GaN Nanowire Arrays for Efficient Optical Read-Out and

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GaN Nanowire Arrays for Efficient Optical Read-Out and Optoelectronic Control of NV Centers in Diamond Martin Hetzl, Jakob Wierzbowski, Theresa Hoffmann, Max Kraut, Verena Zuerbig, Christoph E. Nebel, Kai Müller, Jonathan J. Finley, and Martin Stutzmann Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00763 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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GaN Nanowire Arrays for Efficient Optical Read-Out and Optoelectronic Control of NV Centers in Diamond Martin Hetzl,∗,† Jakob Wierzbowski,† Theresa Hoffmann,† Max Kraut,† Verena Zuerbig,‡ Christoph E. Nebel,‡ Kai Müller,† Jonathan J. Finley,† and Martin Stutzmann∗,† †Walter Schottky Institut and Physics Department, Technische Universität München, 85748 Garching, Germany ‡Fraunhofer Institute for Applied Solid State Physics IAF, 79108 Freiburg, Germany E-mail: [email protected]; [email protected]

Abstract Solid state quantum emitters embedded in a semiconductor crystal environment are potentially scalable platforms for quantum optical networks operated at room temperature. Prominent representatives are nitrogen-vacancy (NV) centers in diamond showing coherent entanglement and interference with each other. However, these emitters suffer from inefficient optical outcoupling from the diamond and from fluctuations of their charge state. Here, we demonstrate the implementation of regular n-type gallium nitride nanowire arrays on diamond as photonic waveguides to tailor the emission direction of surface-near NV centers and to electrically control their charge state in a pi-n nano-diode. We show that electrical excitation of single NV centers in such a diode can efficiently replace optical pumping. By engineering of the array parameters, we find

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an optical read-out efficiency enhanced by a factor of ten and predict a three orders of magnitude stronger lateral NV-NV coupling through evanescently coupled nanowire antennas compared to planar diamond not covered by nanowires, which opens up new possibilities for large-scale on-chip quantum computing applications.

Keywords GaN nanowires, NV centers, photonic crystal coupling, Far-field engineering

Introduction Color centers in diamond such as nitrogen-vacancy (NV) centers are of strong interest as qubits for emergent quantum technologies operating at ambient conditions. 1–4 Owing to the weak spin-orbit coupling of C atoms in the diamond crystal, electron (nuclear) spins of NV centers have coherence times extending up to one second. 5 However, difficulties arise in terms of tunability due to charge state fluctuations, which lead to blinking and spin state instabilities. 6,7 Furthermore, the large refractive index of diamond inhibits an effective optical outcoupling of surface-near emitters. Recently, different micro- and nanostructuring techniques have been applied to the diamond surface to significantly improve the external luminescence yield. Examples include solid immersion lenses (SILs) defined around individual NVs and the formation of diamond nano pyramids and nanowires (NWs). 8–11 Although these approaches have yielded enhanced luminescence signals, the underlying diamond bulk had to be irreversibly modified by etching, or the NV centers had to be placed with nanometer precision into the photonic structures. Due to their rather large spatial extent of several tens of micrometers, SILs are more technologically demanding for scaled NV center networks. Similarly, embedding NV centers within diamond NWs often results in a reduction of the arising life time of their excited states 2


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due to the proximal distance of the NV centers to a large and disordered surface area. 12 Furthermore, the top-down approach to diamond NW synthesis induces non-uniformities especially for downscaling towards hundreds of nanometers for NW diameter and pitch. 13 Lateral Schottky junctions, 14 gated surface termination layers and diamond p-i-n diodes have all proven to be promising techniques to reduce the charge state fluctuations. 7,15 However, due to their in-plane design on the diamond surface, all these approaches require large area structures on the sample or suffer from unsatisfactory optical read-out capabilities. In this article, we present a novel approach to nano-contact individual surface-near NV centers using gallium nitride (GaN) NW antennas grown site-selectively on highly pure, electronic grade bulk diamond. The GaN NWs provide a nanoscale electrical contact to control the NV charge states, locally enhance the light extraction efficiency and, through photonic crystal effects, sculpt the far-field radiation pattern. Thanks to the small footprint of GaN NWs, downscaling towards tens of nanometers is possible, providing the possibility to contact individual NV center/GaN NW complexes. Furthermore, the realization of diamond/GaN nano-diodes facilitates a highly effective electroluminescent excitation of NV centers localized within the area beneath a GaN NW. Numerical and experimental analysis of intensity far-field patterns of NW array-guided emission allows for a precise determination of the crystallographic orientation of individual surface-near NV centers within the diamond substrate. Moreover, modifications of the far-field characteristics via antenna engineering reveal a strongly enhanced lateral coupling of surface-near NV centers via evanescent field coupling, that paves the way for on-chip quantum communication over large networks.

Experimental Section The sample design used in this work is presented in Figure 1a. Single crystalline diamond (111) substrates of type IIb with 100 ppm boron and a rms roughness of ≤0.5 nm were fabri-

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Figure 1: Illustrations of the GaN NW/diamond heterostructures. (a) Schematic sketch of the p-i-diamond/AlN/n-GaN nano-diodes containing NV centers in the vicinity of the diamond surface. (b) Tilted view (45◦ ) SEM images of a hexagonally ordered GaN NW array with a NW diameter D of 220 nm and a pitch P of 500 nm for two different magnifications. cated via the high-pressure high-temperature (HPHT) method at the Technological Institute for Superhard and Novel Carbon Materials (TISNCM) in Moscow, Russia. 50-100 nm epitaxial type IIa intrinsic diamond (111) were grown on the boron-doped diamond via CVD with additionally supplied oxygen during growth to reduce the incorporation of boron from the substrate. 16 NV centers were implanted by CuttingEdge Ions LLC (Los Angeles, USA) via 15N+ ion bombardment with an acceleration voltage of 10 kV, a dose of 1·1011 cm−2 and a tilt angle of 7◦ . This is expected to result in NV centers in a depth of 15-30 nm and a high density of 30-50 NVs µm−2 by extrapolating the data from Ref. 17. In order to form NV centers, the samples were annealed for 4 h at 800◦ C under high vacuum conditions. Prior to group III-nitride growth, the diamond substrates were exposed to O plasma at 200 W for 5 min to remove contamination from the diamond surface and to guarantee a homogeneous surface termination. Previous studies concerning GaN NW growth on diamond have shown the appearance of both polarity types of the GaN wurtzite crystal, which is a sensitive parameter for the electronic properties of a GaN/diamond heterojunction. 18,19 In order to induce the nucleation of the preferential Ga-polar GaN crystal orientation, a 10 nm thin Al-polar AlN layer was grown via MBE under Al-rich conditions and a substrate surface temperature of 720◦ C. Afterwards, the sample was etched for 5 min in concentrated HCl to remove metallic

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Al clusters. N-type doped GaN NWs were grown via selective area epitaxy. For that, a 10 nm thick Ti film was thermally evaporated onto the AlN layer and hexagonal arrays of nano apertures with controlled diameter (D) and pitch (P) were defined via e-beam lithography using a positive e-beam resist to serve as nucleation sites and determine the diameters of the GaN NWs grown by molecular beam epitaxy (MBE). A detailed description of this process can be found in Ref. 20. In order to stabilize the Ti mask, a conversion to titanium nitride (TiN) by N plasma exposure at 400◦ C was conducted within the MBE chamber. N-type GaN NW growth was achieved at a substrate surface temperature of 740◦ C under highly N-rich conditions. Various GaN NW array geometries have been grown on the same diamond plate to allow a direct investigation and comparison of the optical and electronic effects without disturbing sample-to-sample variations. After the NW growth, the TiN mask was removed by a 1 h bath in HF (5%) solution, resulting in the final structure shown in Figure 1a. For future applications, e.g., quantum registers, downscaling towards a single NV center beneath each GaN NW is of particular interest. The existing sample design could easily be adapted by, e.g., using the TiN mask for both implantation of NV centers and nucleation of GaN NWs (see Supporting Information, Fig. S1), which facilitates the otherwise challenging spatial alignment of NV center locations and NW antennas. Figure 1b shows 45◦ -tilted view scanning electron microscopy (SEM) images of as-grown GaN NWs on diamond at two different magnifications. The figure illustrates the large scale growth of the NWs with nearly 100% yield, a uniform shape with a constant pitch of 500 nm, a diameter of 220 nm and a height of 630 nm.

For the electrical contacting of the GaN NWs, 100 nm of amorphous Al2 O3 was deposited as an electrical insulator onto the diamond and the GaN NWs via atomic layer deposition (ALD). The space in between the NWs was filled with S1818 photo resist and was etched by O plasma to reveal the Al2 O3 -coated NW top facets. H3 PO4 etching for 50 s at 100◦ C

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locally removed the Al2 O3 to reveal the conductive GaN NW top facets. Afterwards, the S1818 was removed. Indium tin oxide (ITO) was thermally evaporated and structured via photo-lithography to achieve transparent electrical contacts. To improve the conductivity of the ITO, the sample was thermally annealed for 5 h at 300◦ C. The electrical contact to the p-type diamond was achieved over the backside of the conductive diamond bulk. A schematic sketch of the sample structure can be found in the Supporting Information (Fig. S5b).

All optical measurements were performed with a home-built confocal photoluminescence (PL) microscope. The sample was excited with a frequency-doubled continuous wave Nd:YAG laser with wavelength of λ=532 nm. We employed an objective with a numerical aperture (NA) of 0.75 focusing the excitation laser to a spot (1/e2 contour) of ≈660 nm with a linearized power density of 250 kW/cm2 . The collected luminescence was analyzed by a Jobin Yvon Triax500 spectrometer with a focal length of 550 mm and a 300 lines/mm grating. The spectra were then recorded by a LN2 cooled Horiba Symphony II (open-electrode) c CCD camera. The laser was filtered with a Semrock RazorEdge long-pass filter (LP03-

532RE-25) with an onset at 90 cm−1 redshifted from the laser. We performed all cryogenic measurements with a helium flow cryostat at 10 K. Two-dimensional maps were recorded either with a closed-loop piezo system (Tritor102 CAP, Piezosysteme Jena) or a 4f scanning arrangement with a fast steering mirror (Newport FSM-300-01).

We collected the angular-resolved far-field (Fourier) patterns in the same PL microscope by adding an additional lens in the detection path (see Supporting Information, Fig. S2). Light entering the objective at different angles is focused in the back focal plane (BFP) that is located 15.5 mm inside the objective and a telescope (magnification M=1.6) projects the Fourier image onto a Peltier cooled CCD (Atik 314L+). On the BFP, equal emission angles (even from different spatial positions) are focused onto the same spot. The resulting image is the Fourier transform of the electric field at the focal plane of the objective and is

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equal to the far-field image of the propagating light. 21–23 In our experiments the PL signal was collected by a NA=0.75 objective that corresponds to a maximum collection angle of θmax =sin−1 (NA/nair ). In order to reduce the continuous background signal of NV centers beside GaN NWs and within the diamond bulk, the obtained Fourier images on the NWs were subtracted by a Fourier image outside the NW array. Additional remarks on the working principle of Fourier microscopy are attached in the Supporting Information.

FDTD simulations were performed with the Lumerical finite-difference time-domain (FDTD) Solutions software package, 24 solving Maxwell’s equation iteratively in a leap-frog algorithm. In order to simulate the transmission of the light emission of a single NV center in diamond via a GaN NW array, we used a simulation cell with perfectly matched layer (PML) absorbing boundaries. The simulation cell consists of one dipole emitter, representing the NV center, 15 nm beneath the diamond surface, a hexagonally ordered array of hexagonal GaN NWs with a quantity of 150 to 600 NWs within the cell, depending on the pitch used, and air as the surrounding medium. The far-field of the transmitted light was detected by a planar detection monitor above the GaN NW array. For the simulations in Figure 2d, the statistically more frequent and better transmitted NV center orientation of the dipole emitter of 19.5◦ with respect to the substrate surface has been used. To correlate the simulations with the experimental data, the detection monitor has been limited to an angle of 48.5◦ , equal to a NA=0.75 of the objective used.

Results and Discussion The radiative emission from the NV centers beneath the GaN NW arrays was probed using PL spectroscopy. For a quantitative comparison of enhancement factors, we refer our data to the nonstructured planar surface of diamond. The top view SEM image in Figure 2a

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Figure 2: PL enhancement of NV centers outcoupled via GaN NW arrays. (a) Top view SEM image of a GaN NW array corner with D=220 nm and P=500 nm and (b) PL intensity map of the NV center signal (λ=573-800 nm) of the same region. (c) PL spectra of the NV centers measured for different NW diameters and a fixed P=500 nm at a temperature of 10 K. The NW diameters are given in the figure using the same color as for the curves. (d) Simulated transmission of the NV center luminescence over a GaN NW array as functions of the NW diameter and the pitch. (e) Comparison of the NW diameter-dependent PL intensity of the NV centers for P=300 nm and P=500 nm with the corresponding transmission simulations (dashed lines) at an emission wavelength of 655 nm.

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shows the corner of the sample presented in Figure 1b and Figure 2b shows a false-color PL intensity map recorded at T=10 K and integrating the intensity over the spectral range λ=573-800 nm corresponding to the NV center emission. At every position we observe a distinct enhancement of the NV signal on the GaN NW array compared to the unpatterned region of the substrate. Due to the high NV center density, the PL signal is homogeneous throughout the NW array. A PL intensity map of an array with a large pitch of 4 µm revealing enhanced NV center outcoupling via single GaN NWs is attached in the Supporting Information (Fig. S3a). We also noted a similar enhancement effect for lower NV center densities (see Supporting Information, Fig. S3b), where separated emitters could be detected beneath a GaN NW array.

In order to analyze the influence of the geometry of the NW array on the PL spectra, measurements were performed for different NW diameters. In Figure 2c, we present examples of PL spectra (T=10 K) recorded for different NW diameters and an identical pitch of 500 nm and compare the results with the spectrum in the unpatterned region of the sample. Due to the large bandgap of GaN and the superior crystal quality of MBE-grown NWs, 19,20,25,26 no GaN-related PL contribution could be detected in the spectra, except the Raman lines at ≈550 nm. Both luminescent states of the NV centers are observed, namely the neutral NV0 charge state with the zero-phonon-line (ZPL) at 575 nm and the negative NV− state with the ZPL at 637 nm. 7,27 Since the spectral position of the broad phonon replica peak of the NV centers is located at longer wavelengths (λ ≈655 nm), we identify NV− as being the predominant charge state for the present sample design, where the NV centers are in a close vicinity to the p-type diamond substrate. 15 The PL enhancement factor is strongly influenced by the NW diameter (Fig. 2c) and also by the pitch (not shown), with a maximum intensity increase in the case of the phonon replica emission by a factor of ten to twelve for a NW pitch of 500 nm and a diameter of 220 nm. These values are higher than, or comparable to previous approaches to enhance emission, such as SILs and single diamond NWs. 8–12

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To quantitatively understand the observed PL enhancement effects, we performed FDTD simulations of the transmission (λ=655 nm) as a function of the NW diameter and pitch (Fig. 2d, c.f. Experimental Section). An increased outcoupling of the emission from NV centers occurs for NW pitches larger than 450 nm. For the NW diameter, we find a maximum of outcoupling for a diameter of ≈220 nm. In the literature, this effect has been assigned to the resonant coupling of the light to a specific waveguide mode within the NW photonic cavity. 11 A comparison of the experimental PL data and the simulated NV transmission is plotted in Figure 2e for two different NW pitches of 300 nm and 500 nm. The evolution of the PL signal with the NW diameter is in qualitative agreement with the simulated transmission. In addition, an overall increase of outcoupled light is observed for the larger pitch. In any case, the PL transmission of NV centers through GaN NW arrays is equal or higher compared to the bare diamond surface. In addition to the emission from the NV centers, two sharp emission lines arise from silicon vacancy centers (SiVs) at ≈740 nm (Fig. 2c). 28–30 This type of color center in diamond is also of significant interest for emergent quantum technologies and was most probably incorporated in the 50-100 nm intrinsic diamond layer during chemical vapor deposition (CVD). As for the NV PL signal, the luminescence of the SiVs is enhanced for a GaN NW array grown on top, demonstrating the general applicability of GaN NW arrays as antennas for the optical read-out of color centers in diamond and also on other substrate species containing coherent defect centers, e.g., SiVs in silicon carbide. 31–33

The collection efficiency of the NV luminescence is determined by its far-field radiation pattern, which can be deterministically influenced by the NW array parameters (pitch + diameter), as well as by the orientation of the NV center axis. In order to understand the far-field radiation profiles from single NV centers and ensembles transmitted and guided through the GaN NW arrays, we have used FDTD simulations and have experimentally verified the calculations by performing annularly resolved back focal plane imaging in the

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Figure 3: Far-field radiation patterns of NV centers outcoupled via GaN NW arrays. (a,b) Cross-sectional views of the NV light propagation, (c,d) simulated far-field images and (e,f) measured back focal plane images of a [111]- and a [1¯11]-oriented NV center, respectively. The NV centers are located beneath a NW array with a pitch of P=1000 nm and a NW diameter of D=260 nm. The scales on the left of each far-field image refer to the detection angle Θ of the emission, whereas Φ refers to the rotation angle. The orientations of the NV center axes are indicated by red arrows in (a,b). (g) Measured Fourier image of an ensemble of NV centers beneath a NW array with a pitch of P=600 nm and a NW diameter of D=160 nm and corresponding far-field simulations for different NV center orientations and their statistical superposition. A coordinate system indicating the solid angles is inserted in the figure.

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Fourier plane (see Experimental Section). The dipole axis of the NV center in diamond can occupy four different orientations, corresponding to the four bond directions of the tetrahedral diamond crystal lattice. Based on the diamond (111) surface orientation, this results in a NV center with a dipole parallel to [111] and three tilted by 109.5◦ with respect to the NW axis. The three tilted dipole configurations are each rotated by 120◦ with respect to the [111]-axis of the bulk diamond. Figures 3a-b show cross-sectional images of the simulated electromagnetic field (λ=690 nm), propagating from a NV center 15 nm below the diamond surface through a GaN NW array (1000 nm pitch), for an out-of-plane and a tilted dipole, respectively. The orientation of the NV center is indicated by a red arrow in the figures. Due to the low refractive index contrast of ≈0.8% between GaN (nGaN = 2.371) and diamond (ndiamond = 2.407) at this wavelength, 34,35 an efficient coupling of the light emitted from the NV center upwards into the GaN NW occurs. For an out-of-plane NV center (Fig. 3a), one fraction of the emitted radiations propagates within the NW cavity and is radiated into free space. The other part propagates evanescently along the NW side facets and progressively couples to neighboring NWs. The resulting interference pattern then strongly depends on the NW array geometry: The NW diameter determines the number of guided modes within the NW and the extent of the evanescent field, 36 whereas the pitch defines the interference with neighboring NWs. For the tilted NV center (Fig. 3b) the light coupled into the NW exhibits a set of total reflections along the NW side walls until it couples out over the NW top facet. In this case, next neighbor NW coupling appears, however, with pronounced asymmetric propagation directions due to the distribution of the NV-dipole moment over the four allowed tetrahedral axes. In Figure 3c, the simulated far-field detected above the GaN NW array is plotted for a parallel NV center. Here, the coordinate system is defined by the detection angle Θ, i.e. the angular offset of the emission from the detection axis, and the rotation angle Φ. A clear sixfold symmetry resembling the hexagonal arrangement of the GaN NW array is observed. 23

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For this particular diameter, the six high-intensity regions of the emission are located at angles of Θ ≈ 40◦ . Please note that the maxima in Fourier space are rotated by ∆Φ = 90◦ with respect to the array orientation in real space. However, for no NW the main emission occurs at angles Θ > 50◦ (not shown) due to refraction of the surface being out of the solid angle collected by the microscope used. This result shows that the emission is redirected as a consequence of the GaN NW above the emitter, producing a higher collection efficiency of emitted NV luminescence (c.f. Fig. 2). For the tilted NV center (Fig. 3d), the center of mass of the emission is located mostly close to the optical axis, with an angular offset of Θ ≈ 10◦ due to the angular tilt of the NV-dipole emitter. Consequently, the far-field images of NV centers guided through GaN NW arrays can give detailed information on the orientation of the emitting dipole and, thus, on the crystallographic orientation of individual NV centers within the diamond lattice. In order to verify the simulations experimentally, the far-field radiation from NV centers was measured via Fourier microscopy spectrally integrated over the wavelength range from 680 nm to 700 nm (see Experimental Section). 22,23 The high density of 30-50 NVs µm−2 gives rise to one or more NV centers beneath each GaN NW. Due to the large pitch of 1000 nm for this array our laser could be focussed onto single NWs and, thus, single (or few) NV centers. Examples of two Fourier images are plotted in Figures 3e and 3f. The hexagonal pattern in Figure 3e with high-intensity regions at detection angles of Θ ≈40◦ clearly resembles the simulated far-field of a NV center parallel to the [111] axis (Fig. 3c). As this NV center orientation occurs in diamond (111) with a probability of only 25%, it can be expected that this particular image originates from a single NV center beneath this GaN NW. Accordingly, the concentric intensity spot with an offset of Θ ≈10◦ measured at a different NW (Fig. 3f) can be assigned to a tilted NV center with a rotated in-plane orientation pointing towards Φ ≈-30◦ . Until now, single NV centers have been measured and analyzed via far-field imaging and simulations, in particular for large NW pitches (Figs. 3a-f). For smaller pitches, the finite

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laser spot size of ≈1µm leads to a larger quantity of GaN NWs transmitting the laser to excite ensembles of NV centers. The measured Fourier image of a NV center ensemble is presented in Figure 3g (Experiment). A six-fold pattern with high intensity spots at detection angles of Θ ≈25-30◦ has been observed at any probed site on this array (600 nm pitch, 160 nm NW diameter). In addition, characteristic fringes occur at the edges of the spots, smearing out towards higher detection angles. In order to understand this complex Fourier image, the far-fields of all possible NV orientations have been simulated for this NW array. For a NV center parallel to the [1¯11] axis (lowermost contour), an asymmetric radiation pattern tilted towards Θ = 20◦ in detection angle has been obtained, similar to Figure 3d. Due to a three-fold symmetry of the NV centers along [¯111] and [11¯1], identical but 120◦ -rotated patterns have been simulated (not shown). In distinct contrast, the Fourier pattern of an NV center along the [111] orientation again shows the six-fold symmetry caused by the hexagonal NW array (c.f. Fig. 3c). For an ensemble of optically excited NV centers beneath a GaN NW array, the fractions of different NV center orientations follow an equal statistical distribution and the resulting intensities can be summed up incoherently (Fig. 3g, [111]+[¯111]+[1¯11]+[11¯1]). Six triangular pyramidal structures appear, with intensity maxima towards smaller detection angles at Θ ≈25◦ . For a better comparison with the experimental data (Fig. 3g, Experiment), the superimposed far-field simulation is projected on a plane (Fig. 3g, Simulation). As a result, simulation and experiment agree in detail with each other. The mentioned fringes at higher angles are due to the NV centers oriented in [111] direction, whereas the superposition of the tilted NV centers forms the high-intensity spots of the six-fold symmetry. Additional far-field simulations and Fourier images of NW array parameters with even higher collection efficiencies are attached in the Supporting Information (Fig. S4). Our results demonstrate that GaN NW arrays can efficiently redirect PL from surface-near NV centers into solid angles easily accessible with air immersion objectives. This simplifies optical access to the emission centers and facilitates experiments at low temperatures. In

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addition, one can readily reconstruct the exact crystallographic orientation for individual NV centers from measuring and numerically comparing the far-field emission pattern. We now turn to the influence of an externally applied voltage on the NV center luminescence in a p-i-diamond/n-GaN nano-diode. The combination of p-type diamond and n-type GaN is advantageous compared to pure diamond-based diodes since n-type doping of diamond is still very challenging due to high ionization energies of phosphorus donors. 37–39 After electrically contacting the GaN NW arrays and the p-diamond substrate (see Experimental Section), the current-voltage characteristics of our structures as well as voltage-dependent PL and electroluminescence (EL) from individual NV centers below single GaN NWs have been investigated. In Figure 4a, a current-voltage curve of a p-i-diamond/n-GaN NW diode is plotted, where the n-type GaN NWs have previously been electrically grounded. Note that the measured current refers to the absolute current flowing through a 50x50 µm2 NW ensemble contact. A clear diode behavior can be observed, with a rectification ratio of 103 at 10 V. An ideality factor of 2 to 3.5 indicates recombination current as the main transport mechanism within the junction. 40 This is in agreement with numerical band structure simulations of the heterodiode (c.f. Supporting Information, Fig. S5a), predicting a suppressed electron diffusion from the GaN into the diamond conduction bands. 18 For voltages higher than 10 V, the flowing current is limited by the series resistance of the device RS =2.0 kΩ. Current-voltage measurements on separate parts of the device assign RS to almost equal parts to the ITO conducting paths and the i-p-diamond bulk, whereas the minor part of the applied voltage drops at the actual pin diode interface. For voltages lower than 7 V, a symmetric current profile with a parallel resistance RP =28.5 MΩ indicates the presence of leakage currents. Figure 4b shows voltage-dependent luminescence spectra of NV centers beneath a contacted NW array under optical excitation. Due to the small pitch of 600 nm, numerous NWs have been exposed to the laser, leading to an excitation of ensembles of NV centers. Owing to the low refractive index of ITO (nIT O = 1.72 at λ = 690 nm), 41 deposited at the top and

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Figure 4: Voltage-dependent NV center luminescence from p-i-diamond/n-GaN nano-diodes. (a) Current-voltage measurements of a p-i-diamond/n-GaN nano-diode for an ensemble of contacted NWs with P=600 nm and D=260 nm. (b) Voltage-dependent luminescence spectra originating from a superposition of PL and EL from the nano-diodes under optical and electrical excitation and (c) fitted luminescence intensity ratios of the NV0 and the NV− emissions for NW ensembles and a single NW. (d) Comparison of luminescence spectra of NV centers beneath a single GaN NW under zero and forward bias with and without optical excitation.

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side facets of the GaN NWs for electrical contacting, and its large refractive index contrast to diamond, a similar PL enhancement effect of NV centers guided via GaN NW arrays can be expected. However, FDTD simulations (not shown) indicate a shift of the optimum array parameters for a maximum PL outcoupling efficiency by the order of tens of nanometers, which has to be taken into account in a hybrid PL/EL device. At zero bias, i.e. pure PL, most of the NV centers are in their NV− charge state (c.f. Fig. 2c). For highly negative voltages a similar spectrum has been obtained with, however, a slight decrease in the PL intensity. For voltages higher than ≈+18 V the NV phonon side band gains in intensity, leading to a blueshift of the overall spectrum until saturation occurs for voltages higher than ≈+35 V. Consequently, for high enough forward bias, a shift of the photo-excited phonon replica peak from NV− (peak at ≈682 nm) towards NV0 (peak at ≈613 nm) occurs. 14,27,42 In order to investigate the influence of the applied voltage on the luminescence of the nanodiodes, voltage-dependent measurements in the absence of optical excitation were performed. For this case, pure EL was detected from the NV centers blueshifted compared to PL measurements at zero bias (Supporting Information, Fig. S5c). An approximately linear increase of the EL intensity with applied voltage and a series resistance-limited current in the bias region between 18 and 40 V was observed (Supporting Information, Fig. S6). In addition, no energetic shift of the EL phonon replica peak occurred with increasing voltage. Note that also power-dependent PL measurements between 50 W/cm2 and 2 MW/cm2 were performed, which also indicate no spectral shift of the NV centers for different excitation intensities (not shown). Thus, not the excitation density, i.e. current density or photon flux, but the actual excitation mechanism is important for the resulting NV center charge state distribution. While photo-excitation preferentially induces NV− charge states, electrical excitation leads to an increase of NV0 . This is probably caused by different types of charge carriers involved: In PL, electron-hole-pairs are generated and recombine directly via the NV centers. In contrast, for EL, an asymmetric charge carrier distribution of holes injected from the p-diamond and electrons from the n-GaN is expected due to the large conduction band offset of GaN and

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diamond (Supporting Information, Fig. S5a), leading to an increase of NV0 charge states. Consequently, the observed spectral shift from NV− towards NV0 in voltage-dependent luminescence measurements under optical excitation (Fig. 4b) can be explained by a complex interplay of photo- and electrical excitation of the NV centers within the nano-diodes, which allows a tuning of the NV charge state distribution via laser power and applied voltage. However, full conversion to NV0 emission could not be reached, which holds also true for measurements at single NWs and, thus, single or few NV centers. In order to quantify the degree of charge state control, the voltage-dependent luminescence evolution of the NV charge states from Figure 4b has been fitted by two Gaussian functions with peaks centered at the literature values of the phonon replica peaks for NV0 and NV− , respectively. The resulting voltage-dependent ratios of the integrated luminescence signals of NV0 and NV− peaks are plotted in Figure 4c for two different ensemble measurements on the same NW array (black and blue dots) and a single NW (red dots). For all luminescence measurements and voltages lower than +18 V, a constant intensity ratio of IN V 0 /IN V − of 0 to 0.09 occurs. For higher voltages, i.e. in the high injection regime, a sharp increase of the NV0 charge state appears, which saturates at 30 to 40 V. A maximum ratio of 0.4 was obtained by measuring at NW arrays with a high density of implanted NV centers (black dots). However, for different measurement spots, different saturation levels occur (blue dots). This can have two reasons: Firstly, polarity inversions in the GaN NWs can occur due to an insufficiently homogeneous AlN buffer layer. 43 This would lead to a distortion of the band structure at the GaN NW/diamond interface, 18 such that carrier recombination due to electrical excitation does not occur in the vicinity of the NV centers but on the GaN side of the junction. 18,19 Secondly, the undoped 10 nm thin AlN buffer in between the diamond and the GaN NWs serves as an additional blocking layer for electrons from the GaN conduction band to inhibit tunneling into the NV energy states. Thickness fluctuations of the AlN layer lead to local variations of the electron injection efficiency into the diamond and, thus, to different equilibrium charge states of the NV centers. In both cases, a thicker AlN blocking layer is

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expected to improve the electric control of the charge state of NV centers. Note that the voltage-independent spectral position of the NV center luminescence for pure EL (Fig. 4c, green dots and Supporting Information, Fig. S5c), i.e. in the absence of optical excitation, might be explained by the high voltage required to inject current into the heterodiode. In this voltage regime (>18 V), the quasi Fermi levels within the space charge region of the heterodiode, i.e. the intrinsic diamond region, are expected to show no significant changes anymore. This leads to the constant charge state distribution of the NV centers observed for pure EL. Figure 4d depicts the individual and combined PL/EL response of NV centers beneath a single GaN NW. Here, an energetic blueshift accompanied by an increase of the luminescence signal can be observed when applying a voltage of +40 V on this NW (Fig. 4d, red and blue curves), similar to the ensemble measurements (Fig. 4b). Surprisingly, after turning off the optical pump to measure pure EL (Fig. 4d, red curve), the same spectrum has been observed, apart from the absence of the diamond Raman line at 572 nm. This means that under large enough forward bias, i.e. the saturation regime in Figure 4c, the NV centers beneath the GaN NW in a p-i-diamond/n-GaN nano-diode can efficiently be operated via electrical excitation alone, whereas for additional laser exposure, the NV centers are completely screened from optical excitation. The communication of NV centers with each other, e.g., in form of quantum interference or quantum entanglement of single photons with specific spin states are promising ways towards quantum registers and quantum information processors operating at room temperature. 3,8,44 The concept of GaN NW arrays acting as antennas can be tailored towards this application. By choosing specific boundary conditions for the guided light inside individual NWs, the NV center emission will couple laterally along the array. In Figure 5a, FDTD simulations of the electric field emitted from a [111]-oriented NV center (at x=0) are plotted in a crosssectional view for a planar diamond surface and with additional GaN NW arrays with pitches of 1000 nm and 500 nm placed directly above the NV center. For the purpose of enhancing

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Figure 5: Far-field engineering for lateral NV center communication. (a) Cross-sectional view of the simulated electric field emitted from a [111]-oriented NV center in planar diamond and beneath GaN NW arrays with a NW diameter of 100 nm and pitches of 1000 nm and 500 nm. (b) Simulated horizontal intensity profile of the NV center luminescence at the height of the NV center of -15 nm beneath the diamond surface. the evanescent outcoupling of the guided light through the GaN NW side walls, small NW diameters of the order of 100 nm have been chosen. 45 By using a NW length according to m the Fabry-Pérot condition of L= 2n λ, where m is an integer number and n the refractive

index of GaN at the respective wavelength, the electric field exhibits a node at the end facet, suppressing emission into free space. 46 For a plane surface (Fig. 5a, top), the large refractive index contrast of diamond and air leads to a NV center emission preferentially directed towards the diamond bulk, whereas a lateral propagation of light is strongly suppressed. In contrast, the introduction of a GaN NW array with a pitch of 1000 nm (Fig. 5a, center) leads to an enhanced outcoupling in both axial and lateral directions. As the refractive index of GaN is almost equal to diamond for this wavelength regime, 34,35 an unperturbed transition and funneling of the emission into the NW occurs. Owing to the Fabry-Pérot condition of the NW length, a reduced outcoupling over the upper facet involves an enhanced evanescent field at the side walls, which, in turn, couples to neighboring NWs. The same effect applies for a 500 nm pitch (Fig. 5a, bottom), with even stronger coupling between neighbor NWs.

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Assuming single NV centers beneath each GaN NW (c.f. Supporting Information, Fig. S1b), a photonic coupling from one excited NV center into neighbor NV centers is possible via antenna-to-antenna communication. In order to quantify this, horizontal profiles at the NV center depth of the squared electric field, proportional to the emission intensity, are plotted in Figure 5b. For all NW pitches, a pronounced intensity increase occurs in lateral directions spread over several micrometers compared to planar diamond. In addition, distinct resonances appear at the NW sites, indicating an effective back-action of light into the diamond. This photonic interplay between individual NW antennas gives rise to on-chip NV-NV coupling. In our simulations, we have found the best NV-NV coupling for array pitches of ≈ 500 nm with an enhancement of 500 at the next neighbor sites compared to planar diamond. Due to varying strain fields within the diamond, a fluctuating detuning of the transition energies of excited spin states occurs for individual NV centers. 44 In order to allow coherent spin interactions between different NV centers, compensating localized electric fields can be applied at each NV center via separately contacted p-i-n nano-diodes (c.f. Fig. 4).

Table 1: List of methods for NV center PL enhancement. The enhancement factors refer to the NV center signal from a planar diamond substrate. For the oil immersion lens, the given range is due to different orientations of the NV center. For all other methods, experimental values are used. Method Oil immersion lens SILs

Enhancement factor Advantages factor 3 - 11 Easy handling 6 - 10

Diamond NWs 3 - 13 & umbrellas GaN NW array 10 - 12

Disadvantages

Ref.

Contamination; Simulation no low temperature Simple fabrication Large areas 8 required Same material Challenging 9,10 as substrate n-type doping Far-field engineering; MBE growth This work electrically active required

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Conclusions We successfully showed the implementation of periodic n-type GaN NW arrays on diamond for efficient optical read-out and electrical control of surface-near NV centers. The advantages of GaN- compared to diamond-based structures are a distinctly higher controllability and flexibility in terms of nanostructuring, optical quality and electronic properties. A comparison of different PL enhancement methods for NV centers is listed in Table 1. Waveguiding of NV center emission through GaN NW arrays exhibited remarkable optical properties and tunability. Through tailoring the array geometry, we observed an enhancement of the collected far-field signal by a factor of ≈ 10. Moreover, by measuring the far-field patterns of individual NV centers and ensembles, we could unambiguously identify the orientation and composition of dipoles in the diamond lattice beneath the GaN NW array with FDTD simulations. The injection of electrical current through p-i-diamond/n-GaN nano-diodes and the resulting EL of NV centers was demonstrated to effectively substitute optical pumping. The discrepancy of the charge state distribution for photo- and electro-excited NV centers indicates sample-dependent charge state equilibrium conditions that can be tuned by optimizing the sample structure, e.g., via introducing a charge carrier blocking layer. By adjusting the array parameters, simulations predicted an enhancement of the optical coupling of neighboring NV centers through evanescently coupled GaN NW antennas by almost three orders of magnitude. The combination of the presented techniques, namely downscaling towards single NW antenna/NV center contacts, far-field engineering of the NV center emission for quantum communication and their purely electrical operation are fundamental findings to pave the way for establishing high quantities of on-chip controlled room temperature-operated quantum bits.

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Acknowledgement Financial support by the Deutsche Forschungsgemeinschaft (DFG) via the Forschergruppe 1493, the ExQM PhD programme of the Elite Network of Bavaria, TUM.solar in the frame of the Bavarian Collaborative Research Project "Solar technologies go Hybrid" (SolTec), the Bavarian Academy of Sciences and Humanities and the excellence cluster Nanosystems Initiative Munich (NIM) is gratefully acknowledged. The authors thank Julia Winnerl and Richard Hudeczek for their help with the FDTD simulations and Felix Eckmann for the ALD growth of Al2 O3 .

Supporting Information available Sample design for single NV center/GaN nanowire contacts. Principle of Fourier microscopy. Photoluminescence enhancement of single NV centers. Fourier images of optimized antenna geometries. Sample design and electroluminescence from NV centers in a p-i-diamond/nGaN nano-diode. Voltage- and current-dependent electroluminescence signal.

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GaN Nanowire Arrays for Efficient Optical Read-Out and

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