in HBT
description: experiments, and presents a practical method of deterministically positioning emitters.