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The irradiation effect of MeV protons on diamond-like carbon films
Nuclear Instruments and Methods in Physics Research B71 (1992) 186-190 North-Holland
HB
Beam Interactions with Materials &Atoms
The irradiation effect of MeV protons on diamond-like carbon films Tianmin Wang, Weijie Wang and Buliang Chen
Department of Materiais Science, Lanzhou University, Lanzhou, 730000, Gansu Province, China
Received 29 July 1991 and in revised form 3 January 1992 Diamond-like carbon films, prepared by dual-ion beam sputtering deposition on glass substrate, were irradiated by 1 .5 MeV protons under doses ranging from 1 x 10 1 ; to I x 10 17 ions/cmz . The electrical resistivity, the infrared transmittance and the Raman spectrum were used to characterize the films . It was shown that the irradiation reduced both the film resistivity and the infrared optical transmittancy of specimens obviously and there existed a critical ion dose (1 x 10' ions/cm2 ) for the onset of the reductions . The aging effect of the irradiated films in air was also studied at room temperature and it was found thai higher increases in resistivity appeared for films irradiated by higher doses. It was concluded that the breaking of C-C bonds, C-H bonds and the release of hydrogen atoms are responsible for both the decreases in resistivity and in optical transmittancy. The critical ion dose is a threshold for the release of hydrogen . The aging effect of resistivity was caused by the passivation of dangling bonds in air . 1 . Introduction In recent years, a great deal of research has been done about the influences of ion irradiations on the structure and properties of diamond-like carbon (DLC) films. Differs nt kinds of ions, different ion energies and doses were used to bombard the DLC films [1-6] . Generally, the results obtained showed a similar tendency for the changes of film properties with irradiation . It was reported that [1-4] with the increase of ion irradiation dose, the electrical resistivity of the films reduced greatly and the films changed from light brown to shiny black in color, the optical transparency (particularly in the infrared (IR) region), hardness, internal stress and some other properties of the films were also changed due to irradiations. Structural analyses were also made and it was shown that [1-4] the irradiation of heavy energetic ions on the DLC films caused the breaking of C-H bonds, C-C bonds and the releasing of hydrogen from the films. However, Raman measurements and electron spin resonance measurements for 50 keV C + irradiated DLC films indicated that the hydrogen loss was not accompanied by growth of graphite crystallites [5,6] and it was thus considered that the decrease of resistivity was caused by the damage produced by nuclear collisions as the impinging ion comes to rest . But other investigators [4] concluded that the defects produced by electronic energy loss of the bombarding ions (energy from Dr Wang Weijie. Department of Materials Science, Lanzhou University, Lanzhou 730000, China. Correspondence to:
0168-58'3X/92/$05 .00 rJ 1992 - Elsevier
50 keV to 1 MeV) are responsible for both the hydrogen loss and the change in resistivity . Although so much work has been reported, most of these studies were completed by means of heavy ions, many features of the DLC films under the irradiations of energetic ions were not clearly realized, the transformation mechanism of the films under different post-treat conditions is thus at an early stage and there are still many questions to be investigated further. By comparing different results from the previous reports, we find that the hydrogen incorporated in the as-deposited films played a very important role during the transformation of the films. Considering the important role of hydrogen in DLC films, we have recently irradiated DLC films by highly energetic protons. In the present paper, we will report some important experimental results and discuss the irradiation effect briefly.
2. Experimental The DLC films adopted by us were prepared by the dual-ion beam sputtering method on a glass substrate. The details of the deposition equipment can be found elsewhere [7] . The deposition conditions of the as-deposited films arc as follows: Bombarding ion beam energy : 300 cV, bombarding ion beam current density : 0.32 mA/ cm`, CH 4/(CH 4 + Ar) in bombarding ion source: 80%, sputtering ion (Ar + ) beam energy : 960 cV,
Science Publishers B.V. All rights reserved
187
T. Wang ei al. / Irradiation of diamond-like carbon fibns
sputtering ion beam current density : 6 .4 mA/cm`, Ar flow in sputtering ion source: 0.60 SCCM, pressure in chamber: 2.6 x 10 -2 Pa, substrate : glass with the size of 70 x 20 x 1 .1 mm, ion etching time of bombarding ions on substrate : 5 min, substrate temperature: lower than 50°C, rotating speed of substrate holder : 30 rpm, The as-deposited films were up into small specimens of size 20 X 10 mm and two aluminum electrodes were deposited by evaporation onto the opposite sides of the 20 mm long direction. The uncovered region was then 10 x 10 mm in size. The electrical resistance R . of the 10 x 10 mm area was measured by the two-clectrode method and the square resistivity p o of film was determined by p o = R,, - D/T, where D is the length of the two aluminum electrodes and T is the distance between them. Because in our experiment, D = T = 10 mm, so p o = R, The bulk resistivity p of a film was thus obtained by p = p o d, where d is the thickness of the film determined by ellipsometry at a wavelength of 6328 A. The results indicated that the films on all small specimens are very uniform, their electrical resistivity is 1 .1 x 105 fZ cm and their film thickness is 3500 ± 100 P+ .
The irradiation was performed with an ion accelerator at the Department of Modern Physics, Lanzhou University. The beam size was exactly 10 x 10 mm and the uncovered as-grown regions were then irradiated by 1 .5 MeV protons at different doses ranging from 1 x 10" to 1 x 10" ions/cm`, respectively. From TRIM we estimated that the average range of 1 .5 MeV protons in the film is about 20 wm. Thus the film thickness is much thinner than the penetration depth of protons. During the irradiations, the ion beam density was fixed at 3 WA/cm= and the substrate holder was cooled by water. The measured temperature on the back of the substrate was lower than 60°C. Because the unirradiated DLC films suffered no transformation if heated in vacuum to 200°C for one hour, we believe that a possible temperature rise on the film surface has no influence on the irradiation effects. IR absorption spectra and Raman spectra were measured in atomsphere just before and after the ion irradiation. The exciting laser wavelength is 5145 A in the Raman measurement. After ion irradiation, the resistivity was also determined by the same method as mentioned above . 3. Results Fig . 1 shows the dose dependence of resistivity of the irradiated DLC films . The resistivity was measured immediately after the irradiation . It is evident that when the dose is higher than a critical value (1 x 10' 5
Ê u
N
sr îEr FW
I
I
1X1015
1X1016
1X1017
DOSE (ionslcm2 )
Fig . 1 . Resistivity vs . proton dose of irradiated films. ions/cm= ), the resistivity drops rapidly with increasing dose . When the irradiation dose is very high (1 x 10" ions/cm'- ), the resistivity drops about 2 orders of magnitude . After irradiation, all specimens were exposed in air and the relations between resistivity vs . aging time was also measured. It was observed that an aging effect appeared in all irradiated films . The film resistivity incteased with time . The biggest increase (about one order of magnitude) appeared for films irradiated by both 1 x 10" ions/cm 2 and 1 x 10' 7 ions/cm' doses, as shown in fig. 2 . It can be seen that at the initial aging stage, the increase in the speed of resistivity is much higher, while at the final stage when 50 h have passed by, the increase in speed decreases and the resistivity vs time curves eventually flatten out . The IR transmission spectra of specimens were measured with a Fourier-transform spectrometer. A similar critical dose value for the drop in optical transmission was found. It was observed that compared with that of an as-grown specimen, the IR transmission 105 E v G á r
DOSE =1X1016
S
N
á 3 u
0
20
30
50
AGING TIMC(hr)
70
Fig . 2. Resistivity vs. aging time of films irradiated with a dose of 1 x 10' ° and 1 x 1017 ions/cm= .
T. Wang et al. / Irradiation of diarnorrd-like carbon films 100 90 t!0 70 60 50
1150
40 30
4000 5000 WAVENUMBER (cm'I) Fig . 3. The total IR transmittance vs . wavenumber curves of specimens. 6000
spectra of specimens irradiated with doses of 1 X 10", 1 X 10 '° and 1 X 10' 5 ions/cm =, respectively, have hardly any change. But the spectra of specimens irradiated with doses of 1 X 10' ° ions/cry. = and 1 X 10' 7 ions/cm` exhibit drops of about 25% and 30% in transmission over the range of 6000-4000 cm - ', (see fig . 3) . This means that the ion irradiation under high doses (> I X 10' 5 ions/cm 2 ) would seriously damage the IR optical transmission of DLC films . Because the films were very thin and were deposited on a glass substrate it was very difficult to get separated film layers . But by means of computer, the
1475 RAMAN SHIFT(--1 ) Fig . 5 . Raman spectra of films .
1800
IR transmittance spectra of single film layers were obtained, as illustrated in fig. 4. These spectra can be used to analyse the different C-H stretch bonds . It is clear that below the critical dose (1 X 10" 5 ions/cm`), there arc still several sharp absorption peaks at 2855 CM -I (SP 'CH 3 ), 2920 cm -1 (sp 3 CH, and sp'CH) and 3045 cm - ' (sp`CH). However when the dose is higher than the critical value, these absorption peaks corresponding to sp' C-H bonds and sp` C-H bonds become very weak (1 X 10" ions/cm = ) or even disappear (1 X 10" ions/cm = ). Raman spectra of films are given in fig. 5 . It is easy to find that the Raman spectra of the as-grown film and the ion irradiated films are all composed of two broad bands, namely G-line (the graphite-like peak) and D-line (the diamond-like peak) [8,9] . The rise of all the curves at the lower frequency edge is contributed to by the scattering of the glass substrate . It can be found that with the increase of irradiation dose, the G-line narrows and its peak position shifts toward high frequency edge . At the dose of 1 X 10 17 ions/cm'-, the peak shifts to about 1560 em - ' . 4. Discussion
3600
3000 2400 WAVENUMBER(Crn7 I ) Fig . 4. IR transmittance spectra of all film layers.
As mentioned above, the irradiation of 1 .5 MeV protons on DLC films caused a decrease in electrical resistivity and a drop in the optical transmittance in the IR region . A distinct critical dose (1 X 10' 5 ions/cm`) for the onset of the drop in resistivity and in the optical transmittance was observed. This is similar to the results reported before [1,4] . However, in our experiment, a final or saturation value in resistivity, as reported by others [4], was not achieved . However, it can be deduced from the tendency of the resistivity vs. dose curves that a further increase in the irradiation dose must lead to a further decrease in resistivity. In addition to this, we can also notice that the property changes are coincident with structural changes: with
T. Wang et al. / Irradiation of diamond-like carbon films the increase of irradiation dose, both the IR transmittance curve and the Raman spectrum of the film layer clearly change; when the dose is higher than the same critical value, the absorption peaks in the IR spectra corresponding to sp3 C-H bonds and sp2 C-H bonds become very weak (1 X 10 16 ions/cm 2 ) or even disappear (1 X 10 17 ions/cm=). While the G-peak in the Raman spectrum shifts towards the higher frequency edge. Referring to some publications [1,3,4], we predict the following mechanism to explain the transformation feature of the films under the 1 .5 MeV proton irradiations . When incident ions with high energy pass through the film, electronic excitations occur around their trajectories. As a result many C-C bonds and C-H bonds are broken and many dangling bonds and free atoms are produced . The numbers of both the dangling bonds and the free atoms are larger for films irradiated at higher doses. Among these, some can be recombined but some cannot . Hydrogen atoms are most likely to be released to the vacuum if they are not recaptured by the dangling bonds. If we assume that at lower doses, most of the hydrogen atoms can be recaptured by the dangling bonds, then a large number of broken bonds produced by ion irradiations will remain to be passivated and hence the measured C-H peaks will have no large decreases. However, when the dose is very high, the release of hydrogen atoms occurs, the broken bonds cannot be effectively passivated, then the measured C-H peaks will have large drops . Because of the breaking of C-C bonds and C-H bonds, the release of hydrogen and consequently the changes in bond-types, the properties of the films will have considerable changes. This model is exactly in agreement with our observations described above . According to the model, the value of the critical dose (1 X 10' 6 ions/cm`) in our experiment is a threshold for the release of hydrogen . Raman spectra are useful in determining the C-C bonds in DLC films . It is well known [8,9] that the Raman spectrum of large single-crystal graphite has a single line at about 1580 cm - ' (G-line), while polycrystalline graphite exhibits another line at about 1355 cm`' (D-lines). As Dillon et al. suggested by comparison with theory [8], the as-grown DLC films are all composed of a similar G-line and a disorder induced D-line . Compared with that of 1580 em -1 and 1355 cm - ', the down shifts of G-line indicates the presence of bond-angle disorder, the similarly down-shift of the D-line indicates the presence of some sp' C-C bonds as well as disorder. However, the up-shift of the two lines towards the high frequency edge means that the crystallites are dominated by sp'- C-C over spj C-C bonds. The narrowing of the G-line also means the removal of bond-angle disorder and the increasing dominance of crystallites. In our present work, we find
189
that the peak positions of the G-line and D-line of as-grown film are shifted down to the lower frequency edge . Due to the irradiation, they shift towards the high frequency edge and the G-line narrows. This suggests that (according to Dillon et al. [8]) there exists bond-angle disordered sp 2 C-C bonds and spa C-C bonds in as-grown films, the irradiation of 1 .5 MeV protons lead to the decrease of the bond-angle disorder and the increase of sp 2 C-C bond dominated crystallite size and/or number. This gives us further evidence for the breaking of C-H bonds and C-C bonds and also for the release of hydrogen . Since without these processes, the sp'- C-C peak and sp3 C-C peak in Raman spectrum would not have any changes. In the final process, many dangling bonds of carbon are created in the films . When the films are taken out into the atomsphere, the dangling bonds can make some C = C bonds with neighbors or C = O bonds by capturing oxygen atoms in air, thus leading to an increase in resistivity . Because the films irradiated at higher doses contain a higher density of dangling bonds, so their increases in resistivity should be higher . The observed aging effect in film resistivity confirms this deduction very well . A similar effect was found by Fujimoto et al. [3]. They did not measure the resistivity, but by means of IR spectra, they discovered the formation of C = O bonds and C = C bonds in 12 MeV C3+ irradiated films exposing in air . 5 . Summary In summary, according to our results, there exists a critical ion dose (1 X 10 15 ions/cm`) in irradiating DLC films with 1 .5 MeV protons. Below this dose, the hydrogen atoms produced by ion irradiation could be recaptured effectively by dangling bonds, thus the irradiation could hardly lead to any obvious changes in electrical and optical properties . However, for dose higher than 1 X 10' 5 ions/cm'-, the release of hydrogen atoms occurred and the number of dangling bonds was increased . This in turn led to a big decreases in electrical resistivity and in optical transmittancy . Thus the critical ion dose can be considered as a threshold for the release of hydrogen . Also, with the increase of dose, the bond-angle disorder was reduced and the components of crystallites were increased. The aging effect of resistivity occurred in all irradiated films. This effect can be understood in termo of passivation mechanism at dangling bonds in air. Acknowledgements Prof . Z.Y. Liu at the Modern Physics Department of Lanzhou University, Mrs. F.L . Sheng in Analysis
19(1
T Wnng ƒ .L / 1-uliarion
and Testing Centre of Lanzhou University, and Prof. L.P . Huang, Mr . C.T. Luo, M.K. Li and D.Q . Liu at the Lanzhou Research Institute of Physics are all acknowledged for their kind help during the research .
-b-fl
[4] [5] [6]
References [l] S. Prawer, R. Kalish, M. Adel and V. Richter, Appl. Phys . Lett . 49 (18) (1986) 1157. [21 J.W. Zou, K. Schmidt, K. Reicheit and B. Stritzker, J. Vac. Scn. Technol. A6 (6) (1988) 3103. [31 F. Fujimoto, M. Tanaka, Y. lwata, A. Ootuka, K. Komaki,
[7]
[8j [9]
M. Haba, K. Kobayashi, Nucl. Insir . and Meth. B33 (1988) 792. D.C . Ingram, A.W. McCormick, Nucl. Instr . and Meth . B34 (1988) 68. M.E. Adel, R. Kalish and S. Prawer, J. Appl . Phys . 62 (10) (1987) 4096 . S. Prawer, R. Kalish, M. Adel and V. Richter, J. Appl . Phys . 61 (1987) 4492. T.M. Wang, W.J . Wang, G.A. Liu, L.P. Huang, C.T. Luo, D.Q. Liu, M. Xu and Y.M. Yang, Int. Conf. on Thin Film Physics and Applications, eds. Shi X. Zhou and Young L. Wang, Proc. of SPIE 1519 (1991) 890. R.O. Dillon, J.A. Woollam, V, Katkanant, Phys. Rev. B29 (16) (1986) 3482. H.C. Tsai and D.B . Bogy, J. Vac. Sci . Technol . A5(6) (1987) 3287.
HB
Beam Interactions with Materials &Atoms
The irradiation effect of MeV protons on diamond-like carbon films Tianmin Wang, Weijie Wang and Buliang Chen
Department of Materiais Science, Lanzhou University, Lanzhou, 730000, Gansu Province, China
Received 29 July 1991 and in revised form 3 January 1992 Diamond-like carbon films, prepared by dual-ion beam sputtering deposition on glass substrate, were irradiated by 1 .5 MeV protons under doses ranging from 1 x 10 1 ; to I x 10 17 ions/cmz . The electrical resistivity, the infrared transmittance and the Raman spectrum were used to characterize the films . It was shown that the irradiation reduced both the film resistivity and the infrared optical transmittancy of specimens obviously and there existed a critical ion dose (1 x 10' ions/cm2 ) for the onset of the reductions . The aging effect of the irradiated films in air was also studied at room temperature and it was found thai higher increases in resistivity appeared for films irradiated by higher doses. It was concluded that the breaking of C-C bonds, C-H bonds and the release of hydrogen atoms are responsible for both the decreases in resistivity and in optical transmittancy. The critical ion dose is a threshold for the release of hydrogen . The aging effect of resistivity was caused by the passivation of dangling bonds in air . 1 . Introduction In recent years, a great deal of research has been done about the influences of ion irradiations on the structure and properties of diamond-like carbon (DLC) films. Differs nt kinds of ions, different ion energies and doses were used to bombard the DLC films [1-6] . Generally, the results obtained showed a similar tendency for the changes of film properties with irradiation . It was reported that [1-4] with the increase of ion irradiation dose, the electrical resistivity of the films reduced greatly and the films changed from light brown to shiny black in color, the optical transparency (particularly in the infrared (IR) region), hardness, internal stress and some other properties of the films were also changed due to irradiations. Structural analyses were also made and it was shown that [1-4] the irradiation of heavy energetic ions on the DLC films caused the breaking of C-H bonds, C-C bonds and the releasing of hydrogen from the films. However, Raman measurements and electron spin resonance measurements for 50 keV C + irradiated DLC films indicated that the hydrogen loss was not accompanied by growth of graphite crystallites [5,6] and it was thus considered that the decrease of resistivity was caused by the damage produced by nuclear collisions as the impinging ion comes to rest . But other investigators [4] concluded that the defects produced by electronic energy loss of the bombarding ions (energy from Dr Wang Weijie. Department of Materials Science, Lanzhou University, Lanzhou 730000, China. Correspondence to:
0168-58'3X/92/$05 .00 rJ 1992 - Elsevier
50 keV to 1 MeV) are responsible for both the hydrogen loss and the change in resistivity . Although so much work has been reported, most of these studies were completed by means of heavy ions, many features of the DLC films under the irradiations of energetic ions were not clearly realized, the transformation mechanism of the films under different post-treat conditions is thus at an early stage and there are still many questions to be investigated further. By comparing different results from the previous reports, we find that the hydrogen incorporated in the as-deposited films played a very important role during the transformation of the films. Considering the important role of hydrogen in DLC films, we have recently irradiated DLC films by highly energetic protons. In the present paper, we will report some important experimental results and discuss the irradiation effect briefly.
2. Experimental The DLC films adopted by us were prepared by the dual-ion beam sputtering method on a glass substrate. The details of the deposition equipment can be found elsewhere [7] . The deposition conditions of the as-deposited films arc as follows: Bombarding ion beam energy : 300 cV, bombarding ion beam current density : 0.32 mA/ cm`, CH 4/(CH 4 + Ar) in bombarding ion source: 80%, sputtering ion (Ar + ) beam energy : 960 cV,
Science Publishers B.V. All rights reserved
187
T. Wang ei al. / Irradiation of diamond-like carbon fibns
sputtering ion beam current density : 6 .4 mA/cm`, Ar flow in sputtering ion source: 0.60 SCCM, pressure in chamber: 2.6 x 10 -2 Pa, substrate : glass with the size of 70 x 20 x 1 .1 mm, ion etching time of bombarding ions on substrate : 5 min, substrate temperature: lower than 50°C, rotating speed of substrate holder : 30 rpm, The as-deposited films were up into small specimens of size 20 X 10 mm and two aluminum electrodes were deposited by evaporation onto the opposite sides of the 20 mm long direction. The uncovered region was then 10 x 10 mm in size. The electrical resistance R . of the 10 x 10 mm area was measured by the two-clectrode method and the square resistivity p o of film was determined by p o = R,, - D/T, where D is the length of the two aluminum electrodes and T is the distance between them. Because in our experiment, D = T = 10 mm, so p o = R, The bulk resistivity p of a film was thus obtained by p = p o d, where d is the thickness of the film determined by ellipsometry at a wavelength of 6328 A. The results indicated that the films on all small specimens are very uniform, their electrical resistivity is 1 .1 x 105 fZ cm and their film thickness is 3500 ± 100 P+ .
The irradiation was performed with an ion accelerator at the Department of Modern Physics, Lanzhou University. The beam size was exactly 10 x 10 mm and the uncovered as-grown regions were then irradiated by 1 .5 MeV protons at different doses ranging from 1 x 10" to 1 x 10" ions/cm`, respectively. From TRIM we estimated that the average range of 1 .5 MeV protons in the film is about 20 wm. Thus the film thickness is much thinner than the penetration depth of protons. During the irradiations, the ion beam density was fixed at 3 WA/cm= and the substrate holder was cooled by water. The measured temperature on the back of the substrate was lower than 60°C. Because the unirradiated DLC films suffered no transformation if heated in vacuum to 200°C for one hour, we believe that a possible temperature rise on the film surface has no influence on the irradiation effects. IR absorption spectra and Raman spectra were measured in atomsphere just before and after the ion irradiation. The exciting laser wavelength is 5145 A in the Raman measurement. After ion irradiation, the resistivity was also determined by the same method as mentioned above . 3. Results Fig . 1 shows the dose dependence of resistivity of the irradiated DLC films . The resistivity was measured immediately after the irradiation . It is evident that when the dose is higher than a critical value (1 x 10' 5
Ê u
N
sr îEr FW
I
I
1X1015
1X1016
1X1017
DOSE (ionslcm2 )
Fig . 1 . Resistivity vs . proton dose of irradiated films. ions/cm= ), the resistivity drops rapidly with increasing dose . When the irradiation dose is very high (1 x 10" ions/cm'- ), the resistivity drops about 2 orders of magnitude . After irradiation, all specimens were exposed in air and the relations between resistivity vs . aging time was also measured. It was observed that an aging effect appeared in all irradiated films . The film resistivity incteased with time . The biggest increase (about one order of magnitude) appeared for films irradiated by both 1 x 10" ions/cm 2 and 1 x 10' 7 ions/cm' doses, as shown in fig. 2 . It can be seen that at the initial aging stage, the increase in the speed of resistivity is much higher, while at the final stage when 50 h have passed by, the increase in speed decreases and the resistivity vs time curves eventually flatten out . The IR transmission spectra of specimens were measured with a Fourier-transform spectrometer. A similar critical dose value for the drop in optical transmission was found. It was observed that compared with that of an as-grown specimen, the IR transmission 105 E v G á r
DOSE =1X1016
S
N
á 3 u
0
20
30
50
AGING TIMC(hr)
70
Fig . 2. Resistivity vs. aging time of films irradiated with a dose of 1 x 10' ° and 1 x 1017 ions/cm= .
T. Wang et al. / Irradiation of diarnorrd-like carbon films 100 90 t!0 70 60 50
1150
40 30
4000 5000 WAVENUMBER (cm'I) Fig . 3. The total IR transmittance vs . wavenumber curves of specimens. 6000
spectra of specimens irradiated with doses of 1 X 10", 1 X 10 '° and 1 X 10' 5 ions/cm =, respectively, have hardly any change. But the spectra of specimens irradiated with doses of 1 X 10' ° ions/cry. = and 1 X 10' 7 ions/cm` exhibit drops of about 25% and 30% in transmission over the range of 6000-4000 cm - ', (see fig . 3) . This means that the ion irradiation under high doses (> I X 10' 5 ions/cm 2 ) would seriously damage the IR optical transmission of DLC films . Because the films were very thin and were deposited on a glass substrate it was very difficult to get separated film layers . But by means of computer, the
1475 RAMAN SHIFT(--1 ) Fig . 5 . Raman spectra of films .
1800
IR transmittance spectra of single film layers were obtained, as illustrated in fig. 4. These spectra can be used to analyse the different C-H stretch bonds . It is clear that below the critical dose (1 X 10" 5 ions/cm`), there arc still several sharp absorption peaks at 2855 CM -I (SP 'CH 3 ), 2920 cm -1 (sp 3 CH, and sp'CH) and 3045 cm - ' (sp`CH). However when the dose is higher than the critical value, these absorption peaks corresponding to sp' C-H bonds and sp` C-H bonds become very weak (1 X 10" ions/cm = ) or even disappear (1 X 10" ions/cm = ). Raman spectra of films are given in fig. 5 . It is easy to find that the Raman spectra of the as-grown film and the ion irradiated films are all composed of two broad bands, namely G-line (the graphite-like peak) and D-line (the diamond-like peak) [8,9] . The rise of all the curves at the lower frequency edge is contributed to by the scattering of the glass substrate . It can be found that with the increase of irradiation dose, the G-line narrows and its peak position shifts toward high frequency edge . At the dose of 1 X 10 17 ions/cm'-, the peak shifts to about 1560 em - ' . 4. Discussion
3600
3000 2400 WAVENUMBER(Crn7 I ) Fig . 4. IR transmittance spectra of all film layers.
As mentioned above, the irradiation of 1 .5 MeV protons on DLC films caused a decrease in electrical resistivity and a drop in the optical transmittance in the IR region . A distinct critical dose (1 X 10' 5 ions/cm`) for the onset of the drop in resistivity and in the optical transmittance was observed. This is similar to the results reported before [1,4] . However, in our experiment, a final or saturation value in resistivity, as reported by others [4], was not achieved . However, it can be deduced from the tendency of the resistivity vs. dose curves that a further increase in the irradiation dose must lead to a further decrease in resistivity. In addition to this, we can also notice that the property changes are coincident with structural changes: with
T. Wang et al. / Irradiation of diamond-like carbon films the increase of irradiation dose, both the IR transmittance curve and the Raman spectrum of the film layer clearly change; when the dose is higher than the same critical value, the absorption peaks in the IR spectra corresponding to sp3 C-H bonds and sp2 C-H bonds become very weak (1 X 10 16 ions/cm 2 ) or even disappear (1 X 10 17 ions/cm=). While the G-peak in the Raman spectrum shifts towards the higher frequency edge. Referring to some publications [1,3,4], we predict the following mechanism to explain the transformation feature of the films under the 1 .5 MeV proton irradiations . When incident ions with high energy pass through the film, electronic excitations occur around their trajectories. As a result many C-C bonds and C-H bonds are broken and many dangling bonds and free atoms are produced . The numbers of both the dangling bonds and the free atoms are larger for films irradiated at higher doses. Among these, some can be recombined but some cannot . Hydrogen atoms are most likely to be released to the vacuum if they are not recaptured by the dangling bonds. If we assume that at lower doses, most of the hydrogen atoms can be recaptured by the dangling bonds, then a large number of broken bonds produced by ion irradiations will remain to be passivated and hence the measured C-H peaks will have no large decreases. However, when the dose is very high, the release of hydrogen atoms occurs, the broken bonds cannot be effectively passivated, then the measured C-H peaks will have large drops . Because of the breaking of C-C bonds and C-H bonds, the release of hydrogen and consequently the changes in bond-types, the properties of the films will have considerable changes. This model is exactly in agreement with our observations described above . According to the model, the value of the critical dose (1 X 10' 6 ions/cm`) in our experiment is a threshold for the release of hydrogen . Raman spectra are useful in determining the C-C bonds in DLC films . It is well known [8,9] that the Raman spectrum of large single-crystal graphite has a single line at about 1580 cm - ' (G-line), while polycrystalline graphite exhibits another line at about 1355 cm`' (D-lines). As Dillon et al. suggested by comparison with theory [8], the as-grown DLC films are all composed of a similar G-line and a disorder induced D-line . Compared with that of 1580 em -1 and 1355 cm - ', the down shifts of G-line indicates the presence of bond-angle disorder, the similarly down-shift of the D-line indicates the presence of some sp' C-C bonds as well as disorder. However, the up-shift of the two lines towards the high frequency edge means that the crystallites are dominated by sp'- C-C over spj C-C bonds. The narrowing of the G-line also means the removal of bond-angle disorder and the increasing dominance of crystallites. In our present work, we find
189
that the peak positions of the G-line and D-line of as-grown film are shifted down to the lower frequency edge . Due to the irradiation, they shift towards the high frequency edge and the G-line narrows. This suggests that (according to Dillon et al. [8]) there exists bond-angle disordered sp 2 C-C bonds and spa C-C bonds in as-grown films, the irradiation of 1 .5 MeV protons lead to the decrease of the bond-angle disorder and the increase of sp 2 C-C bond dominated crystallite size and/or number. This gives us further evidence for the breaking of C-H bonds and C-C bonds and also for the release of hydrogen . Since without these processes, the sp'- C-C peak and sp3 C-C peak in Raman spectrum would not have any changes. In the final process, many dangling bonds of carbon are created in the films . When the films are taken out into the atomsphere, the dangling bonds can make some C = C bonds with neighbors or C = O bonds by capturing oxygen atoms in air, thus leading to an increase in resistivity . Because the films irradiated at higher doses contain a higher density of dangling bonds, so their increases in resistivity should be higher . The observed aging effect in film resistivity confirms this deduction very well . A similar effect was found by Fujimoto et al. [3]. They did not measure the resistivity, but by means of IR spectra, they discovered the formation of C = O bonds and C = C bonds in 12 MeV C3+ irradiated films exposing in air . 5 . Summary In summary, according to our results, there exists a critical ion dose (1 X 10 15 ions/cm`) in irradiating DLC films with 1 .5 MeV protons. Below this dose, the hydrogen atoms produced by ion irradiation could be recaptured effectively by dangling bonds, thus the irradiation could hardly lead to any obvious changes in electrical and optical properties . However, for dose higher than 1 X 10' 5 ions/cm'-, the release of hydrogen atoms occurred and the number of dangling bonds was increased . This in turn led to a big decreases in electrical resistivity and in optical transmittancy . Thus the critical ion dose can be considered as a threshold for the release of hydrogen . Also, with the increase of dose, the bond-angle disorder was reduced and the components of crystallites were increased. The aging effect of resistivity occurred in all irradiated films. This effect can be understood in termo of passivation mechanism at dangling bonds in air. Acknowledgements Prof . Z.Y. Liu at the Modern Physics Department of Lanzhou University, Mrs. F.L . Sheng in Analysis
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T Wnng ƒ .L / 1-uliarion
and Testing Centre of Lanzhou University, and Prof. L.P . Huang, Mr . C.T. Luo, M.K. Li and D.Q . Liu at the Lanzhou Research Institute of Physics are all acknowledged for their kind help during the research .
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[4] [5] [6]
References [l] S. Prawer, R. Kalish, M. Adel and V. Richter, Appl. Phys . Lett . 49 (18) (1986) 1157. [21 J.W. Zou, K. Schmidt, K. Reicheit and B. Stritzker, J. Vac. Scn. Technol. A6 (6) (1988) 3103. [31 F. Fujimoto, M. Tanaka, Y. lwata, A. Ootuka, K. Komaki,
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M. Haba, K. Kobayashi, Nucl. Insir . and Meth. B33 (1988) 792. D.C . Ingram, A.W. McCormick, Nucl. Instr . and Meth . B34 (1988) 68. M.E. Adel, R. Kalish and S. Prawer, J. Appl . Phys . 62 (10) (1987) 4096 . S. Prawer, R. Kalish, M. Adel and V. Richter, J. Appl . Phys . 61 (1987) 4492. T.M. Wang, W.J . Wang, G.A. Liu, L.P. Huang, C.T. Luo, D.Q. Liu, M. Xu and Y.M. Yang, Int. Conf. on Thin Film Physics and Applications, eds. Shi X. Zhou and Young L. Wang, Proc. of SPIE 1519 (1991) 890. R.O. Dillon, J.A. Woollam, V, Katkanant, Phys. Rev. B29 (16) (1986) 3482. H.C. Tsai and D.B . Bogy, J. Vac. Sci . Technol . A5(6) (1987) 3287.