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In situ optimization of co-implantation and substrate temperature conditions for nitrogen-

vacancy center formation in single-crystal diamonds

View the table of contents for this issue, or go to the journal homepage for more

2011 New J. Phys. 13 035022

(http://iopscience.iop.org/1367-2630/13/3/035022)

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T h e o p e n – a c c e s s j o u r n a l f o r p h y s i c s

New Journal of Physics

In situ optimization of co-implantation and substratetemperature conditions for nitrogen-vacancy centerformation in single-crystal diamonds

J Schwartz, P Michaelides, C D Weis and T Schenkel1

Ion Beam Technology Group, Accelerator and Fusion Research Division,Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USAE-mail: T Schenkel@LBL.gov

New Journal of Physics 13 (2011) 035022 (8pp)Received 4 January 2011Published 23 March 2011Online at http://www.njp.org/doi:10.1088/1367-2630/13/3/035022

Abstract. In this paper, we present first results of in situ characterizationof nitrogen vacancy (NV)-center formation in single-crystal diamonds afterimplantation of low-energy nitrogen ions (7.7 keV), co-implantation ofhydrogen, helium and carbon ions and in situ annealing. Diamond sampleswere implanted either at room temperature or at 780 ◦C. We found that dynamicannealing during co-implantation enhances NV-center formation by up to 25%.

Contents

1. Introduction 22. Experimental setup 23. Results and discussion 34. Conclusion 8Acknowledgments 8References 8

1 Author to whom any correspondence should be addressed.

New Journal of Physics 13 (2011) 0350221367-2630/11/035022+08$33.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

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1. Introduction

Spins of color centers in diamond, in particular the negatively charged nitrogen vacancy (NV)center (NV−), are promising quantum bit candidates and they can make possible advancedsensing techniques [1]–[3]. Reliable formation of NV-centers [4]–[6], their placement withhigh spatial resolution [4, 5, 7, 13] and the engineering of spin environments for optimalcoherence times and spectral stability are formidable challenges [8]–[10]. Implantation ofnitrogen ions into single-crystal diamonds with low nitrogen background followed by annealingto >600 ◦C is a simple technique for the formation of some NV− centers [4, 5, 11]. NVcenters form during annealing, when nitrogen atoms are incorporated into substitutional sitesin the diamond crystal lattice [12] and vacancies bind to substitutional nitrogen atoms. Thecharge states of the NV center (NV+, NV0, NV−) are determined by the local band structureand can be strongly affected e.g. by a bath of surrounding substitutional nitrogen atoms (P1centers), by other impurities and by surface effects [8, 9, 16]. NV-formation efficiencies,i.e. the number of NV centers formed per implanted nitrogen atom, reported so far are wellbelow unity [5]–[7] for low-energy implantations (<30 keV), where range straggling is smallenough to allow high placement accuracy [4, 5, 7]. Adding vacancies by co-implantation of ionspecies other than nitrogen can enable the capture of them into stable NV centers and has beenshown to enhance NV-formation efficiencies [4, 6, 14]. The addition of vacancies is limited bydamage accumulation, which degrades the crystal lattice irrevocably above a damage thresholdof about 1022 vacancies cm−3. Annealing diamonds with damage levels above this thresholdleads to irrevocable graphitization, whereas annealing at damage levels below the threshold re-crystallizes the diamond lattice [15]. Dynamic annealing, i.e. annealing during co-implantationwhere samples are heated to temperatures above about 600 ◦C [15], can increase the conversionof substitutional nitrogen with vacancies into stable NV centers if the processing conditionsfor favorable kinematics of vacancy generation, dynamic damage repair and low loss ratesfor already formed NV centers can be identified. We have configured an in situ processingand characterization chamber that allows systematic studies of ion implantation and dynamicannealing conditions for enhanced NV-center formation efficiencies.

2. Experimental setup

Figure 1 shows a schematic diagram of our experimental setup. Ion beams enter a vacuumchamber with a base pressure of 10−7 torr after mass selection in an ExB filter. Beam intensitieswere <10 nA cm−2 and the typical dose rate was ∼3 × 109 ion cm−2 s−1, which is low enoughto avoid dose rate effects and beam-induced heating. The beam energy was set to 7.7 keV forall ion species used in this study and we used a 14N+ ion fluence of 1013 cm−2 throughout.Co-implantations were performed with hydrogen, helium and carbon ions. The ranges of theseions in diamond as calculated with SRIM [17] are 60 nm (H), 40 nm (He), 13 nm (C) and11.5 nm (N). All implantations were performed at normal incidence. SRIM does not track thecrystal structure of samples, and channeling effects for the implants at normal incidence willbroaden the range profiles for H and He co-implants, but only minimally for C and N implantsat 7.7 keV [7]. Diamond samples were mounted on a goniometer with a heatable sample holderin a temperature range up to 1000 ◦C. We selected an annealing temperature of 780 ◦C inthis study. Vacancies become mobile and damage recovery during annealing is effective fortemperatures above about 600 ◦C [12, 15], while temperatures above 950 ◦C can lead to surface

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Figure 1. Schematic diagram of the experimental setup with an ion source, amass analyzer (ExB filter), a vacuum chamber, a goniometer with a heatablesample holder and a fiber coupled excitation laser and spectrometer.

graphitization. In some ambients, diamond surfaces can also etch during annealing. We didnot observe reduced photoluminescence (PL) intensities or other evidence for surface etchingof diamond samples during extended annealing at 780 ◦C for up to 2 h in vacuum. Followingimplantation and annealing, NVcenters are excited with a fiber-coupled laser (532 nm,300 mW) and PLspectra are collected in a fiber-coupled spectrometer. The samples used wereelectronic grade single-crystal chemical vapour deposition diamonds (100) with a nitrogenbackground <5 ppb from Element 6.

Figure 2(a) shows a series of spectra from a diamond sample after implantation of nitrogenat room temperature and after a series of in situ annealing cycles. In figure 2(b), we showspectra for co-implantation of carbon ions after nitrogen implantation while the sample washeated to 780 ◦C during co-implantation. The spectra show the well-known features, includingthe diamond Raman line close to the NV0 zero-phonon line at 575 nm, the NV− line at 637 nmand the vibronic sideband from the NV− centers. The intensities of NV-center lines change dueto co-implantation and annealing; however, spectral feature changes, e.g. linewidth changes ofthe NV− zero-phonon line, due to damage accumulation [14], could not be observed in theseroom temperature PL measurements.

3. Results and discussion

Figure 3 shows the evolution of NV-center PL counts during in situ annealing and for a series ofco-implantation fluences where the co-implanted species was helium. NV− and NV0 intensitieswere extracted from peak intensities in spectra, such as the ones shown in figure 2. The error barshown resulted from the laser stability and the alignment uncertainty in repeated measurementsof the same implant spots. Trends for NV− and NV0 intensities were similar and in figure 3 weadded intensities from NV− and NV0 lines. NV intensities first increase for co-implantation ofonly 1011 cm−2 He ions and then begin to decrease significantly for a co-implantation fluence of1013 cm−2. Annealing recovers the PL intensity following co-implantation with 1013 He cm−2

after only 10 min. Extended annealing to 2 h did not show further changes in the NV PLintensity after co-implantation with 1012 He cm−2. Annealing in specific ambients can lead toetching of diamond surfaces. The fact that PL intensities were constant for samples annealed at780 ◦C for 2 h after co-implantation with 1012 He cm−2 indicates that no surface etching tookplace in these experiments. Surface etching would reduce PL intensities, especially for therelatively shallow nitrogen implants used in this study. On the other hand, we did not performcontrol experiments with e.g. step edge or repeated depth profiling measurements to exclude

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Figure 2. Examples of PL spectra (room temperature) from diamond samplesfollowing implantation of (a) nitrogen ions (1013 cm−2) and for a series ofannealing times, (b) for N implantation followed by co-implantation of carbonions with the sample at 780 ◦C during implantation (1011, 1012 and 1013 cm−2).The red curve in (b) shows the background spectrum from a diamond samplethat was implanted with C only, without prior N implantation.

any surface etching. If there was surface etching, then the loss of material and NV centerswould have to be compensated for with increasing NV conversion during extended annealingto account for the observed constant PL intensities, which seems unlikely. The constant PLintensity during a 2 h anneal also indicates that diffusion of NV centers is minimal at 780 ◦C invacuum.

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Figure 3. PL intensities from NV centers (NV− plus NV0) as a function ofannealing time and for a series of co-implantation fluences for co-implantationwith helium ions (7.7 keV, normal incidence) at room temperature. Note that co-implantation with 1013 cm−2 He+ ions decreases the NV count significantly butannealing for only 10 min recovers most of the NVs. Representative error barsare shown for the 1012 cm−2 co-implant series. Lines are a guide to the eye.

Figure 4 shows the evolution of NV PL intensities, in this case for co-implantation ofcarbon ions, where the diamond sample was heated to a temperature of 780 ◦C during co-implantation. Co-implantation increases the NV PL signal, indicating that additional vacanciesare captured by substitutional nitrogen atoms to form stable NV centers. For the NV count toincrease, already formed NVs must be stable during co-implantation and annealing. We notethat PL is only sensitive to NV− and NV0 charge states of NV centers. Tracking these, we findthe trends for both PL visible NV charge states to be similar. The co-implantation species andthe substrate temperature affect optimal fluences for enhanced NV formation. Above a criticalfluence, further co-implantation decreases the NV intensities. At a first glance, this is to beexpected for diamond from many years of futile attempts to achieve reliable n-type doping.Lattice damage accumulates and solid phase epitaxial re-growth, which is so routinely used forsilicon, does not occur in diamond under readily achievable laboratory conditions of temperatureand pressure [12, 15]. We find that co-implantation of 1012 cm−2 carbon ions into hot diamondsenhances NVPL intensities by about 25%.

The histogram in figure 5 summarizes the results for co-implantation of hydrogen, heliumand carbon ions as a function of fluence and for samples at room temperature or heated to 780 ◦Cduring co-implantation. The results are normalized to the nitrogen-only implants. Annealingafter co-implantation, without prior nitrogen implantation, led to only a negligible NV PL signalowing to the low nitrogen concentration in the substrates (figure 2(b)). The absolute conversion

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Figure 4. PL intensities from NV centers (NV− and NV0) as a function ofannealing time and for a series of co-implantation fluences for co-implantationwith carbon ions (7.7 keV, normal incidence) and with diamonds heated to780 ◦C during co-implantation. Representative error bars are shown for theN-only implant series (1013 cm−2, 7.7 keV, normal incidence). Lines are a guideto the eye.

efficiencies were not measured in these studies. From previous studies, we estimate these tobe of the order of 5% [5, 7]. Further optimization of the setup will allow PL measurements atlow temperature, where relative emission intensities in the NV0 and NV− zero-phonon lines aremuch enhanced. Also, counting single centers with a high-resolution optical microscope willenable absolute calibration of the efficiency of implanted N to PL active NV-center conversion.

The damage threshold for graphitization of diamond during annealing above a temperatureof 600 ◦C is about 1022 vacancies cm−3 [12, 14, 15]. We observe a drop in the NVPL intensityfor the co-implantation of 1013 ions cm−2, consistently for H, He and C ions. For C ions,ranges are about the same as for N ions at the same implant energy and channeling doesnot play a significant role at the relatively low implant energy of 7.7 keV [7]. With SRIM,we can estimate that each carbon ion forms about 56 vacancies in the diamond sample(assuming a density of 3.5 cm−3 and a displacement energy of 35 eV). The resulting damagedensity is only about 4 × 1020 vacancies cm−3, a factor of 25 below the damage thresholdfor graphitization. Also, dynamic annealing [12, 15] at the low dose rates used here (co-implantations were performed with beam intensities <10 nA mm−2) can be expected to preventdamage accumulation altogether. For He and H co-implantations, we find similar trends as for Cin the fluence dependence of NV PL intensities. The damage levels and profiles for these lighterions are, however, very different from that for C. SRIM cannot predict the damage profiles forH and He co-implantations under channeling conditions very well. Yet it is clear from SRIM

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Figure 5. Histogram summarizing the NV PL intensity evolution for the co-implantation of hydrogen, helium and carbon ions (all 7.7 keV, normal incidence)into diamond samples held at room temperature or heated to 780 ◦C duringimplantation. A representative error bar is shown for the N-only implant in thehydrogen co-implantation at room temperature series.

simulations even without consideration of channeling effects that damage levels, i.e. the numberof vacancies created at the shallow positions of the implanted nitrogen ions, are much reducedcompared with C co-implants. For C, the vacancy density at the peak of the nitrogen depthprofile of 11.5 nm is 3 vacancies ion−1 nm−1, for He it is ten times lower and for H it is only0.02 vacancy ion−1 nm−1. Both the total number and the densities of vacancies are much lowerfor H and He compared with C ions. This finding points towards another mechanism that leadsto reduced NV PL intensities, other than damage accumulation in the diamond lattice. Thefact that the observed fluence dependences of NV PL intensities for H, He and C co-implantswith their very different energy loss profiles [17] are similar indicates that target ionization andaccumulation of charged defects in the diamond lattice lead to ionization of NV centers and isresponsible for the drop in NV PL intensities for co-implantation fluences above 1012 cm2 for H,He and C ions. For C ions, the drop is greatest, consistent with the combined effects of localizedionization and lattice damage. For H and He co-implants, the much lower lattice damage levelsfor co-implantation under channeling conditions are offset by ionization effects, which convertNV centers to dark states, e.g. NV+, leading to the observed drop in NV PL intensities. There isalso a possibility that chemical effects could play a role for H [18] and He implants. However,the range of these lighter ions is much greater (60 nm for H and 40 nm for He) than that of theimplanted nitrogen (peak range of 11.5 nm) at the same implant energy of 7.7 keV used for allion species, so this seems very unlikely.

We speculate that further optimization of NV formation will be possible if we evaluatea broader range of implantation temperatures, ion implant energies, charge compensation andannealing conditions.

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4. Conclusion

We have studied NV formation in single-crystal CVD diamonds in an in situ processing cham-ber as a function of co-implantation and annealing conditions. Co-implantation adds vacancies,which can be captured by substitutional nitrogen atoms to form NV centers, and NV-center for-mation is enhanced up to a critical co-implantation fluence. Co-implantation into hot substratesunder conditions of dynamic annealing aids NV formation. We discussed the effects of damageaccumulation and the accumulation of charges in the diamond lattice due to electronic energyloss of ions during co-implantation as the mechanisms responsible for the decrease of NV PLintensities for co-implantation fluences above 1012 cm−2. Further optimization of processingconditions has the potential to enable NV-center formation with enhanced reliability.

Acknowledgments

We thank David Toyli and David Awschalom for technical assistance and collaborative efforts.This work was supported by the Laboratory Directed Research and Development Programof the Lawrence Berkeley National Laboratory under US Department of Energy contract no.DE-AC02-05CH11231 and through the DARPA Quest program.

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