Laser Photonics Rev. 7, No. 5, L61–L65 (2013) / DOI 10.1002/lpor.201300065

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Abstract Engineering nanostructures from the bottom up en-ables the creation of carefully sculpted complex structures thatare not accessible via top down fabrication techniques, in par-ticular, complex periodic structures for applications in photonicsand sensing. In this work a proof of principle that bottom up ap-proach can be adopted and utilized for sculpting devices fromdiamond is proposed and demonstrated. A realization of peri-odic structures is achieved by growing nanoscale single crystaldiamond through a defined pattern. Optical wave-guiding of anarrow band emission attributed to the SiV defects in diamondis demonstrated by overgrowth on a thin diamond membrane. Inaddition, an array of hexagonal microdisks with diameter sizesranging from 1 to 4 μm is demonstrated. The bottom up ap-proach for diamond opens up new avenues for devices fabrica-tion and sculpting three dimensional structures.

Bottom-up engineering of diamond micro-and nano-structures

Igor Aharonovich1,2,∗, Jonathan C. Lee1, Andrew P. Magyar1,3, David O. Bracher1,and Evelyn L. Hu1

Controlled engineering of materials into ordered periodicarrays at the nanoscale enables manipulation of the inter-action between light and matter [1–3]. In particular, real-ization of nanophotonic devices such as photonic crystalcavities enables investigation of quantum effects includ-ing enhancement of spontaneous emission rates, as wellas practical applications in sensing, optical nonlinearityand lasing [4, 5]. To achieve optimal performance, theoptical cavities have to be physically isolated to confinelight at the nanoscale. Therefore, their fabrication requiresthin, wavelength-sized suspended membranes and typi-cally demands lithographic procedures involving multipleetching steps. Indeed, the majority of devices for photon-ics, sensing and nano-electronics have been engineeredpredominantly from materials such as silicon, silica andGaAs.

Recently, diamond has emerged as a promising platformfor these applications owing to its exceptional physical andmechanical properties, chemical inertness and its abilityto host ultra-bright single photon emitters [6–11]. How-ever, diamond is not amenable to routine, scalable nano-engineering and processing, mainly due to lack of a nativeoxide and its chemical stability. Recent reports on gener-ating devices by ion implantation [12], bonding approach[13] or angled etching [14] showed excellent geometricalstructures, however, the final devices still require long re-

1 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 02138, USA2 School of Physics and Advanced Materials, University of Technology Sydney, Broadway, NSW, 2007, Australia3 Center for Nanoscale Systems, Harvard University, Cambridge, MA, 02138, United States∗Corresponding author(s): e-mail: igor.aharonovich@uts.edu.au

active ion etching (RIE) steps that can hinder the quality ofthe material (e.g. surface roughness of the top and the sidewalls). Furthermore, some device geometries are not read-ily accessible via a directional dry etch. In this work, wepropose and demonstrate a bottom up approach that can beadopted and utilized to controllably build diamond micro-and nano-structures. A realization of periodic structuresand optical wave-guiding is achieved by growing nanoscalesingle crystal diamond through a defined pattern.

In nature, bottom up self-assembly yields complex pho-tonic structures [15–18]. Such structures have been mim-icked synthetically with materials including polymers andsilica spheres [2,15]. Bottom up approaches have also beenutilized for silicon [19] and GaAs [20], however, expandingthis approach to diamond is challenging due to the complexconditions required for growth. Unlike other semiconduc-tors, diamond cannot be grown by molecular beam epi-taxy, on a sacrificial non-diamond substrate that can be eas-ily etched or undercut [21]. Furthermore, diamond growthoccurs in three dimensions and, consequently, growth ofhigh aspect ratio structures (e.g. nanowires) is challenging.Finally, diamond growth occurs at elevated temperaturesunder high plasma densities, inhibiting the use of poly-mers as a mask during growth. Herein, we realize periodicmicro- and nano-structures out of single crystal diamondwith excellent optical and structural properties fabricated

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L62 I. Aharonovich et al.: Bottom-up engineering of diamond nanostructures

Figure 1 (a) SiO2 hard mask is patterned on the top [100] dia-mond surface. (b) A single crystal nanodiamond array is grownthrough the patterned SiO2 hard mask. (c) Example of a differentpattern after SiO2 removal – exposing a perfectly aligned peri-odic array of diamond nanostructures. (d) An individual diamondnanocrystal is formed from overgrowing the SiO2 mask.

by growth using microwave-enhanced plasma chemical va-por deposition (MPCVD).

The bottom up structure is grown through a silica tem-plate on a single crystal diamond substrate, [100] oriented.Figure 1a shows a scanning electron microscope (SEM)image of the mask. The patterned structure aims to limitthe diamond growth laterally and allows faster growth in thevertical direction. The growth of the diamond through thepattern is carried out in a MPCVD system. Figure 1b showsan SEM image of the diamond sample with the patternedmask after the growth. The growth replicates the periodic-ity of the pattern with excellent precision. Each patternedaperture yields only one diamond crystal. By employing ahigh hydrogen to methane ratio and relatively high pres-sure (∼60 torr), secondary nucleation is suppressed and thediamond nucleation occurs only on the exposed diamondsurface. After growth, the SiO2 mask is removed by im-mersing the sample in hydrofluoric acid. Figure 1c shows anexample of another pattern with square pattern and smallerfeatures after the hydrofluoric acid treatment. Inhomoge-neous nucleation is suppressed and the grown crystals arefaceted, having uniform size and morphology while form-ing a perfect long-range array of nanoscale crystals. In theinset of figure 1c is a higher magnification image of severaldiamond nanostructures.

One of the major advantages of the bottom-up approach,particularly for diamond, is the ability to engineer morecomplex geometries that are not possible with top downetching. Figure 1d shows an example of a single crystal di-amond pyramid grown through an aperture on top of a sin-gle crystal diamond. The top of the pyramid is suspendedabove the diamond surface, forming an isolated photonicconstituent. This geometry is achieved in two stages: firstthe lateral growth is confined by the aperture and diamondgrows vertically. Second, the diamond reaches the height ofthe mask and grows both laterally and vertically, formingthe pyramid geometry. A similar strategy can be used togrow diamond bridges and other undercut geometries, and

indicates the possibility of growing more complex geome-tries such as inverse opal structures.

While the diamond nanostructures above are fabricatedon the surface of bulk diamond, for many photonic applica-tions, an isolated structure (i.e. a thin suspended membrane,waveguides or microrings) rests on a low index material –such as SiO2. To demonstrate the suitability of this bottomup technique for practical photonic applications, waveguidestructures and hexagonal microdisk [22] are patterned. Toprovide the necessary optical isolation, thin diamond mem-branes are utilized as templates.

Diamond membranes, 1.7 μm thick, are produced byion implantation and lift-off, as reported previously [23].A thin layer of SiO2 (300 nm) is deposited on the mem-brane using PECVD and the desired pattern is defined by e-beam lithography. The membrane is then introduced into anMPCVD reactor for the “bottom up” growth of the waveg-uides and microdisks. After the growth, the hybrid deviceis flipped over and the original membrane template is re-moved by RIE in oxygen plasma, leaving behind only thegrown photonic structures. The final plasma step does notlimit the integrity of the device since the final etch step toremove the original diamond membrane does not degradethe grown structures. Furthermore, any damage is local-ized on the top surface of the structure rather than on theedges, minimizing overlap between potential RIE damageand the modes of the device. Finally, very thin ∼10 nmdiamond membranes could be used as a template, and notsignificantly alter the optical properties of devices and thuseliminate the need for any RIE. However, challenges re-main to produce uniform thin diamond membranes at thisscale.

Figure 2a illustrates this procedure. The inset offigure 2a shows an optical photograph of the stand-alonediamond devices (bright features), resting on a SiO2 sub-strate. Figure 2b and 2c show SEM images of a diamondwaveguide and an hexagonal microdisks array, respectively.Microdisks resonators with diameter sizes ranging from 1to 4 μm were grown, while the waveguides have a width of1 μm and a varied length of 10 – 30 μm.

The optical properties of the grown resonators werecharacterized using a 532 nm excitation laser at room tem-perature. Figure 2d shows a photoluminescence (PL) spec-trum recorded from one of the microdisks, showing a nar-rowband emission centered at 738 nm, which is attributedto the SiV defect. No whispering gallery modes were ob-served from the resonators. This may be due to opticallosses to the underlying substrates since the vertical dis-tance between disk and substrate is only 300 nanometers.In the future, a different choice of mask thickness and bet-ter optical isolation of the microcavity can mitigate thisproblem. Recently the SiV defect attracted considerable at-tention, due to its narrow emission line, short excited statelifetime and exceptional brightness [10,24]. Hence, the bot-tom up fabrication of resonators containing this defect is apromising route towards practical realization of integrateddiamond photonics. Remarkably, no other color centers aregrown into the optical resonator, endowing it with a highsignal to noise ratio and an efficient filtering. Note that in

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Figure 2 (a) Schematic illustration of the bottom up approach to generate optical resonators out of diamond membranes. A thin layerof SiO2 (blue) is deposited on a diamond membrane (yellow). The pattern is defined by e-beam lithography and the membrane istransferred for the regrowth. Then, the structure is flipped and the original diamond membrane is removed by RIE, leaving behindpristine single crystal diamond resonators. The inset shows an optical photograph of the stand-alone diamond devices (bright features)Optical image of the bottom-up cavities and waveguides. Bright features are the diamond resonators, while the surrounding is theSiO2 substrate. (b, c) SEM images of an array of hexagonal resonators and a diamond waveguide (d) PL spectrum recorded from oneof the resonators using 532 nm excitation. Inset, Raman spectrum showing a sharp line at 1332 cm−1, characteristic of a first orderdiamond Raman line.

our case ensembles of SiV defects are incorporated intothe structures resulting in a bright PL signal. To reduce theamount of the SiV formed in the diamond, one may use adifferent mask (e.g. alumina), position the membranes onother substrates (e.g. sapphire or iridium) and replace thetraditional quartz bell jar of the CVD reactor with a stainlesssteel one.

A Raman spectrum recorded from the same microdisk isreported in the inset of figure 2d. A sharp line at 1332 cm−1

(2.5 cm−1 Full Width at Half Maximum), the first orderdiamond Raman line, is clearly seen, indicating the highquality of the grown resonator.

The fabricated waveguides are further characterizedthrough a transmission measurement. Figure 3a shows anoptical image of the diamond waveguide. To carry out thetransmission measurement, the excitation and collectionspots were separated, as indicated by the black and redarrow, respectively. Figure 3b shows the resulting PL char-acterization from this measurement. The transmitted SiVsignal through the waveguide is clearly visible, indicating

Figure 3 (a) Optical image of the bottom up engineered dia-mond waveguide. (b) Transmission spectrum recorded from thiswaveguide with excitation and detection positions as indicated bythe black and red arrows, respectively.

a properly working waveguide. Our results are comparablewith diamond waveguides fabricated by direct ion implanta-tion or reactive ion etching techniques [25–27]. Some lossesare expected due to imperfect fabrication of the waveguide,scattering and absorption due to internal defects as wellas the underlying silica substrate. Nevertheless, this is anunprecedented demonstration of bottom up engineering ofphotonic constituents from diamond.

The bottom up growth of diamond microstructures isa powerful tool to generate homogeneous diamond struc-tures with excellent optical performance. There are severaladvantages of the bottom up approach for fabricating struc-tures from diamond. First, this approach enables generationof photonic constituents that did not undergo any ion oretching induced damage during the process, and thereforeis expected to yield the best quality material. Second, em-ploying the bottom up approach, nanodiamonds with nar-row size distribution and homogeneous geometries can beengineered – useful for photonic and sensing applications.Finally, the bottom up technique has the potential to engi-neer various color centers within the nanodiamonds (e.g.by positioning the membrane on other substrates like sap-phire or irridium and/or flowing a precursor gas during thegrowth, as was demonstrated with nickelocene [28]). Forcontrolled nitrogen incorporation, one may use the deltadoping technique where the nitrogen gas is introduced fora very brief time during the growth [29]. The generation ofcolor centers does not require ion implantation, and hencethe whole device is not subject to ion damage.

In summary we demonstrate the bottom up growth oforiented, single crystal diamond nanostructures. Realiza-tion of a periodic array of single crystal nanodiamondson top of a bulk crystal diamond is a highly sought afterfeature for applications in sensing and anti-reflection coat-ings. We further envision that pillar structures grown fromdiamond may be extremely promising for applications inbio-sensing. The bio-compatibility of diamond, concurrentwith surface pillars hosting optically active defects, can be

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L64 I. Aharonovich et al.: Bottom-up engineering of diamond nanostructures

utilized for inter cellular electrical signals [30]. Thin dia-mond membranes were utilized to grow periodic structures,as well as hexagonal optical resonators and waveguides.This work opens new avenues for exploring novel fabri-cation techniques with diamond, to take advantage of theease of growth of one-dimensional structures [31], and en-ables more complex 3D structures that are not accessiblevia traditional top down approaches.

Experimental details

Methods: for the bottom up growth on top of a single crystaldiamond, [100] oriented diamond samples, with nitrogenconcentration <1 parts per million were purchased fromElement 6TM. The samples were first cleaned in a mix-ture (1:1:1 sulfuric–perchloric–nitric-acid) and rinsed withwater. 300 nm of SiO2 was deposited on the top surfaceto serve as a hard mask using plasma enhanced chemi-cal vapor deposition. E-beam lithography was used to pat-tern a periodic structure, which is transferred to the SiO2mask. Polymethyl methacrylate (PMMA C6) from Micro-Chem Inc. was used to pattern the SiO2. Inductively cou-pled plasma RIE in fluorine environment was used to etchthe pattern into the SiO2 and expose the diamond surface.The patterned diamond growth was performed in a SEKIMPCVD reactor under the following conditions: pressure60 torr, microwave plasma 950 W, CH4:H2 400:4 stan-dard cubic centimeters per minute for 10 minutes. No ex-ternal heating source was used and the temperature was∼850 ◦C as read by a pyrometer. Thin diamond membranes(1.7 μm) were fabricated by ion implantation and liftoffas described previously [23]. Masking and regrowth of thediamond membranes were similar to the process describedabove for the single crystal sample.

Characterization: the Raman measurements were car-ried out in room temperature using 532 nm laser excita-tion in a confocal Raman microscope (LabRAM ARAMIS,Horiba Jobin-Yvon) with typical spatial resolution on theorder of 1μm.

The waveguide transmission measurements were per-formed using a custom built confocal microscope usinga 532 nm continuous wave (CW) diode laser through a100 ×, 0.9 numerical aperture objective. The emission wascollected through the same objective and directed into aspectrometer. The laser light was filtered using a dichroicmirror.

Acknowledgements. The authors acknowledge T.L. Liu andThomas Babinec for helpful discussions and Prof David Clarkefor the access to the Raman facilities. The financial support of theDARPA under the Quantum Entanglement Science and Tech-nology (QuEST) Program. This work was performed in part atthe Center for Nanoscale Systems (CNS), a member of the Na-tional Nanotechnology Infrastructure Network (NNIN), which issupported by the National Science Foundation under NSF AwardNo. ECS-0335765. CNS is part of the Faculty of Arts and Sci-ences at Harvard University. Dr Aharonovich is the recipient of anAustralian Research Council Discovery Early Career ResearchAward (project number DE130100592).

Received: 9 May 2013, Revised: 2 June 2013,Accepted: 17 June 2013

Published online: 11 July 2013

Key words: Diamond, bottom up, nanophotonics, nanostruc-tures.

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