Diamond structure recovery during ion irradiation at elevated temperatures

Diamond structure recovery during ion irradiation at elevated temperatures

Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx Contents lists available at ScienceDirect Nuclear Instruments and Methods i...

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Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Diamond structure recovery during ion irradiation at elevated temperatures Alec Deslandes a,⇑, Mathew C. Guenette a, Kidane Belay b, Robert G. Elliman b, Inna Karatchevtseva a, Lars Thomsen c, Daniel P. Riley a, Gregory R. Lumpkin a a b c

Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, Sydney, Australia Research School of Physics and Engineering, The Australian National University, Canberra 0200, Australia Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia

a r t i c l e

i n f o

Article history: Received 29 September 2014 Received in revised form 27 May 2015 Accepted 14 July 2015 Available online xxxx Keywords: Diamond Ion irradiation Temperature Raman spectroscopy X-ray spectroscopy

a b s t r a c t CVD diamond is irradiated by 5 MeV carbon ions, with each sample held at a different temperature (300– 873 K) during irradiations. The defect structures resulting from the irradiations are evident as vacancy, interstitial and amorphous carbon signals in Raman spectra. The observed variation of the full width at half maximum (FWHM) and peak position of the diamond peak suggests that disorder in the diamond lattice is reduced for high temperature irradiations. The dumbbell interstitial signal is reduced for irradiations at 873 K, which suggests this defect is unstable at these temperatures and that interstitials have migrated to crystal surfaces. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy results indicate that damage to the diamond structure at the surface has occurred for room temperature irradiations, however, this structure is at least partially recovered for irradiations performed at 473 K and above. The results suggest that, in a high temperature irradiation environment such as a nuclear fusion device, in situ annealing of radiation-created defects can maintain the diamond structure and prolong the lifetime of diamond components. Crown Copyright Ó 2015 Published by Elsevier B.V. All rights reserved.

1. Introduction Diamond is the carbon allotrope that offers the best resistance to radiation damage due to the strong binding energy of its lattice. Due to CVD diamond’s versatility for applications in extreme radiation environments it has been proposed for use in nuclear fusion devices as windows through which heating power can be delivered [1], as radiation detectors [2,3], and as a protective coating on high ion flux components such as the divertor [4]. The diamond material of these fusion components will be exposed to irradiation with energetic particles, including 14 MeV neutrons produced by deuterium–tritium (D–T) reactions. Due to the cost and time required to reach appreciable doses of relevant neutron irradiations, a rational choice to simulate the damage caused by the 14 MeV neutrons from the D–T fusion reaction is that of the most damaging knock-on atoms, i.e. MeV carbon atoms. High energy particle bombardment of diamond produces damage in the form of vacancies and interstitial atoms. With enough damage, the diamond structure can be lost and with annealing, the damaged region will ⇑ Corresponding author. E-mail address: [email protected] (A. Deslandes).

graphitise [5,6]. In a nuclear fusion environment, materials will potentially have a temperature of several hundred Kelvin, and damage due to the neutron flux is expected to be of the order of 1 displacement per atom (dpa) [7], hence there is a need to investigate the irradiation of diamond at elevated temperatures. There have been several decades of investigation into the effect of sample temperature during irradiation of diamond with high energy particles, much of which is summarised in a review by Prins [8] and the references therein. The motivation of much of this earlier work was the use of ion implantation for doping of diamond [9,10], and diamond growth [11,12]. Elevated sample temperatures during implantation were found to drive interstitials to the surface, but resulted in agglomeration of vacancies in extended defects [12]. These extended defects were not ideal for doping applications, and the temperature-enhanced mobility meant the dopants could also diffuse from their desired location [13]. The technique of cold implantation followed by rapid anneal was instead adopted [14], and much of the work in literature henceforth has applied this technique. Prawer and Kalish found the activation energy for defect diffusion in the ion track thermal spike during hot implants to be 0.2 eV [15], whereas post-implantation annealing experiments found a value for C interstitial diffusion to be 1.3 eV [16].

http://dx.doi.org/10.1016/j.nimb.2015.07.058 0168-583X/Crown Copyright Ó 2015 Published by Elsevier B.V. All rights reserved.

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Kalish et al. used Raman spectroscopy of post-implantation annealed diamond to monitor the dynamics of defects, and showed that above 900 K a diamond dumbbell interstitial peak at 1630 cm 1 disappeared, whereas beyond 1100 K a vacancy-related peak at 1490 cm 1 reduced in intensity [17]. Similar Raman results were found by Orwa et al., who also used polarisation of the laser light to show a broad skewed peak centred at 1245 cm 1 that was assigned to amorphous carbon [18]. In irradiated polycrystalline films, a luminescent band ascribed to the H3 centre was observed around 1250 cm 1 [19]. Partial electron yield (PEY) Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is a surface sensitive (typically less than 5 nm) synchrotron-based technique that shows the local bonding environment of the absorbing element. Previously, it was demonstrated that NEXAFS is sensitive to formation of defective structure in diamond bombarded with 30 keV Xe ions [20]. For fusion applications, NEXAFS has been used to investigate carbon ion irradiation of diamond [21], and exposure of diamond to hydrogen plasma [22]. NEXAFS studies also showed that surface termination can effect core-level bulk excitons [23], and diamond graphitised by implantation can be partially recovered by post-implantation annealing [24]. An understanding of the damage resulting from irradiation at different temperatures is of interest for diamond applications in nuclear fusion devices, to aid the understanding of component failure and lifetimes. In this work we use Raman and NEXAFS spectroscopy to investigate the damage structures arising in diamond as a result of high energy ion irradiation at a range of temperatures between 300 K and 873 K.

2. Experimental Chemical vapour deposited polycrystalline diamond of grade TM-100 was obtained from Element 6 Ltd. The diamond samples were free standing 10 mm  10 mm  0.25 mm plates and were used as-received. Samples were irradiated with 5 MeV C2+ ions, using a 5 lA broad beam, in a system pumped with oil-free vacuum pumps and operating vacuum of <1  10 7 Torr. Each sample was irradiated to a fluence of 1  1017 ions cm 2, which was monitored by integrating the drain current from the sample. Sample temperature was monitored using a thermocouple attached to the sample holder, which was interfaced with the sample heater. Different samples were irradiated at 300 K, 473 K, 673 K and 873 K. The sample irradiated at 300 K exhibited a temperature rise of approximately 20 K throughout the irradiation, which is considered to be negligible for the purposes of this experiment. SRIM estimates of the range and damage profiles for 5 MeV C2+ irradiation of diamond have previously been reported [21]. The predicted range was 2.4 lm. The predicted damage created by 1  1017 ions cm 2 is 1 dpa, averaged over the range, assuming displacement energy of 50 eV. Raman spectra were collected using a Renishaw inVia Raman spectrometer equipped with an Argon ion laser (514 nm) and a Peltier cooled CCD detector. Stokes shifted Raman spectra were collected in the range of 100–2000 cm 1 with a spectral resolution of 1.7 cm 1 for the 1800 lines mm 1 grating that was used. The spot size was approximately 1.5 lm for 50 magnification. The probing depth for this wavelength in diamond is of the order of a few lm. Six spectra were collected from different locations near the centre of each sample. Each spectra was background subtracted by fitting a polynomial to the boundary of the region of interest, before fitting or averaging. Peaks were fit using a Voigt function, with constraints to the peak positions on the order of ±25 cm 1.

Near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements were performed at the Soft X-ray Spectroscopy beamline at the Australian Synchrotron [25]. Details of the experimental setup and normalisation process can be found elsewhere [22]. An incident photon energy range of 270–320 eV was used corresponding to the carbon K-edge. Samples were mounted at an angle of 45° to the incident X-ray beam. The NEXAFS signal was collected in partial electron yield (PEY) mode with the cut-off voltage set to 135.5 V. The mean free paths of electrons at this energy are of the order of a few nm, resulting in a highly surface sensitive measurement. Carbon K-edge NEXAFS peak assignments are taken from Guenette et al. and the references therein [22]. The photon energy calibration and normalisation process was carried out as per the methods outlined by Watts et al. [26,27]. The sp2 fraction of the surface (<5 nm depth) was calculated by applying the well-established method used for electron energy loss spectroscopy (EELS) measurements [28] to the NEXAFS data, details of which can be found elsewhere [21]. 3. Results Fig. 1 shows Raman spectra of diamond irradiated at different temperatures and an unirradiated sample. Each spectrum has been background subtracted, normalised to the height of the diamond peak, and the figure shows the averaged spectrum for each set of irradiation conditions. The spectrum of the unirradiated sample exhibits a sharp diamond peak at 1333 cm 1, and a broad disordered carbon peak around 1500 cm 1. After irradiation peaks are evident at 1630 cm 1, which are assigned to a carbon interstitial diamond dumbbell defect. Vacancy-related peaks are evident at 1490 cm 1 and 1420 cm 1 [17]. There is broadening of the diamond peak about 1333 cm 1 following irradiation, which indicates variations to the diamond bonding state that are introduced by a distribution of defective diamond bonds. For wavenumbers less than the 1333 cm 1 diamond peak there appears to be either an asymmetric broadening of the diamond peak, indicative of diamond bonds that are distorted by the presence of nearby defects, or this signal may originate from a defect state such as the H3 centre [19]. The broad signal of disordered carbon around 1500 cm 1 appears to be greatest for the sample irradiated at 300 K, but with increasing irradiation temperature it decreases to be approximately half of this intensity for the samples irradiated at 673 K and 873 K. The Raman spectra of the diamond irradiated at 300 K and 473 K exhibit the greatest differences when compared to that

Fig. 1. Raman spectra of diamond irradiated at different temperatures.

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of the unirradiated diamond. Out of the irradiated diamond samples, the Raman spectrum of the diamond irradiated at 873 K bears the closest resemblance to that of the unirradiated diamond, but still suggests significant modification to the diamond structure has occurred. Spectra were fitted to examine changes to specific peaks caused by irradiation at different temperatures, with Fig. 2 (left) showing the fit for diamond irradiated at 473 K. Fig. 2 (right) shows the variation with irradiation temperature in the peak position (xD) and FWHM of the diamond peak which is nominally at 1333 cm 1 for pristine diamond. Each data point is the mean of the fitted results for the collected spectra from each set of irradiation conditions, and the error bars show the standard error. The FWHM is observed to increase for the samples irradiated at 300 K and 473 K compared to the unirradiated sample. The FWHM is then observed to decrease for the sample irradiated at 873 K. The peak position for the unirradiated diamond is observed to be 1333 cm 1 and then decreases to 1326 cm 1 for the irradiations performed at 300 K, before returning to 1331 cm 1 for the highest temperature irradiation at 873 K. Fig. 3 shows NEXAFS spectra of the unirradiated and irradiated diamond. Strong diamond NEXAFS features can be observed in the spectrum of the unirradiated diamond; namely the diamond exciton at 289.1 eV and the diamond second bandgap at 302 eV. Following irradiation at 300 K, a significant increase in the sp2-related peak at 285.0 eV can be observed, along with a disappearance of the diamond NEXAFS features. Irradiations at higher temperatures yield different results; slight hints of the diamond exciton are visible in the spectra of the samples irradiated at 473 K and 673 K, and this feature becomes clearer in the sample irradiated at 873 K. However, the exciton signal is relatively weak compared to the diamond reference. The diamond second bandgap is clearly visible for all samples irradiated at temperatures above 300 K. This indicates that irradiating at 473 K and above reduces the amount of disorder compared to irradiations at 300 K. The sp2 fraction, shown in Table 1, for the room temperature irradiated sample is 21.1%, and remains at a similar level for both the 473 K and 673 K irradiations. The sp2 fraction decreases to 15.4 % for the sample irradiated at 873 K. [email protected] bonds can be observed via the 288.5 eV peak for the sample irradiated at 300 K. This peak is not clearly visible for any sample irradiated at 473 K and above. Some C–H bonding can be observed for all samples from the shoulder peak at 287 eV.

4. Discussion The Raman spectra show that the sample temperature during irradiation affects the structure of ion-damaged diamond. The observed decrease with temperature of the disorder signal around 1500 cm 1 indicates that after irradiation at elevated temperature there are fewer displaced atoms or vacancies contributing to local disorder of the diamond structure. The peaks indicative of vacancy and interstitial defects are observed for all irradiation temperatures. The vacancy-related peak at 1490 cm 1 appears consistently for all irradiated samples, which suggests the vacancy defects are stable up to 873 K. The sample irradiated at 873 K exhibits a decreased interstitial defect signal compared to samples irradiated at lower temperatures, suggesting fewer dumbbell defects. Dumbbell defects have been shown to be less stable at these elevated temperatures, and post-implantation annealing has shown that at higher temperatures, this defect signal disappears [17,18]. The increased mobility of individual interstitials at higher temperatures may allow for them to be lost to grain boundaries or other fixed sinks [12], resulting in less interstitials available to form dumbbell defects. Annihilation of interstitials with vacancies

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appears to be a less likely explanation, as the peak originating from vacancy defects is unaffected by temperature. The vacancy signal behaviour suggests that higher temperatures are required to mobilise vacancies, and here there is no observation of evidence for the agglomeration of vacancies into extended defects as has been previously reported [12]. The Raman shift of the diamond peak is observed to decrease in wavenumber following irradiation. The change in position is greatest at the lowest irradiation temperatures, however, for the irradiation performed at 873 K the diamond peak position is similar to that of the unirradiated diamond. The decrease of the diamond peak position with irradiation is indicative of increased tensile stress in the diamond lattice [19]. The fact that the peak position is closer to that of unirradiated diamond at higher temperatures suggests that the stress is relieved at higher irradiation temperatures. The FWHM of the diamond peak is increased for the irradiated samples, which indicates a broadened distribution to the vibrations of diamond chemical bonds. The decrease of the FWHM for elevated irradiation temperatures shows that the width of this distribution is reduced and that in situ annealing has occurred to recover the diamond structure. The NEXAFS results are indicative of elevated irradiation temperatures affecting the defect structure at the surface. The sp2 fraction is less for the sample irradiated at 873 K compared to those irradiated at lower temperatures. This is indicative of less ion-damaged defect states relaxing to sp2 structure, and shows that the sp3 nature is retained to a greater degree. The diamond exciton and second band gap were not observed for the room temperature irradiated sample, but were evident for irradiations at elevated temperatures. Their observation at elevated temperatures indicates that the long-range diamond structure that produces these signals is maintained, likely by annealing of distortions to the lattice that affect the long-range order. Previous isotopic experiments showed C implanted in diamond at elevated temperature migrates to the surface [29]. The NEXAFS results suggests that for irradiations at elevated temperature a disordered layer is not created near the surface but instead the diamond structure is at least partially retained, likely incorporating mobilised C interstitials that have diffused to the surface. While the irradiation progresses, the diamond structure at the surface can also be damaged by the impinging MeV ions. The Raman and NEXAFS results provide complementary characterisation of damage to the bulk and the surface. NEXAFS as the surface sensitive technique gives information on the damage created by deposition of energy via inelastic processes. On the other hand, Raman spectroscopy, with a probing depth of few microns in diamond, gives summary information on both inelastic and elastic interactions of MeV ions with diamond. The results indicate that the damage created in the bulk and at the surface have different responses to temperature. For these MeV irradiations, at the surface most of the energy is transferred from the ions to the diamond via electronic stopping, whereas nuclear stopping dominates over the order of lm resulting in the creation of many more vacancies. The damage created at the surface may thereby be more readily annealed due to the lower number of displacements per atom arising from electronic stoping near the surface. This is supported by the NEXAFS data, as even at a relatively moderate sample temperature of 473 K, the diamond second bandgap and exciton signals exhibit recovery which is then maintained for samples irradiated at 673 K or 873 K. On the other hand, the Raman spectra show the greatest recovery occurs for irradiation at 873 K, with limited recovery observed at 473 K. The recovery of the damage in the bulk is likely limited by mobility of defect species such as interstitials and vacancies. The results show that in situ annealing of damage occurs for diamond irradiated at elevated temperatures, hence radiation

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Fig. 2. (a) Fitted Raman spectrum of diamond irradiated at 473 K, (b) peak position and FWHM of the diamond peak for diamond irradiated at different temperatures.

the dumbbell interstitial signal in Raman spectra. NEXAFS spectra also show that diamond structure is better maintained during irradiation at elevated temperature via a decreased sp2 fraction, retained diamond exciton, and strong retention of the second-band gap signals. These results show that in a high temperature irradiation environment diamond components may maintain structural integrity and exhibit prolonged performance, compared to the damage accumulated during irradiation at low temperature. Acknowledgments

Fig. 3. NEXAFS spectra of diamond irradiated at various temperatures.

Table 1 sp2 fraction of the near surface region for diamond irradiated at various temperatures. Sample

sp2 fraction

Diamond reference 300 K 473 K 673 K 873 K

0.024 0.211 0.197 0.203 0.154

tolerance tests of components should be carried out at the temperatures expected for operation to better predict their lifetimes. This in situ recovery of structure will delay the onset of irradiation-induced graphitisation, and prolong the advantageous bulk properties of diamond such as its excellent thermal conductivity. The surface is observed to retain diamond structure if irradiated at elevated temperatures. It could thereby be expected that diamond’s abilities to withstand erosion [30] and retain less hydrogen [21] compared to graphite may be maintained at elevated temperatures when confronting the challenging plasma and radiation damage conditions in a nuclear fusion reactor. 5. Conclusion In situ healing of defects and disorder for diamond irradiated at high temperature is observed via the narrowing of the diamond peak, decrease in the broad disordered signal, and decrease in

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