Deterministic Quantum Emitter Formation in Hexagonal Boron Nitride

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Deterministic Quantum Emitter Formation in Hexagonal Boron Nitride via Controlled Edge Creation Joshua Ziegler, Rachael Klaiss, Andrew Blaikie, David Joseph Miller, Viva Horowitz, and Benjamin J Aleman Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00357 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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Deterministic Quantum Emitter Formation in Hexagonal Boron Nitride via Controlled Edge Creation Josh Ziegler†, Rachael Klaiss †, Andrew Blaikie†, David Miller†, Viva R. Horowitz§, Benjamín J. Alemán† †

Department of Physics; Material Science Institute; Center for Optical, Molecular, and Quantum Science, University of Oregon, Eugene, Oregon 97403 § Department

of Physics, Hamilton College, Clinton, New York 13323 *E-mail:

[email protected]

Keywords: Quantum emitter, 2D material, hexagonal boron nitride, focused ion beam

Quantum emitters (QEs) in 2D hexagonal boron nitride (hBN) are extremely bright and are stable at high temperature and under harsh chemical conditions. Because of they reside within an atomically thin 2D material, these QEs have a unique potential to couple strongly to hybrid optoelectromechanical and quantum devices. However, this potential for coupling has been underexplored because of challenges in nanofabrication and patterning of hBN

ACS Paragon Plus Environment

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QEs. Motivated by recent studies showing that QEs in hBN tend to form at edges, we use a focused ion beam (FIB) to mill an array of patterned holes into hBN. Using optical confocal microscopy, we find arrays of bright, localized photoluminescence that match the geometry of the patterned holes. Furthermore, second-order photon correlation measurements on these bright spots reveal that they contain single and multiple QEs. By optimizing the FIB parameters, we create patterned single QEs with a yield of 31%, a value close to Poissonian limit. Using atomic force microscopy to study the morphology near emission sites, we find that single QE yield is highest with smoothly milled holes on unwrinkled hBN. This technique dramatically broadens the utility and convenience of hBN QEs, and achieves a vital step toward the facile integration of the QEs into large-scale photonic, plasmonic, nanomechanical, or optoelectronic devices.

Introduction: Sources of single photons are a key component of many emerging quantum information technologies such as quantum computation, communication, and sensing.1–3 However, the more commonly used quantum emitters (QEs), such as cold atoms and spontaneous parametric downconversion sources, require complex setups that limit their viability for widespread use. In contrast, solid-state QEs require significantly simpler setups because they can operate in ambient conditions.4,5 This attribute, combined with the possibility of high quantum efficiency6–8 and integrability with on-chip devices,9,10 suggests them as the ideal class of QEs for large-scale implementation of quantum information technologies. Recent studies have discovered that hexagonal boron nitride (hBN), a large-bandgap 2D material, possesses QEs that are ultrabright and stable under ambient conditions.11–19 Moreover, because of hBN’s atomic-scale thickness and extreme mechanical strength,20 its QEs will couple


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strongly to photonic, plasmonic, and optomechanical modes, making them especially useful for hybrid quantum devices. However, to reliably integrate hBN QEs into these hybrid devices, a nanofabrication method to control both the creation and location of the QE (i.e. deterministic creation) is desperately needed. It has been observed that QEs tend to occur stochastically at edges or regions of high curvature,14,15 suggesting two routes toward deterministic creation of QEs in hBN. Recently, hBN QEs were created by inducing curvature with nanopillars.18,19 This technique achieved a yield of single QEs that is promising (~10%),19 but the technique’s potential for integration is significantly limited because it requires a structured substrate and thin hBN material. In this letter, we propose and demonstrate a method to deterministically create QEs through the creation of edges in hBN. We create these edges by patterned milling of hBN using a gallium focused ion beam (FIB). By optimizing the milling and annealling11,12,14 parameters, we obtain a 31% yield of single QEs. We also explore morphology dependence of QE formation and find that they form best with uniform milling on relatively smooth CVD hBN. Our technique dramatically broadens the scope of applications of these QEs due to its high-yield and versatility, being independence from a structured substrate. Results and Discussion: To create QEs in our hBN via edge creation, we transfer few-layer CVD hBN (Graphene Supermarket) onto SiO2 and then use FIB to mill holes into the hBN, thereby forming edges at the hole perimeter. We do not perform an additional irradiation to activate QEs. The required dose to remove hBN material was in the range 10-13 C/µm2 to 10-10 C/µm2, with beam energies from 5-30 keV. Initial tests showed that energies of either 10 or 20 keV and milling doses of 1pC/µm2 were close to optimum for see QE creation, as inferred by 𝑔2(0) < 0.5 (see Figure 1b), so we studied two near-optimal conditions within this parameter range in greater depth. In one region of our


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sample (Region 1), we used the FIB milling conditions of 10 keV and 1 pC/µm2, while in another region (Region 2) we used 20 keV and 10 pC /µm2. In order to generate a high density of single, optically addressable QEs with enough perimeter for QEs to form, we FIB patterned arrays of 500 nm diameter circular holes with a center-to-center separation of 1 µm in each region. An atomic force microscope (AFM) image of these holes is shown in Figure 1a, where the FIB was operated at 20 keV and 10pC/µm2. The depth of the holes for this FIB condition was approximately 4 nm or 12 hBN atomic layers. After milling, hBN

Figure 1: (a) AFM image of a FIB

flakes were annealed in oxygen at 850º C. This process

milled region of hBN, with a line cut

serves to activate QEs11,12 while simultaneously

shown below. (b) Example of 𝑔2(𝑡)

removing carbon deposited by the FIB or residual

data for a single QE in Region 2

organic material. This annealing step is usually

showing 𝑔2(0) = 0.33, well below the

performed

nitrogen)

antibunching threshold for a single

environment,11–15 but annealing in oxygen typically

QE. (left) 60 ns time window centered

doesn’t affect QEs in hBN.12 Although we are reporting

about 𝑡 = 0. (right) 800 ns window of

results from CVD hBN, we also tested exfoliated hBN

the same data.

in

an

inert

(argon

or

(HQGraphene) but the samples appeared to have a very high native defect density, which frustrated efforts to characterize single QEs (see Supplementary Information [SI]).


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Figure 2: Confocal, AFM and representative photoluminescence spectra for QEs made with various FIB parameters. Top Row (Region 1): Low dose, low energy. Bottom row (Region 2): High dose, high energy. (a) Confocal scan showing high QE visibility. (b) AFM image showing rough, non-uniform milling (c) Representative photoluminescence spectrum showing a single emission line with a broad background. (d) Confocal scan showing high QE visibility. (e) AFM scan showing uniform milling and smooth sidewalls. (f) Representative photoluminescence spectrum showing a QE spectrum with low background. The hole milling process under both FIB conditions is effective at deterministic patterning of localized photoluminescence (PL). By spatially mapping the PL of milled regions of the hBN flakes using a home-built confocal microscope, we find that FIB milling in Regions 1 and 2 both result in arrays of bright, highly visible spots with a periodicity matching that of the patterned features (see Figure 2a,d). The contrast between the bright spots and the surrounding region between the bright spots is high (> 20:1). These surrounding regions have a PL ~5-10 times lower than that regions of unmilled CVD hBN far from the milled sites (Figure S2b), which also have


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many randomly scattered sites of localized PL. The relatively low background near the patterned bright spots may be due to the long Gaussian tails of the FIB spot exposing areas near milled holes to a low ion dose, which is known to remove fluorescent organic surface residues and photoactive defects.16 Thus, it appears that the reduction of background PL is a convenient side-effect of the milling process. We note that, although FIB patterning clearly generates patterned PL, it is unknown if PL sources are localized inside the etched holes, on the hole edges, or in the unmilled regions between the edges. Techniques such as scanning near-field optical microscopy (SNOM), which overlay optical and topographical images, may help resolve this question. When we applied the conditions used in Regions 1 and 2 to wrinkled regions of CVD hBN and exfoliated hBN, we observed extremely low pattern visibility and we were unable to resolve single QEs. We suspect that the failure of our technique in wrinkled regions may be related to high compressive strain indicated by the wrinkles, while the failure in exfoliated hBN may be due to the high native defect density of the sample. (See the SI for optical data, AFM images, and further discussion of these regions (Figure S1, Figure S3)). Altogether, the flat regions of CVD hBN milled under either FIB condition generate spatially localized, high-contrast PL emission sites. To assess the single-photon purity of individual bright spots formed by FIB milling, we perform antibunching measurements (see Methods for details) and obtain second-degree correlation function data (𝑔2(𝑡)), as seen in Figure 1b. The antibunching data is fit by a three-level model to determine the antibunching depth of each milled site, as well as other photo-physical properties of the emitters (see Methods). The average number of photons was determined from 𝑔2(0) from the 1

equation 𝑔2(0) = 1 ― 𝑛, where n is the average number of photons emitted.21 A site was denoted as zero average photon number if there was no PL signal from the site. All antibunching data is summarized in Figure 3, showing lines at antibunching values corresponding to 2, 3, and 4 average


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photons. The cutoffs correspond to the number of QEs for ideal single photon emission, which would have zero probability of two photon emission and no background light. Even

without

ideal

single

QEs,

a

𝑔2(0) < 0.5

unambiguously indicates a single QE. We use this cutoff to classify sites as hosting single QEs, and do not attempt to

Figure

perform any background subtraction. This is primarily

degree of antibunching for both

because any significant background must be local, as

regions of FIB-treated CVD hBN

indicated by the high contrast confocal image, and is further

(Region

motivated by the inherent difficulty of accurately

antibunching depth for each

deconvolving background and signal light.

region

3:

Distributions

2).

Violin

showing

the

plot

of

of

larger

The FIB milling approach is effective at fabricating

overall degree of antibunching

patterned single QEs. To assess the antibunching of milled

in region 2. Top axis labels show

regions and the effectiveness of making single QEs, we

average

characterized rows that appeared representative and

corresponding to antibunching

characterized all FIB milled sites in those rows. In Region 1

values.

photon

numbers

we performed antibunching characterization on two adjacent rows of 9 sites. In Region 2 we characterized four rows of 9 sites; we also chose three adjacent rows as well as one row 20 µm above those three to check that the results were roughly consistent across the region. In Region 1, we measured an average antibunching depth of 𝑔2(0) = 0.78 ± 0.15, corresponding to an average photon number of 4.5. In this region, we found only one site exhibiting the antibunching signature of a single QE (𝑔2(0) < 0.5) out of 18 measured FIB milled holes, although all 16 that showed PL exhibited some amount of antibunching and 13


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had antibunching characteristic of < 4 QEs. On the other hand, in Region 2, we measured an average antibunching depth of 𝑔2(0) = 0.57 ± 0.19, corresponding to an average photon number of 2.3. In this region, 11 out of 36 of milled holes had the antibunching characteristic of a single QE, demonstrating a single QE creation yield of 31%. Moreover, of the 34 of 36 holes in Region 2 that showed PL, all exhibited some amount of antibunching, demonstrating a QE creation yield of 94%. Our 31% yield of sites with single QEs is close to the theoretical maximum of 36.8% for a Poissonian QE creation process. The single QE yield is ~ 2.5 times greater than the yield achieved by nanopillar strain engineering.19 Combining the site areal density (1 site per 1 μm2) with the single QE yield, we calculate a single QE areal density of ~1 per 3 μm2. For a typical size hBN sheet (~ 50 μm × 50 μm), this density would create over 800 individually addressable single QEs. In order to estimate the amount of FIB milled edge necessary to create single QEs using our processing parameters, we use the hole circumference (~1.5 μm) and the single QEs yield (~1 per 3 holes) to find a single QE linear density of ~1 per 5 μm of ion-milled edge. In addition to exhibiting markedly different yield of single QE, Regions 1 and 2 also differ in their PL spectra. Typical spectra for sites in Region 1 (𝑔2(0) > 0.5) and Region 2 (𝑔2(0) < 0.5) are shown in Figure 2c and Figure 2f, respectively (see Methods for measurement details). The sharp peak at 2.27 eV corresponds to the silicon Raman peak for 532 nm excitation. Spectra from Region 1 tend to exhibit a single sharp emission line, but often have a broad background. Combined with the high pattern contrast in confocal images for this region, this suggests that the milling parameters in Region 1 create a large amount of background PL localized to the milling site. This background is at least one of the factors contributing to the low photon purity in this region. However, it is possible that there are multiple QEs contributing to a single sharp emission line. The very low amount of antibunching measured in Region 1 suggests that both factors likely


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contribute. In Region 2, we again often see a single primary emission line but the background is considerably lower compared to Region 1. Therefore, the higher energy and larger dose used to mill Region 2 is likely more effective at removing fluorescent contaminants and defects, and thereby leads to a lower background PL and a higher photon purity. Combined with the demonstrated high amount of antibunching in Region 2, this data shows that the high energy, high dose FIB parameters are preferred for QE creation. Another key difference between Region 1 and 2 is the surface topography near the emission sites. Using AFM to measure the topography (see Methods), we find that the holes in Region 1 (low energy, low dose) are poorly-defined, non-uniform, and rough (see Figure 2b) with an arithmetic mean roughness, Ra, of 0.6 nm in the center of the hole. In contrast, the holes in Region 2 (high energy, high dose) have a well-defined circular shape and a smooth profile (Figure 2e) with a depth ~5 nm over a lateral distance of 200 nm, and an arithmetic mean roughness, Ra, of 0.23 nm in the center of the hole. The roughness seen in Region 1 provides further evidence that fluorescing contaminants may not be thoroughly etched away under these FIB milling conditions. Through the formation of many deep pits, the milling in Region 1 created a large amount of in-plane edges on which QEs could form, in agreement with the hypothesis that edges are responsible for QE formation in hBN. Our AFM measurements suggest that a smooth, uniformly milled hole

Figure 4: Zero phonon line energies across all emitters for which

there

was

a

clearly

identifiable central emission line showing a similar distribution of energies compared to previous observations of hBN quantum

is optimal for single QE formation.

emitters.


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As compared to other techniques to activate QEs in hBN, the QEs created through FIB milling were similar in terms of the distribution of zero phonon lines (ZPLs), PL intensity, and optical stability. The distribution of ZPLs of these QEs, shown in Figure 4, was very similar to QEs found in CVD hBN suspended on nanopillars.19 In the setup we used to take antibunching measurements we a filter light with energy above 2.25 eV, however the phonon sideband from those emitters was below the cutoff and allowed through to our detectors. In PL spectra, we find phonon sidebands at 150 ± 23 meV from their respective zero phonon lines, consistent with previous observations.11–15 To further compare our QEs to those created by other methods, we measure PL intensity vs. power for our emitters in Region 2 and fit that data to a first-order saturation model,22 𝐼(𝑃) = 𝐼∞𝑃/(𝑃 + 𝑃𝑠𝑎𝑡) (see Figure S2c). We find that our brightest emitter has an 𝐼∞ of roughly 2.6 Mcps, on par with other hBN quantum emitters.11–15 Across the 11 single QEs, roughly 33% were optically stable for the entire measurement duration – a few hours of illumination at 80 µW excitation power. Of these 11, only one bleached entirely and stopped emitting PL. The remaining six exhibited some blinking without fully bleaching. This blinking and bleaching behavior is similar to what is observed in hBN QEs23,24 created using other methods, suggesting that our FIB milling procedure is at least as effective for creating optically-stable emitters. The FIB milling approach produces a high density of individually addressable single QEs. Leveraging our ability to generate large quantities of single QEs, we study the PL blinking dynamics of large numbers of hBN QEs in a relatively uniform surrounding environment (e.g. substrate, charge, strain). We measure the probability distribution of PL intensity for 10 single QEs as histograms binned by brightness at fixed 80 µW excitation laser power (Fig. 5a). See SI for PL intensity versus time. While some QEs show a single peak (QE 1 to 6), others (QE7-10) show two peaks, suggesting a bright and dim state. Furthermore, some have a narrow distribution centered


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near a single intensity, while others are broader and even exhibit long tails. To characterize the blinking behavior of a single QE with distinct bright (or “on”) and dim (or “off”) states – that is, a QE that exhibits PL distributions with two non-overlapping peaks in Fig 4a – we measure the probability that a given blink will last a certain duration of time. Performing this analysis on Q10, we specify the dim state as PL intensity < 17 kCounts/s and the bright state as PL intensity > 19 kCounts/s for QE10 at this excitation power. This type of analysis is only appropriate for two-state emitters with a clear threshold (e.g. QE9 and 10). The probability density data for the dim (Fig 5b) and bright (Fig 5c) states show that short blinks occur more often than long-lasting blinks. We fit these data to a dominant power law, probability ∝ 𝑡 ―2 for QE10 over the timescales shown, where t is the state duration, consistent with the power law observed in other quantum emitters,25–29 and in contrast to systems that show an exponential behavior with a single characteristic time.30–33 The dominant power law we observe rules out a simple charging model34, which

Figure 5: (a) PL intensity of the single QEs. The last one, labeled QE10, shows blinking behavior that we analyze in b and c. (b) Probability density of the dim state lifetime. The offset dashed grey line is a power law fit to the data with power 2.06 ± 0.08. (c) Probability density of the bright state lifetime. The offset dashed grey line is a power law fit to the data with power 2.02 ± 0.05.

would predict an exponential distribution, but leaves a variety of blinking models as possibilities,33 Figure 4: a) PL intensity of the single QEs. The last one, labeled QE11, shows blinking behavior that we analyze in b and c. b) Probability density of the dim state lifetime. The offset dashed grey line is a power law fit to ACS Paragon Plus Environment the data with power 2.06 ± 0.08. c) Probability density of the bright state lifetime. The offset dashed grey line is a power law fit to the data

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including modified charging models, where the barrier between two states can vary35. The bright state of QE9 does not show a simple power law fit over the same time scale (see SI). It may be possible to mitigate PL intensity fluctuation through the use of a passivating layer,23 or a more pristine substrate.24 However, this analysis, together with the ability to generate many single QEs, may help to shed light on the mechanisms governing PL instabilities in hBN QEs by showing that something other than simple charging is causing this blinking. While we have found a combination of dose, energy, and defocus which yields high quality QEs, it may be possible to push the single photon purity and density of these QEs higher through further exploration of the rich parameter space available with FIB milling. Beyond the simple parameters we explored, changing FIB defocus or angle may reduce roughness of milled hBN36,37 and result in a reduced background PL. Due to the relative independence of QE formation to ion type,14 a helium FIB or electron beam could be used to perform this milling in a less destructive way38–40 and enable greater single photon purity. Our approach achieved a single QE density of 0.33/μm2, which is largely determined by the hole array density (1/μm2). Thus, a simple way to increase the single QE density may be to decrease the hole spacing, perhaps while altering hole geometry to maintain a constant perimeter. It should also be possible to expand this technique beyond electron or ion beam milling by using a combination of photolithography and reactive ion etch (RIE) processes41 to pattern holes in hBN. Such a generalization of this technique would dramatically lower the barrier for further studies and applications of hBN QEs. The formation process of these QEs is difficult to ascertain because the defect structure is still poorly understood.42 However, our AFM measurements show that the FIB process does not result in high curvature, suggesting that the process of QE formation via edge creation is distinct from the high curvature method previously demonstrated.19 The dual QE generation pathways (i.e. edges


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or high curvature) may be due to local band structure shifts due to strain43 or edge relaxation.44,45 It may also be that high strain or edges allow for local reconstruction of the hBN,46,47 leading to formation of optically-active Stone-Wales-like defects. A future study correlating high resolution optical images with topographical images (i.e. using SNOM or colocalized optical/AFM) could provide valuable insight into the nature of QE formation in hBN. Conclusion: In this work, we have used FIB milling to generate patterned single QEs in hBN with a yield of 31%. Our FIB-based fabrication method fills in the crucial need to create and control the location of single QEs in hBN, thus enabling the integration of these QEs into chip-scale plasmonic, photonic, and optomechanical devices for quantum information applications in ambient conditions. Our technique will provide large numbers of individually addressable single hBN QEs for QE-based sensing applications,48 and significantly lowers the barrier for studying the physics of hBN QEs and allows for more expansive surveys of their properties. Methods and Materials: Sample fabrication and surface characterization. The samples used in this study were prepared by polymer transfer49 of ~15 layer CVD hBN purchased from Graphene Supermarket. Samples were annealed in oxygen at 850C for a half hour both before and after focused ion beam milling in order to remove hydrocarbon contaminants. Focused ion beam milling was performed on a FEI Helios Dual-Beam gallium FIB with beam parameters set as noted above. Before milling, the sample and chamber were plasma cleaned with air for five minutes to remove residual hydrocarbons. AFM measurements were performed on a Bruker Dimension FastScan Atomic Force Microscope operated in PeakForce mode.


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Optical measurements. We performed initial confocal scans and antibunching experiments of QEs in a home-built confocal microscope equipped with a 532 nm solid state laser (OptoEngine), 100x 0.7 NA objective and two avalanche photodiodes (Micro Photon Devices) in a Hanbury Brown-Twiss setup. All optical measurements were performed at an excitation power of roughly 80 µW. Time-correlated single photon counting was performed on a PicoQuant TimeHarp 260. These measurements were used to determine if PL sources were QEs, and also yielded their average photon number, non-radiative and radiative lifetimes, and bunching amplitude. These parameters were extracted from antibunching measurements by fitting our data |𝑡|

to a simple

model21

for a three-level system:

𝑔2(𝑡)

2

= 1 ― 𝜌 +𝜌²[1 ― (1 ― 𝑎)𝑒

―𝜏

1

|𝑡|

+𝑎𝑒

―𝜏

2

],

where 𝑎 is the bunching amplitude, 𝜏1 is the non-radiative lifetime, 𝜏2 is the radiative lifetime, and 𝜌2 = 1 ― 𝑔2(0) where 𝑔2(0) is the degree of antibunching. Photon number in a given milled hole 1

was determined by binning 𝑔2(0) values according to 𝑔2(0) = 1 ― 𝑛. We measured the spectra of QEs using a commercial Witec Raman spectrometer equipped with a Peltier-cooled Andor iDus CCD.

Author contributions. JZ and BA conceived and designed the experiment. JZ and RK performed sample fabrication. JZ, RK, and VH performed characterization. AB, DM, and JZ designed and built the optical apparatus. DM and JZ developed the 2D material transfer techniques. JZ and VH analyzed the data. All authors contributed to writing the manuscript. BA supervised the work. Supporting Information. For an example of spectra with a peak wavelength below our filters, AFM data on unmilled CVD and exfoliated hBN, characterization of QEs on exfoliated hBN,


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and details on the analysis performed on the PL intensity variation can be found in the Supporting Information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGEMENTS The authors thank Rudy Resch, Kara Zappitelli, Brittany Carter, and Georgios Tsekenis for discussion of this work. This work was supported by the University of Oregon and the National Science Foundation (NSF) under grant No. DMR-1532225. The authors acknowledge facilities and staff at the Center for Advanced Materials Characterization in Oregon, and the use of the University of Oregon’s Rapid Materials Prototyping facility, funded by the Murdock Charitable Trust. References: (1)

Aharonovich, I.; Englund, D.; Toth, M. Solid-State Single-Photon Emitters. Nat. Photonics 2016, 10, 631–641.

(2)

Rudolph, T. Why I Am Optimistic about the Silicon-Photonic Route to Quantum Computing. APL Photonics 2016, 2, 1–19.

(3)

Schlussel, Y.; Lenz, T.; Rohner, D.; Bar-Haim, Y.; Bougas, L.; Groswasser, D.; Kieschnick, M.; Rozenberg, E.; Thiel, L.; Waxman, A.; et al. Widefield Imaging of Superconductor Vortices with Electron Spins in Diamond. Phys. Rev. Appl. 2018, 10, 1–6.


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(4)

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Deterministic Quantum Emitter Formation in Hexagonal Boron Nitride

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