Sub-2 Kelvin characterization of nitrogen-vacancy centers in silicon carbide nanopillars

Quantum emitters in semiconductor substrates is a rapidly maturing field for quantum information technologies. However, many of the ambitious plans for nanophotonic-based quantum computers require significant work on the integration of on-chip single-photon sources and detectors ¹. There is a myriad of implementations of quantum emitters that can be integrated with nanophotonic devices: semiconductor quantum dots, color centers in diamond, silicon, and silicon carbide, and more recently in 2D materials like hBN or CrCl₃ as well as rare earth ions implanted into semiconductors ^{2, 3, 4, 5, 6}. In particular, silicon carbide has a number of useful optical properties such as the large electronic bandgap of $3.26$ eV that allows for a wide variety of color centers and the availability of high quality, affordable, large-scale wafers and as such has attracted attention as a quantum information platform ^{7, 8, 9, 10}. Specifically, the NV center in 4H-SiC has been characterized at room temperature as a single-photon source with a lifetime of 2.1 - 2.8 ns ^{11, 12}. Additionally, coherent control of NV centers has been demonstrated on both ensembles and single emitters at room temperature and 10 K ^{11, 13}. Further, work is being done to enable the transfer of electron-spin polarization to nearby nuclear spins ¹⁴ which could enable quantum memory technology. Single digit temperature characterization and integration into nanophotonic devices are two areas that had previously remained unexplored in the NV center parameter space until this work.

In parallel with demonstrations of the 4H-SiC NV center’s potential for quantum communication, two significant developments in commercially available lab equipment occurred: closed-cycle, optical cryostats that reliably achieve temperatures below 2 K from companies like Montana Instruments and ICEoxford, and superconducting nanowire single-photon detectors (SNSPDs) that have high sensitivity ($\geq 80\%$), low dark counts ($<100$ Hz), and low recovery time ($\sim 50$ ns) with operating temperatures above 2 K from companies like Quantum Opus and Single Quantum. The potential combination of these two technologies has led to new opportunities for quantum optics groups that work in the single-photon regime. We introduce the ICECAP, an Integrated Cryogenic system for Emission, Collection And Photon-detection, as the first such incorporated system. We then use it to demonstrate low single-digit temperature spectral properties of NV centers integrated into 4H-SiC nanopillars for the first time. This work represents a significant step toward the NV center in 4H-SiC as a scalable, telecom-compatible, nanophotonic platform.

Color centers in bulk emit light in all directions which results in significant losses when photons are collected via an objective lens. For the purposes of quantum information processing, enhanced collection, achievable by, for example, integration with, nanopillars, would impact the color center’s rate as a single-photon source, entanglement distribution success rate, as well as the sensitivity of the spin-qubit readout.

We use the finite-difference time-domain (FDTD) package in Ansys Lumerical to simulate the outcoupling properties of SiC nanopillars at the emission wavelengths of the NV color center in 4H-SiC. Nanopillars with a height of 1 $\mu$m and diameters varying from 300-1100 nm were simulated, with a dipole emitter positioned at the lateral and longitudinal center of the nanopillar. We calculate the light collection efficiency of the color center emission for an objective with numerical aperture of 0.85, positioned above the nanopillar. We observe that the nanopillar guides the dipole emission towards the top of the nanopillar as shown in Figure 1a. The light collection efficiency varies as a function of nanopillar diameter and the dipole source orientation inside the nanopillar, as shown in Figure 1b. We find that the nanopillars boost the light collection efficiencies around five-fold for axial and more than ten-fold for basal color centers when compared to $\sim$2% value for an emitter at the same depth (500 nm below the surface) in the bulk.

The 4H-SiC samples are first commercially implanted (CuttingEdge Ions, LLC) with ¹⁴N⁺ ions at an energy of 375 keV and a dose of 1 $\times$ 10¹⁴ cm^{-2 12} to generate a peak nitrogen concentration at a depth of 500 nm, as calculated by Stopping and Range of Ions in Matter (SRIM) simulations. The implants are then activated by annealing the samples in a 1-inch Lindberg Blue tube furnace at 1050 ^∘C in nitrogen atmosphere for 60 minutes ¹². We then pattern arrays of circular holes in a 350 nm PMMA layer, with diameters ranging from 300-1100 nm, using e-beam lithography. A 5 nm titanium adhesion layer followed by a 50 nm nickel hard mask layer are then e-beam evaporated and lifted off to transfer the circular patterns to this metal layer. Nanopillars are etched into SiC through inductively coupled plasma reactive ion etching (ICP-RIE) involving SF₆ and O₂ chemistry ¹⁵. The SF₆ and O₂ flow rates are maintained at a ratio of 4:1 to achieve an etch rate of $\sim$300 nm/min and a pillar height of 1 $\mu$m. The metal layer is then removed by etching the samples in Transene’s nickel and titanium etchants. Scanning electron microscope (SEM) images of the fabricated nanopillars are shown in Figure 1c-d.

Traditionally, a 4f confocal microscope for single-photon cryogenic measurements of nanophotonic systems, like the one illustrated in Figure 2a, consists of a few core parts: a pump laser, steering optics, the 4f optics^***4f optics consist of a mirror, two lenses each with focal length f, and a microscope objective. The mirror is placed as the starting point. The first lens is placed 1f away from the mirror and the second lens is placed 2f from the first lens. Finally the microscope objective is placed 1f from the second lens giving a total length of 4f., sample cryostat, and single-photon detectors. Current commercially available single-photon detectors, such as single-photon avalanche diodes, that are housed at room temperature typically have collection efficiencies at about $50\%$ or lower and dark counts of $<100$ Hz. These figures of merit can be improved upon by using SNSPDs. However, SNSPDs have typically worked at $T<2$ K, which come with monetary costs of buying and maintaining an extra cryostat in addition to the time cost of maintaining and the physical lab space required to house the extra equipment. For optically accessible non-dilution cryostats, the typical operating temperatures have been $\geq 3$ K which has precluded incorporating SNSPDs directly into the optical cryostat. Optical dilution refrigerators exist, but they generally require microscope objectives with much longer working distances and lower numerical aperture, making single-photon collection more difficult.

The cryostat used in this work is a Montana Instruments Cryostation xp100 model with a 2-inch diameter sample space and a base temperature of 1.56 K. The microscope objective is internal to the vacuum shroud which allows for the use of an objective with numerical aperture of $0.85$ to maximize collection efficiency. The SNSPDs (Figure 2b), fiber feedthrough (Figure 2c), and SNSPD control electronics used are from Quantum Opus and are maximally efficient at 1310 nm with an operating temperature of 2.5 K or below.

The sample in the cryostat is optically accessible without the use of fiber optics, which allows for relatively easy use of confocal microscopy techniques. Its low temperature capabilities also make prototyping on-chip integrated SNSPDs much easier and will make possible in-situ comparisons between on-chip experiments and commercially available SNSPDs. The sensitivity and recovery time of SNSPD devices built into the cryostat allow for fast experiments, like the lifetime measurements reported in this paper that comprised of millions of photon detections and were taken in a single hour. Further, this technique can also be expanded toward fully inside-the-cryostat detection by coupling light from a photonic device to a tapered fiber ^{16, 17} and subsequently directly coupling the fiber to the SNSPD inside the chamber.

To characterize the NV centers in the fabricated nanopillars, we perform photoluminescence (PL) measurements in ICECAP. First, we confirm the presence of the four zero-phonon lines (ZPLs) that correspond to the four possible nonequivalent lattice sites for the NV center in 4H-SiC. Figure 3a has the results of one such measurement performed at 1.56 K with a pump laser of 785 nm; the ZPLs agree with the literature values ^{18, 19, 20, 21}. For all of the measurements reported in this work, we use an 1150 nm longpass filter to remove the signal from the intrinsic divacancy color centers.

As a test of the SNSPDs, we focus on an area of the chip that has both fabricated nanopillars and an unetched area, and perform a 2D PL scan across it. Figure 3b shows that not only are the single-photon detectors detecting signal from the PL scan, but also that there appears to be enhanced collection from the pillars compared to the unetched bulk, as expected from the simulations reported in the previous section. The completely etched area in between these two regions of interest shows no light collection because the etching depth is greater than the color center implantation depth, so any NV centers that were present have been etched away.

To check that the enhanced collection mentioned above includes photons emitted from NV centers, we compare measured NV center ZPL photon counts extracted from the PL spectra from the 600 nm pillars and from the bulk unetched area shown in Figure 3a. Each peak is fitted to a Lorentzian and the error reported is the standard error of the given parameter. The percent increase in amplitudes for each ZPL can be seen in Table 1. We see statistically significant collection enhancement for all 5 peaks measured. Notably, this demonstrated collection enhancement follows the simulated pattern in Figure 1b. The axial ZPL counts (1179 nm and 1222 nm) are enhanced about five-fold, while the basal ZPLs (1175 nm and 1243 nm) achieve 14- to 28-fold collection efficiency enhancement. The 1173 nm peak has been occasionally misclassified as due to a tungsten impurity ²², but further studies have ruled out the tungsten lines ²³ and confirmed the $kh$-like polarization behavior of this peak and reclassified it as an NV center peak ²⁴.

Another important conclusion drawn from the PL spectra comparison study is that very little ZPL broadening, if any at all, was observed. Table 1 reports the ratio of the widths of each of the Lorentzians calculated from the nanopillar PL spectrum to those calculated from the bulk spectrum in the relative broadening row. Therefore, even if the fabrication methods do induce small amounts of strain into the surrounding bulk material, NV centers are robust to it.

Additionally, we measure the optical lifetime of all 4 NV center orientations collectively in the 600 nm diameter nanopillars and also the lifetimes of the $kk$ and $hk$ orientations using bandpass filters formed from rotating both a long-pass filter (Thorlabs FELH1250) and a short-pass filter (Edmund Optics 89-675 1250nm SP) under white light excitation until only the regions of interest were passed. Plots of these measurements are shown in Figure 4. Each decay is fitted to a biexponential function $A_{1}\exp(-t/\tau_{1})+A_{2}\exp(-t/\tau_{2})$, where $A_{1}$, $A_{2}$, and $\tau_{1}$ are fitting parameters and $\tau_{2}\approx 0.54$ ns is fixed as the decay rate of the pump laser pulse reflecting off surfaces inside the sample chamber, which was measured separately. This is necessary because these back-reflections of pump light are simultaneous with color center emission. While this back-reflection signal is significantly attenuated by a 1150 nm longpass filter prior to detection, it is still present in the data due to its vastly greater initial brightness relative to the NV center emission.

We find a collective lifetime $\tau=(2.5\pm 0.1)$ ns, which is in proximity of the previously reported value $\tau=(2.7\pm 0.01)$ ns of ensemble NV center lifetimes at a higher temperature of 20 K ¹¹. The individual ZPL lifetimes are $2.85\pm 0.01$ ns and $2.21\pm 0.01$ ns for the $kk$ and $hk$ ZPLs respectively. This disagrees slightly with the values reported for single NV centers at room temperature which range between 2.3 ns and 2.5 ns for three axial- and three basal- labelled NV centers; the specific orientation of the sites ($hk$ vs $kh$ or $kk$ vs $hh$) is not explicitly mentioned in this work ²⁵. This variation may be due to differences in laser power ¹², temperature, or a sampling anomaly.

In this work we demonstrate, to our knowledge, the first integration of 4H-SiC NV^- centers into nanopillars and report the spectral properties of NV center ensembles at temperatures below 2 K. These nanopillars display enhanced collection, up to 28-fold, relative to bulk measurements which will enable SiC NV center applications as efficient single photon sources and spin-photon entangling interfaces. The measured lifetimes and spectral properties agree well with literature results for both ensemble and single NV centers at higher temperatures in bulk samples, and no significant spectral broadening from nanofabrication-induced strain or surface proximity was observed. The success of these measurements paves the way for more complex nanophotonic structures like the on-chip beamsplitters or integration with on-chip single-photon detectors ^{26, 27}.

In this work we also developed new instrumentation, named ICECAP, that incorporates SNSPDs directly into the cryogenic sample chamber for spectroscopic characterization of NIR quantum emitters. The versatility of this system should be of particular interest to industry partners. Turn-key ICECAP instruments could conceivably be produced for less than the cost of a separate optical cryostat and SNSPD cryostat combination. In terms of future quantum workforce, movement toward more monolithic systems would lead to technicians and engineers needing training on fewer instruments. Integrated instruments like ICECAP will also lead to a decrease in energy, helium, and total physical footprint costs of future QIST centers. Furthermore, the combination of the ICECAP system, refined fabrication techniques, and the favorable properties of NV centers in 4H-SiC as spin-1 color centers that emit single photons in the near infrared makes for a rapidly emerging quantum information technology and science workhorse.

We acknowledge support from NSF CAREER (Award 2047564). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work was partially supported by the UC Davis Physics REU program under NSF grant PHY2150515.

The authors acknowledge Dr. Tevye Kuykendall from the Inorganic Nanostructures Facility at the Molecular Foundry for his valuable support with the annealing furnaces. We thank Amy Conover and Dr. Aaron Miller from Quantum Opus and Ian Durnford and Taylor Bohach from Montana Instruments for their support when incorporating the SNSPDs and cryostat into a single system. Part of this study was carried out at the UC Davis Center for Nano and Micro Manufacturing (CNM2).