γ radiation thermoluminescence performance of HFCVD diamond films

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 248 (2006) 103–108 www.elsevier.com/locate/nimb

c radiation thermoluminescence performance of HFCVD diamond films S. Gaste´lum a, E. Cruz-Zaragoza b, R. Mele´ndrez a, V. Chernov a, M. Barboza-Flores a

a,*

Centro de Investigacio´n en Fı´sica de la Universidad de Sonora, Apdo. Postal 5-088, Hermosillo, Sonora 83190, Mexico b Instituto de Ciencias Nucleares UNAM, Apdo. Postal 70-54, 04510 Me´xico, DF, Mexico Received 28 November 2005; received in revised form 10 March 2006 Available online 2 May 2006

Abstract Polycrystalline chemically vapor deposited (CVD) diamond films have been proposed as detectors and dosimeters of ionizing radiation with prospective applications in high-energy photon dosimetry applications. We present a comparison study on the thermoluminescence (TL) properties of two diamond film samples grown by the hot filament CVD method having thickness of 180 and 500 lm and exposed to c radiation in the 1–300 Gy dose range. The 180 lm thick sample deposited on silicon substrate displayed a TL glow curve peaked at 145 C. The 500 lm, which was a free standing sample, exhibited higher intensity and a well defined first order kinetics TL glow peak around 289 C. Both diamond samples showed a linear dose behavior in the 1–50 Gy range and sublinear behavior for higher doses. The 180 and 500 lm samples presented about 80% and 30% TL losses in a 24 h period, respectively, with both samples showing excellent TL reproducibility. The results indicate that the 500 lm CVD diamond film exhibited a good TL behavior adequate for c radiation dosimetry.  2006 Elsevier B.V. All rights reserved. PACS: 78.60.Kn; 85.60.Gz Keywords: Diamond films; Chemical vapor deposition; Dosimetry; Thermoluminescence

1. Introduction Polycrystalline CVD diamond films have been successfully tested as a passive thermoluminescence (TL) detector and radiation dosimeter. In fact, the main TL features and dose–response of doped and non-doped CVD diamond were investigated for ionizing a, b, c and X-ray and non-ionizing UV radiation [1–13]. The advantage of CVD diamond is the relatively low cost as compared to natural diamond as well as the possibility of producing thickness of the order of few to hundreds lm. In addition, impurities may be added during the growing process of the CVD films, which allows the opportunity to change the TL main features as well as the dose behavior. Impurities in the *

Corresponding author. Tel.: +52 662 259 2156; fax: +52 662 212 6649. E-mail address: [email protected] (M. BarbozaFlores). 0168-583X/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.006

CVD films permit to have localized trapping states inside the wide band gap (5.5 eV) of diamond. The TL glow curve depends on the distribution of localized trapping states since the electron–holes pairs created immediately after irradiation exposure are trapped by those trapping levels. Nowadays, it is possible to produce CVD polycrystalline diamond films with TL properties comparable to those found in LiF commercial dosimeters [12–17]. In view of the fact of the extraordinary properties of CVD diamond like tissue equivalence, chemically inert, radiation hard, non-toxic and high spatially resolution, it has been also investigated in relation to radiotherapy applications involving high-energy photons and/or electrons with sensitivity and TL response comparable to those observed in natural diamond [11,17–19]. Therefore, CVD diamond is considered as a very promising material for online radiotherapy medical applications. However, the state of the art in CVD diamond does not allow yet producing high

S. Gaste´lum et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 103–108

quality CVD diamond that permits accurate dose determination in a very small size area, dose rate independence as well as stable and reproducible TL response. The TL properties depend mainly on the number and types of defects produced by impurities or to intrinsic defects formation during growing. Therefore, further investigation is required to find a suitable CVD diamond fabrication conditions that allow growing good quality CVD diamond with tailored TL and dosimetric properties. In the present paper, we report the TL characterization results obtained in two CVD diamond films, deposited on silicon substrate and free standing, grown by the hot filament technique having 180 and 500 lm thickness, exposed to c radiation in the 1–300 Gy dose range.

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The CVD diamond films were grown by the hot filament technique using CH4(5 sccm)–CO(10 sccm)– H2(400 sccm) gas mixture on silicon substrate. Two film samples with thickness of 500 and 180 lm and identified as HF01 and HF02, respectively, were studied in a freestanding (500 lm, HF01) form and as grown (180 lm, HF02) in its original silicon substrate. The HF01 sample was subjected to a CrO3–H2SO4 etching treatment at 750 C for half an hour in order to remove the substrate producing a free standing diamond sample. The samples were exposed to c radiation using a MDS Nordion Gammacell 200 60Co source providing a 0.67 Gy/min dose rate. The thermoluminescence measurements were performed in darkness in a Harshaw TLD 3500 reader using a heating rate of 2 C/s. Raman spectra were obtained using an Almega XR Dispersive Raman Spectrometer equipped with a thermoelectrically cooled CCD detector. The excitation source was a 5 mW 532 nm radiation provided by a frequency-doubled Nd:YVO4 laser. Scanning electron microscope (SEM) images were obtained using a JEOL SEM system equipped with an Oxford EDS analyzer.

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3. Results and discussion

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The main TL features of CVD diamond film exposed to c radiation as a function of the dose is displayed in Fig. 1. The diamond sample HF01, which is 500 lm thick has about one order of magnitude higher TL efficiency than the diamond sample HF02 with thickness of 180 lm. The TL glow curve of the HF01 and HF02 samples are peaked around 289 and 145 C, respectively. The TL behavior shown in Fig. 1 possesses a major difference respect to previous TL results in CVD diamond films exposed to beta and c radiation, as discussed in two important works [3,14]. In those investigations the TL glow curve is composed of low temperature glow peaks around 100–180 C and TL glow peaks located around 200–350 C. In the present CVD diamond HF01 and HF02 samples the TL glow curve is composed of a main TL glow curves structure peaked at

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Fig. 2. (a) TL glow curves (crosses) of 20 Gy c irradiated HF01 and (b) HF02 CVD diamond films. Solid lines are the TL peaks found with a deconvolution procedure. Bold lines represent the sum of all the TL peak components.

S. Gaste´lum et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 103–108

289 and 145 C, respectively. A home made curve fitting process, based upon a non-linear least-squares (Levenberg–Maquart) deconvolution procedure, indicated that the HF01 TL glow curve is well fitted with a single, first order kinetics, peak centered at 290 C. The fitting program is capable of simultaneously processing up to ten first or second order kinetics TL peaks for which, the maximum temperature Tm (K), activation energy E (eV) and frequency factor S (s 1) for each TL peak are determined. The kinetics parameter for the HF01 samples were Tm = 290 C, E = 0.76 eV and S = 4.13 · 105 s 1. On the other hand, the glow peak deconvolution analysis of the

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HF02 sample determined that the low-temperature glow curve shape is composed of two peaks. The first being first order kinetics peaked at 120 C and the second located at 157 C following a second order kinetics. The activation energies for these peaks were E = 0.57 and E = 0.89 eV, respectively, and the frequency factors were S = 1.88 · 103 and S = 3.7 · 104 s 1, respectively. Fig. 2 shows the actual experimental TL glow curve data along with the fitted glow curve components for the HF01 and HF02 diamond samples. It is pertinent to mention that the kinetics parameters of the c-irradiated HF01 and HF02 samples differ from to those found on the same samples but

Fig. 3. (a) Scanning electron microscopy micrograph of the HF02 CVD diamond sample. (b) Raman spectrum of HF02 and HF01 (inset) CVD diamond samples.

S. Gaste´lum et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 103–108

irradiated with b and UV-irradiation [5]. The activation energies values of CVD and natural diamond seem to depend on the type of diamond (doped and undoped) as well as on the kind of radiation used to induce the TL [5,23–25]. Therefore, a great deal of investigation is still required to fully understand the nature of the trapping localized states produced by irradiation in the different types of doped and undoped CVD diamond. In spite of the uncertain nature of a particular CVD diamond TL glow curve structure, it is frequently attributed to the existence of impurities accidentally introduced during the growing process, morphological structure and existing defects. The effects of impurities concentrations like boron and nitrogen on CVD diamond films have been also thoroughly investigated. Small boron doping concentration changes the TL glow curve shape and increases sensitivity [20,21]. On the other hand, nitrogen impurities seem to produce a TL quenching effect and particular TL emission due to the impurity [22]. Furthermore, it has been demonstrated that thickness in CVD diamond films has a strong impact on the TL glow curve shape. Borchi et al. [26], reported the TL glow curve results on 10 CVD diamond samples of thickness between 350 and 900 lm. The samples displayed a very different TL glow curve shape characterized with a semi-isolated high-temperature peak around 300 C or a low-temperature glow curve structure around the 50–250 C. Similar results were reported by Furetta et al. in two CVD diamond samples [27]. Fig. 3(a) shows the SEM micrograph of the diamond sample HF02 and Fig. 3(b) displays the Raman spectrum of both HF01 and HF02 diamond sample. We should recall that the main difference between the HF01 and the HF02 samples here investigated is that the HF01 is a free standing sample with the silicon substrate removed. The HF02 Raman spectroscopy indicated the presence of a prominent diamond (sp3 bonding) 1332 cm 1 peak. However, the Raman spectrum showed in addition a contribution from the 1580 cm 1 graphite band and the presence of amorphous sp2 carbon asymmetrical broad band in the region 1000– 1600 cm 1. It is known that natural diamond displays a characteristic TL glow curve of similar intensities around the 25–175 C and 175–350 C temperature intervals; precisely at which the HF02 and HF01 glow peaks appear. Therefore, the glow curve shape of the as grown HF02 may have some contribution from diamond, diamond like carbon or some form of carbon bounded to the silicon substrate. The difference between the HF02 and the free standing HF01 (chemically etched) samples can be clearly seen in the Raman spectrum displayed in Fig. 3(b). The inset undoubtedly indicates the graphitization of the diamond film. It is noteworthy to observe that the etching procedure actually removed most of the amorphous carbon phases leaving a highly ordered graphite region on the HF01 diamond sample (G band at 1580 cm 1), along with the main 1332 cm 1 diamond peaks overlapped with the 1350 cm 1 disordered D graphite band. These Raman graphitic bands at 1580 and 1350 cm 1, called D and G bands, respectively

have been recently the subject of investigation in CVD diamond [28]. The distinctive features exhibited by the HF01 and HF02 samples have a major effect on the TL glow curve shape due perhaps to the differences in morphological defects, grain size, texture, surface structure and impurities concentrations, as pointed out by some authors [23,26]. These TL glow curve differences prevail in samples fabricated under similar CVD reactor and growing conditions. The main HF01 and HF02 TL glow curve features observed in Figs. 1 and 2, have been also observed in a series of CVD diamond films [26,27]. Other TL measurements, in free standing CVD diamond [29] and CVD diamond, grown on undoped and boron doped Si substrates [23], have found the existence of highly intense TL glow peaks in the low-temperature side of the glow curve, as well as at the high-temperature side of the glow curve, either as single or well structured TL glow curve containing several peaks. It is pertinent to comment on the low 10 3–10 5 s 1 frequency factors obtained for the diamond HF01 and HF02 samples through the fitting deconvolution procedure. Unusually low frequency factors are characteristic of localized transitions in which radiative recombination can take place without a transition of the electron into the conduction band. In this case, the electron is thermally stimulated into an excited state from which a transition into the recombination center is permitted. Low frequency factors for diamond have already being reported by many authors. For instance, Borchi et al. [26] obtained values in the range of 107–109 s 1and 104–109 s 1 in another CVD diamond samples by Borchi et al. [8]. Benabdesselam et al. [29] reported a value of 4.2 · 106 s 1 in a free standing polycrystalline CVD diamond. We also used the Chen’s and the initial rise methods [30] to analyze the CVD diamond TL diamond data. Chen’s method provided averaged values

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S. Gaste´lum et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 103–108

of E = 0.9 eV and S = 9 · 105 s 1 for the sample HF01, while the initial rise method described a process with E = 0.67 eV. Compared to the values of E = 0.76 eV and S = 4.13 · 105 s 1 obtained with our TL glow curve fitting deconvolution procedure we confirmed that our deconvolution program works adequately. Of course, further investigation is necessary to gather additional information about the physical meaning of very low frequency factors. It is known that the TL performance of some CVD diamond film is comparable to commercial LiF dosimeters [1,7–9]. The HF01 sample presents a well-defined TL glow peaked around 290 C that is at higher temperature than those found in the TLD-100 commercial dosimeter. Low-temperature TL glow peaks means charge

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carriers trapped in localized states that are instable compared to those trapped at high-temperature trapping states. Trapping state instabilities may imply that a part of the trapped charge carriers could possible recombine radiatively or non-radiatively. It gives rises in both cases to TL losses, called TL fading, which eventually may affect the dosimetric performance of the dosimeter. Fig. 4 illustrates the TL fading characteristics of the HF01 and HF02 CVD diamond samples as compared to commercial LiF:Ti, Mg (TLD 100) dosimeters. The TL fading losses were monitored over a period of 24 h; the previously irradiated samples were stored at room temperature in darkness for a given time followed by a TL readout. The HF01 diamond sample and the TLD100 dosimeter both experienced about 30% TL losses from its initial values during the 24 h of storage, although the HF02 sample losses 80% of its original TL in the same period of time. The effect of repetitive use of the CVD samples is depicted in Fig. 5 which shows the TL integrated signal as a function of the reading-out cycle number. The HF02 samples exhibit an extremely good TL stability during the read-out cycles and a deviation of about 5% was observed for the HF01 sample. The lower part of Fig. 5, shows the TL dose–response for both CVD diamond samples; the total integrated TL is plotted as a function of irradiation dose in the range of 1–300 Gy. Low dose linearity is good for dosimetric application in radiotherapy.

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The TL performance of CVD diamond samples here studied is good enough to envisage future investigation to improve CVD diamond quality adequate for dosimeter applications. Although, the HF01 and HF02 CVD diamond samples were tested for c radiation, the samples behaves quite well for b and UV radiation [5]. The results allow evaluating the good perspective of CVD diamond in TL dosimeter applications. Low dose linearity around the 10–20 Gy is adequate for clinical and radiotherapy purposes. In addition, diamond is remarkable advantageous in relation to medical and radiotherapy applications, since it is chemically inert to body fluids, non-toxic and tissue equivalent. All of these properties make CVD diamond a promising TLD material and worth to continuous and systematic investigations.

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Fig. 5. (a) TL of the HF01 (500 lm) and HF02 (100 lm) CVD diamond samples as a function of the read-out cycle (upper). The samples were exposed each time to 20 Gy of c radiation and the TL reading of 2 C/s was performed afterward; (b) The lower curve represents the dose behavior of the same samples and the inset refers to the low 1–50 Gy dose behavior.

The authors are indebted to Dr. B. Gan for growing the samples and Prof. C. Furetta for enthusiastic and enlighten discussions on the TL glow curve deconvolution procedures. We acknowledge financial support from Oficina de Colaboracio´n Interinstitucional UNAM, SEP and Conacyt Grants No. 36521, 37641 and 32069.

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