Towards practical applications of quantum emitters in boron nitride

We demonstrate quantum emission capabilities from boron nitride structures which are relevant for practical applications and can be seamlessly integrated into a variety of heterostructures and devices. First, the optical properties of polycrystalline BN films grown by metalorganic vapour-phase epitaxy are inspected. We observe that these specimens display an antibunching in the second-order correlation functions, if the broadband background luminescence is properly controlled. Furthermore, the feasibility to use flexible and transparent substrates to support hBN crystals that host quantum emitters is explored. We characterise hBN powders deposited onto polydimethylsiloxane films, which display quantum emission characteristics in ambient environmental conditions.

www.nature.com/scientificreports/ From the point of view of optical characterisation of quantum light sources in hBN, the spectral properties of such emitters are quite well understood. Further progress in the field will likely depend on a possibility to tailor the parameters of the hBN samples and to realise novel ideas for incorporation of hBN quantum emitters into structures of practical importance. In this work we explore these concepts by (i) characterising the hBN samples grown by the method of MOVPE when using different process parameters and (ii) demonstrating that hBN powder emitters may act as efficient quantum emitters on flexible and transparent polydimethylsiloxane (PDMS) films in ambient atmosphere. The reported observations are relevant for the progress in development of technologically sound BN structures with the focus on quantum emission capabilities in a broad range of environmental conditions.

Quantum emitters in MOVPE-grown polycrystalline hBN structures
hBN polycrystalline films have been grown via MOVPE on sapphire supports using process parameters as listed in Table 1. For the optical study, the hBN structures were relocated onto Si/SiO 2 substrates via pick-up transfer techniques utilising polydimethylsiloxane membranes. Generally, the photoluminescence (PL) spectra of our MOVPE hBN samples measured under conditions of the below bandgap excitation take form of broad emission bands. Such response of wide gap materials in visible spectral range may be related to ensembles of deeply bound defect states. Most importantly, the high-resolution μPL mapping reveals locations where individual narrow emission lines arise. Although all investigated samples reveal such narrow line emitters, the potential to isolate single PL resonances is limited by the apparent intensity of the broadband contribution. Representative spectra for samples S1-S3, showing signature of individual quantum emitters in our MOVPE hBN, are presented in Fig. 1. It is quite clear that the feasibility of an observation of individual defects in the optical spectra varies strongly across the investigated samples. Interestingly, the intensity of the background appears to be mostly related to the character of the crystallites that form the hBN layers, as may be seen in scanning electron microscopy images shown in Fig. S1 in Supplementary Information.
A general inspection of the properties of quantum emitters in the MOVPE-grown hBN shows similar characteristics to those displayed by their counterparts in mechanically exfoliated layers and powders [11][12][13][14][15] . Representative results of the optical characterisation of the narrow line emitting centres in our samples are summarised in Fig. 2. Individual emitters exhibit a large degree of linear polarization of emitted light, which is indicative of an intrinsic anisotropy of the states partaking in the recombination processes. The angular dependence of the emission intensity is given as: Here, I A is the amplitude of the angle (α)-dependent oscillations, α 0 is the phase shift with respect to an arbitrarily chosen axis of the linear polarizer and I 0 is the minimal intensity (displayed by the emitter when intrinsic polarisation axis is perpendicular to the axis of the polarizer). The luminescent centre, whose emission spectra and angle-dependent emission intensity is presented in Fig. 2a,b, displays a polarisation degree of: Characteristically for hBN quantum emitters, the second-order correlation function, obtained in Hanbury Brown and Twiss configuration, is dominated by a short time-scale antibunching 25 in combination with a longer time-scale bunching. The shape of the bunching can be reproduced well by a bi-exponential decay function, so that the experimental data is well accounted for by the following formula: with characteristic times t 1 = 1.3 ± 0.1 µs , t 2 = 77 ± 1 µs , t 3 = 1.2 ± 0.3 ns and bunching amplitudes The emergent anti-bunching is characterised by an amplitude that constitutes 70 ± 3% of the maximum number of counts.
The parameters describing the g (2) (τ) function can also be altered by varying the excitation conditions. Figure 3 presents the dependence of the photoluminescence spectra and the correlation functions on the excitation power. Comparatively, this data demonstrates the decrease of the broadband background emission in the timeintegrated spectra in conjunction with the decrease of the bunching amplitude for lower excitation powers. These two effects consequently decrease the value of g (2) (0). Particularly, in the regime of low excitation power (0.1 mW focused to a spot of about a micron diameter in our experiment), the broadband background contribution and  Table 2.

Quantum emitters in hBN powders on a flexible and transparent substrate
The appearance of quantum emitters in different forms of hBN, especially the polycrystalline structures, allows their seamless integration with a variety of modern technologies. Particularly important are optoelectronic structures and devices operating on transparent and/or flexible substrates [26][27][28] . Here, we demonstrate the quantum emission capabilities of hBN powders deposited onto polydimethylsiloxane (PDMS) films. Our micro-optical characterisation of such samples provides a proof of concept that quantum emitters in hBN may operate in ambient conditions on polymer films in a comparable fashion as on commonly used non-conductive crystal substrates. In Fig. 4, we present a comparison between the room-and cryogenic-temperature optical response of the hBN powder deposited on a PDMS film under 2.410 eV (514.4 nm) laser excitation. In both cases, a doublet of Raman lines from PDMS polymer 29 is conspicuous around 2920 cm −1 combined with a narrower hBN Raman resonance at 1356 cm −1 . In the room temperature spectrum, presented in Fig. 4a, an additional broader line appears at 2.288 eV. At low temperature, the hBN emission resonances become significantly narrower, demonstrating that PDMS film may transfer heat efficiently even at cryogenic conditions, consequently allowing the hBN powder to thermalize.
In Fig. 4c, the most robust additional narrow resonances are seen at 1.843 eV and 2.269 eV. More examples of photoluminescence spectra, illustrating a broad spectral range wherein emission resonances from hBN powder may be found, are presented in S. I. in Fig. S3. A very good stability on the timescale of hundreds of seconds is Figure 1. The optical images of Si/SiO 2 substrates supporting transferred MOVPE hBN are confronted with typical μPL spectra taken on locations displaying emission from single defects for samples S1 (a,b), S2 (c,d) and S3 (e,f), respectively. The low temperature (5 K) μPL spectra were measured under 514 nm laser excitation with the power on the sample 300 μW for samples S1, S2 and 50 μW for sample S3. All studied samples host quantum emitters. However, for the specimen with strong broadband background it is infeasible to find suitable excitation conditions (e.g., resonant laser energy or linearly polarised excitation aligned with the intrinsic anisotropy axis of the emitter 13 ), so that the narrow lines would be well pronounced (sample S3).  Fig. 4b,d. The most relevant for practical aspects of a useful quantum emitter is the presence of an antibunching in photon correlation function in ambient conditions. It is indeed observable for the individual emission resonances displayed by hBN powder on PDMS samples, as we demonstrate in Fig. 5. Notably, the quantum emission

Summary
We have explored technological aspects of utilising hBN quantum emitters as building blocks compatible with a broad range of modern technologies. Our MOVPE polycrystalline BN films display well-isolated and robust narrow-line resonances characterised by deep antibunching in a second-order correlation function. Such observation demonstrates that high-quality solid-state single quantum emitters do not necessarily require high-quality single crystals. Furthermore, there appears to be no stringent requirements for the substrates in order to achieve quantum emission from hBN structures in ambient conditions. As a proof of principle, we verify this notion by inspecting the optical response of quantum emitters in hBN on PDMS films, which may play a pivotal role in fabrication of 2D heterostructures and devices.

Methods
Crystal growth. The BN crystals were grown on two-inch sapphire wafers via MOVPE method using an Aixtron CCS 3 × 2″ system. Triethylborane (C 6 H 15 B, TEB) and ammonia (NH 3 ) were used as the active agents acting as a source of boron and nitrogen, respectively. The growth temperature was controlled in situ by ARGUS Thermal Mapping System and for all samples was set to 1050 °C. The differences between the studied samples arise from the variation of growth parameters: carrier gas (nitrogen or hydrogen), reactor pressure and the volume of ammonia. The dependence of the properties and surface morphology on growth parameters has been Table 2. The fitting results for the experimental g (2) (τ) functions from Fig. 3 are presented. For this data, A 2 parameter was set to 0 as the bunching shape could be reproduced with a single time constant. The other parameters were fitted with the exception of bunching amplitude for the g (2) (τ) function obtained under the condition of 0.1 mW laser excitation, for which the bunching amplitude is negligible.  www.nature.com/scientificreports/ further described in Ref. 34 . All samples were grown in the conditions of ammonia insufficiency which promotes not only formation of nitrogen vacancies but also carbon incorporation into the boron nitride structure. Such growth strategies may open new pathways for controllable formation of specific luminescent centres. Relatively large amount of TEB results in a very effective material synthesis. Dense nucleation on the sapphire surface and appearance of defects during the growth rule out formation of epitaxial, continuous layer well aligned with the sapphire substrate and for this reason obtained boron nitride has a polycrystalline form.
Sample preparation. The MOVPE-grown BN layers were transferred from the initial sapphire substrates to commonly used in optical spectroscopy Si/SiO 2 substrates, allowing better visualisation of the BN layered via optical contrast methods. PDMS-based pick-up transfer technique was used to relocate the boron nitride crystals between various supports.
Optical spectroscopy. The optical spectra were measured in a standard back-scattering geometry under a single-frequency 514.4 nm (2.410 eV) laser excitation. The laser beam was focused on the surface of the sample down to a spot of about 1 μm diameter with a long working distance 50 × magnification objective. The signal from the sample was spectrally resolved with a 500 mm spectrometer and detected by a charge-coupled device camera, producing an optical spectrum which consists of a combination of Raman and photoluminescence resonances. A flow helium cryostat equipped with a cold finger was used to cool the samples down to 5 K temperature. The optical spectra resolved by the linear polarisation were measured with a λ/2 waveplate followed by a linear polarised located in the detection path of the optical setup. The photon correlation measurements were performed in a Hanbury Brown and Twiss configuration with the continuous wave 514.4 nm laser acting as an excitation source. Two avalanche photodiodes with a response time of 200 ps were used to detect the single photons and generate electrical signals. The delay between the electrical pulses was measured with a specialised correlation card module.  www.nature.com/scientificreports/