Diamond and Related Materials 8 (1999) 1642–1647www.elsevier.com/locate/diamond

Diamond-like carbon nanocrystals formed by implanting fusedquartz and sapphire (a-Al2O3) with carbon ions

J.O. Orwa *, J.C. McCallum, S. Prawer, K.W. Nugent, D.N. JamiesonSchool of Physics, Microanalytical Research Centre, The University of Melbourne, Parkville, Victoria 3052, Australia

Accepted 30 November 1998

Abstract

In an attempt to synthesize diamond nanocrystals 1 MeV carbon ions were implanted into fused quartz and sapphire to severaldifferent doses and annealed in a furnace at 1100°C for different durations in different annealing ambients. We observe a peak inthe optical absorption spectrum at around 5 eV in carbon-implanted quartz and sapphire samples after thermal annealing. Westudied the behaviour of the absorption peak as a function of carbon ion dose, annealing time and ambient. Raman spectroscopyusing the 514.5 nm line of an argon ion laser was used to elucidate the nature of the chemical bonding in the carbon-implantedsamples. Several sharp hitherto unknown Raman peaks are observed in carbon-implanted and annealed sapphire which areconsistent with the presence of diamond-like carbon clusters. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Annealing; Ion implantation; Nanocrystals; Substrate

1. Introduction since the possibility exists of forming sp2- as well assp3-bonded clusters. In this study we look at the forma-tion of carbon clusters in fused quartz and sapphire asRecently, there have been many studies involving the

synthesis and characterization of nanometer-sized clus- a function of ion dose, annealing time and annealingenvironment. In previous work we have shown that theters of various materials embedded by ion implantation

in both amorphous and crystalline substrates [1]. An use of deep ion implantation to bury damage beneath arelatively undamaged diamond cap can promoteattractive property of these clusters is an observed

variation in their bandgaps with cluster size which allows the regrowth of sp3- rather than sp2-bonded carbonstructures. Hence in the present work, we havefor the tuning of luminescence wavelengths according

to the desired application. In the case of diamond employed MeV ion implantation to bury a layer of Cdeep inside fused quartz and sapphire matrices.nanoclusters, the added characteristic of negative

electron affinity [2] promises that these materials mayeventually find use as cold cathode emitters [3]. Thisstudy aims to investigate the possibility of synthesizing

2. Experimentaldiamond nanocrystals by implanting fused quartz andsapphire with carbon and annealing in a furnace under

1 MeV carbon ions were implanted into fused quartzforming gas, oxygen and argon ambients. This work isand sapphire to doses of 1×1017 cm−2, 2×1017 cm−2motivated by the previous successful synthesis of siliconand 5×1017 cm−2 at room temperature. An additionaland germanium nanocrystals in both fused quartz andimplant of the same energy was made into quartz to asapphire using ion implantation (Ref. [1] and referencesdose of 5×1016 cm−2. Each implant uniformly coveredtherein). In these studies, the nanocrystals were foundan area of 1 cm2 and had a projected range (rangeto be randomly oriented in the amorphous quartz matrixstraggling) estimated using transport of ions in matterand oriented with the host matrix in sapphire. The(TRIM) code [4] of Rp(DRp)=1.45(±0.12) mm forsituation with carbon, however, is more complicatedquartz and Rp(DRp)=0.88(±0.08) mm for sapphire. Asa control, 1 MeV argon ions were implanted into quartz* Corresponding author. Tel.: +61 3-9344-5081;to a dose of 1×1017 cm−2 and subjected to identicalfax: +61 3-9347-4783.

E-mail address: joo@physics.unimelb.edu.au (J.O. Orwa) annealing treatments as the carbon-implanted samples.

0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved.PII: S0925-9635 ( 99 ) 00044-8

1643J.O. Orwa et al. / Diamond and Related Materials 8 (1999) 1642–1647

Although the projected range (range straggling) of theargon ions was only 0.90(±0.12) mm, the damage cre-ated at the end of range by the implanted dose wascomparable to damage created by a dose of around4×1017 C cm−2. Each implanted sample was cut intoseveral smaller pieces for thermal annealing at 1100°Cfor 1, 4 and 16 h in flowing forming gas (4% hydrogenin argon) and oxygen ambients. Other carbon-implantedquartz samples were annealed in flowing argon for 1and 13.5 h. Absorption spectroscopy was performed onall samples using a Cary 5 spectrophotometer in thedouble beam configuration using unimplanted substratesas references. To identify the nature of the carbonstructures formed we performed Raman spectroscopy.The Raman signal was excited by 2 mW of the 514.5 nmline of an argon-ion laser focused to a 1 mm spot anddetected using a DILOR XY Confocal micro-Ramanspectrometer with optical multi-channel collection. Theintegration time for each spectrum was 300 s. Selected

Fig. 1. Absorption coefficient versus energy for 1 MeV carbon ionssamples were subjected to Rutherford backscatteringimplanted into fused quartz to a dose of 5×1017 cm−2 and annealed

spectrometry (RBS) and Secondary ion mass spectro- for various durations at 1100°C in forming gas. The spectrum of anscopy (SIMS) to measure the distribution of carbon in unannealed sample is included for comparison.the samples.

Fig. 2 shows the absorption coefficient versus wave-length for carbon-implanted sapphire samples that have

3. Results been annealed for 1 h in forming gas. The spectrum ofan unannealed sample implanted to a dose of

While the argon-implanted quartz remained optically 2×1017 cm−2 is shown for comparison and is representa-clear, all the carbon as-implanted samples appeared tive of all unannealed samples. The peak observed atdark to the naked eye. Assuming that the argon creates around 5 eV in the annealed samples is clearly absentthe same amount of damage in quartz as carbon, the in the unannealed sample. In addition, we did not see adarkening can be attributed to the presence of carbon peak at 5 eV in the absorption spectrum of a sapphireand not just to ion induced damage. sample implanted to a dose of 1×1017 cm−2 with 3 MeV

3.1. Absorption Measurements

Fig. 1 shows absorption spectra for fused quartzsamples that have been implanted to a dose of5×1017 cm−2 and annealed in forming gas for 1, 4, and16 h. A spectrum of a sample that was implanted withargon to a dose of 1×1017 cm−2 and annealed undersimilar conditions for 1 h is shown for comparison. Thepeak observed at around 5.2 eV in the annealed carbonimplants only appears as a shoulder in the as-implantedsample and is absent in the argon implant. The absenceof the peak in the argon implant suggests that the peakis related to the presence of carbon and does not arisefrom damage alone. Previous studies on radiationdamage to quartz show that several damage centers areproduced which give rise to absorption peaks in the UVfrom 200 to 400 nm but these all disappear upon annea-ling at 950°C [5]. We therefore propose that the observedpeak is due to clusters of carbon that result from the

Fig. 2. Absorption coefficient versus energy for 1 MeV carbon ionsannealing process. The absence of a definite peak in the implanted into a-Al2O3 to a range of doses and annealed for 1 h atunannealed carbon-implanted sample thus points to the 1100 oC in forming gas. The spectrum for an unannealed sample

implanted to a dose of 2×1017 cm−2 is shown for comparison.lack of any significant clustering before annealing.

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Au ions and annealed for 1 h at 1100°C. Known radia-tion-damage-related absorption bands in sapphire occurat around 6.1 and 4.7 eV [6 ] and hence do not coincidewith the peak we observe. As in the case of fused quartzwe conclude that the peak observed at 5 eV is due tocarbon clusters produced in the sapphire matrix uponannealing.

By fitting the absorption peaks with Lorentzian func-tions we have been able to extract information onvariations of absorption with sample treatment parame-ters. Fig. 3 shows the variation of absorption coefficientwith dose for sapphire and quartz samples annealed for1 and 4 h in forming gas. In all cases, the absorptioncoefficient increases linearly with dose. This is under-standable as the absorption is expected to increase withan increase in the carbon content. It is also notablefrom the plots that for sapphire, the absorption coeffi-cient increases with annealing time while for fused quartz

Fig. 4. Absorption coefficient versus annealing time for quartz andthe opposite is the case. A similar trend was observedsapphire samples annealed in various annealing ambients.

for samples annealed in argon and oxygen. These trendsof variation of absorption coefficient with annealingtime are seen much more clearly in Fig. 4. For the caseof oxygen, the absorption coefficient was zero after 16 hof annealing. SIMS and RBS measurements showedthat this decrease is correlated with loss of carbon fromthe fused quartz samples, presumably due to diffusion.No evidence of carbon loss was observed in sapphire.On the contrary, a slight increase in absorption coeffi-cient was observed with annealing time.

In Fig. 5 we show the variation of the absorptionpeak energy with ion dose for carbon-implanted fusedquartz and sapphire samples annealed in forming gasfor 1 and 4 h. Although the general behaviour of thepeak energy as a function of dose appears similar forfused quartz and sapphire samples, the peak energiesfor quartz are somewhat higher for the corresponding

Fig. 5. Peak energy versus ion dose for carbon-implanted fused quartzand a-Al2O3 samples annealed for 1 and 4 h at 1100°C in forming gas.

doses. We comment on possible reasons for these differ-ences below.

A summary of the absorption data is as follows.$ The absorption coefficient of the peak observed at

around 5 eV increases linearly with carbon ion dosefor both quartz and sapphire implants.

$ The absorption coefficient of the peak decreases ifthe amount of carbon is reduced as is the case whencarbon is lost by diffusion.

$ The peak appears in the same energy range for quartzand sapphire samples.

$ The peak is not seen in quartz samples that havebeen implanted with argon and annealed.

$ The peak is observed as a weak shoulder in unan-Fig. 3. Absorption coefficient versus dose for carbon ions implantednealed samples of both carbon-implanted quartz andinto fused quartz and a-Al2O3 and annealed for 1 and 4 h in form-

ing gas. carbon-implanted sapphire.

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$ The peak is not observed for sapphire implanted withgold and annealed at 1100°C.

$ The peak position shifts to higher energy with increas-ing dose for both quartz and sapphire samples.From the above points we conclude that the peak is

due to the presence of clusters of carbon. Because ofthe high absorption energy, close to the band gap ofdiamond, we propose that the clusters responsible maybe diamond like, i.e. comprising a significant proportionof the C atoms in the sp3 configuration.

3.2. Raman measurements

Having established from absorption data that thepeak at 5 eV is due to carbon clusters, we turn to Ramanspectroscopy to determine the chemical bonding in theclusters. Fig. 6 shows Raman spectra of carbonimplanted into fused quartz to a dose of 5×1017 cm−2and annealed at 1100°C for various durations in forminggas. The single Raman peak in the unannealed sample Fig. 7. Raman spectra for carbon-implanted a-Al2O3 samplesat around 1550 cm−1 indicates that the carbon exists (5×1017 cm−2) annealed for various durations in forming gas. The

spectrum for an unannealed sample is included for comparison. Themainly in amorphous form. After 1 h annealing in thespectra have been vertically displaced for clarity.various environments, the Raman D and G peaks

located at 1350 and 1580 cm−1, respectively, appear,suggesting that the implanted carbon has formed into quite different. The sapphire samples annealed in form-

ing gas show a broad, weak G peak centered aroundsp2-bonded micro-clusters. The decrease in the full widthat half maximum of the G peak with annealing time is 1617 cm−1. Two sharp peaks which increase in intensity

with annealing duration are observed at 1299 andan indication of the growth of the sp2-bonded structuresinto larger and more well-ordered clusters. 1392 cm−1 superimposed on the broad, weak D peak.

Other notable sharp but weak peaks are seen at 1050Raman results for carbon-implanted sapphire areshown in Fig. 7. Although the spectrum of the and 1700 cm−1. The peak at 1040 cm−1 is of special

interest as it is often observed in nanocrystalline chemi-as-implanted sample is very similar to that obtained forcarbon as-implanted quartz, the annealed spectra are cally vapour-deposited diamond [7,8]. The peak has

recently been reported in diamond nanoclusters about2 nm in size [9]. By comparing the Raman spectraobtained using the 514.7 and 487.96 nm lines of thelaser, we have determined that the peaks are not relatedto luminescence. The peaks were also absent fromsamples of sapphire implanted with gold. We considercontribution to the Raman spectra from damage relatedcenters in sapphire to be unlikely at wavenumbers above1000 cm−1 because of the large masses involved inpossible defect bonding pairs. We therefore concludethat the sharp peaks originate from non-graphitic clus-ters of implanted carbon which are formed when thesamples are annealed.

A summary of the Raman data is thus as follows.$ The as-implanted carbon exists mainly in amorphous

form as seen from the single peak at around1550 cm−1 in both quartz and sapphire.

$ The quartz samples show a clear development of theD and G peaks with dose and annealing time suggest-ing the formation of micro-polycrystalline graphite.

Fig. 6. Raman spectra for carbon-implanted fused quartz samples$ A small peak is observed at around 1040 cm−1 in(5×1017 cm−2) annealed for various durations in forming gas. The

sapphire implying that nanocrystalline diamond mayspectrum of an unannealed sample is included for comparison. Thespectra have been vertically displaced for clarity. be present.

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$ In addition to showing both the D and the G peaks, ature is only 53% of the melting temperature. Thereforethe mobility of the defects and implanted carbon isthe sapphire samples show a rich spectrum of sharp

peaks above 1000 cm−1 which we believe are related expected to be higher for quartz than sapphire, makingout-diffusion of carbon much more likely for the caseto the presence of clusters of non-graphitic carbon.of fused quartz.

Next, we address the question of why the Ramanspectra of the C-implanted sapphire and fused quartz4. Discussionare so different. For fused quartz the dominant bondingappears to be in the form of micro-polycrystalline graph-We have presented clear evidence that the peak

observed at 5 eV in the absorption spectra is due to the ite, as is evidenced by the observation of strong RamanD and G peaks. Although there is evidence that suchpresence of carbon clusters. However, the origin of the

peak is uncertain. Fig. 5 shows that the peak position structures are also present for the case of C-implantedsapphire, they do not appear to be the dominant bondingincreases with increasing ion dose. This is an unexpected

result because increasing the ion dose is expected to type. Channeling RBS experiments of the sapphiresamples revealed that although annealing for up to 4 hresult in the formation of larger clusters which, accord-

ing to predictions from quantum confinement, should at 1100°C is insufficient to completely restore the crys-tallinity of the Al2O3 matrix, some repair of the latticehave smaller band gaps. We observe the opposite trend

thus suggesting that the transition corresponding to the has occurred. Thus, the carbon clusters are still locatedwithin a defective sapphire matrix, and possibly con-5 eV peak is not simply due to band to band transition

in a diamond nano-cluster. Rather, we speculate that strained to specific locations within the defective matrix.This may constrain the clusters to grow under conditionsthe peak at 5 eV is due to transitions between defect

states associated with the nano-clusters. The fact that of high pressure more favourable for the formation ofsp3-bonded structures.we see an absorption peak and not an absorption edge

further suggests that the absorption is dominated by We note, however, that it is difficult to determine thetrue extent of the existence of sp3-bonded clusters usingdefects. The defects may be associated with the surface

of the clusters, or may be due to interstitial defects (such visible Raman spectroscopy since the sp2-bonded carbonatoms are subject to resonance enhancement in theas the so-called split-interstitial or dumbbell defect) or

vacancy or vacancy clusters. The energy levels of these Raman cross section due to the similarity in the laserwavelength with the sp2 carbon energy gap of ~2 eVdefects may well depend on the cluster size. A simple

picture based on quantum confinement will probably be [7]. This results in the visible Raman spectrum display-ing a far greater sensitivity to sp2- than to sp3-bondedinadequate to explain the experimental data.

We now turn to the differences between the behaviour carbon clusters. To overcome this problem, UV Ramanusing a 244 nm excitation has recently been employedof carbon implanted into fused quartz and sapphire. In

the former case, annealing in oxygen leads to the loss and the signature of sp3-bonded clusters has beenobserved [7]. Thus the next stage in this research willof carbon from the fused quartz, (with concomitant

decrease in the absorption coefficient). Such out-diffu- be to perform UV Raman spectroscopy and transmis-sion electron microscopy on the C-implanted fusedsion of carbon does not occur from sapphire. There are

at least two possible reasons for this. Firstly, after the quartz and sapphire samples.implantation, many broken Si–O and Al–O bonds arepresent. Upon annealing there will be a competitionbetween the formation of C–O bonds and Si–O bonds(for the case of quartz) and between C–O bonds and 5. ConclusionAl–O bonds (for the case of sapphire). Since the Al–Obond in Al2O3 is much stronger than the Si–O bond in We have shown that when carbon is implanted into

fused quartz and sapphire and thermally annealed atSiO2, the probability for the formation of C–O or CO2complexes in SiO2 is much higher than in Al2O3. Once 1100°C carbon nanoclusters are created which give rise

to peaks in the optical absorption spectrum at aroundformed, CO or CO2 can readily diffuse out of the matrix.Such a mechanism for loss of carbon from the SiO2 5 eV. Visible Raman spectroscopy confirms the presence

of sp2-bonded carbon clusters in C-implanted andmatrix is consistent with the observation that the lossof carbon is much more rapid when the fused quartz is annealed fused quartz. For C-implanted sapphire, such

sp2-bonded clusters are also present, but in addition, Cannealed in oxygen rather than argon or forming gas.A second related reason for the difference between in other bonding configurations is present as is evidenced

by sharp peaks in the Raman spectrum at 1299 andthe behaviour of carbon in quartz and in sapphire relatesto the annealing temperature. For quartz, the annealing 1392 cm−1. The results are consistent with the formation

of carbon clusters containing C in both sp2 and sp3temperature of 1100°C is about 68% of the meltingtemperature whereas for Al2O3 the annealing temper- bonding configurations.

1647J.O. Orwa et al. / Diamond and Related Materials 8 (1999) 1642–1647

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