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Radiation induced luminescence processes in c-BN
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Radiation Measurements 38 (2004) 615 – 618 www.elsevier.com/locate/radmeas
Radiation induced luminescence processes in c-BN L. Trinklera;∗ , B. Berzinaa , M. Benabdesselamb , P. Iacconib , L. B+tter-Jensenc , K. Atobed a Physics
of Defects in Ionic Materials, Institute of Solid State Physics, University of Latvia, Kengaraga St., 8, Riga LV-1058, Latvia b University of Nice-Sophia Antipolis, LPES-CRESA, Parc Valrose, Nice 06108, France c Ris* National Laboratory, Roskilde DK-4000, Denmark d Department of Physics, Faculty of Science, Naruto University of Education, Japan Received 11 November 2003; received in revised form 11 November 2003; accepted 5 January 2004
Abstract Spectral properties of cubic boron nitride have been studied using methods of photoluminescence (PL), X-ray excited luminescence (XL), thermoluminescence (TL) and optically stimulated luminescence. It is found that emission of cubic boron nitride is presented by 4 subbands, their relative yield is determined by the excitation type: blue, green (dominant) and red bands are observed in PL, ultraviolet, blue (dominant), green and red bands—in XL. Three thermal peaks are found in TL curves in the 0 –700◦ C temperature range, their presence and intensity depend on radiation type used. A tentative correspondence between thermal peaks and emission bands is found. c 2004 Elsevier Ltd. All rights reserved. Keywords: Cubic boron nitride; Photoluminescence; Thermoluminescence; Optically stimulated luminescence
1. Introduction Cubic boron nitride (c-BN) belongs to III–V nitride materials, which have already found a broad application range in electronics, optoelectronics and other >elds due to their outstanding features. c-BN is a wide band gap material (6:4 eV), extremely hard and chemically stable. Production of single crystal c-BN is a complicated and expensive process. Most of the studies deal with thin layers or polycrystalline samples. Usually samples contain uncontrolled intrinsic defects and impurities, which a@ect the properties of the material. Our previous paper (Berzina et al., 2002) was devoted to characterisation of c-BN ceramics using methods of optical spectroscopy. Basing on the properties of photoluminescence (PL) and UV-induced optically stimulated luminescence (OSL) it was concluded that defect centers present in the material produce recombination luminescence. In the
∗ Corresponding author. Tel.: +371-725-3592; fax: +371-713-2778. E-mail address: [email protected] (L. Trinkler).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.010
present paper, we continue studies of this material adding methods of thermally stimulated luminescence (TL) with the aim to determine properties of the luminescence centres and to estimate the present samples for practical application in dosimetry.
2. Materials and equipment c-BN used in studies were the ceramics samples produced in Japan. According to producers they consist of grains with almost completely cubic phase. Samples have square shape 4 × 4 × 0:5 mm3 , they are of greenish-grey colour and opaque. Most of the spectral measurements were ful>lled in Latvia, using the self-made experimental set-up, consisting of a deuterium lamp LDD 400 (Russia) used as an ultraviolet radiation (UVR) source, halogen lamp used for optical stimulation, two monochromators for selection of emission wavelength located in the excitation and luminescence or stimulation channels, a number of lenses and optical >lters, a photomultiplier tube and a recorder. An X-ray lamp (40 kV, 15 mA) was used for X-ray excited
616
L. Trinkler et al. / Radiation Measurements 38 (2004) 615 – 618 1.2
Luminescence intensity, a.u.
Luminescence intensity, a.u.
1.2 5
1
3
0.8
4 2
0.6 0.4
1
0.2 0 200
300
400
500
600
700
Wavelength, nm
Fig. 1. Spectral properties of c-BN ceramics: PL excited at 270 nm from the fresh sample (1), the 6 months aged sample (2), the 12 months aged sample after heating up to 600◦ C (3); OSL excited at 270 nm and stimulated by infrared light (4); XL (5).
luminescence (XL). In OSL emission and excitation spectra each point was obtained in a single “UV irradiation—optical stimulation” cycle. All spectra obtained are corrected for instrumental spectral response. Some TL and OSL measurements were ful>lled in RisH National Laboratory, using a RisH model TL/OSL DA-12 reader with linear heating and built-in blue LED array (470 nm) and 90 Y= 90 Sr beta source (0:1 Gy=s). UV irradiation for TL measurements was carried out using the Sol2 lamp (K. Honley GmbH), simulating solar emission spectrum on the ground level. Application of the UV >lter resulted in selection of the excitation light in the 300 –400 nm spectral range. TL was recorded through a glass >lter—either blue (300 –500 nm) or UV (240 –400 nm) one. A UV photometer PMA 2200 (Solar Light Co) was used to measure absolute values of UV irradiance. TL measurements were ful>lled also in University of Nice using a computer-controlled TL equipment with linear heating. A xenon lamp was used for UV irradiation of a sample, the excitation wavelength was selected by a monochromator. TL emission was recorded with a photomultiplier tube. The spectral region of the TL emission was selected with interference >lters of two kinds having bandpass 416 ± 10 and 506 ± 7 nm. All spectral measurements were carried out at RT, TL measurements applied to thermal region above RT with the heating rate 2◦ C=s.
3. Experimental results Photoluminescence emission of c-BN ceramics shown on Fig. 1 covers a broad spectral range: 350 –700 nm. PL spectrum of a fresh c-BN sample measured in our previous work (Berzina et al., 2002) has a well-pronounced peak at 500 nm (curve 1). Very large width and complex shape of PL emission spectrum allows proposing that it consists of
1
2
3
1
0.8
0.6
0.4
0.2
0 200
4
250
300
350
400
Wavelength, nm
Fig. 2. Excitation spectra of PL for lum : 360 nm (1), 500 nm (2), 660 nm (3) and OSL for lum 500 nm (4) from c-BN.
several overlapping subbands. The PL spectrum of an aged sample obtained after 6 months of storage (curve 2) has a more pronounced long wavelength part compared to that of a fresh sample. Heating up to 600◦ C and quenching of the 12 months aged sample caused ever higher increase of the long wavelength part of the spectrum (curve 3). PL excitation features are shown in Fig. 2. It is seen that the shape of an excitation spectrum depends on the emission region selected; it also proves that PL emission from c-BN contains several subbands, each with its own characteristic excitation spectrum. Analysing spectral properties one can assume that there are at least 3 subbands in PL emission: a blue one around 400 nm excited in a narrow band at 250 nm (Fig. 2, curve 1), a green one peaking at 500 nm and excited in a broad spectral region 200 –370 nm with a dominant peak at 270 nm (Fig. 2, curve 2) and a red one at 600 –700 nm having a broad and di@use excitation spectrum (Fig. 2, curve 3). Contribution of the red luminescence band increases with storage time and heating, as it is seen from PL emission spectrum of the aged and heated samples. Studies of OSL in c-BN are hampered by a very weak luminescence signal. Preliminary studies show that OSL emission is observed in the 400 –550 nm spectral range with a maximum at 480 nm (Fig. 1, curve 4). Its excitation spectrum contains no pronounced bands (Fig. 2, curve 4) but has a shoulder at 250 nm and sharply increases in the short wavelength side. XL (Fig. 1, curve 5) is also characterised with a broad spectrum, which is shifted to short wavelengths compared to the PL spectrum and has a maximum around 400 nm. The shape of the XL spectrum implies a complex structure containing several luminescence subbands. Some of these subbands may be the same as in the PL spectrum (400 – 700 nm). Their relative contribution into the integral XL yield is di@erent from the PL case—now the 400 nm subband is dominant. Besides the XL spectrum contains also a subband in the 280 –320 nm, which is not observed in PL. Similar spectrum centred at 400 nm was observed in cathodoluminescence of c-BN (Manfredotti et al., 2001).
L. Trinkler et al. / Radiation Measurements 38 (2004) 615 – 618 80000
5000 1
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60000 50000
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TL yield, counts
617
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We have not measured the luminescence spectrum excited by beta rays but we could assume that it should be the same as the luminescence spectrum excited by X-rays and cathode emission. In RisH National Laboratory thermoluminescence from c-BN was studied using two types of irradiation—UVR and beta rays (Fig. 3, curves 1, 3 and 2, 4, respectively). In both cases two kinds of >lters—blue (Fig. 3, curves 1, 2) and UV (Fig. 3, curves 3, 4) were used for the luminescence recording. In all cases TL signal from the irradiated c-BN is observed at temperatures up to 700◦ C (Fig. 3). TL curve 1 induced by the UVR and recorded through the blue >lter contains two clearly distinguished thermal peaks at 180◦ C and around 530 –550◦ C, labelled as T1 and T3 , correspondingly. Using of the UV >lter instead of the blue >lter (curve 3) not only decreases the TL yield but also almost completely eliminates the low temperature peak T1 . The beta radiation induced TL curve recorded through the blue >lter (curve 2) is characterised with a dominant peak T3 at 530◦ C and much weaker peaks T2 at 360◦ C and T1 180◦ C. If the beta-induced TL is measured using the UV >lter (curve 4), than the T1 peak is not observed, the T3 peak decreases drastically but the T2 peak, though also smaller, becomes dominant. The TL measurements in University of Nice were carried out irradiating the sample by UVR at 280 nm and recording the TL emission through interference >lters: 416 and 506 nm (Fig. 4, curves 1 and 2, respectively). In general the TL curves are similar to those obtained in RisH National Laboratory: the same T1 and T3 peaks are clearly seen in the UVR-induced TL curves. The T3 peak is dominant in both curves, while the T1 peak is relatively better distinguished in curve 2. Fig. 5 shows the results of the combined OSL and TL measurements, ful>lled in RisH National Laboratory. Curves 1 and 3 are obtained by heating of the c-BN sample after
200
300
400
500
600
700
Temperature, C
Fig. 4. TL curves from c-BN after UV irradiation (280 nm) recorded through interference >lters: 416 nm (1) and 506 nm (2).
20000
TL yield , counts
Fig. 3. TL curves from c-BN obtained after irradiation with UVR recorded through blue >lter (1) and UV >lter (3) and after irradiation with beta rays (3 Gy) recorded through blue >lter (2) and UV >lter (4). UVR irradiance 2900 W=cm2 , UVR dose received 173 mJ=cm2 .
100
15000 1 10000
2
T2 T3 3
T1
5000
4 0 0
100
200
300
400
500
600
700
Temperature, C
Fig. 5. E@ect of optical bleaching on TL from c-BN. TL curves obtained just after irradiation with UVR (1) and beta rays (3) are compared with those obtained after exposure to blue light (470 nm) of the sample irradiated by UV (2) and beta (4) radiation. TL is recorded through the UV >lter.
the UV and beta irradiation, respectively, using the UV >lter for luminescence recording. If the irradiated sample is >rst illuminated by a blue LED until the complete termination of the OSL signal and only then heated, the observed TL curves change their shape (curve 2—for the UV irradiation, curve 4—for beta irradiation): the low temperature peaks T1 and T2 are bleached out, while the T3 thermal peak remains unchanged. The process of the TL fading was studied only in the case of the UV irradiation. Measuring the TL signal from the preliminary UVR-exposed sample after storage during 24 h at room temperature it was found that the T1 peak had faded by 50% approximately, while the T3 peak practically had not changed. Fading of the T2 peak was not studied. 4. Discussion and conclusions Our earlier work (Berzina et al., 2002) has already shown that luminescence in c-BN is caused by recombination processes. Results of the present work allow developing of
618
L. Trinkler et al. / Radiation Measurements 38 (2004) 615 – 618
this conception. Analysing PL and XL spectra totally 4 luminescence bands could be distinguished: UV, blue, green and red. Their relative contribution into the luminescence yield depends on the excitation type. Excitation of the defect levels inside the band gap by UVR produces the dominant green band and the weak blue and red bands in the PL spectrum. Excitation by ionising radiation, which is followed by band-to-band transitions and probably exciton processes results in emergence of the dominant blue band and weaker UV band, weak green and red bands are also observed. The number of centres responsible for the red luminescence increases during aging and thermal treatment, as it is seen from the PL spectrum evolution during ageing of the sample. Though the UVR-induced OSL spectrum practically coincides with the green PL band, the OSL excitation spectrum is quite di@erent from that of PL excitation. Its sharp increase in the short wavelength side allows supposing that besides excitation of local defect levels (in the 250 –350 nm region) exciton processes are also involved in OSL generation. TL from c-BN was studied after UVR and beta ray irradiation. Shape of the TL curves depends on the radiation type implying di@erent luminescence mechanisms. Presence of several thermal peaks, each with its own characteristic emission, in the TL curves implies participation of at least three di@erent types of charge carrier trap centres. Regretfully, up to now we had no chance to measure directly spectra of TL emission due to the low intensity of the TL signal. One can only try to >nd a correspondence between thermal peaks and emission bands basing on the analysis of transmission of the >lters used and emission spectra of c-BN. We could ascribe the T1 peak to the green band because it is much more intensive under UV irradiation compared to beta irradiation and not observed through the UV >lter. Small contributions of the T1 peak into the TL curve recorded through the interference >lter 416 nm (Fig. 4, curve 1) could be explained by overlapping of the broad luminescence bands. Emission spectrum in the T3 peak seems to contain both blue and green subbands, because this peak is present in all TL curves recorded through di@erent >lters. The T2 peak is dominant when the beta-induced TL is observed through the UV >lter, that is why we ascribe it to the most short wavelength subband 280 –320 nm, observed only under ionising radiation. The red emission band cannot be observed in the TL process because the >lters used cut it o@. The T3 peak, which evidently corresponds to the deepest trap level, is the most stable: is not bleached by illumination with blue light (contrary to T1 and T2 peaks) and is not subjected to fading during storage of an irradiated sample at RT.
The obtained results are not suPcient for elucidating of the defect types responsible for luminescence processes. Using literature data based on theoretical calculations (Gorchica et al., 1998; Gubanov et al., 1996, 1997), boron and nitrogen vacancies (VB and VN ), antisites (BN , NB ) or impurities can be mentioned as potential candidates for such centers. Estimating the present samples of c-BN for practical application in dosimetry the following features could be mentioned. c-BN produces OSL and TL signal after exposure to UVR and ionising radiation. TL is observed in a thermal region up to 700◦ C with 3 thermal peaks. The T1 peak is subjected to signi>cant fading but the stable T3 peak is located at too high temperatures, not suitable for practical application (fading of T2 was not studied). The main drawback of this material is a very low intensity of luminescence signal, observed in PL, OSL and TL. Besides, the number of the centres responsible for the red luminescence is not constant but depends on ageing and thermal treatment of the sample. Notwithstanding that these particular samples of c-BN are not suitable for application in dosimetry, the studies should be continued considering controlled doping of the material. Acknowledgements We are grateful for assistance in PL measurements to R. Krutohvostov. This research was sponsored by Supported by EC Center of Excellence CAMART of Institute of Solid State Physics (contract No ICA1-CI-2000-7007) and Programme of Latvian-French cooperation in science and technology OSMOSE. References Berzina, B., Trinkler, L., Atobe, K., 2002. Spectral characteristics of native defects in BN. Phys. Stat. Sol. (c) 0 (1), 421–424. Gorchica, I., Svane, A., Christensen, N.E., 1998. Native defects and carbon impurity in cubic BN. MRS Internet J. Nitride Semicond. Res. 3, 48. Gubanov, V.A., Pentaleri, E.A., Fong, C.Y., Klein, B.M., 1997. Electronic structure of defects and impurities in III–V nitrides: II Be, Mg and Si in cubic boron nitride. Phys. Rev. B 56 (20), 13077–13086. Gubanov, V.A., Lu, Z.W., Barry, M.K., Fong, C.Y., 1996. Electronic structure of defects and impurities in III–V nitrides: vacancies in cubic boron nitride. Phys. Rev. B 53 (8), 4377–4385. Manfredotti, C., Vittone, E., Lo Giudice, A., et al., 2001. Ionoluminescence in CVD diamond and in cubic boron nitride. Diamond Relat. Mater. 10, 568–573.
Radiation Measurements 38 (2004) 615 – 618 www.elsevier.com/locate/radmeas
Radiation induced luminescence processes in c-BN L. Trinklera;∗ , B. Berzinaa , M. Benabdesselamb , P. Iacconib , L. B+tter-Jensenc , K. Atobed a Physics
of Defects in Ionic Materials, Institute of Solid State Physics, University of Latvia, Kengaraga St., 8, Riga LV-1058, Latvia b University of Nice-Sophia Antipolis, LPES-CRESA, Parc Valrose, Nice 06108, France c Ris* National Laboratory, Roskilde DK-4000, Denmark d Department of Physics, Faculty of Science, Naruto University of Education, Japan Received 11 November 2003; received in revised form 11 November 2003; accepted 5 January 2004
Abstract Spectral properties of cubic boron nitride have been studied using methods of photoluminescence (PL), X-ray excited luminescence (XL), thermoluminescence (TL) and optically stimulated luminescence. It is found that emission of cubic boron nitride is presented by 4 subbands, their relative yield is determined by the excitation type: blue, green (dominant) and red bands are observed in PL, ultraviolet, blue (dominant), green and red bands—in XL. Three thermal peaks are found in TL curves in the 0 –700◦ C temperature range, their presence and intensity depend on radiation type used. A tentative correspondence between thermal peaks and emission bands is found. c 2004 Elsevier Ltd. All rights reserved. Keywords: Cubic boron nitride; Photoluminescence; Thermoluminescence; Optically stimulated luminescence
1. Introduction Cubic boron nitride (c-BN) belongs to III–V nitride materials, which have already found a broad application range in electronics, optoelectronics and other >elds due to their outstanding features. c-BN is a wide band gap material (6:4 eV), extremely hard and chemically stable. Production of single crystal c-BN is a complicated and expensive process. Most of the studies deal with thin layers or polycrystalline samples. Usually samples contain uncontrolled intrinsic defects and impurities, which a@ect the properties of the material. Our previous paper (Berzina et al., 2002) was devoted to characterisation of c-BN ceramics using methods of optical spectroscopy. Basing on the properties of photoluminescence (PL) and UV-induced optically stimulated luminescence (OSL) it was concluded that defect centers present in the material produce recombination luminescence. In the
∗ Corresponding author. Tel.: +371-725-3592; fax: +371-713-2778. E-mail address: [email protected] (L. Trinkler).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.010
present paper, we continue studies of this material adding methods of thermally stimulated luminescence (TL) with the aim to determine properties of the luminescence centres and to estimate the present samples for practical application in dosimetry.
2. Materials and equipment c-BN used in studies were the ceramics samples produced in Japan. According to producers they consist of grains with almost completely cubic phase. Samples have square shape 4 × 4 × 0:5 mm3 , they are of greenish-grey colour and opaque. Most of the spectral measurements were ful>lled in Latvia, using the self-made experimental set-up, consisting of a deuterium lamp LDD 400 (Russia) used as an ultraviolet radiation (UVR) source, halogen lamp used for optical stimulation, two monochromators for selection of emission wavelength located in the excitation and luminescence or stimulation channels, a number of lenses and optical >lters, a photomultiplier tube and a recorder. An X-ray lamp (40 kV, 15 mA) was used for X-ray excited
616
L. Trinkler et al. / Radiation Measurements 38 (2004) 615 – 618 1.2
Luminescence intensity, a.u.
Luminescence intensity, a.u.
1.2 5
1
3
0.8
4 2
0.6 0.4
1
0.2 0 200
300
400
500
600
700
Wavelength, nm
Fig. 1. Spectral properties of c-BN ceramics: PL excited at 270 nm from the fresh sample (1), the 6 months aged sample (2), the 12 months aged sample after heating up to 600◦ C (3); OSL excited at 270 nm and stimulated by infrared light (4); XL (5).
luminescence (XL). In OSL emission and excitation spectra each point was obtained in a single “UV irradiation—optical stimulation” cycle. All spectra obtained are corrected for instrumental spectral response. Some TL and OSL measurements were ful>lled in RisH National Laboratory, using a RisH model TL/OSL DA-12 reader with linear heating and built-in blue LED array (470 nm) and 90 Y= 90 Sr beta source (0:1 Gy=s). UV irradiation for TL measurements was carried out using the Sol2 lamp (K. Honley GmbH), simulating solar emission spectrum on the ground level. Application of the UV >lter resulted in selection of the excitation light in the 300 –400 nm spectral range. TL was recorded through a glass >lter—either blue (300 –500 nm) or UV (240 –400 nm) one. A UV photometer PMA 2200 (Solar Light Co) was used to measure absolute values of UV irradiance. TL measurements were ful>lled also in University of Nice using a computer-controlled TL equipment with linear heating. A xenon lamp was used for UV irradiation of a sample, the excitation wavelength was selected by a monochromator. TL emission was recorded with a photomultiplier tube. The spectral region of the TL emission was selected with interference >lters of two kinds having bandpass 416 ± 10 and 506 ± 7 nm. All spectral measurements were carried out at RT, TL measurements applied to thermal region above RT with the heating rate 2◦ C=s.
3. Experimental results Photoluminescence emission of c-BN ceramics shown on Fig. 1 covers a broad spectral range: 350 –700 nm. PL spectrum of a fresh c-BN sample measured in our previous work (Berzina et al., 2002) has a well-pronounced peak at 500 nm (curve 1). Very large width and complex shape of PL emission spectrum allows proposing that it consists of
1
2
3
1
0.8
0.6
0.4
0.2
0 200
4
250
300
350
400
Wavelength, nm
Fig. 2. Excitation spectra of PL for lum : 360 nm (1), 500 nm (2), 660 nm (3) and OSL for lum 500 nm (4) from c-BN.
several overlapping subbands. The PL spectrum of an aged sample obtained after 6 months of storage (curve 2) has a more pronounced long wavelength part compared to that of a fresh sample. Heating up to 600◦ C and quenching of the 12 months aged sample caused ever higher increase of the long wavelength part of the spectrum (curve 3). PL excitation features are shown in Fig. 2. It is seen that the shape of an excitation spectrum depends on the emission region selected; it also proves that PL emission from c-BN contains several subbands, each with its own characteristic excitation spectrum. Analysing spectral properties one can assume that there are at least 3 subbands in PL emission: a blue one around 400 nm excited in a narrow band at 250 nm (Fig. 2, curve 1), a green one peaking at 500 nm and excited in a broad spectral region 200 –370 nm with a dominant peak at 270 nm (Fig. 2, curve 2) and a red one at 600 –700 nm having a broad and di@use excitation spectrum (Fig. 2, curve 3). Contribution of the red luminescence band increases with storage time and heating, as it is seen from PL emission spectrum of the aged and heated samples. Studies of OSL in c-BN are hampered by a very weak luminescence signal. Preliminary studies show that OSL emission is observed in the 400 –550 nm spectral range with a maximum at 480 nm (Fig. 1, curve 4). Its excitation spectrum contains no pronounced bands (Fig. 2, curve 4) but has a shoulder at 250 nm and sharply increases in the short wavelength side. XL (Fig. 1, curve 5) is also characterised with a broad spectrum, which is shifted to short wavelengths compared to the PL spectrum and has a maximum around 400 nm. The shape of the XL spectrum implies a complex structure containing several luminescence subbands. Some of these subbands may be the same as in the PL spectrum (400 – 700 nm). Their relative contribution into the integral XL yield is di@erent from the PL case—now the 400 nm subband is dominant. Besides the XL spectrum contains also a subband in the 280 –320 nm, which is not observed in PL. Similar spectrum centred at 400 nm was observed in cathodoluminescence of c-BN (Manfredotti et al., 2001).
L. Trinkler et al. / Radiation Measurements 38 (2004) 615 – 618 80000
5000 1
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4000
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TL yield, counts
617
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We have not measured the luminescence spectrum excited by beta rays but we could assume that it should be the same as the luminescence spectrum excited by X-rays and cathode emission. In RisH National Laboratory thermoluminescence from c-BN was studied using two types of irradiation—UVR and beta rays (Fig. 3, curves 1, 3 and 2, 4, respectively). In both cases two kinds of >lters—blue (Fig. 3, curves 1, 2) and UV (Fig. 3, curves 3, 4) were used for the luminescence recording. In all cases TL signal from the irradiated c-BN is observed at temperatures up to 700◦ C (Fig. 3). TL curve 1 induced by the UVR and recorded through the blue >lter contains two clearly distinguished thermal peaks at 180◦ C and around 530 –550◦ C, labelled as T1 and T3 , correspondingly. Using of the UV >lter instead of the blue >lter (curve 3) not only decreases the TL yield but also almost completely eliminates the low temperature peak T1 . The beta radiation induced TL curve recorded through the blue >lter (curve 2) is characterised with a dominant peak T3 at 530◦ C and much weaker peaks T2 at 360◦ C and T1 180◦ C. If the beta-induced TL is measured using the UV >lter (curve 4), than the T1 peak is not observed, the T3 peak decreases drastically but the T2 peak, though also smaller, becomes dominant. The TL measurements in University of Nice were carried out irradiating the sample by UVR at 280 nm and recording the TL emission through interference >lters: 416 and 506 nm (Fig. 4, curves 1 and 2, respectively). In general the TL curves are similar to those obtained in RisH National Laboratory: the same T1 and T3 peaks are clearly seen in the UVR-induced TL curves. The T3 peak is dominant in both curves, while the T1 peak is relatively better distinguished in curve 2. Fig. 5 shows the results of the combined OSL and TL measurements, ful>lled in RisH National Laboratory. Curves 1 and 3 are obtained by heating of the c-BN sample after
200
300
400
500
600
700
Temperature, C
Fig. 4. TL curves from c-BN after UV irradiation (280 nm) recorded through interference >lters: 416 nm (1) and 506 nm (2).
20000
TL yield , counts
Fig. 3. TL curves from c-BN obtained after irradiation with UVR recorded through blue >lter (1) and UV >lter (3) and after irradiation with beta rays (3 Gy) recorded through blue >lter (2) and UV >lter (4). UVR irradiance 2900 W=cm2 , UVR dose received 173 mJ=cm2 .
100
15000 1 10000
2
T2 T3 3
T1
5000
4 0 0
100
200
300
400
500
600
700
Temperature, C
Fig. 5. E@ect of optical bleaching on TL from c-BN. TL curves obtained just after irradiation with UVR (1) and beta rays (3) are compared with those obtained after exposure to blue light (470 nm) of the sample irradiated by UV (2) and beta (4) radiation. TL is recorded through the UV >lter.
the UV and beta irradiation, respectively, using the UV >lter for luminescence recording. If the irradiated sample is >rst illuminated by a blue LED until the complete termination of the OSL signal and only then heated, the observed TL curves change their shape (curve 2—for the UV irradiation, curve 4—for beta irradiation): the low temperature peaks T1 and T2 are bleached out, while the T3 thermal peak remains unchanged. The process of the TL fading was studied only in the case of the UV irradiation. Measuring the TL signal from the preliminary UVR-exposed sample after storage during 24 h at room temperature it was found that the T1 peak had faded by 50% approximately, while the T3 peak practically had not changed. Fading of the T2 peak was not studied. 4. Discussion and conclusions Our earlier work (Berzina et al., 2002) has already shown that luminescence in c-BN is caused by recombination processes. Results of the present work allow developing of
618
L. Trinkler et al. / Radiation Measurements 38 (2004) 615 – 618
this conception. Analysing PL and XL spectra totally 4 luminescence bands could be distinguished: UV, blue, green and red. Their relative contribution into the luminescence yield depends on the excitation type. Excitation of the defect levels inside the band gap by UVR produces the dominant green band and the weak blue and red bands in the PL spectrum. Excitation by ionising radiation, which is followed by band-to-band transitions and probably exciton processes results in emergence of the dominant blue band and weaker UV band, weak green and red bands are also observed. The number of centres responsible for the red luminescence increases during aging and thermal treatment, as it is seen from the PL spectrum evolution during ageing of the sample. Though the UVR-induced OSL spectrum practically coincides with the green PL band, the OSL excitation spectrum is quite di@erent from that of PL excitation. Its sharp increase in the short wavelength side allows supposing that besides excitation of local defect levels (in the 250 –350 nm region) exciton processes are also involved in OSL generation. TL from c-BN was studied after UVR and beta ray irradiation. Shape of the TL curves depends on the radiation type implying di@erent luminescence mechanisms. Presence of several thermal peaks, each with its own characteristic emission, in the TL curves implies participation of at least three di@erent types of charge carrier trap centres. Regretfully, up to now we had no chance to measure directly spectra of TL emission due to the low intensity of the TL signal. One can only try to >nd a correspondence between thermal peaks and emission bands basing on the analysis of transmission of the >lters used and emission spectra of c-BN. We could ascribe the T1 peak to the green band because it is much more intensive under UV irradiation compared to beta irradiation and not observed through the UV >lter. Small contributions of the T1 peak into the TL curve recorded through the interference >lter 416 nm (Fig. 4, curve 1) could be explained by overlapping of the broad luminescence bands. Emission spectrum in the T3 peak seems to contain both blue and green subbands, because this peak is present in all TL curves recorded through di@erent >lters. The T2 peak is dominant when the beta-induced TL is observed through the UV >lter, that is why we ascribe it to the most short wavelength subband 280 –320 nm, observed only under ionising radiation. The red emission band cannot be observed in the TL process because the >lters used cut it o@. The T3 peak, which evidently corresponds to the deepest trap level, is the most stable: is not bleached by illumination with blue light (contrary to T1 and T2 peaks) and is not subjected to fading during storage of an irradiated sample at RT.
The obtained results are not suPcient for elucidating of the defect types responsible for luminescence processes. Using literature data based on theoretical calculations (Gorchica et al., 1998; Gubanov et al., 1996, 1997), boron and nitrogen vacancies (VB and VN ), antisites (BN , NB ) or impurities can be mentioned as potential candidates for such centers. Estimating the present samples of c-BN for practical application in dosimetry the following features could be mentioned. c-BN produces OSL and TL signal after exposure to UVR and ionising radiation. TL is observed in a thermal region up to 700◦ C with 3 thermal peaks. The T1 peak is subjected to signi>cant fading but the stable T3 peak is located at too high temperatures, not suitable for practical application (fading of T2 was not studied). The main drawback of this material is a very low intensity of luminescence signal, observed in PL, OSL and TL. Besides, the number of the centres responsible for the red luminescence is not constant but depends on ageing and thermal treatment of the sample. Notwithstanding that these particular samples of c-BN are not suitable for application in dosimetry, the studies should be continued considering controlled doping of the material. Acknowledgements We are grateful for assistance in PL measurements to R. Krutohvostov. This research was sponsored by Supported by EC Center of Excellence CAMART of Institute of Solid State Physics (contract No ICA1-CI-2000-7007) and Programme of Latvian-French cooperation in science and technology OSMOSE. References Berzina, B., Trinkler, L., Atobe, K., 2002. Spectral characteristics of native defects in BN. Phys. Stat. Sol. (c) 0 (1), 421–424. Gorchica, I., Svane, A., Christensen, N.E., 1998. Native defects and carbon impurity in cubic BN. MRS Internet J. Nitride Semicond. Res. 3, 48. Gubanov, V.A., Pentaleri, E.A., Fong, C.Y., Klein, B.M., 1997. Electronic structure of defects and impurities in III–V nitrides: II Be, Mg and Si in cubic boron nitride. Phys. Rev. B 56 (20), 13077–13086. Gubanov, V.A., Lu, Z.W., Barry, M.K., Fong, C.Y., 1996. Electronic structure of defects and impurities in III–V nitrides: vacancies in cubic boron nitride. Phys. Rev. B 53 (8), 4377–4385. Manfredotti, C., Vittone, E., Lo Giudice, A., et al., 2001. Ionoluminescence in CVD diamond and in cubic boron nitride. Diamond Relat. Mater. 10, 568–573.