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Comparative study of microstructure in a-CxN1–x films deposited by radiofrequency magnetron sputtering
Diamond and Related Materials 10 Ž2001. 1156᎐1159
Comparative study of microstructure in a-C x N1 ᎐ x films deposited by radiofrequency magnetron sputtering O. Durand-Drouhin, M. Lejeune, M. Clin,U , M. Benlahsen, J. Henocque, A. Zeinert Laboratoire de Physique de la Matiere ` Condensee, ´ Faculte´ des Sciences d’Amiens, 33 rue Saint Leu, Amiens Cedex 80039, France
Abstract In this work, optical properties and the microstructure of non-hydrogenated amorphous carbon nitride Ža-C x N1 ᎐ x . have been investigated on films deposited by a reactive r.f. magnetron sputtering source with a graphite target. The r.f. power applied to the target varies between 15 and 350 W, leading to a negative target voltage range approximately y94 to y795 V. A combination of Raman spectroscopy measurements and infrared ŽIR. absorption experiments, which give information about the local microstructure and optical transmission measurements in the UV-Visible and near IR and photothermal deflection spectroscopy from which we determined the refractive index n and the optical gap E04 are applied to fully characterise the films in their as-deposited state. The variation of the I DrIG ratio indicates that the microstructure of the samples shows a critical change in the local bonding of the C atoms, for deposition parameters corresponding to a target voltage of y400 V and a nitrogen pressure approximately 4᎐5 Pa. Photoluminescence spectra show a maximum at approximately 650 nm and the photoluminescence efficiency presents a maximum as a function of the r.f. power in the same range as the maximum of the I DrIG ratio. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Amorphous carbon; Nitrides; Sputtering; Characterization
1. Introduction Carbon nitride ŽCN. films have generated great interest due to their promising properties, such as superhardness more than diamond, theoretically predicted by Liu and Cohen w1x on the C 3 N4 structure. Various deposition methods have been used to synthesise this hypothetical phase w2x unfortunately, no indubitable result revealing the existence of C 3 N4 crystals has been obtained. Nevertheless, amorphous carbon nitride films deserve studying with regard to the correlation between the optoelectronic properties and the microstructure
which is not well understood, particularly the role of nitrogen incorporation in the material. The aim of the present work is therefore to specify the relationships between microstrustural investigations obtained from IR and Raman spectroscopies and optoelectronic properties deduced from optical transmission and photoluminescence measurements. This study was carried out on different series of samples synthesised by r.f. magnetron sputtering of a graphite target in pure nitrogen plasma.
2. Experimental U
Corresponding author. Tel.: q33-32282-7664; fax: q33-322827891. E-mail address: [email protected] ŽM. Clin..
CN films were synthesised by r.f. Ž13.56 MHz. magnetron sputtering of a graphite target, using nitrogen plasma as the reactant gas. Four series of samples were
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 4 8 2 - 9
O. Durand-Drouhin et al. r Diamond and Related Materials 10 (2001) 1156᎐1159
grown at working pressure between 2 and 5 Pa and the substrate holder was kept at room temperature by water cooling. The samples were deposited at floating substrate holder potential by varying the r.f. power applied to the target between 15 and 350 W, corresponding to a r.f. power density range approximately 0.34᎐7.95 W cmy2 , leading to a negative target bias voltage range approximatelyy 94 to y795 V. The distance between the target and substrate was 7 cm. The substrates were Si wafer, glass and quartz, cleaned though a degreasing process in ultrasonic baths of trichloroethylene, acetone and methanol. All substrates were further cleaned by sputter etching to remove residual contaminants. The thickness of the deposited films, determined using a Dektak 3 profilometer, was approximately 1 m. To determine the refractive index and optical gap E04 , optical transmission experiments were carried out using a CARY 5E UV-Vis-NIR spectrophotometer. Photoluminescence ŽPL. measurements were performed with a cw argon laser at an excitation wavelength of 488 nm. Microstructural characterisation of the films by IR absorption and Raman spectroscopy measurements were respectively obtained using a Fourier transform IR NICOLET spectrometer in the range of 400᎐4000 cmy1 and a DILOR Z24 triple monochromator using the 514.5-nm line of an argon ion laser. All these measurements were applied to the samples in their as-deposited state.
3. Results and discussion Before starting the discussion on the Raman results, it is important to point out that the Raman spectrum of microcrystalline graphite is characterised by two peaks at 1350 and 1570 cmy1 w3x. The G band which appears at 1570 cmy1 is due to the E2g symmetric vibration on sp 2 carbon sites observed in crystalline graphite, while the D band at 1350 cmy1 is attributed to the A1g mode which is sensitive to the size of the graphitic microdomains and expresses the degree of disorder regard to graphite like structure. The relative integrated intensity ratio I D rIG is related to the size and the number of the graphitic domains w4x. Fig. 1 shows Raman spectrum obtained for a CN film deposited at working pressure of 4 Pa and target voltage of y500 V. This spectrum exhibits two overlapping bands, the D band at 1380 cmy1 and the G band at 1575 cmy1 . The variation of the relative integrated intensity ratio I D rIG as a function of the target voltage is presented in Fig. 2. The films deposited at working pressure of 2, 3 and 4 Pa presents qualitatively the same tendency with a maximum approximately y400 V, while it is a minimum for films prepared at nitrogen pressure of 5 Pa.
1157
Fig. 1. Example of decomposition into gaussian components of the G- and D-band of the Raman spectrum obtained for film prepared at nitrogen pressure of 4 Pa and target voltage of y500 V.
The analysis of the IR spectra obtained in the range 800᎐3600 cmy1 gives information about the C᎐C and C᎐N bonding. The IR spectra can be separated in three regions, below 1800 cmy1 the peaks centred approximately 1600 cmy1 are characteristic of the CsC and CsN double bond. In the range 1800᎐2500 cmy1 the band centred approximately 2200 cmy1 which does not appear in amorphous carbon films is attributed to the stretching vibration of the nitrile groups, due to the incorporation of nitrogen atoms into the films. The band of approximately 2350 cmy1 can be attributed to the O᎐C᎐O antisymmetric stretching vibration of CO 2 in air. Above 2500 cmy1 , the wide band observed at 3400 cmy1 indicates a contamination of the samples by oxygen or water Žwhich corresponds to the O᎐H bondstretching mode.. The broad band which appears in the range 800᎐1800 cmy1 can be decomposed into sub-bands at 1220, 1400,
Fig. 2. Variation of the integrated intensity ratio I D rIG vs. target voltage for CN films prepared at different nitrogen pressure.
1158
O. Durand-Drouhin et al. r Diamond and Related Materials 10 (2001) 1156᎐1159
Fig. 3. Decompositon into gaussian components of the IR spectrum in the range 800᎐1800 cmy1 for film prepared at nitrogen pressure of 3 Pa and target voltage of y640 V.
Fig. 4. Decomposition into gaussian components of the IR spectrum in the nitrile stretching mode region for film prepared at nitrogen pressure of 3 Pa and target voltage of y640 V.
1570 and 1650 cmy1 as shown in Fig. 3. The peak at approximately 1220 cmy1 is attributed to the sp 3 C᎐N bonds w4x, while the band at approximately 1650 cmy1 can be assigned to the CsN double bonds w5x. The modes observed at 1400 and 1570 cmy1 are Raman active and correspond, respectively, to the D band due to the disorder and size of the graphitic domains and the G band due to the carbon᎐carbon vibration in the aromatic rings. These bands which are IR forbidden for amorphous carbon, become IR active when the symmetry of the graphitic rings is broken due to the incorporation of nitrogen atoms into the CN films w5x. The integrated intensity ratio I D rIG vs. target voltage, deduced from IR results shows qualitatively the same variation than that deduced from the Raman spectra, in agreement with the model proposed by Kaufman et al. w6x. The integrated absorption of the nitrile bands in the stretching mode region Žwhich corresponds to the C⬅N triple bonds. shows a decrease as the r.f. power, i.e the negative target voltage increases, suggesting a more important incorporation of nitrogen atoms into the films at low r.f. power. This stretching nitrile band is decomposed into Gaussian components in Fig. 4. The sub-bands observed at 2180 and 2235 cmy1 are assigned to the vibrations of nitrile groups bonded to different configurations. The refractive index measured at 2 m is constant with the increase of the target voltage and equal to 1.8 for all the samples and does not depend on the working pressure. On the contrary, the optical gap E04 decreases from 2.2 to 1.65 eV with the increase of the target voltage ŽFig. 5.. The decrease of optical gap with the increasing target voltage appears to be associated with the structural and composition changes in the
films. The optical gap is reduced with the lowering of the nitrogen content. Various photoluminescence ŽPL. measurements were carried out on different series of samples. Fig. 6 exhibits the PL emission spectra at room temperature obtained at an excitation wavelength of 488 nm for the 3-Pa series. The irradiation density was about 10 mW cmy2 . At this value no fatigue was observed. All the spectra are featureless with a maximum approximately at 650᎐700 nm. PL measurements with a pulsed nitrogen laser Ž337 nm. combined with a dye exhibit no significant change in the spectral shape between 337 and 450 nm. Although in our films, hydrogen was not intentionally incorporated, the shape and the maximum of the PL spectra are similar to previous published data on hydrogenated a-CN x :H films w7᎐9x. We determined the PL efficiency as a function of the r.f. power,
Fig. 5. Variation of the optical gap E04 vs. target voltage for different nitrogen pressure.
O. Durand-Drouhin et al. r Diamond and Related Materials 10 (2001) 1156᎐1159
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4. Conclusion
Fig. 6. Photoluminescence emission spectra at room temperature for films prepared at nitrogen pressure of 3 Pa, for different r.f. power Ži.e. target voltage.. The excitation wavelength was 488 nm. The weak structure approximately 525 nm is due to imperfect laser line compensation. The spectra are normalised to the film thickness.
normalised to the absorption coefficient at the excitation energy. goes though a maximum for a r.f. power of approximately 50 W and decreases strongly beyond 100 W. Films grown above 200 W do not show any PL. As it can be observed in a-C:H the temperature dependence of the PL intensity is moderate between 77 and 300 K, the PL intensity in our samples decreases monotonously by a factor of 2. Time resolved photoluminescence performed at 2 K revealed very short decay times in the range of 50᎐100 ps when excited by a pulsed Ar laser at 514 nm. The decay time does not change significantly within the spectral range of the emission Ž550᎐800 nm. and was similar when a pulsed dye laser Ž650 nm. was used. The similar behaviour concerning the spectral response and the decay time, compared to what has been observed in a-C:H films w10,11x indicate that the origin of the PL in our CN films may be the same than in amorphous carbon and the presence of nitrogen does not modify significantly the recombination processes. In this context, it is interesting to note that the maximum in the PL efficiency as a function of the r.f. power for the 3-Pa series is in the same range as the maximum of the integrated intensity ratio I D rIG curve vs. target voltage, suggesting that the organisation of the sp 2 clusters plays the predominant role for the PL.
The shape modification of the Raman and IR spectra as a function of r.f. power and nitrogen working pressure is probably due to changes in size and number of Csp 2 and Csp 3 sites and composition of the films. The optical gap behaviour reflects these changes which do not affect the density of films as suggested by the refractive index which remains constant whatever the deposition parameters. IR results show that the bonded nitrogen content present in our samples is all the more important as they are prepared at high r.f. power, while the organisation of the sp 2 clusters seems to be very sensitive to the energy of ion bombardment of the growing samples, with a critical behaviour for a r.f. power of approximately 110 W which correspond to a target voltage of y400 V.
Acknowledgements The authors gratefully acknowledge the help of Ph. Lavallard ŽGPS, Paris. and C. Barthou ŽLOS, Paris. for the PL measurements. This work was financially supported by the Interreg II programme. References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x
A.Y. Liu, M.L. Cohen, Science 245 Ž1989. 841. S. Muhl, J.M. Mendez, Diam. Relat. Mater. 8 Ž1999. 1809. F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 Ž1976. 1126. M.R. Wixom, J. Am. Ceram. Soc. 73 Ž1990. 1973. P. Hammer, N.M. Victoria, F. Alvarez, J. Non-Cryst. Solids 227᎐230 Ž1998. 645. J.H. Kaufman, S. Metin, D.D. Saperstein, Phys. Rev. B 39 Ž1989. 13053. S. Kobayashi, S. Nozaki, H. Morisaki, M. Susumu, Jpn. J. Appl. Phys. 36 Ž1997. 5187. N.I. Klyui, Yu.P. Piryatinskii, V.A. Semenovich, Mater. Lett. 35 Ž1998. 334. N. Mutsukura, K. Akita, Diam. Relat. Mater. 9 Ž2000. 761. W. Lormes, M. Hundhausen, L. Ley, J. Non-Cryst. Solids 227᎐230 Ž1998. 570. J. Robertson, G. Amaratunga, J. Appl. Phys. 80 Ž1996. 2998.
Comparative study of microstructure in a-C x N1 ᎐ x films deposited by radiofrequency magnetron sputtering O. Durand-Drouhin, M. Lejeune, M. Clin,U , M. Benlahsen, J. Henocque, A. Zeinert Laboratoire de Physique de la Matiere ` Condensee, ´ Faculte´ des Sciences d’Amiens, 33 rue Saint Leu, Amiens Cedex 80039, France
Abstract In this work, optical properties and the microstructure of non-hydrogenated amorphous carbon nitride Ža-C x N1 ᎐ x . have been investigated on films deposited by a reactive r.f. magnetron sputtering source with a graphite target. The r.f. power applied to the target varies between 15 and 350 W, leading to a negative target voltage range approximately y94 to y795 V. A combination of Raman spectroscopy measurements and infrared ŽIR. absorption experiments, which give information about the local microstructure and optical transmission measurements in the UV-Visible and near IR and photothermal deflection spectroscopy from which we determined the refractive index n and the optical gap E04 are applied to fully characterise the films in their as-deposited state. The variation of the I DrIG ratio indicates that the microstructure of the samples shows a critical change in the local bonding of the C atoms, for deposition parameters corresponding to a target voltage of y400 V and a nitrogen pressure approximately 4᎐5 Pa. Photoluminescence spectra show a maximum at approximately 650 nm and the photoluminescence efficiency presents a maximum as a function of the r.f. power in the same range as the maximum of the I DrIG ratio. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Amorphous carbon; Nitrides; Sputtering; Characterization
1. Introduction Carbon nitride ŽCN. films have generated great interest due to their promising properties, such as superhardness more than diamond, theoretically predicted by Liu and Cohen w1x on the C 3 N4 structure. Various deposition methods have been used to synthesise this hypothetical phase w2x unfortunately, no indubitable result revealing the existence of C 3 N4 crystals has been obtained. Nevertheless, amorphous carbon nitride films deserve studying with regard to the correlation between the optoelectronic properties and the microstructure
which is not well understood, particularly the role of nitrogen incorporation in the material. The aim of the present work is therefore to specify the relationships between microstrustural investigations obtained from IR and Raman spectroscopies and optoelectronic properties deduced from optical transmission and photoluminescence measurements. This study was carried out on different series of samples synthesised by r.f. magnetron sputtering of a graphite target in pure nitrogen plasma.
2. Experimental U
Corresponding author. Tel.: q33-32282-7664; fax: q33-322827891. E-mail address: [email protected] ŽM. Clin..
CN films were synthesised by r.f. Ž13.56 MHz. magnetron sputtering of a graphite target, using nitrogen plasma as the reactant gas. Four series of samples were
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 4 8 2 - 9
O. Durand-Drouhin et al. r Diamond and Related Materials 10 (2001) 1156᎐1159
grown at working pressure between 2 and 5 Pa and the substrate holder was kept at room temperature by water cooling. The samples were deposited at floating substrate holder potential by varying the r.f. power applied to the target between 15 and 350 W, corresponding to a r.f. power density range approximately 0.34᎐7.95 W cmy2 , leading to a negative target bias voltage range approximatelyy 94 to y795 V. The distance between the target and substrate was 7 cm. The substrates were Si wafer, glass and quartz, cleaned though a degreasing process in ultrasonic baths of trichloroethylene, acetone and methanol. All substrates were further cleaned by sputter etching to remove residual contaminants. The thickness of the deposited films, determined using a Dektak 3 profilometer, was approximately 1 m. To determine the refractive index and optical gap E04 , optical transmission experiments were carried out using a CARY 5E UV-Vis-NIR spectrophotometer. Photoluminescence ŽPL. measurements were performed with a cw argon laser at an excitation wavelength of 488 nm. Microstructural characterisation of the films by IR absorption and Raman spectroscopy measurements were respectively obtained using a Fourier transform IR NICOLET spectrometer in the range of 400᎐4000 cmy1 and a DILOR Z24 triple monochromator using the 514.5-nm line of an argon ion laser. All these measurements were applied to the samples in their as-deposited state.
3. Results and discussion Before starting the discussion on the Raman results, it is important to point out that the Raman spectrum of microcrystalline graphite is characterised by two peaks at 1350 and 1570 cmy1 w3x. The G band which appears at 1570 cmy1 is due to the E2g symmetric vibration on sp 2 carbon sites observed in crystalline graphite, while the D band at 1350 cmy1 is attributed to the A1g mode which is sensitive to the size of the graphitic microdomains and expresses the degree of disorder regard to graphite like structure. The relative integrated intensity ratio I D rIG is related to the size and the number of the graphitic domains w4x. Fig. 1 shows Raman spectrum obtained for a CN film deposited at working pressure of 4 Pa and target voltage of y500 V. This spectrum exhibits two overlapping bands, the D band at 1380 cmy1 and the G band at 1575 cmy1 . The variation of the relative integrated intensity ratio I D rIG as a function of the target voltage is presented in Fig. 2. The films deposited at working pressure of 2, 3 and 4 Pa presents qualitatively the same tendency with a maximum approximately y400 V, while it is a minimum for films prepared at nitrogen pressure of 5 Pa.
1157
Fig. 1. Example of decomposition into gaussian components of the G- and D-band of the Raman spectrum obtained for film prepared at nitrogen pressure of 4 Pa and target voltage of y500 V.
The analysis of the IR spectra obtained in the range 800᎐3600 cmy1 gives information about the C᎐C and C᎐N bonding. The IR spectra can be separated in three regions, below 1800 cmy1 the peaks centred approximately 1600 cmy1 are characteristic of the CsC and CsN double bond. In the range 1800᎐2500 cmy1 the band centred approximately 2200 cmy1 which does not appear in amorphous carbon films is attributed to the stretching vibration of the nitrile groups, due to the incorporation of nitrogen atoms into the films. The band of approximately 2350 cmy1 can be attributed to the O᎐C᎐O antisymmetric stretching vibration of CO 2 in air. Above 2500 cmy1 , the wide band observed at 3400 cmy1 indicates a contamination of the samples by oxygen or water Žwhich corresponds to the O᎐H bondstretching mode.. The broad band which appears in the range 800᎐1800 cmy1 can be decomposed into sub-bands at 1220, 1400,
Fig. 2. Variation of the integrated intensity ratio I D rIG vs. target voltage for CN films prepared at different nitrogen pressure.
1158
O. Durand-Drouhin et al. r Diamond and Related Materials 10 (2001) 1156᎐1159
Fig. 3. Decompositon into gaussian components of the IR spectrum in the range 800᎐1800 cmy1 for film prepared at nitrogen pressure of 3 Pa and target voltage of y640 V.
Fig. 4. Decomposition into gaussian components of the IR spectrum in the nitrile stretching mode region for film prepared at nitrogen pressure of 3 Pa and target voltage of y640 V.
1570 and 1650 cmy1 as shown in Fig. 3. The peak at approximately 1220 cmy1 is attributed to the sp 3 C᎐N bonds w4x, while the band at approximately 1650 cmy1 can be assigned to the CsN double bonds w5x. The modes observed at 1400 and 1570 cmy1 are Raman active and correspond, respectively, to the D band due to the disorder and size of the graphitic domains and the G band due to the carbon᎐carbon vibration in the aromatic rings. These bands which are IR forbidden for amorphous carbon, become IR active when the symmetry of the graphitic rings is broken due to the incorporation of nitrogen atoms into the CN films w5x. The integrated intensity ratio I D rIG vs. target voltage, deduced from IR results shows qualitatively the same variation than that deduced from the Raman spectra, in agreement with the model proposed by Kaufman et al. w6x. The integrated absorption of the nitrile bands in the stretching mode region Žwhich corresponds to the C⬅N triple bonds. shows a decrease as the r.f. power, i.e the negative target voltage increases, suggesting a more important incorporation of nitrogen atoms into the films at low r.f. power. This stretching nitrile band is decomposed into Gaussian components in Fig. 4. The sub-bands observed at 2180 and 2235 cmy1 are assigned to the vibrations of nitrile groups bonded to different configurations. The refractive index measured at 2 m is constant with the increase of the target voltage and equal to 1.8 for all the samples and does not depend on the working pressure. On the contrary, the optical gap E04 decreases from 2.2 to 1.65 eV with the increase of the target voltage ŽFig. 5.. The decrease of optical gap with the increasing target voltage appears to be associated with the structural and composition changes in the
films. The optical gap is reduced with the lowering of the nitrogen content. Various photoluminescence ŽPL. measurements were carried out on different series of samples. Fig. 6 exhibits the PL emission spectra at room temperature obtained at an excitation wavelength of 488 nm for the 3-Pa series. The irradiation density was about 10 mW cmy2 . At this value no fatigue was observed. All the spectra are featureless with a maximum approximately at 650᎐700 nm. PL measurements with a pulsed nitrogen laser Ž337 nm. combined with a dye exhibit no significant change in the spectral shape between 337 and 450 nm. Although in our films, hydrogen was not intentionally incorporated, the shape and the maximum of the PL spectra are similar to previous published data on hydrogenated a-CN x :H films w7᎐9x. We determined the PL efficiency as a function of the r.f. power,
Fig. 5. Variation of the optical gap E04 vs. target voltage for different nitrogen pressure.
O. Durand-Drouhin et al. r Diamond and Related Materials 10 (2001) 1156᎐1159
1159
4. Conclusion
Fig. 6. Photoluminescence emission spectra at room temperature for films prepared at nitrogen pressure of 3 Pa, for different r.f. power Ži.e. target voltage.. The excitation wavelength was 488 nm. The weak structure approximately 525 nm is due to imperfect laser line compensation. The spectra are normalised to the film thickness.
normalised to the absorption coefficient at the excitation energy. goes though a maximum for a r.f. power of approximately 50 W and decreases strongly beyond 100 W. Films grown above 200 W do not show any PL. As it can be observed in a-C:H the temperature dependence of the PL intensity is moderate between 77 and 300 K, the PL intensity in our samples decreases monotonously by a factor of 2. Time resolved photoluminescence performed at 2 K revealed very short decay times in the range of 50᎐100 ps when excited by a pulsed Ar laser at 514 nm. The decay time does not change significantly within the spectral range of the emission Ž550᎐800 nm. and was similar when a pulsed dye laser Ž650 nm. was used. The similar behaviour concerning the spectral response and the decay time, compared to what has been observed in a-C:H films w10,11x indicate that the origin of the PL in our CN films may be the same than in amorphous carbon and the presence of nitrogen does not modify significantly the recombination processes. In this context, it is interesting to note that the maximum in the PL efficiency as a function of the r.f. power for the 3-Pa series is in the same range as the maximum of the integrated intensity ratio I D rIG curve vs. target voltage, suggesting that the organisation of the sp 2 clusters plays the predominant role for the PL.
The shape modification of the Raman and IR spectra as a function of r.f. power and nitrogen working pressure is probably due to changes in size and number of Csp 2 and Csp 3 sites and composition of the films. The optical gap behaviour reflects these changes which do not affect the density of films as suggested by the refractive index which remains constant whatever the deposition parameters. IR results show that the bonded nitrogen content present in our samples is all the more important as they are prepared at high r.f. power, while the organisation of the sp 2 clusters seems to be very sensitive to the energy of ion bombardment of the growing samples, with a critical behaviour for a r.f. power of approximately 110 W which correspond to a target voltage of y400 V.
Acknowledgements The authors gratefully acknowledge the help of Ph. Lavallard ŽGPS, Paris. and C. Barthou ŽLOS, Paris. for the PL measurements. This work was financially supported by the Interreg II programme. References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x
A.Y. Liu, M.L. Cohen, Science 245 Ž1989. 841. S. Muhl, J.M. Mendez, Diam. Relat. Mater. 8 Ž1999. 1809. F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 Ž1976. 1126. M.R. Wixom, J. Am. Ceram. Soc. 73 Ž1990. 1973. P. Hammer, N.M. Victoria, F. Alvarez, J. Non-Cryst. Solids 227᎐230 Ž1998. 645. J.H. Kaufman, S. Metin, D.D. Saperstein, Phys. Rev. B 39 Ž1989. 13053. S. Kobayashi, S. Nozaki, H. Morisaki, M. Susumu, Jpn. J. Appl. Phys. 36 Ž1997. 5187. N.I. Klyui, Yu.P. Piryatinskii, V.A. Semenovich, Mater. Lett. 35 Ž1998. 334. N. Mutsukura, K. Akita, Diam. Relat. Mater. 9 Ž2000. 761. W. Lormes, M. Hundhausen, L. Ley, J. Non-Cryst. Solids 227᎐230 Ž1998. 570. J. Robertson, G. Amaratunga, J. Appl. Phys. 80 Ž1996. 2998.