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Corinne Steiner, Rebecca Rahmel, Frank Volmer, Rika Windisch, Lars H. Janssen, Patricia Pesch, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Florian Libisch, Bernd Beschoten, Christoph Stampfer, Annika Kurzmann

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[Current-induced brightening of vacancy-related emitters in hexagonal boron nitride](https://mdr.nims.go.jp/datasets/c12644c8-4448-4ed5-880b-29816be0fcc9)

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Current-induced brightening of vacancy-related emitters in hexagonal boron nitridePHYSICAL REVIEW RESEARCH 7, L032037 (2025)LetterCurrent-induced brightening of vacancy-related emitters in hexagonal boron nitrideCorinne Steiner ,1,2,* Rebecca Rahmel ,1 Frank Volmer ,1,3 Rika Windisch ,4 Lars H. Janssen ,1 Patricia Pesch,1Kenji Watanabe ,5 Takashi Taniguchi ,6 Florian Libisch ,4 Bernd Beschoten ,1Christoph Stampfer ,1,2 and Annika Kurzmann 1,7,†1JARA-FIT and 2nd Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany2Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich, 52425 Jülich, Germany3AMO GmbH, Advanced Microelectronic Center Aachen (AMICA), 52074, Aachen, Germany4Institute for Theoretical Physics, TU Wien, 1040 Wien, Austria5Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan6Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan72nd Institute of Physics, University of Cologne, 50937 Köln, Germany(Received 25 February 2025; accepted 28 June 2025; published 19 August 2025)We perform photoluminescence measurements on vacancy-related emitters in hexagonal boron nitride (hBN)that are notorious for their low quantum yields. The gating of these emitters via few-layer graphene electrodesreveals a reproducible, gate-dependent brightening of the emitter, which coincides with a change in the directionof the simultaneously measured leakage current across the hBN layers. At the same time, we observe thatthe relative increase of the brightening effect scales linearly with the intensity of the excitation laser. Bothobservations can be explained in terms of a photo-assisted electroluminescence effect. Interestingly, emitters canalso show the opposite behavior, i.e., a decrease in emitter intensity that depends on the gate leakage current. Weexplain these two opposing behaviors by different concentrations of donor and acceptor states in the hBN andshow that precise control of the doping of hBN is necessary to gain control over the brightness of vacancy-relatedemitters by electrical means. Our findings contribute to a deeper understanding of vacancy-related defect emittersin hBN that is necessary to make use of their potential in quantum information processing.DOI: 10.1103/cd62-5hq8Introduction. Hexagonal boron nitride (hBN) has emergedas a promising host material for quantum emitters [1–5]. Es-pecially vacancy-related defects, e.g., defects consisting of avacancy adjacent to a carbon substitutional, show significantpotential for quantum information processing, as they can pos-sess a triplet ground state and spin-conserving excited states[6–10]. In this respect, spin-lattice relaxation times of defectspins in the microsecond range [11,12] and room-temperatureoptical initialization and readout of triplet defect states havealready been demonstrated [13]. However, whereas multi-carbon substitutional defects (e.g., carbon dimers and trimers)[14–16], can result in emitters exhibiting high brightness evenat room temperature [17–22], vacancy-related defects are no-torious for their low quantum yields [10,23]. In fact, the lowbrightness of vacancy-related defects has so far preventedsecond-order correlation (g(2)) measurements and thus theunambiguous confirmation of their single photon emission[10]. Therefore, methods for the brightening of such dark*Contact author: corinne.steiner@rwth-aachen.de†Contact author: kurzmann@ph2.uni-koeln.dePublished by the American Physical Society under the terms of theCreative Commons Attribution 4.0 International license. Furtherdistribution of this work must maintain attribution to the author(s)and the published article’s title, journal citation, and DOI.emitters are of high interest. An effective method for tuningthe emission energies and intensities of bright emitters wasestablished by the integration of hBN into van der Waalsheterostructures, which allow the use of optically transparentfew-layer graphene gates [24–26]. This enables the precisecontrol of the local electric field by applying voltages to thegates while maintaining optical access to the emitters. Thereported variations in emitter intensity in response to the ap-plied electric field have been attributed to several mechanisms,such as gate-induced (dis)charging of the emitter’s charge-transition levels or of nearby defects [24–26]. These findingshighlight the complex interplay between the local electronicenvironment and the optical properties of hBN emitters. Athorough understanding of this interplay might be crucial forpaving the way towards quantum devices and sensors basedon hBN emitters [5,10,22,27].Here, we investigate the brightening of a dark hBN emitterthat we assign to be a vacancy-carbon-substitutional (V-C)type defect by analyzing its phonon side bands (PSBs).Furthermore, we demonstrate that a gate-voltage-inducedleakage current across the hBN layers appears to be re-sponsible for the brightening effect. Our findings can beexplained by a model that includes photo-assisted electro-luminescence as the underlying mechanism of the reportedintensity variations. Additionally, our results enable us toassign the different gate-dependent intensity variations ob-served in different emitters to differences in the doping2643-1564/2025/7(3)/L032037(10) L032037-1 Published by the American Physical Societyhttps://orcid.org/0009-0002-5581-8750https://orcid.org/0009-0006-0951-5010https://orcid.org/0000-0003-3526-2687https://orcid.org/0000-0003-2101-4025https://orcid.org/0009-0007-3463-3112https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-1467-3105https://orcid.org/0000-0001-5641-9458https://orcid.org/0000-0003-2359-2718https://orcid.org/0000-0002-4958-7362https://orcid.org/0000-0001-5947-0400https://ror.org/04xfq0f34https://ror.org/02nv7yv05https://ror.org/01sd0e661https://ror.org/04d836q62https://ror.org/026v1ze26https://ror.org/026v1ze26https://ror.org/00rcxh774https://crossmark.crossref.org/dialog/?doi=10.1103/cd62-5hq8&domain=pdf&date_stamp=2025-08-19https://doi.org/10.1103/cd62-5hq8https://creativecommons.org/licenses/by/4.0/CORINNE STEINER et al. PHYSICAL REVIEW RESEARCH 7, L032037 (2025)5855959.0 -9.00 -9.09.0 9.0ε (eV)2.0502.1001(d)(e)600605-9.00-10100(c)2.0752.125V   (V)GV   (V)G(a)(b) 100 50 100-50I (pA)(g)λ (nm)580 600 620050100(f)590610SiOhBN hBN⁺FLGFLGA I E532 nmVGSMUT = 77 K00.51.0Î      (a.u.)ZPLÎ (cts/s)λ (nm)Î (a.u.)Î (a.u.)0 0.42-0.42E (V/nm)TGBGBGTG0.51.00Î      (a.u.)ZPL I (pA)0100-5050E (V/nm)0 0.42 -0.42 0.42 0.42-0.42 -0.42210 μm3 μm90 450 810    = 0 V  = 5 VGGVVZPL30 meVε (eV)2.04 1.982.102.16FIG. 1. (a) Schematic cross section of the few-layer graphene-gated hBN sample and its operating principle. The structure consists ofan hBN emitter layer (hBN+) and an hBN capping layer that are encapsulated in few-layer graphene (FLG) and stacked onto a Si/SiO2substrate. A source measurement unit (SMU) is used to apply a voltage (VG) to the FLG gates and to simultaneously measure the leakagecurrent I through the hBN. (b) Optical image of the sample. The outlines of the top and back gate are highlighted with white and yellowlines, respectively. (Zoom-in) Spatially resolved PL signal of the dual-gated area. Bright localized spots indicate the observable emitters inthe hBN. (c) Comparison of a high intensity emitter spectrum at VG = 5 V, corresponding to E = 0.23 V/nm, (magenta) and a low-intensityspectrum at VG = 0 V (cyan). The ZPL and two additional side modes are visible and show gate-dependent intensity variations. Both spectrawere measured at 77 K with a laser power of 200 µW. (d) The extracted zero phonon line (ZPL) intensity (ÎZPL) of one emitter scales with themeasured leakage current (color-coded to show its dependence on the applied external electric field E ). (e) Illustration of how the gate voltageis swept in an alternating sequence over time between negative and positive values. (f) Corresponding PL spectra of the emitter during repeatedgate voltage sweeps measured at T = 77 K and with a laser power of 200 µW. The emitter intensity is plotted as function of the applied gatevoltage and electric field, respectively. The dashed gray lines mark the turning points of the voltage sweeps. The emitter shows a reproduciblelinear Stark shift and voltage-dependent changes in intensity. The blue and magenta arrows mark the spectra depicted in (c). (g) ZPL intensityobtained by taking the area of a Lorentzian function fitted to the ZPL peak from Fig. 1(f) (blue points). The blue shaded areas mark the electricfield ranges where the emitter intensity is increased. The red curve shows the leakage current that was measured simultaneously to the PLsignal. The dotted line indicates the zero-line of the current.levels of the hBN. Therefore, our method of simultaneouslymeasuring gate-dependent photoluminescence (PL) and gateleakage currents provides an additional and useful methodfor the characterization of both the hBN host material and itsemitters.Experimental methods. The structure of the graphene-gatedhBN samples used for this Letter is shown schematically inFig. 1(a). The samples consist of two hBN flakes that areencapsulated in few-layer graphene (FLG) flakes, resulting ina plate capacitor geometry. They were stacked and placed ontoa Si++/SiO2 (285 nm) substrate using a dry transfer method[28–30]. Prior to the stacking, one hBN flake was thermallyannealed (see the Supplemental Material [31] for processdetails) to induce emitters into the hBN flake [17] and ishereafter referred to as the emitter layer (hBN+). The secondhBN flake acts as a capping layer, separating the surface ofthe emitter layer from the FLG in order to prevent quenchingof surface emitters caused by direct contact with graphene[32,33]. The FLG flakes serve as transparent, electrostaticgates, referred to as top gate (TG) and back gate (BG) andwere contacted with lithographically defined Cr/Au contacts.Applying a gate voltage VG between these two gates, i.e.,electrodes, via a source measure unit [SMU, see Fig. 1(a)]allows us to study emitters in hBN under the influence ofthe induced out-of-plane electric field. Additionally, we canprobe the voltage-induced leakage current that flows betweenthe two electrodes through the hBN layers. Figure 1(b) showsan optical image of such a sample with yellow and whitelines highlighting the outlines of the back and top gate, re-spectively. The zoom-in shown in the right panel of Fig. 1(b)depicts the spatially resolved photoluminescence signal of theblack-marked area. Localized emission centers are visible asspatially confined bright yellow spots.Results. We now focus on a specific emitter located withinthe dual-gated area of the sample, which exhibits a zerophonon line (ZPL) at approximately 587 nm (2.11 eV), seeFig. 1(c). The first thing to note is the very low brightnessof the emitter of only up to around 120 counts per secondat an excitation power of 200 µW and a laser spot size ofabout 500 nm. Such a low quantum yield is characteristicL032037-2CURRENT-INDUCED BRIGHTENING OF … PHYSICAL REVIEW RESEARCH 7, L032037 (2025)of vacancy-related defects [10,23]. Further support for theassignment of the emitter as a vacancy-related defect comesfrom the examination of the phonon side bands (PSBs), whichcan be observed in PL measurements conducted at a temper-ature of 77 K (see the Supplemental Material [31] for 4 Kdata of the emitter). These PSBs are separated from the ZPLby an energy difference of around 30 meV, which is muchlower than the typical energy separation of 160 ± 20 meVcommonly observed for bright emitters [1,3,14,17,18,34,35].This reduced energy separation corresponds well with defectbreathing modes of vacancy-related defects in hBN [36,37].Because of the large spectral weight of these breathing modes[38], vacancy-related defects are not expected to show pro-nounced PSBs in the normally observable energy range.Moreover, the specific positions of the PSBs in the spectrumshown in Fig. 1(c) are found to be in agreement with theo-retical predictions for a vacancy-carbon substitutional defect[36,38] suggesting that our experimentally observed emitter isindeed hosted by a V-C defect type (for further discussion, seethe Supplemental Material [31]).Next, we focus on the gate voltage dependence of theemission properties of this dark emitter. As seen in Fig. 1(c),the intensity of the emitter varies significantly between thetwo depicted spectra recorded at VG = 0 V and VG = 5 V. Toinvestigate the brightening effect in more detail, we sweep theapplied gate voltage VG in repeated cycles between –9 V and9 V, as illustrated in Fig. 1(e). During these cycles, we si-multaneously measure the leakage current flowing through theFLG/hBN/FLG structure and record PL spectra of the emitter.The color-coded plot in Fig. 1(f) shows these spectra as afunction of the resulting electrical field, which was calculatedfrom the applied gate voltage according to the Lorentz lo-cal field approximation [24,39] [E = ((εhBN + 2) × VG)/(3t ),with the permittivity εhBN = 3.4 and the combined thicknesst = 39 nm of both hBN layers]. Over the repeated sweepcycles of the gate voltage we observe a linear Stark shift of0.54 ± 0.04 nm/(V/nm) and notably a clear and reproduciblebrightening of the emitter’s ZPL and PSBs. This linear Starkshift is consistent with an emitter exhibiting a permanent elec-tric dipole moment, a consequence of the defect’s geometrybreaking the inversion symmetry with respect to the hBNlattice plane [24,40–42].During the upward sweeps of the gate voltage, the emitterintensity increases abruptly before returning to its originalintensity during the sweep back down to zero gate voltage.This is exemplary shown in Fig. 1(c), which depicts twospectra at VG = 0 V and VG = 5 V that correspond to linecuts in Fig. 1(f) (see blue and magenta arrows). A directcomparison of the peak intensities, which were obtained byfitting a Lorentzian curve to the measured spectra, revealsthat the intensity increases by up to a factor of six. It isnoteworthy that this increase only happens at positive gatevoltages. Within the voltage range from VG = 0 V to –9 V noincrease in intensity is observed. Furthermore, by comparingthe gate voltage values where the emitter switches its inten-sity from dark to bright with the values where the intensityswitches back to its darker state it is apparent that the intensityvariations follow a hysteretic behavior (see the SupplementalMaterial [31] for further discussion). In order to explore theinterplay of the observed intensity variations with the leakage(a)1.0(b)0.50.0580 600λ (nm)620Î/Î      (a.u.)ZPL6543210 100P (μW)20030 meVZPL30 meVGhBN2DV  = 0VV  = 5VGGA       /AlowhighFIG. 2. (a) Comparison of the two spectra shown in Fig. 1(c)with their intensity normalized to 1. For the sake of clarity, peaksrelated to the Raman signals of hBN (574 nm [43]) and graphene (Gpeak at 581 nm and 2D peak at 620 nm [44]), which do not scale havebeen labeled. (b) Ratio between the bright and dark ZPL amplitudesas a function of the power of the excitation laser (laser spot size ofaround 500 nm). The dashed gray line serves as a guide to the eyeand depicts ideal linear behavior.current through the hBN, the extracted peak intensities of theZPL (blue points) are plotted alongside the simultaneouslymeasured current (red curve) in Fig. 1(g). This comparisonreveals that the intensity variations of the emitter are directlylinked to the current I through the hBN. Specifically, thetransitions from negative to positive current directions (seethe intersections between the red curve and the gray dashedline, the latter depicting the I = 0 pA level) coincide closelywith the emitter switching between its dark and bright state.This observation becomes even more evident when the ZPLintensity is plotted as a function of the leakage current, asdepicted in Fig. 1(d). A distinct transition from low to highZPL intensity is observed around zero leakage current, em-phasizing the role of the current in modulating the brightnessof the emitter. Note that the measured leakage current consistsof multiple contributions, leading to a nonperfect coincidencebetween the direction change of the measured current andthe intensity increase (see the Supplemental Material [31]).We note that, except for the shift in wavelength caused by theStark effect, the ZPLs as well as the PSBs match each otherfor both the dark and the bright states, as is evident whencomparing spectra of the emitter with the ZPL normalizedto 1 as shown in Fig. 2(a). Furthermore, we identify that alaser-induced photoexcitation process must play a crucial rolein the brightening effect of the emitter distinguishing it froma pure electroluminescence effect [45,46]. For this, Fig. 2(b)shows the ratio of the ZPL amplitudes in the bright and darkstates as a function of laser excitation power, which revealsthat the brightening effect becomes increasingly pronouncedat higher laser powers. We are not aware of any electro-static (dis)charging effects, which were previously attributedto intensity variations of bright hBN emitters [24–26], thatwould depend on laser power in such a manner. Moreover,the features of the observed brightening are not consistentwith an intensity modulation induced by a quantum confinedStark effect (QCSE), as the latter is expected to be continuousin electric field and not dependent on the excitation power[41,47] (see the Supplemental Material [31] for detailedL032037-3CORINNE STEINER et al. PHYSICAL REVIEW RESEARCH 7, L032037 (2025)V  = 0 V,  I = 0 pATG hBN BGμTG2.3 eV6 eV>2.1 eV μBGhBN+G(a) V  > 0 V,  I > 0 pAμTGμBGG(b) V  < 0 V,  I < 0 pATG hBN BGhBN+μBGμTGG(c)CBVBεTG hBN BGhBN+FIG. 3. Model that links changes in the charge transport across the hBN layers to the intensity variations of the emitter in the hBN+layer. The top gate (TG) and the bottom gate (BG) are made of few-layer graphene. (a) At VG = 0 V, no charge transport through the hBNoccurs and the emitter can only be excited by optical absorption in the hBN+ layer. (b) For VG > 0, photoexcited electrons (filled circles) andholes (open circles) from the gate electrodes, (shaded areas in the band structure of the FLG), undergo a photo-assisted field emission intocarbon-related donor and acceptor states, depicted by blue and red arrows, respectively. Electrons and holes flow towards the emitter in thehBN+ via laser-induced Poole-Frenkel like emission (blue and red curved lines) and recombine over the energy levels of the emitter, creatingthe brightening effect caused by electroluminescence. (c) For VG < 0, electrons injected by the back gate electrode scatter into mid-gap statesof the emitter layer before reaching the emitter, suppressing the electroluminescence effect.discussion). It can thus be concluded that the brightening ofthe dark emitter necessitates a photoexcitation process thatis also closely linked to the leakage current across the hBNlayers, as shown in Fig. 1(d).Based on the above arguments, we present a model (seeFig. 3) that proposes a photo-assisted injection of chargecarriers— driven by the interplay of laser excitation and gatevoltage—as the mechanism responsible for the brightening ofthe emitter.We approximate the gapless band structure of the FLGtop and bottom gates (TG and BG) by two parabolic bands[48]. At zero gate voltage, the respective chemical potentialsμTG,BG of the two gates are positioned at the charge neutralitypoints [Fig. 3(a)]. Under laser illumination (ε = 2.3 eV), elec-trons in the gate electrodes are excited from the valence intothe conduction band [indicated by the blue and green shadedareas in Fig. 3(a)], creating photoexcited electrons and holesup to 1.15 eV away from the chemical potentials. We note thatscattering processes among these photoexcited charge carriersmay push a small fraction of them to even higher energy states[49–51]. We now discuss the mechanism by which chargecarriers can be injected from the FLG electrodes into the hBNlayers. First, we note that the electric field strength at whichthe leakage current across the hBN begins to show a stronglynonlinear increase in Fig. 1(g) is rather small in comparisonto reference samples that are not showing a current-inducedbrightening effect (see the Supplemental Material [31]). Weattribute this low threshold voltage to the onset of photo-assisted field emission [52,53] into carbon monomer defect(CB, CN) states present in the hBN [54,55] (see the Supple-mental Material [31] for detailed discussion). These defectscan form both donor and acceptor states based on whether thecarbon atom replaces a boron (CB) or a nitrogen atom (CN)[56–61], as illustrated by the discrete energy states (grey andblue short dashes) in the hBN band gap in Fig. 3. Dependingon the calculation method [62], the corresponding energystates are predicted to lie anywhere from a few hundred meVup to 1.5 eV below the conduction band or above the valenceband edge [56–61]. Furthermore, carbon monomers have beenproposed to conduct gate leakage currents across the hBNlayers [54,55]. We therefore assume that under the applicationof a gate voltage [see Fig. 3(b)], photoexcited charge carriersfrom the gate electrodes undergo photo-assisted field emission[52,53] into these carbon monomer states [depicted by thehorizontal, blue, and red arrows in Fig. 3(b)]. The rate of sucha photo-assisted field emission is expected to depend nonlin-early on the electric field strength and linearly on the laserintensity, as described by Fowler-Nordheim theory [52,53].The predicted nonlinear dependence on the electric field isobserved in the current plotted in Fig. 1(g) at higher gate volt-ages. The predicted linear dependence on the laser intensity isconsistent with the laser power dependence shown in Fig. 2(b)(see also a previous study from some of us in Ref. [63]),supporting that the brightening effect is indeed caused bya photo-assisted electroluminescence effect. Because of thecontinuous laser illumination, charge carriers get continuouslyinjected into the defect donor and acceptor states, allowingthem to follow the externally applied gate electric field byhopping between different defect states via Poole-Frenkel likeemission [64] (represented by the curved blue and red linesfor donor and acceptor states, respectively; full circles repre-sent electrons, hollow ones holes). Note that this excitationmechanism and the corresponding photo-induced transport ofcharges between hBN and adjacent two-dimensional materi-als are in accordance with studies on photo-doping effectsin van der Waals heterostructures using hBN as a dielectriclayer [65–69]. The injection of both electrons into the hBNfrom one gate electrode and holes into the hBN+ from theother electrode [see Fig. 3(b)] leads to an explanation for theobserved brightening of the emitter’s ZPL and its connectionto the leakage current across the hBN. The brightening ef-fect can be understood in terms of photoexcited electron-holepairs that recombine over the energy levels of the emitterand thus create additional electroluminescence—an effect thatL032037-4CURRENT-INDUCED BRIGHTENING OF … PHYSICAL REVIEW RESEARCH 7, L032037 (2025)we denote as photo-assisted electroluminescence [see orangecurved arrow in Fig. 3(b)]. We support this proposed mech-anism by theoretical simulations that can be found in theSupplemental Material [31].Next, we focus on the asymmetry of the brightening effectwith respect to the gate voltage polarity and the resultinginduced leakage current. As we will explain in the following,this asymmetry is a direct consequence of the asymmetryin the sample structure and can be described by theoreticalsimulations using a rate equation model (see the SupplementalMaterial [31]). Because of the emitter generation process byannealing, it is likely that additional vacancy complexes (e.g.,multivacancy or vacancy-substitutional defects) have beencreated in the hBN+ that are not present in the untreated hBNlayer. These defects may lack radiative recombination chan-nels or be quenched because of their proximity to the FLGelectrode [32,33] but they provide extra energy states withinthe hBN+. Importantly, in contrast to carbon monomer defectspresent in both hBN layers—which create single donor oracceptor energy states—vacancy complexes can exhibit en-ergy levels with a series of occupied and unoccupied statesextending over the entire band gap range at the location ofthe defect (see the thin-stacked lines spanning over the hBN+band gap in Fig. 3 and the more detailed discussion in the Sup-plemental Material [31]) [6,7,38,56–59,70,70–73]. With this,the observed gate voltage asymmetry can now be explainedunder the assumption that the energy levels of these vacancycomplexes that are only present in the hBN+ may allow foradditional charge relaxation channels with differing relaxationrates for electrons and holes. At negative gate voltages, holesfrom the TG are injected into the hBN layer and can reachthe emitter in the hBN+ by hopping between acceptor states(as for the positive gate voltage case). However, electronsinjected from the BG now have additional relaxation channelswithin the hBN+, making it less likely that they will reach theupper emitter level related to the optical emission. Figure 3(c)provides a simplified illustration of this process by showingthat holes do not relax before reaching the emitter while elec-trons do [compare red lines in Fig. 3(b) with the blue lines inFig. 3(c)]. In fact, as we discuss in the Supplemental Material[31], it is sufficient that only a fraction of holes does notrelax before reaching the emitter. Provided that the relaxationof excited electrons into mid-gap states effectively preventsthe occupation of the upper energy level of the emitter, theelectroluminescence effect can be captured adequately by thepresented model.Interestingly, we also find hBN emitters that do not exhibita brightening but rather show a current (gate-voltage) induceddecrease in their emission intensity. Such an emitter is pre-sented in Fig. 4. The emitter is located in a second samplewith the same stacking order as shown in Fig. 1(a). Boththe position of the ZPL at 590.7 nm and the strength of thelinear Stark shift of s = 0.73 ± 0.01 nm/(V/nm) are similarto the other emitter [compare Fig. 1(f) with Fig. 4(a)]. How-ever, at positive electric field values, the emitter intensity nowdecreases in contrast to the previously discussed increase inintensity. Analogous to the first emitter, the change in intensity[blue data points in Fig. 4(b)] coincides with the direction ofthe leakage current switching from negative to positive values[red line in Fig. 4(b)] [74].V  = 0 V,  I = 0 pAG(c) V  > 0 V,  I > 0 pAG(d)0.51.0800-40040000 0.22 -0.236.5 -6.82.0950592591590-0.23 -0.23 -0.230.22 0.22 0.22-6.8 -6.8 -6.86.5 6.5 6.5(a)(b)10T = 4 K2.1012.0980V  (V)GÎ (a.u.)ε (eV)λ (nm)Î      (a.u.)ZPL I (pA)E (V/nm)TG hBN BGhBN+ TG hBN BGhBN+μBGμTGμTGμBGCBVBεFIG. 4. Gate voltage dependence of emitter intensity and leakagecurrent of a second emitter in a different sample with a ZPL at590.7 nm measured at 4 K with a laser power of 50 µW. (a) Emitterintensity as a function of the applied electric field. The emitterexhibits a reproducible linear Stark shift and voltage-dependentchanges in intensity. (b) Integrated ZPL intensity of Lorentz fittedpeaks from (a). The red curve depicts the simultaneously measuredgate leakage current. The blue shaded areas mark the electric fieldranges where the emitter intensity is decreased. As for the first emit-ter, a clear dependence of the intensity change on the gate leakagecurrent is observed. (c), (d) Adapted schematic of the previouslydiscussed model for this emitter. All notations like in Fig. 3. Panel(c) depicts the case for VG = 0. The chemical potentials of the FLGelectrodes are shifted towards the hBN valence band correspondingto the case of p-doped hBN. (d) For VG > 0 electron injection andthus electron-hole recombination in the emitter levels is blocked.Occasional de-occupation of the lower emitter layer blocks its PLemission.We now demonstrate that our model allows us to under-stand this darkening effect by assuming different doping inthe hBN layers of the two samples. For the first emitter,we assumed a rough balance of donor and acceptor states(Fig. 3), which resulted in the chemical potentials of the FLGelectrodes being aligned approximately to the center of theband gap of the hBN layers. Such an alignment can indeedbe observed in several angle-resolved photoemission spec-troscopy (ARPES) or scanning tunneling spectroscopy (STS)studies [57,71,75]. For the second emitter, we now assume p-doped hBN for which other studies have shown a pronouncedshift of the chemical potentials towards the valence band ofthe hBN [76–78] [see Fig. 4(c) for VG = 0]. This p-dopingrequires an excess of acceptor states near the valence bandedge that leads to a shift in the band alignment of the hBNL032037-5CORINNE STEINER et al. PHYSICAL REVIEW RESEARCH 7, L032037 (2025)relative to the FLG electrodes, where the chemical potentialsof the electrodes are now no longer in the center of the hBNband gap, but shifted closer to the hBN valence band edge.As a result, the dielectric strength of a structure with sucha band alignment would be reduced as it is indeed observedfor the second sample (see the Supplemental Material [31]).Based on this change in band alignment, the darkening of theemitter can now be understood from Fig. 4(d). Here, the bandalignment corresponds to p-doped hBN and is depicted fora positive gate voltage VG > 0, for which this emitter showsa decrease in intensity [in contrast to the brightening effect,compare Fig. 1(f) and Fig. 3(b)]. Because of the shift of thechemical potential towards the valence band, the top gate canno longer inject electrons into donor states of the hBN cappinglayer [see red cross symbol in Fig. 4(d)]. Instead, the wholeleakage current over the hBN is carried via holes over statesnear the valence band edge (see curved red arrows). The lackof electron injection inhibits electron-hole pair recombinationvia the energy levels of the emitter, i.e., there is no photo-assisted electroluminescence effect. Instead, the lower levelof the emitter occasionally becomes unoccupied because ofholes that enter the lower emitter level, visualized in Fig. 4(d).These holes have a long residence time within this level be-cause of the potential barrier formed by the low defect densitycapping layer. During such times, the emitter can no longer beoptically excited, leading to an effective decrease in emitterintensity. For negative voltages, holes may still enter the loweremitter level but are not stuck there because the defect densitywithin the emitter layer provides only small potential barrierstowards the next energy level. Thus, the photoluminescenceof the emitter is not blocked and the intensity does not change(see the Supplemental Material [31] for detailed analysis ofthe VG < 0 case).Conclusions. In summary, we have demonstrated that theintensity of V-C type hBN emitters can be modulated byphoto-assisted electroluminescence. For this, the necessaryleakage current is injected into the hBN layers from few-layergraphene gates via photo-assisted field emission. Depend-ing on the doping level of the hBN layers, we propose thatthe leakage current can either consist of both electrons andholes or only of holes. The first case leads to the possibil-ity of electron-hole pairs recombining at the location of theemitter, resulting in its brightening, whereas the second caseresults in the sporadic de-occupation of the emitter’s lowerenergy level, resulting in the emitter getting even darker.Our findings highlight the necessity for a reliable bench-marking of hBN crystals [79,80] and the need for a carefuldetermination of their doping levels in order to achieve elec-tronic control over the brightness of dark quantum emittersin hBN.Acknowledgments. The authors thank F. Lentz and S. Trel-lenkamp for their support in sample fabrication. Funded bythe Deutsche Forschungsgemeinschaft (DFG, German Re-search Foundation) under Germany’s Excellence Strategy -Cluster of Excellence Matter and Light for Quantum Com-puting (ML4Q) EXC 2004/1 - 390534769 and by the FederalMinistry of Education and Research (BMBF) and the Min-istry of Culture and Science of the German State of NorthRhine-Westphalia (MKW) under the Excellence Strategy ofthe Federal Government and the Länder. K.W. and T.T. ac-knowledge support from the JSPS KAKENHI (Grants No.21H05233 and No. 23H02052) and World Premier Interna-tional Research Center Initiative (WPI), MEXT, Japan. Thisresearch was funded in part by the Austrian Science Fund(FWF) [10.55776/DOC142, 10.55776/COE5]. Fabrication ofthe samples was supported by the Helmholtz Nano Facility(HNF) at the Forschungszentrum Jülich [98].The authors declare no competing interests.Data availability. The data supporting the findings of thisstudy are openly available [99].[1] T. T. Tran, K. Bray, M. J. Ford, M. 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