Generation of Spin Defects by Ion Implantation in Hexagonal Boron Nitride Nai-Jie Guo,1, 2 Wei Liu,1, 2, ∗ Zhi-Peng Li,1, 2 Yuan-Ze Yang,1, 2 Shang Yu,1, 2 Yu Meng,1, 2 Zhao-An Wang,1, 2 Xiao-Dong Zeng,1, 2 Fei-Fei Yan,1, 2 Qiang Li,1, 2 Jun-Feng Wang,1, 2 Jin-Shi Xu,1, 2 Yi-Tao Wang,1, 2, y Jian-Shun Tang,1, 2, z Chuan-Feng Li,1, 2, x and Guang-Can Guo1, 2 1CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, P.R.China 2CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, P.R.China (Dated: May 26, 2021) Abstract: Optically addressable spin defects in wide-bandage semiconductors as promising sys- tems for quantum information and sensing applications have attracted more and more attention recently. Spin defects in two-dimensional materials are supposed to have unique superiority in quantum sensing since their atomatic thickness. Here, we demonstrate that the negatively boron − charged vacancy (VB ) with good spin properties in hexagonal boron nitride can be generated by ion implantation. We carry out optically detected magnetic resonance measurements at room tem- − perature to characterize the spin properties of VB defects, showing zero-filed splitting of ∼ 3.47 − GHz. We compare the photoluminescence intensity and spin properties of VB defects generated by different implantation parameters, such as fluence, energy and ion species. With proper parameters, − we can create VB defects successfully with high probability. Our results provide a simple and prac- ticable method to create spin defects in hBN, which is of great significance for integrated hBN-based devices. Keywords: 2D materials, hexagonal boron nitride, negatively boron charged vacancy, spin prop- erties, ion implantation Solid-state spin defects have attracted widespread at- fects have great photostability and exhibit excellent spin − tention as promising quantum systems in the last decades properties at room temperature [24]. Besides, the VB [1], and have numerous applications in quantum infor- defects have a triplet ground state (S = 1) and can be mation [2, 3] and quantum sensing [4, 5]. Some promi- initialized, manipulated and optically read out at room nent systems have been studied extensively, including temperature, showing the potential for spin-based quan- the nitrogen-vacancy (NV) center [6{9] and the silicon- tum information and sensing applications [24, 27]. vacancy center [10, 11] in diamond, the divacancy center In this context, we demonstrate a new way to generate [12, 13] and the silicon-vacancy center [14, 15] in silicon − VB defects in hBN crystal by ion implantation process carbide, etc. Although these defects have many remark- using an ion implanter. At present, it is reported that able properties, such as long room-temperature spin co- − the VB defects can be generated by high-dose neutron ir- herence time [16], there are some intrinsic limitations due radiation [24], focused ion beam (FIB) implantation [25] to the three-dimensional nature of the materials. For ex- and femtosecond laser writing [26]. Compared with these ample, it is difficult to make the spin defects close to the approaches, ion implantation is more convenient to im- sample surface, which affects the accuracy of the sensor plement and the process is gentle and controllable. More- [17]. over, the ion implanter is commercially available and has a variety of available ion sources [31]. With appropriate Recently, the emergence of spin defects in two- − energy and fluence of implanted ions, we created VB de- dimensional materials and van der Waals crystals pro- fects successfully using an ion implanter, which exhibit vides a remedy to the limitations of three-dimensional good contrast in optically detected magnetic resonance materials. One of the most outstanding materials is hexa- (ODMR) results. genal boron nitride (hBN) that possesses a wide bandgap arXiv:2105.12029v1 [quant-ph] 25 May 2021 In the experiment, we used a commercially avail- and a variety of atom-like defects, which makes hBN be- able monocrystalline hBN sample purchased from HQ come a superb quantum system for single photon emit- Graphene with ∼ 1 mm lateral size. The monocrys- ters [18{23] and spin-addressable systems at room tem- talline hBN was exfoliated with tape into 10-100 nm- perature. Currently, one of the most researches on spin thick flakes, which were transferred onto a silicon sub- defects are about the negatively boron charged vacancy strate later. The sample was then put into an ion (V−) that consists of a missing boron atom replaced by B implanter (IonImplantatation-CETC-M56100) and the an extra electron in the hBN crystal [24{30]. The V− de- B hBN flakes were implanted with parallelized ion beams over a large area. Through ion imolatation process we − created VB defects successfully. The process is schemat- ically shown in Fig.1(a). The high-energy ions break the ∗Electronic address: [email protected] yElectronic address: [email protected] B-N bonds in the hBN lattice and knock out boron atoms, zElectronic address: [email protected] leaving negatively charged vacancies behind. The pho- xElectronic address: cfl[email protected] toluminescence (PL) and spin properties of the defects 2 （a） （b） ES 3B1 MS _ V B 1A1 Optical excitation Intersystem crossing Photoluminescence mS = ±1 2E D m = 0 S GS 3A1 （c） （d） 0 600 -1 500 ） -2 ν1 ν2 % a.u.) 400 （ （ -4 300 -3 200 Contrast Intensity -5 100 -6 0 3600 650 700 750 800 850 900 950 1000 3300 3400 3500 3700 Wavelength（nm） MW frequency（MHz） − 14 2 Fig. 1: Generation of VB defects by implanting nitrogen ions with the energy of 30 keV and the fluence of 1 × 10 ions/cm in hBN. (a) Schematic of ion implantation process. Alternating boron (red) and nitrogen (blue) atoms form crystalline hexagonal − structure of an hBN monolayer. Implanted nitrogen (green) ions knock out boron atoms from the hBN lattice to generate VB − 3 3 defects. (b) Simplified VB energy-level diagram and the transitions among the ground state ( A1), the excited state ( B1) and 1 the metastable state ( A1). (c) Photoluminescence (PL) spectrum of the implanted sample at room temperature, showing an emission centered at ∼ 820 nm. (d) ODMR measurement of the spin defects generated by ion implantation without external magnetic filed. The red line is fitted by two-Lorentzian function, where ν1 ∼ 3405 MHz and ν2 ∼ 3548 MHz. are characterized by using a confocal microscope system ODMR spectrum is fitted by two-Lorentzian function, combined with a microwave system. We use a 532-nm shown in Fig.1(d). The result indicates that the fluores- laser to excite the defects with laser power of 4.7 mW, a cence signal drops when the microwave filed oscillates at 0.5 N.A. objective (Olympus) to focus on the sample and ν1 ∼ 3405 MHz and ν2 ∼ 3548 MHz, which is consistent − collect the fluorescence by a 9-µm-core-diameter fiber to with ODMR spectra of VB defects measured in previous an avalanche photodiode, and a copper wire with a di- works [24, 29]. Fig.1(b) shows that the ms = ±1 excited − ameter of 20 µm that is close to the implanted sample as state of VB center is more likely to return ms = 0 ground an antenna to deliver a microwave filed [29, 32]. state through the nonradiative intersystem crossing, so − With the setup described above, we first characterize VB spin will be polarized into ms = 0 ground state un- the PL spectrum of the defects generated by implanting der continuous laser exciting. When the microwave fre- nitrogen ions with the energy of 30 keV and the fluence quency is in resonance with the splittings between ground of 1 × 1014 ions/cm2, shown in Fig.1(c). The implanted state sublevels, photons in ms = 0 state will be pumped into ms = ±1 state, leading to a decrease in fluores- samples exhibit strong PL emission ranging from 700 − to 1000 nm and centered around 820 nm, which is the cence intensity [29]. The VB center has a triplet ground − state (S = 1) with a zero-filed splitting (ZFS) described characteristic of VB centers and consistent with reported V− defects created by neutron irradiation, FIB and laser by the parameters D and E. The resonance frequen- B cies ν and ν in ODMR spectrum can be represented writing [24{26]. Besides, the V− defects that we create 1 2 B by ν = D=h ± pE2 + (gµ B)2, where h is the Planck are stable for a long time at room temperature. 2;1 B constant, g is the Land´e factor, µB is the Bohr magneton To further verify that the defects generated by ion and B is the static magnetic filed [24]. In the absence of − implantation are VB centers, we perform optically de- external magnetic filed, the ZFS parameters D and E can tected magnetic resonance (ODMR) measurements at be given by D=h = (ν1 + ν2)=2 and E=h = (ν2 − ν1)=2. room temperature. ODMR measurements are carried out In our experiment, we find D=h = 3475 ± 5 MHz and by scanning the frequency of microwave filed from 3250 E=h = 70 ± 5 MHz. These results show promising spin to 3750 MHz without external magnetic filed and the 3 （a） （b） 0 600 1×1013 ions/cm2 1×1014 ions/cm2 500 1×1015 ions/cm2 1×1016 ions/cm2 1×1013 ions/cm2 400 -6 0 300 200 Intensity (a.u.) 13 2 100 3.2×10 ions/cm -6 ） 0 0 % 650 700 750 800 850 900 950 1000 （ Wavelength（nm） （c） 85 1×1014 ions/cm2 Contrast -6 80 0 75 3.2×1014 ions/cm2 70 -5 E (MHz) 0 65 60 1×1015 ions/cm2 55 -4 13 13.5 14 14.5 15 3300 3400 3500 3600 3700 Fluence (lg(ions/cm2)) MW frequency（MHz） Fig.