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Graphite-like bonding induced in hydrogenated amorphous carbon films with high nitrogen content
~
Solid State Communications, Vol. 84, No. 11, pp. 1025-1027, 1992. Printed in Great Britain.
0038-1098/9255.00+ .00 Pergamon Press Ltd
GRAPHITE-LIKE BONDING INDUCED IN HYDROGENATED AMORPHOUS CARBON FILMS WITH HIGH NITROGEN CONTENT D. Mendoza,* J. Aguilar-Hern£ndez,'*and G. Contreras-Puente*Instituto de Investigaciones en Materiales, Universidad Nacional Aut6noma de MSxico, Apartado Postal 70-360, Coyoac£n 04510, M6xico D. F. MEXICO **Escuela Superior de Fisica y Matem~iticas, Instituto Polit~cnico Nacional, 07738, Mdxico D. F. MEXICO. (Received 27 July 1992 by A. A. Maradudin)
In the present work we analyze the effect of nitrogen incorporation on the structural properties of hydrogenated amorphous carbon films. Photoluminescence, Raman and optical studies indicate that the high nitrogen content induces the formation of sixfold fused rings clusters (graphite layers).
The study of the effect on the physical properties of hydrogenated amorphous carbon films (a-C:H) by the introduction of nitrogen has been undertaken by a number of groups) -s Depending on the deposition conditions the nitrogenated a-C:H films (a-C:H(N)) present a variety of properties; Jones and Stewart, 1 Han and Feldman, 2 and Amir and Kalish s found that the optical energy gap (Eg) diminishes as the nitrogen content increases; although Torng, e t al. 4 found the opposite behavior. It has also been found that nitrogen stabilize the diamond s p a bonding; 4 Kaufman, e t al. z concluded that nitrogen does not dramatically affect the film structure but it breaks symmetry in the sp ~ domains causing the Raman-active G (graphitic) and D (disordered) bands to become infrared active, finally it has also been seen that nitrogen increases the structural order in the a-C:H(N) films3 In a previous work 6 we found a linear relationship between the energy gap and the width of the Urbach edge (E0), and concluded that E0 can be taken as a measure of the disorder in the a-C:H(N) films introduced by the incorporation of nitrogen. In the present work we report photoluminescence (PL), Raman and optical measurements of a-C:H(N) films. We find that the integrated intensity and the full width at half maximum of the PL spectra, as well as the optical band gap, are not monotonic functions of the nitrogen content, we conclude that a high content of nitrogen induces the formation of graphitic domains. The a-C:H(N) films were deposited by rf glow discharge decomposition of a mixture of methane and nitrogen onto Corning 7059 glass substrates for optical measurements, and onto crystalline silicon for photoluminescence and Raman measurements. The conditions for the deposition were as follows: gas pressure in the deposition chamber equal to 300 mTorr, power dissipation of 252 m W / c m , methane flow (f,~) equal to 10 sccm and a nitrogen flow (f~) from 0 to 40 sccm. We use the quantity f=fJ(f~+fm) as a measure of the relative nitrogen flow introduced into the deposition chamber. For thickness
measurements we used Coming substrates partially co,~ ered with the a-C:H(N) film, a thin aluminum film was then evaporated to improve the accuracy of the thickness measurement using a Fizeau interferometer. Transmittance in the visible range was obtained using a Shimadzu UV-260 spectrometer. The photoluminescence and Raman spectra were measured on films of ,,,1500)t of thickness at room temperature using the 4880)i line of the Ar ion laser (50 mW). The optical absorption (a) for photon energies greater than 2.5 eV follows the Tauc relationship aE=B(E-Eg) 2, where the parameter B - l is related to the bandwidth 2 and E is the photon energy. In figure 1 we show the plots of Eg and B -1 as a function of the relative nitrogen flow (f). Figure 2-a shows the PL spectra for three 2.5
20
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R e l a t i v e N i t r o g e n Flow (f) F i g . 1. Optical band gap (Eg) and the parameter B -1 as a function of tim relative nitrogen flow (f). 1025
1026
GRAPHITE-LIKE BONDING 12000 M"'%
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Fig, 2. Fig. (a) shows the photoluminescence spectra (PL) as a function of photon energy for three different values of the relative nitrogen flow (f), (b) shows the integrated PL intensity (I) and the full width at half maximum of the PL spectra (W) as a function of f.
different values of f and in figure 2-b the integrated PL (I) and the full width at half maximum (W) is shown. We see that the PL peak is located at approximately 2.1 eV, this value is similar to the measured value of Eg; from these results we infer that the PL arises from transitions between shallow conduction and valence localized tail states. The non-radiative transitions are mediated by deep states, possibly carbon dangling bonds. For low values of f there are many non-radiative states that make the PL peak small, however as f increases nitrogen saturates some of the carbon dangling bonds ~ and the PL peak increases (fig. 2); these results are consistent with reported electron spin resonance measurements. 7 At this stage nitrogenation may reduce the internal strain facilitating s p a bonding and increases the optical gap 4 in a
Vol. 84, No. ! 1
similar manner as hydrogen, s'9 The integrated PL intensity continues to increase as f increases until a critical point is reached at f~0.5, where it then decreases; this non-monotonic behavior is also found for Eg around the same value of f. The decrease of the PL intensity for f>0.5 will be explained later. For f greater than 0.5 we propose the following explanation. Once nitrogen has saturated many of the carbon dangling bonds, excess of nitrogen begins to be incorporated in sixfold graphite rings3 or indeed may even promote the formation of sixfold rings, l° Figure 3 shows the Raman spectra for three films with different values of f. From this figure we can identify the G (graphitic) and the D (disordered) bands. The G band shows a systematic shift from ,,~1533 cm -1 (f=0) to ,,-1570 cm -~ (f=0.8), the D band for the undoped film (f=0) is very faint compared to that for the most heavily sample (f=0.8) and shifts from ~1370 cm -1 to ~1440 cm -1. Note that, due to the high photoluminescence background for the sample with f=0.5 (see fig. 2), the measurement of the Raman signal becomes difficult; however the existence of the G band is clear. Kaufman, e t al. a found a similar shift for the G band from the undoped to the most heavily doped sample; although they did not find significant change in the G band to D band intensity ratio. The shift to higher energies of the G and the D bands and the increase of the intensity of the D band, compared to the G band, was found in a-C:H subjected to thermal annealing and explained in terms of formation of graphitic domains. 11 Our Raman measurements give support to a graphitic layer formation process in a-C:H(N) films with higher nitrogen content. The physical properties of the carbonaceous films have been seen to depend on the deposition conditions as well as the precursor gas used to obtain the films, as and this fact may be the reason for which Kaufman, et al. a do not find an appreciable change in the G
2000 G D
.1500
1000 =
500 f=0.5
0 1000
v f'~
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1200 1400 1600 Wave N u m b e r ( l / e m )
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Fig. 3. Raman spectra for three samples with different values of f. The simbols G and D correspond to the graphitic and the disordered bands, respectively. The plots are translated upwards for clarity.
Vol. 84, No. 11
GRAPHITE-LIKE BONDING
band to D band intensity ratio. Additional support for the idea that for f>0.5 nitrogen concentrations induce graphitic layer growth comes from measurements of the energy gap (fig. 1). We found that for f>0.5 the optical energy gap decreases. Robertson and O'Reilly s developed a model for a-C:H and found that the value of E9 is controlled by the extent of the graphitic phase, and its value is given by E g ~ l / x / ~ where N is the number of the sixfold rings fused in compact clusters (graphite layers); therefore a decrease of Eu indicates an increase in the size of the graphitic layers. On the other hand, thermal annealing experiments show that the optical transmittance of the heaviest doped sample decreases less abruptly than that for the less doped films. 6 This fact may be explained supposing that the heaviest doped sample shows a more developed stage of the graphitic phase in the as-deposited state. Returning to the photoluminescence spectra, the decrease of the PL intensity, for f>0.5, may be explained by one or both of the following possibilities. The energy needed for defect creation, Ed , in sixfold fused ring clusters diminishes as the number of rings increases, s if high nitrogen content in a-C:H induces the formation of sixfold rings, then one would expect the creation of many defects that may be non-radiative recombination centers. On the other hand, it is argued s that the low doping efficiency in a-C:H is due to strong autocompensation, in which doping is accompanied by an increase of the defect density, this kind of defect may be a second path for non-radiative recombination. This last argument may be true for the case when f<0.5, but due to the low value of Ed for large clusters this effect would be more important for f>0.5, s Finally it should be noted that there is experimental evidence of a decrease of the photoluminescence intensity in a-C:H samples subjected to thermal annealing, la and it is known that thermal annealing promotes the graphitic phase growth. T M In conclusion, to explain photoluminescence, Raman, and optical measurements in a-C:H(N) films, we propose
1027
that small quantities of nitrogen promote the sps bonding, and higher nitrogen content induces the formation of clusters of sixfold fused rings (sp ~ bonding configuration).
Acknowledgements-We thank Stephen Muhl for useful comments and Miguel Angel Canseco for optical measurements. REFERENCES 1. D. I. Jones and A. D. Stewart, Phil. Mag. B 46, 423 (1982). 2. H.-X. Han and B. J. Feldman, Solid State Commun. 65, 921 (1988). 3. J. H. Kaufman, S. Metin, and D. D. Saperstein, Phys. Rev. B 39, 13053 (1989). 4. C. J. Torng, J. M. Sivertsen, J. H. Judy, and C. Chang, J. Mater. Res. 5, 2490 (1990). 5. O. Amir and R. Kalish, J. Appl. Phys. 70, 4958 (1991). 6. D. Mendoza, submitted for publication. 7. S. Lin, K. Noonan, B. J. Feldman, D. Min, and M. T. Jones, Solid State Commun. 80, 101 (1991). 8. J. Robertson and E. P. O'Reilly, Phys. Rev. B 35, 2946 (1987). 9. J. Tersoff, Phys. Rev. B 44, 12039 (1991). 10. J. J. Cuomo, P. A. Leary, D. Yu, W. Reuter, and M. Frisch, J. Vac. Sci. Technol. 16, 299 (1979). 11. R. O. Dillon, J. A. Woollam, and V. Katkanant, Phys. Rev. B 29, 3482 (1984). 12. See for example: Amorphous Hydrogenated Carbon Films, European Materials Research Society Symposia Proceedings, Strasburg, 1987. P. Koidl and P. Oelhafen, Eds. (Les Editions de Physique, Les Ulis Cedex, France, 1987). 13. S. Lin and B. J. Feldman, Solid State Commun. 80, 371 (1991). 14. B. Dischler in reference 12.
Solid State Communications, Vol. 84, No. 11, pp. 1025-1027, 1992. Printed in Great Britain.
0038-1098/9255.00+ .00 Pergamon Press Ltd
GRAPHITE-LIKE BONDING INDUCED IN HYDROGENATED AMORPHOUS CARBON FILMS WITH HIGH NITROGEN CONTENT D. Mendoza,* J. Aguilar-Hern£ndez,'*and G. Contreras-Puente*Instituto de Investigaciones en Materiales, Universidad Nacional Aut6noma de MSxico, Apartado Postal 70-360, Coyoac£n 04510, M6xico D. F. MEXICO **Escuela Superior de Fisica y Matem~iticas, Instituto Polit~cnico Nacional, 07738, Mdxico D. F. MEXICO. (Received 27 July 1992 by A. A. Maradudin)
In the present work we analyze the effect of nitrogen incorporation on the structural properties of hydrogenated amorphous carbon films. Photoluminescence, Raman and optical studies indicate that the high nitrogen content induces the formation of sixfold fused rings clusters (graphite layers).
The study of the effect on the physical properties of hydrogenated amorphous carbon films (a-C:H) by the introduction of nitrogen has been undertaken by a number of groups) -s Depending on the deposition conditions the nitrogenated a-C:H films (a-C:H(N)) present a variety of properties; Jones and Stewart, 1 Han and Feldman, 2 and Amir and Kalish s found that the optical energy gap (Eg) diminishes as the nitrogen content increases; although Torng, e t al. 4 found the opposite behavior. It has also been found that nitrogen stabilize the diamond s p a bonding; 4 Kaufman, e t al. z concluded that nitrogen does not dramatically affect the film structure but it breaks symmetry in the sp ~ domains causing the Raman-active G (graphitic) and D (disordered) bands to become infrared active, finally it has also been seen that nitrogen increases the structural order in the a-C:H(N) films3 In a previous work 6 we found a linear relationship between the energy gap and the width of the Urbach edge (E0), and concluded that E0 can be taken as a measure of the disorder in the a-C:H(N) films introduced by the incorporation of nitrogen. In the present work we report photoluminescence (PL), Raman and optical measurements of a-C:H(N) films. We find that the integrated intensity and the full width at half maximum of the PL spectra, as well as the optical band gap, are not monotonic functions of the nitrogen content, we conclude that a high content of nitrogen induces the formation of graphitic domains. The a-C:H(N) films were deposited by rf glow discharge decomposition of a mixture of methane and nitrogen onto Corning 7059 glass substrates for optical measurements, and onto crystalline silicon for photoluminescence and Raman measurements. The conditions for the deposition were as follows: gas pressure in the deposition chamber equal to 300 mTorr, power dissipation of 252 m W / c m , methane flow (f,~) equal to 10 sccm and a nitrogen flow (f~) from 0 to 40 sccm. We use the quantity f=fJ(f~+fm) as a measure of the relative nitrogen flow introduced into the deposition chamber. For thickness
measurements we used Coming substrates partially co,~ ered with the a-C:H(N) film, a thin aluminum film was then evaporated to improve the accuracy of the thickness measurement using a Fizeau interferometer. Transmittance in the visible range was obtained using a Shimadzu UV-260 spectrometer. The photoluminescence and Raman spectra were measured on films of ,,,1500)t of thickness at room temperature using the 4880)i line of the Ar ion laser (50 mW). The optical absorption (a) for photon energies greater than 2.5 eV follows the Tauc relationship aE=B(E-Eg) 2, where the parameter B - l is related to the bandwidth 2 and E is the photon energy. In figure 1 we show the plots of Eg and B -1 as a function of the relative nitrogen flow (f). Figure 2-a shows the PL spectra for three 2.5
20
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R e l a t i v e N i t r o g e n Flow (f) F i g . 1. Optical band gap (Eg) and the parameter B -1 as a function of tim relative nitrogen flow (f). 1025
1026
GRAPHITE-LIKE BONDING 12000 M"'%
(a) ..,
"* f=0.5 d
8000
g
k •
:
k
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Relative N i t r o g e n Flow (f)
Fig, 2. Fig. (a) shows the photoluminescence spectra (PL) as a function of photon energy for three different values of the relative nitrogen flow (f), (b) shows the integrated PL intensity (I) and the full width at half maximum of the PL spectra (W) as a function of f.
different values of f and in figure 2-b the integrated PL (I) and the full width at half maximum (W) is shown. We see that the PL peak is located at approximately 2.1 eV, this value is similar to the measured value of Eg; from these results we infer that the PL arises from transitions between shallow conduction and valence localized tail states. The non-radiative transitions are mediated by deep states, possibly carbon dangling bonds. For low values of f there are many non-radiative states that make the PL peak small, however as f increases nitrogen saturates some of the carbon dangling bonds ~ and the PL peak increases (fig. 2); these results are consistent with reported electron spin resonance measurements. 7 At this stage nitrogenation may reduce the internal strain facilitating s p a bonding and increases the optical gap 4 in a
Vol. 84, No. ! 1
similar manner as hydrogen, s'9 The integrated PL intensity continues to increase as f increases until a critical point is reached at f~0.5, where it then decreases; this non-monotonic behavior is also found for Eg around the same value of f. The decrease of the PL intensity for f>0.5 will be explained later. For f greater than 0.5 we propose the following explanation. Once nitrogen has saturated many of the carbon dangling bonds, excess of nitrogen begins to be incorporated in sixfold graphite rings3 or indeed may even promote the formation of sixfold rings, l° Figure 3 shows the Raman spectra for three films with different values of f. From this figure we can identify the G (graphitic) and the D (disordered) bands. The G band shows a systematic shift from ,,~1533 cm -1 (f=0) to ,,-1570 cm -~ (f=0.8), the D band for the undoped film (f=0) is very faint compared to that for the most heavily sample (f=0.8) and shifts from ~1370 cm -1 to ~1440 cm -1. Note that, due to the high photoluminescence background for the sample with f=0.5 (see fig. 2), the measurement of the Raman signal becomes difficult; however the existence of the G band is clear. Kaufman, e t al. a found a similar shift for the G band from the undoped to the most heavily doped sample; although they did not find significant change in the G band to D band intensity ratio. The shift to higher energies of the G and the D bands and the increase of the intensity of the D band, compared to the G band, was found in a-C:H subjected to thermal annealing and explained in terms of formation of graphitic domains. 11 Our Raman measurements give support to a graphitic layer formation process in a-C:H(N) films with higher nitrogen content. The physical properties of the carbonaceous films have been seen to depend on the deposition conditions as well as the precursor gas used to obtain the films, as and this fact may be the reason for which Kaufman, et al. a do not find an appreciable change in the G
2000 G D
.1500
1000 =
500 f=0.5
0 1000
v f'~
(~4~
1200 1400 1600 Wave N u m b e r ( l / e m )
"
1800
Fig. 3. Raman spectra for three samples with different values of f. The simbols G and D correspond to the graphitic and the disordered bands, respectively. The plots are translated upwards for clarity.
Vol. 84, No. 11
GRAPHITE-LIKE BONDING
band to D band intensity ratio. Additional support for the idea that for f>0.5 nitrogen concentrations induce graphitic layer growth comes from measurements of the energy gap (fig. 1). We found that for f>0.5 the optical energy gap decreases. Robertson and O'Reilly s developed a model for a-C:H and found that the value of E9 is controlled by the extent of the graphitic phase, and its value is given by E g ~ l / x / ~ where N is the number of the sixfold rings fused in compact clusters (graphite layers); therefore a decrease of Eu indicates an increase in the size of the graphitic layers. On the other hand, thermal annealing experiments show that the optical transmittance of the heaviest doped sample decreases less abruptly than that for the less doped films. 6 This fact may be explained supposing that the heaviest doped sample shows a more developed stage of the graphitic phase in the as-deposited state. Returning to the photoluminescence spectra, the decrease of the PL intensity, for f>0.5, may be explained by one or both of the following possibilities. The energy needed for defect creation, Ed , in sixfold fused ring clusters diminishes as the number of rings increases, s if high nitrogen content in a-C:H induces the formation of sixfold rings, then one would expect the creation of many defects that may be non-radiative recombination centers. On the other hand, it is argued s that the low doping efficiency in a-C:H is due to strong autocompensation, in which doping is accompanied by an increase of the defect density, this kind of defect may be a second path for non-radiative recombination. This last argument may be true for the case when f<0.5, but due to the low value of Ed for large clusters this effect would be more important for f>0.5, s Finally it should be noted that there is experimental evidence of a decrease of the photoluminescence intensity in a-C:H samples subjected to thermal annealing, la and it is known that thermal annealing promotes the graphitic phase growth. T M In conclusion, to explain photoluminescence, Raman, and optical measurements in a-C:H(N) films, we propose
1027
that small quantities of nitrogen promote the sps bonding, and higher nitrogen content induces the formation of clusters of sixfold fused rings (sp ~ bonding configuration).
Acknowledgements-We thank Stephen Muhl for useful comments and Miguel Angel Canseco for optical measurements. REFERENCES 1. D. I. Jones and A. D. Stewart, Phil. Mag. B 46, 423 (1982). 2. H.-X. Han and B. J. Feldman, Solid State Commun. 65, 921 (1988). 3. J. H. Kaufman, S. Metin, and D. D. Saperstein, Phys. Rev. B 39, 13053 (1989). 4. C. J. Torng, J. M. Sivertsen, J. H. Judy, and C. Chang, J. Mater. Res. 5, 2490 (1990). 5. O. Amir and R. Kalish, J. Appl. Phys. 70, 4958 (1991). 6. D. Mendoza, submitted for publication. 7. S. Lin, K. Noonan, B. J. Feldman, D. Min, and M. T. Jones, Solid State Commun. 80, 101 (1991). 8. J. Robertson and E. P. O'Reilly, Phys. Rev. B 35, 2946 (1987). 9. J. Tersoff, Phys. Rev. B 44, 12039 (1991). 10. J. J. Cuomo, P. A. Leary, D. Yu, W. Reuter, and M. Frisch, J. Vac. Sci. Technol. 16, 299 (1979). 11. R. O. Dillon, J. A. Woollam, and V. Katkanant, Phys. Rev. B 29, 3482 (1984). 12. See for example: Amorphous Hydrogenated Carbon Films, European Materials Research Society Symposia Proceedings, Strasburg, 1987. P. Koidl and P. Oelhafen, Eds. (Les Editions de Physique, Les Ulis Cedex, France, 1987). 13. S. Lin and B. J. Feldman, Solid State Commun. 80, 371 (1991). 14. B. Dischler in reference 12.