Phys. Status Solidi B, 1–3 (2012) / DOI 10.1002/pssb.201248384 p s sb

statu

s

soli

di

www.pss-b.comph

ysi

ca

basic solid state physics

Investigation of NV centers in

diamond nanocrystallites and nanopillars

Emil Petkov1, Cyril Popov*,1, Torsten Rendler2, Christo Petkov1, Florian Schnabel1, Helmut Fedder2,Sang-Yun Lee2, Wilhelm Kulisch1, Johann Peter Reithmaier1, and Jorg Wrachtrup2

1 Institute of Nanostructure Technologies and Analytics (INA), Center for Interdisciplinary Nanostructure Science and Technology,

University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany2 3. Physikalisches Institut, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany

Received 13 July 2012, revised 14 August 2012, accepted 16 August 2012

Published online 13 September 2012

Keywords nanocrystalline diamond, nanopillars, NV centers

* Corresponding author: e-mail popov@ina.uni-kassel.de, Phone: þ49 561 804 4205, Fax: þ49 561 804 4136

Nitrogen-vacancy (NV) centers were incorporated during hot

filament chemical vapor deposition of diamond nanocrystallites

and nanocrystalline diamond (NCD) films. From the latter

nanopillars with different diameters were prepared applying

electron beam lithography and inductively coupled plasma

reactive ion etching. The deposition of either single crystallites

or closed films was controlled by the nucleation density on the

silicon substrates and the process duration. Optical investiga-

tions revealed the presence of ensembles of color centers in both

nanostructures. An enhancement of the fluorescence emission

by an order of magnitude was observed after the structuring of

the NCD films.

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Diamond is a material with many exceptional properties:it is the hardest known material, possesses the highestYoung’s modulus, a high thermal conductivity, a wideoptical band gap, etc. Due to these outstanding character-istics it finds applications in diverse fields of science andtechnology [1]. In the last decade, diamond has emerged as aunique platform for novel applications, e.g., in quantuminformation technology (QIT) or in magnetometry on ananoscale, using one of the most common luminescentdefects in its lattice, namely the nitrogen-vacancy (NV) colorcenters which emit in the visible range with absolutephotostability at room temperature [2]. In negatively chargedNV centers (NV�) the electronic spin can be easilyinitialized, manipulated, and read out [3, 4] showing thelongest spin decoherence time in solid-state systems atroom temperature (T2¼ 1.8 ms) [5], additionally theyexhibit optical coherent effects such as electromagneticallyinduced transparency (EIT) [6, 7] and coherent populationtrapping [8], etc. The creation of NV centers in diamondcan be accomplished by ion implantation either startingfrom nitrogen-rich native type Ib diamond implanting,e.g., gallium, carbon, or helium to generate vacancies, orfrom nitrogen-poor type IIa diamond implanting nitrogenions which additionally create vacancies along theirtracks. In both cases, subsequent annealing above 600 8C

is required to enhance the migration of the vacanciestoward the substitutional N sites in the diamond latticeand hence the formation of NV centers. In order toexploit the outstanding properties of the NV centers byincreasing both the photon emission yield and the collectionefficiency of the emitted photons, the NV centers shouldbe embedded in an optical cavity, e.g., in all-diamonddevices like nanopillars [9], photonic crystals [10],microrings [11], etc. In these cases either N implantationwas implemented before or after the structuring for theformation of the NV centers or they were incorporatedduring diamond growth.

In the present work, we have investigated NV centers indiamond nanocrystallites and nanopillars, formed during thegrowth process. Single nanocrystallites and nanocrystallinediamond (NCD) films, from which nanopillars were etched,have been deposited by hot filament chemical vapordeposition (HFCVD) [12]. The substrate temperature was840 8C (applying a resistive heater), the working pressure25 mbar, and the filament temperature ca. 2000 8C, asdetermined with an optical pyrometer. The gas mixture usedwas 1% methane in hydrogen with flow rates of 5 sccm CH4

and 500 sccm H2. No nitrogen was added to the gas mixturethus relying on the background pressure in the reactor forthe creation of NV centers. The deposition time was varied

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 E. Petkov et al.: NV centers in diamond nanocrystallites and nanopillarsp

hys

ica ssp st

atu

s

solid

i b

between 30 and 180 min to obtain either single nanocrys-tallites or nanocrystalline films.

Silicon wafers were used as substrates after ultrasonicpretreatment in a suspension of n-pentane and diamondpowders with different grain sizes (ultra-disperse diamondpowder, 3–5 nm, and nanodiamond powder, 250 nm) in orderto control the nucleation density: a high nucleation density(> 1010 cm�2) was obtained for the layers when a mixture ofboth diamond fractions mentioned above was used in thepretreatment suspension, but a low one (ca. 108 cm�2) for theindividual crystals applying only the nanodiamond powder.The usual duration of the pretreatment was 1 h.

The nanocrystalline layers obtained for the abovementioned pretreatment and deposition conditions wereclosed and columnar. The film thickness was about 600 nm,the calculated growth rate 3.3 nm min�1. We have observedpyramidal (111) structures of well-facetted crystals withcolumnar growth starting from distinct nucleation sites onthe silicon surface. The nucleation density (>1010 cm�2)achieved during the pretreatment step ensures the formationof diamond nanocrystallites, which in the course of thedeposition grow until they coalesce to form a closed film,thereafter growing only in height. X-ray diffraction andRaman spectroscopy were used to control the quality ofthe NCD films [12]. In order to obtain single diamondnanocrystallites, the pretreatment aimed at a lower nuclea-tion density (ca. 108 cm�2) and the deposition time wasreduced to 30 min. SEM pictures reveal diamond crystalliteswith sizes in the range of 80–100 nm randomly distributedon the substrate (Fig. 1a), i.e., the deposition rate was thesame as for the closed films.

Diamond nanopillars were fabricated from the NCD filmsusing a top–down approach with the following steps: (i)deposition of a resist, (ii) definition of the nanopillars in theresist by electron beam lithography, (iii) evaporation of 5 nmTi (adhesive layer) and 200 nm Au, (iv) lift-off process of thesacrificial resist layer, (v) inductively coupled plasma reactiveion etching (ICP-RIE) with oxygen for the formation of thediamond nanopillars, and (vi) removal of the Au mask withaqua regia. The etching recipe optimized with respect to theetching rate and quality of the pillars included a r.f. power of200 W, an ICP power of 1000 W, an O2 flow of 10 sccm, aworking pressure of 5 mTorr and a substrate temperature of

Figure 1 (online color at: www.pss-b.com) SEM images of (a)single diamond nanocrystallites grown on a Si substrate and (b)diamond nanopillars fabricated by ICP-RIE.

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

30 8C; the measured etching rate in this case was ca.100 nm min�1. Arrays of diamond pillars with diametersfrom 200 nm up to 1 mm and with a distance of 5 mm betweenthe pillar centers were fabricated (Fig. 1b). The larger pillarsshowed good quality with smooth and vertical walls, but withdecreasing size the pillars became slightly tapered.

The CVD grown diamond nanocrystallites and nano-crystalline films are H-terminated due to the saturation ofthe dangling bonds at the surface with hydrogen atoms fromthe gas phase. Simulations of the electronic band structurehave revealed a band bending for such diamond surfaces,inducing a p-type conductive surface layer due to thetwo-dimensional hole gas formed. This leads to a depletionof electrons in the nitrogen-vacancies close to the surfaceand the formation of predominantly NV0 centers [13]. Inorder to change the charge state of the shallow NV centersa surface modification is required which will renderthe diamond surface, e.g., O-terminated and enhance thefraction of NV� centers. For this purpose prior the opticalmeasurements we have subjected all samples to O2

microwave plasma treatment at 100 W input power and0.7 mbar working pressure for 10 min.

Samples with diamond nanocrystallites and nanopillarswere investigated with an confocal microscope (50� airobjective, 0.95NA). The excitation of the NV centers wasachieved with a 532 nm CW diode pumped solid-statelaser (Coherent, Compass). The fluorescence was collectedthrough the same objective and filtered from the excitationlight using a long-pass interference filter. The second-orderautocorrelation function g(2)(t) was measured using twoavalanche photodiodes in a Hanbury–Brown–Twiss con-figuration.

The confocal images of single diamond nanocrystallitesshowed bright spots randomly distributed on the surface(Fig. 2a). They were selected by their brightness; for eachseries at least five spots were targeted and their fluorescencespectra taken. The room-temperature spectra revealedthe presence of NV� centers (zero-phonon line (ZPL)at 637 nm), together with NV0 (ZPL at 575 nm) and SiVcenters (ZPL at 737 nm), the latter most probably due toincorporation of Si atoms stemming from the substrate(Fig. 2b). In some of the PL spectra, a dispersion of the peakposition of the color centers was observed which can be

Figure 2 (online color at: www.pss-b.com) (a) Confocal fluores-cence image from single diamond nanocrystallites. (b) Typicalphotoluminescence spectrum of the brightest spots at room temper-ature.

www.pss-b.com

Phys. Status Solidi B (2012) 3

Original

Paper

Figure 3 (online color at: www.pss-b.com) (a) Confocal fluores-cence image of an array of diamond nanopillars (inset: SEM top-view). (b) Photoluminescence spectrum taken from pillars with200 nm diameter.

attributed to the shift of the characteristic ZPLs in diamondnanocrystallites owing to their small size [14].

Several crystallites exhibiting strong fluorescence werefurther analyzed to establish the number of the color centerspresent in them. The antibunching from the crystallitesmeasured showed g(2)� 0.5 at t¼ 0, indicating the presenceof two or more color centers in each crystallite. The statisticaldistribution of NV centers in crystallites with different sizeshas been previously investigated; it was shown that thenumber of emitters per nanocrystallite increases with thecrystal size and is higher than 1 for crystallites larger than70 nm [15].

Finite-difference time-domain (FDTD) simulationshave demonstrated that the coupling efficiency betweenan NV center and the waveguide mode of a diamondnanowire with a diameter of 200 nm is higher than 0.85 (ifthe polarization is perpendicular to the nanowire axis) [16].We performed optical measurements of diamond nanopillarswith diameters of 500 and 200 nm. The fluorescence imagesof the arrays revealed bright spots that corresponded to thenanopillars (Fig. 3a). The photon intensity of the brightestspots (1� 106 counts per second at a pumping power of38 mW) was about one order of magnitude higher than thatof a bulk NCD film left unstructured as a marker, and evenhigher for the nanopillars with 200 nm diameter. The PLspectra taken from the latter at room temperature revealedpeaks of NV0 and NV� color centers (Fig. 3b), but nopresence of SiV centers. Second-order correlation histogramin this case showed g(2)(0)¼ 0.89 indicating that the pillarcontains more than one emitter.

In summary, we have demonstrated two differentapproaches to produce diamond nanostructures with NVcenters incorporated during the growth: (i) bottom–up:HFCVD of single diamond nanocrystallites and (ii) top–down: nanopillars structured from NCD films by electronbeam lithography and reactive ion etching. The optical

www.pss-b.com

measurements of the single nanocrystallites and nanopillarsrevealed signals of NV0 and NV� indicating the creation ofensembles of NV centers during the deposition process.

Acknowledgements The authors would like to acknowledgethe financial support of the German Federal Ministry of Education andResearch (BMBF) under the Program ‘‘Quantum Communication’’(Projects QuaHL-Rep and QuOReP).

References

[1] P. May, Philos. Trans. R. Soc. Lond. A 358, 473 (2000).[2] C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, Phys.

Rev. Lett. 85, 290 (2000).[3] M. V. Dutt, L. Childress, L. Jiang, E. Togan, J. Maze,

F. Jelezko, A. S. Zibrov, P. R. Hemmer, and M. D. Lukin,Science 316, 1312 (2007).

[4] A. Batalov, C. Zierl, T. Gaebel, P. Neumann, I.-Y. Chan,G. Balasubramanian, P. R. Hemmer, F. Jelezko, and J. Wrachtrup,Phys. Rev. Lett. 100, 077401 (2008).

[5] G. Balasubramanian, P. Neumann, D. Twitchen, M. Markham,R. Kolesov, N. Mizuochi, J. Isoya, J. Achard, J. Beck, J. Tissler,V. Jacques, P. Hemmer, F. Jelezko, and J. Wrachtrup, NatureMater. 8, 383 (2009).

[6] C. Wei and N. B. Manson, Phys. Rev. A 60, 2540 (1999).[7] E. A. Wilson, N. B. Manson, and C. Wei, Phys. Rev. A 67,

023812 (2003).[8] C. Santori, D. Fattal, S. M. Spillane, M. Fiorentino, R. G.

Beausoleil, A. D. Greentree, P. Olivero, M. Draganski, J. R.Rabeau, P. Reichart, B. C. Gibson, S. Rubanov, D. N. Jamieson,and S. Prawer, Opt. Express 14, 7986 (2006).

[9] B. Hausmann, T. Babinec, J. Choy, J. Hodges, S. Hong,I. Bulu, A. Yacoby, M. Lukin, and M. Loncar, New J. Phys.13, 045004 (2011).

[10] J. Riedrich-Moller, L. Kipfstuhl, C. Hepp, E. Neu, C. Pauly,F. Mucklich, A. Baur, M. Wandt, S. Wolff, M. Fischer,S. Gsell, M. Schreck, and C. Becher, Nature Nanotechnol.7, 69 (2012).

[11] A. Faraon, P. E. Barclay, C. Santori, K. M. C. Fu, and R. G.Beausoleil, Nature Photon. 5, 301 (2011).

[12] W. Kulisch, C. Petkov, E. Petkov, C. Popov, P. N. Gibson,M. Veres, R. Merz, B. Merz, and J. P. Reithmaier, Phys. StatusSolidi A, published online, DOI: 10. 1002/pssa.201228270(2012).

[13] M. V. Hauf, B. Grotz, B. Naydenov, M. Dankerl, S. Pezzagna,J. Meijer, F. Jelezko, J. Wrachtrup, M. Stutzmann, F. Reinhard,and J. A. Garrido, Phys. Rev. B 83, 081304(R) (2011).

[14] Y. Shen, T. M. Sweeney, and H. Wang, Phys. Rev. B 77,033201 (2008).

[15] J. R. Rabeau, A. Stacey, A. Rabeau, S. Prawer, F. Jelezko,I. Mirza, and J. Wrachtrup, Nano Lett. 7, 3433 (2007).

[16] B. Hausmann, M. Khan, Y. Zhang, T. Babinec, K. Martinick,M. McCutcheon, P. Hemmer, and M. Loncar, Diamond Relat.Mater. 19, 621 (2010).

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim