- Email: [email protected]
Surface sensitive electron spectroscopy study of diamond films prepared by hot filament-assisted CVD
Surface Science 270 (1992) 265-271
science
. :..t::.~ : i;j: :;. .....: :., .::..::. .:.‘::.:.,,:: .,.,.::,.’ ... .::..; j;: ....:.:.... .:...,..).., +,; ::,,. “I4:.:.::.:I::::,.I..~.:: .j:.:; . :$“:.:.:.:.:~.::.:: ..... ...... .‘.
North-Holland
: ..
Surface sensitive electron spectroscopy study of diamond films prepared by hot filament-assisted CVD R.C.
Cinti,
Lahoraroirc
Received
17 March
Diamond electron.
B.S. Mathis and A.M. Bonnot
d*E~ude.c des Propri&%
1992; accepted
films prepared
electron
des Solides. CNRS,
for publication
IO September
on natural
photoelectron
diamond
which can be explained
spcctroscopies.
and with graphite
chemical
vapor deposition
have been investigated
Results arc discussed in comparison diamond
surfacrs
by Auger
with previous measure-
data. It is shown that film surfaces have original
in a simple model where perturbed
1. Introduction
electron
structure
must be implicated.
tained on natural diamond by other authors and to those WC have obtained on natural graphite.
Synthesis of diamond films with chemical vapor deposition techniques (CVD), operating at low pressure where diamond is the metastable state of solid carbon, involves complex mechanisms associating: - Decomposition of a hydrocarbon, creating a carbonaceous deposit where carbon atoms with sp’ type bonding, like in diamond, and carbon atoms in sp2 configuration, like in graphite, coexist. - The etching of the deposition by atomic hydrogen, which preferentially attacks atoms with sp2 type bonding, thus principally leaving atoms in the tetrahedral coordination of the diamond. As most of these processes occur on the surface of the growing material, it is of great importancc to get a bcttcr knowledge of the characteristics of its superficial layer. Our study is conccrncd with this subject. In this way, we have studied diamond films deposited on silicon substrates with electron spectroscopy techniques, well adapted to investigate surface properties. Auger electron spectroscopy (AES), energy electron loss spectroscopy (EELS) and ultra-violet photoelectron spectroscopy (UPS) were used. In the discussion, we have compared our results to those obCW).3Y-S02X/UZ/$OS.(W) 0
B.P. 166. MO-12 Grcwohlc~ Cedex OS! Frww
1YY2
by the use of a hot filament-assisted
energy loss and UV
ments done principally induced properties
Electroniques
2. Experimental The diamond films have been prepared by the hot filament-assisted technique. The details of the apparatus have been described elsewhere [I]. Typically, the synthesis conditions were a OS-2 at% methane proportion in hydrogen at a total flow rate of 20 seem and a total pressure of 3000 Pa, a 2OWC filament temperature and a 700900°C substrate temperature. A typical scanning electron micrograph and a Raman spectrum of diamond films synthesized at 7WC with a 2 ~01% methane concentration are shown in fig. 1. The micrograph shows wellfacctted pm sized crystals which completely cover the silicon substrate. The Raman spectrum, mcasured with a 406.7 nm Kr laser excitation radiation focused on a _ lo(l pm” arca, shows the characteristic diamond line at 1332 cm ’ with a 5 cm -’ full width at half maximum and no tract of a graphite-like contribution. Increasing the dcposition temperature up to 900°C deteriorates the diamond film quality: the full width at half maximum of the Raman diamond line increases up to
1992 - Elscvicr Science Publishers B.V. All rights resewed
266
R.C. Cinri et al. / AES, EELS and UPS of synthetic diamond films A
IA + .-
--
c 3 2-l
kl L
+ .-
I
1320
1360
1440
Raman Fig. 1. Scanning
electron
microscopy
micrograph
1500
1560
Shift
Cm-l
and Raman spectrum obtained methane concentration.
10 cm-’ and the graphite-like bands become clearly observed even though their intensity remains much lower than that of the diamond line.
on diamond
films prepared
at 700°C with a 2 ~01%
No significant differences in our measurements could be correlated to the variations of the Raman spectra within this synthesis condition range.
I
150
250 KINETIC
350 ENERGY
450
c
(eV)
Fig. 2. (a)-(c) Auger line shape for three different samples, (d) Auger line for an ion-bombarded sample. For the (b)-(d) spectra, only the fine structure region is shown. Experimental conditions: Ei = 1.8 keV, I, = 1 PA, 3 V peak-to-peak modulation.
R.C. Cinti et al. / AES, EELS
The photoemission measurements (UPS) have been obtained with a 21.2 eV He I excitation, supplied by a simple discharge lamp. Because of the geometry of our experimental set-up, the photoelectrons were collected in a small solid angle (+ 2”), normal to the surface of the sample. They were analyzed with a good energy resolution (- 100 meVf with a home-made analyzer. The AES and EELS measurements have been obtained with a four grid retarding system, an electron gun at normal incidence and the standard phase sensitive detection method. Spectra have been measured both in the direct and derivative modes with a 3 V modulation voltage. Measurements correspond to a 1.8 keV primary electron beam for AES and to a 350 eV primary electron beam for EELS. Measurements and annealing were carried out in ultrahigh vacuum conditions, with a residual pressure below lo-’ Pa. The samples were heated by electron bombardment of the MO sample holder on which the Si supported diamond films were clipped. Before any measurement, the samples were outgassed for 30 min at 450°C in order to evaporate the weakly adsorbed impurities.
3. Results and discussion 3.1. AES results Figs. 2a-2c present typical Auger spectra measured on three diamond films. The spectrum (a) recorded in a large kinetic energy range shows no trace of residual impurities on the film surface. The slight differences observed from one sample to another, principally in the relative intensities of the two structures at - 12 and -21 eV from the principal minimum, are not significant and give a good example of our experimental dispersion. Within these limits, our spectra have the characteristic shape of those obtained on natural diamond [2-41 and on diamond films [S-7]. We have not observed noticeable variations of this characteristic diamond shape, either from sample to sample or after annealing to a temperature of 94o”C, which modifies largely the EELS and UPS results as will be shown below. On the other
and UPS
of synthetic
267
diamond firms
a
* 80 60 40 20 ENERGY LOSS (eV) Fig. 3. Energy loss derivative a normal incidence electron Diamond films, Cc) cleaved bombarded diamond film.
0
1
spectra near the elastic peak for beam of 3.50 eV energy. (a), (b) natural graphite, Cd) Ar ion3 V peak-to-peak m~ulation.
hand, bombardment of samples during a few minutes with 600 eV, 1 PA/cm’ argon ions, radically modifies the shape of the Auger spectrum (fig. 26) which then looks quasi-identical to that of natural cleaved graphite or that of the same graphite sample ion bombarded for 2 h to avoid eventual directional effects. This sensitivity of the diamond film surface to argon ion bombardment, revealed by the characteristic shapes of the Auger spectra, has already been observed [5] on diamond films prepared by microwave plasma enhanced CVD. Because of the very weak variations in the energy position of the Auger structures with the allotropic forms of solid carbon [8], and because of possible charging effects in diamond [Z], we have not attempted to exploit the chemical shift. 3.2. EELS results Figs. 3a and 3b show the loss spectra in the neighborhood of the elastic peak obtained on two
268
R.C. Cinti et al. / AES, EELS and UPS
diamond films. As the experimental conditions were very similar to those used on the natural diamond in ref. [2], the comparison of the respective data is very interesting. In the energy loss domain > 20 eV, the EELS spectra of diamond films are very similar to those of natural diamond. The two structures attributed [9,10] to bulk and surface plasmons at about -33 and - 24 eV, respectively, appear in a similar manner in the two materials. On the contrary, in the proximity of the elastic peak, the energy loss spectra differ strongly. While natural diamond only shows a weak structure at about -5 eV, which has been attributed [2,10] to the onset of transitions at the indirect energy gap, our diamond films present a strong and well-defined structure at about - 7 eV. This energy loss can be compared to that we have observed at the same energy on natural graphite (fig. 3c) and on ionbombarded film (fig. 3d). We think it has the same origin because our experimental procedure (outgassing, ultrahigh vacuum, etc.) avoids, with good confidence, any carbonaceous adsorbed gas phases which could give r + n-* transitions in this energy range [ll]. On pure graphite, this energy loss has been observed by many authors [2,7,10,12,13]. In complete angular resolved EELS studies [12,13] it was detected between 6.4 and 7 eV and ascribed to a r electron plasmon excitation, well predicted by the dielectric theory from the calculated band structure of graphite [141. As a last remark, we can note that a similar structure has also been observed on an ion-bombarded natural diamond sample [15] and on new C,,, material [ 161. Thus, the energy loss spectra of our diamond films are rather complex: they are nearly identical to that of natural diamond in the plasmon energy domain and show a graphite-like character in the neighborhood of the elastic peak. It would be interesting at this step of our work to know in what manner this double character, diamond and graphite, is distributed in our film material. More precisely, if (a) the two carbon phases separately coexist at the film surface (for example, considering the typical film morphology shown in fig. 1, a graphitic intergranular region coexisting with pure diamond well faceted crystals), or if (b) the whole
of synthetic diamond films
a
7
-
b
Fig. 4. Energy loss N(E) spectra by valence band excitations. (a) Measured on diamond film, (b) measured on cleaved graphite. (Normal incidence, 350 eV, 3 V peak-to-peak modu lation.)
film material surface has the double character (then a partial graphitic nature must be attributed to diamond grains in spite of their apparent perfection). Our results do not give a direct answer to this question although good arguments can be put forward to reject hypothesis (a), which considered separated contributions. The first argument proceeds from the perfect diamond-like shape of the EELS spectra of our diamond films in the energy range > 20 eV. If the film spectra were a simple combination of pure diamond plus pure graphite spectra, the diamond specific shape could not appear so clearly due to the smearing by the graphite contribution. The second argument comes from the comparison of the relative intensities in EELS spectra measured in the non-derivative mode and with the same experimental conditions, in our diamond films (fig. 4a) and in natural graphite (fig. 4b). For the two materials, the intensities of the plasmon peaks and of the -7 eV structures are very similar. This behavior tends also to rule out the first hypothesis with separated contributions. Effectively, with two contributions (pure diamond and pure graphite intergranularly located), one could detect on diamond film only a fraction of the -7 eV intensity measured on natural graphite, a fact clearly not
269
R.C. Cinti et al. / AES, EELS and UPS of synthetic diamond films
observed in our case. The last argument proceeds from the EELS behavior of films annealed at increasing temperature. Fig. 5 shows the spectrum of an outgassed sample (fig. 5a) and its evolution with successive 10 min annealings at 610, 830 and 940°C (figs. 5b, 5c and 5d, respectively). As early as 61O”C, a modification of the plasmon region is observed. With increasing temperature, it is transformed to the characteristic shape of graphite. Conversely, the -7 eV structure does not vary (in the non-derivative spectra, not shown here, there is no noticeable variation of its intensity). Such an evolution strongly suggests a graphitization of the film surface. Then, the constancy of the - 7 eV structure during this process confirms that this graphitic peak is not correlated to a graphitic rate on the surface and that it is an intrinsic character of our diamond phase. Is this graphitic character due to microscopic graphite inclusions unifo~~y distributed on our sample material? We have no definitive experimental proof to support or reject this assumption. However, the coexistence in our spectra both of pure diamond contributions (typical bulk and surface plasmons) and only partial
C
* Fig.
6. UPS
, 15
,
I
-10 -5 BINDING ENERGY &,
energy
distribution
energy for a 212 eV He1 excitation. cleaved natural graphite,
cures
versus
initial
(a), (bf Diamond
(d) Ar ion-bombarded
state
films, (c)
graphite.
graphite contribution (the -7 eV structure only, but not the -26 eV typical plasmon) tends to rule out such a situation. 3.3. UPS results
ENERGY LOSS (eV) Fig. 5. Energy
loss derivative
nealed sample.
(a) Just after an outgassing at 450°C.
after 10 min annealing
spectra
at 610,830
measured
on an an(b)-(d)
and 940°C. respectively.
Figs. 6a and 6b show two spectra measured on two diamond films. The slight differences between them are representative of the dispersion in the measurements from sample to sample. They show a maximum at about -8.5 eV below E,, with a weak shouIder on its low energy side and a wide plateau extending towards the Fermi level where the emission falls to zero. The variation between the samples is essentially concerned with this plateau of emission. As we have verified that our measurements do not depend on the collection angle of the photoelectrons, which indicate absence of any texture effects in the films, our results can be directly compared to those of Pate [3], obtained with integrated photoemission on natural diamond. In the two works, the maximum peak located at -8.5 eV, with its shoulder at lower energy, appears in a similar way. On the
270
R.C. Cinti et at. / AES, EELS
I *-
’ e 1 I -10 -5 15 BINDING ENERGY &,
Fig. 7. UPS spectra measured 45O”C, (b)-(d)
annealed EDC
on a sample. (a) Outgassed
at
10 min at 610, 830 and 94O”C, (e)
from ion-bombarded
graphite.
contrary, natural diamond does not usually show the plateau of emission we observe in films. Emission in this binding energy range exists in graphite (our measurements: figs. 6c and 6d, and ref. f9]) and has been attributed to the rr electronic states of sp2 hybridized orbitals. It is also observed in natural diamond, but after annealing up to about 950°C and has been attributed to r surface states of the reconstructed (111) diamond face as the consequence of the desorption of hydrogen which normally fills the dangling bonds I31. Observation of emission originating in graphitic 5- type electron states in our samples is consistent with the EELS results discussed above. This agreement is also found in the evolution of UPS spectra upon annealing at high temperature. Fig. 7 shows the transformations observed after successive 10 min annealing steps at 610, 830 and 940°C. As for EELS spectra, already at 610°C an evolution is detected as a shift of the maximum towards low energies and an increasing emission in the Z- state region. This emission increases again after heating at 830-940°C and tends to
and UPS of synthetic diamond fiims
iook like that observed in disordered graphite. We interpret this evolution by a progressive graphitization of the surface, as we did for EELS results. To close the discussion, let us come back to the problem raised up above, concerning the dual character of our film material. We have seen by EELS results that two distinct carbon phases could be ruled out with a good confidence. Besides, UPS results show a diamond characteristic with extra x type emission, which can be compared to the surface state emission observed on natural diamond reconstructed surfaces. Considering these behaviors, a coherent situation can be imagined where the superficial zone accessible by our surface sensitive techniques is largely dominated by the perturbed (c~stallographi~ally and electronically) diamond crystallite surfaces. Taking into account our results, these perturbations must be imaginated clearly more active, electronically, than simple surface reconstructions. In this way, they could explain, by a growing effect, the sensibility of our material to graphitization that we observe at a temperature as low as * 600°C well below the temperature needed to transform pure natural diamond.
4. Conclusions In summary, we have shown that surface sensitive electron spectroscopies (AES, UPS and EELS) can be fruitfully used for the analysis of synthetic diamond film surfaces. Our main conclusions are: - AES measurements on our samples always give the specific peak shape of natural diamond. It does not enable the detection of important thermally induced transformations in fitm surfaces seen by our other spectroscopies. - EELS and UPS measurements show that the material has a complex electronic structure coupling specific diamond characteristics and specific graphite characteristics, although the two separate carbon phases are proved improbable. - The film surfaces graphitize at temperatures as low as - 600°C clearly below the limit where natural diamond is transformed.
R. C. Cinti et al. / AES, EELS and UPS of synthetic diamond films
- All terpreted electronic diamond
these behaviors can be consistently inin a hypothetical model where large perturbations must be imagined on the crystallites forming the films.
Acknowledgements
The support of this research by KODAK and DRET is gratefully acknowledged. References [l] [2] [3] [4]
A.M. Bonnot, Phys. Rev. B 41 (1990) 6040. P.G. Lurie and J.M. Wilson, Surf. Sci. 65 (1977) 476. B.B. Pate, Surf. Sci. 165 (1986) 83. Y. Mitsuda, T. Yamada, T.J. Chuang, H. Seki, R.P. Chin, H.Y. Huang and Y.R. Shen, Surf. Sci. Lett. 257 (1991) L633.
[5] B.E.
271
Williams and J.T. Glass, J. Mater. Res. 4 (1989) 373. [6] H.J. Steffen, C.D. Roux, D. Marton and J.W. Rabalais, Phys. Rev. B 44 (1991) 3981. [7] T.J. Moravec and T.W. Orent, J. Vat. Sci. Technol. 18 (1981) 226. 181F.R. McFeely, S.P. Kowalczyk, L. Ley, R.G. Cavell, R.A. Pollak and D.A. Shirley, Phys. Rev. B 9 (1974) 5268. [91 J. Robertson, Adv. Phys. 35 (1986) 317. [lOI SD. Berger, D.R. McKenzie and P.J. Martin, Philos. Mag. Lett. 57 (1988) 285. [ill A. Dilks, H.V. Richardson and A.M. Bradshaw, in: Electron Spectroscopy, Vol. 4, Eds. C.R. Brundle and A. Baker (Academic Press, New York, 1981) p. 171, 312. and P. [I21 U. Diebold, A. Preisinger, P. Schattschneider Varga, Surf. Sci. 197 (1988) 430. [131 L.S. Caputi, G. Chiarello, A. Santaniello, E. Colavita and L. Papagno, Phys. Rev. B 34 (1986) 6080. 1141 R.C. Tatar and S. Rabii, Phys. Rev. B 25 (1982) 4126. D51 S.V. Pepper, Surf. Sci. 123 (1982) 47. J.J. Pireaux, P.A. Thiry, R. Caudano, [161 G. Gensterblum, J.P. Vigneron, Ph. Lambin and A. Lucas, Phys. Rev. Lett. 67 (1991) 2171.
science
. :..t::.~ : i;j: :;. .....: :., .::..::. .:.‘::.:.,,:: .,.,.::,.’ ... .::..; j;: ....:.:.... .:...,..).., +,; ::,,. “I4:.:.::.:I::::,.I..~.:: .j:.:; . :$“:.:.:.:.:~.::.:: ..... ...... .‘.
North-Holland
: ..
Surface sensitive electron spectroscopy study of diamond films prepared by hot filament-assisted CVD R.C.
Cinti,
Lahoraroirc
Received
17 March
Diamond electron.
B.S. Mathis and A.M. Bonnot
d*E~ude.c des Propri&%
1992; accepted
films prepared
electron
des Solides. CNRS,
for publication
IO September
on natural
photoelectron
diamond
which can be explained
spcctroscopies.
and with graphite
chemical
vapor deposition
have been investigated
Results arc discussed in comparison diamond
surfacrs
by Auger
with previous measure-
data. It is shown that film surfaces have original
in a simple model where perturbed
1. Introduction
electron
structure
must be implicated.
tained on natural diamond by other authors and to those WC have obtained on natural graphite.
Synthesis of diamond films with chemical vapor deposition techniques (CVD), operating at low pressure where diamond is the metastable state of solid carbon, involves complex mechanisms associating: - Decomposition of a hydrocarbon, creating a carbonaceous deposit where carbon atoms with sp’ type bonding, like in diamond, and carbon atoms in sp2 configuration, like in graphite, coexist. - The etching of the deposition by atomic hydrogen, which preferentially attacks atoms with sp2 type bonding, thus principally leaving atoms in the tetrahedral coordination of the diamond. As most of these processes occur on the surface of the growing material, it is of great importancc to get a bcttcr knowledge of the characteristics of its superficial layer. Our study is conccrncd with this subject. In this way, we have studied diamond films deposited on silicon substrates with electron spectroscopy techniques, well adapted to investigate surface properties. Auger electron spectroscopy (AES), energy electron loss spectroscopy (EELS) and ultra-violet photoelectron spectroscopy (UPS) were used. In the discussion, we have compared our results to those obCW).3Y-S02X/UZ/$OS.(W) 0
B.P. 166. MO-12 Grcwohlc~ Cedex OS! Frww
1YY2
by the use of a hot filament-assisted
energy loss and UV
ments done principally induced properties
Electroniques
2. Experimental The diamond films have been prepared by the hot filament-assisted technique. The details of the apparatus have been described elsewhere [I]. Typically, the synthesis conditions were a OS-2 at% methane proportion in hydrogen at a total flow rate of 20 seem and a total pressure of 3000 Pa, a 2OWC filament temperature and a 700900°C substrate temperature. A typical scanning electron micrograph and a Raman spectrum of diamond films synthesized at 7WC with a 2 ~01% methane concentration are shown in fig. 1. The micrograph shows wellfacctted pm sized crystals which completely cover the silicon substrate. The Raman spectrum, mcasured with a 406.7 nm Kr laser excitation radiation focused on a _ lo(l pm” arca, shows the characteristic diamond line at 1332 cm ’ with a 5 cm -’ full width at half maximum and no tract of a graphite-like contribution. Increasing the dcposition temperature up to 900°C deteriorates the diamond film quality: the full width at half maximum of the Raman diamond line increases up to
1992 - Elscvicr Science Publishers B.V. All rights resewed
266
R.C. Cinri et al. / AES, EELS and UPS of synthetic diamond films A
IA + .-
--
c 3 2-l
kl L
+ .-
I
1320
1360
1440
Raman Fig. 1. Scanning
electron
microscopy
micrograph
1500
1560
Shift
Cm-l
and Raman spectrum obtained methane concentration.
10 cm-’ and the graphite-like bands become clearly observed even though their intensity remains much lower than that of the diamond line.
on diamond
films prepared
at 700°C with a 2 ~01%
No significant differences in our measurements could be correlated to the variations of the Raman spectra within this synthesis condition range.
I
150
250 KINETIC
350 ENERGY
450
c
(eV)
Fig. 2. (a)-(c) Auger line shape for three different samples, (d) Auger line for an ion-bombarded sample. For the (b)-(d) spectra, only the fine structure region is shown. Experimental conditions: Ei = 1.8 keV, I, = 1 PA, 3 V peak-to-peak modulation.
R.C. Cinti et al. / AES, EELS
The photoemission measurements (UPS) have been obtained with a 21.2 eV He I excitation, supplied by a simple discharge lamp. Because of the geometry of our experimental set-up, the photoelectrons were collected in a small solid angle (+ 2”), normal to the surface of the sample. They were analyzed with a good energy resolution (- 100 meVf with a home-made analyzer. The AES and EELS measurements have been obtained with a four grid retarding system, an electron gun at normal incidence and the standard phase sensitive detection method. Spectra have been measured both in the direct and derivative modes with a 3 V modulation voltage. Measurements correspond to a 1.8 keV primary electron beam for AES and to a 350 eV primary electron beam for EELS. Measurements and annealing were carried out in ultrahigh vacuum conditions, with a residual pressure below lo-’ Pa. The samples were heated by electron bombardment of the MO sample holder on which the Si supported diamond films were clipped. Before any measurement, the samples were outgassed for 30 min at 450°C in order to evaporate the weakly adsorbed impurities.
3. Results and discussion 3.1. AES results Figs. 2a-2c present typical Auger spectra measured on three diamond films. The spectrum (a) recorded in a large kinetic energy range shows no trace of residual impurities on the film surface. The slight differences observed from one sample to another, principally in the relative intensities of the two structures at - 12 and -21 eV from the principal minimum, are not significant and give a good example of our experimental dispersion. Within these limits, our spectra have the characteristic shape of those obtained on natural diamond [2-41 and on diamond films [S-7]. We have not observed noticeable variations of this characteristic diamond shape, either from sample to sample or after annealing to a temperature of 94o”C, which modifies largely the EELS and UPS results as will be shown below. On the other
and UPS
of synthetic
267
diamond firms
a
* 80 60 40 20 ENERGY LOSS (eV) Fig. 3. Energy loss derivative a normal incidence electron Diamond films, Cc) cleaved bombarded diamond film.
0
1
spectra near the elastic peak for beam of 3.50 eV energy. (a), (b) natural graphite, Cd) Ar ion3 V peak-to-peak m~ulation.
hand, bombardment of samples during a few minutes with 600 eV, 1 PA/cm’ argon ions, radically modifies the shape of the Auger spectrum (fig. 26) which then looks quasi-identical to that of natural cleaved graphite or that of the same graphite sample ion bombarded for 2 h to avoid eventual directional effects. This sensitivity of the diamond film surface to argon ion bombardment, revealed by the characteristic shapes of the Auger spectra, has already been observed [5] on diamond films prepared by microwave plasma enhanced CVD. Because of the very weak variations in the energy position of the Auger structures with the allotropic forms of solid carbon [8], and because of possible charging effects in diamond [Z], we have not attempted to exploit the chemical shift. 3.2. EELS results Figs. 3a and 3b show the loss spectra in the neighborhood of the elastic peak obtained on two
268
R.C. Cinti et al. / AES, EELS and UPS
diamond films. As the experimental conditions were very similar to those used on the natural diamond in ref. [2], the comparison of the respective data is very interesting. In the energy loss domain > 20 eV, the EELS spectra of diamond films are very similar to those of natural diamond. The two structures attributed [9,10] to bulk and surface plasmons at about -33 and - 24 eV, respectively, appear in a similar manner in the two materials. On the contrary, in the proximity of the elastic peak, the energy loss spectra differ strongly. While natural diamond only shows a weak structure at about -5 eV, which has been attributed [2,10] to the onset of transitions at the indirect energy gap, our diamond films present a strong and well-defined structure at about - 7 eV. This energy loss can be compared to that we have observed at the same energy on natural graphite (fig. 3c) and on ionbombarded film (fig. 3d). We think it has the same origin because our experimental procedure (outgassing, ultrahigh vacuum, etc.) avoids, with good confidence, any carbonaceous adsorbed gas phases which could give r + n-* transitions in this energy range [ll]. On pure graphite, this energy loss has been observed by many authors [2,7,10,12,13]. In complete angular resolved EELS studies [12,13] it was detected between 6.4 and 7 eV and ascribed to a r electron plasmon excitation, well predicted by the dielectric theory from the calculated band structure of graphite [141. As a last remark, we can note that a similar structure has also been observed on an ion-bombarded natural diamond sample [15] and on new C,,, material [ 161. Thus, the energy loss spectra of our diamond films are rather complex: they are nearly identical to that of natural diamond in the plasmon energy domain and show a graphite-like character in the neighborhood of the elastic peak. It would be interesting at this step of our work to know in what manner this double character, diamond and graphite, is distributed in our film material. More precisely, if (a) the two carbon phases separately coexist at the film surface (for example, considering the typical film morphology shown in fig. 1, a graphitic intergranular region coexisting with pure diamond well faceted crystals), or if (b) the whole
of synthetic diamond films
a
7
-
b
Fig. 4. Energy loss N(E) spectra by valence band excitations. (a) Measured on diamond film, (b) measured on cleaved graphite. (Normal incidence, 350 eV, 3 V peak-to-peak modu lation.)
film material surface has the double character (then a partial graphitic nature must be attributed to diamond grains in spite of their apparent perfection). Our results do not give a direct answer to this question although good arguments can be put forward to reject hypothesis (a), which considered separated contributions. The first argument proceeds from the perfect diamond-like shape of the EELS spectra of our diamond films in the energy range > 20 eV. If the film spectra were a simple combination of pure diamond plus pure graphite spectra, the diamond specific shape could not appear so clearly due to the smearing by the graphite contribution. The second argument comes from the comparison of the relative intensities in EELS spectra measured in the non-derivative mode and with the same experimental conditions, in our diamond films (fig. 4a) and in natural graphite (fig. 4b). For the two materials, the intensities of the plasmon peaks and of the -7 eV structures are very similar. This behavior tends also to rule out the first hypothesis with separated contributions. Effectively, with two contributions (pure diamond and pure graphite intergranularly located), one could detect on diamond film only a fraction of the -7 eV intensity measured on natural graphite, a fact clearly not
269
R.C. Cinti et al. / AES, EELS and UPS of synthetic diamond films
observed in our case. The last argument proceeds from the EELS behavior of films annealed at increasing temperature. Fig. 5 shows the spectrum of an outgassed sample (fig. 5a) and its evolution with successive 10 min annealings at 610, 830 and 940°C (figs. 5b, 5c and 5d, respectively). As early as 61O”C, a modification of the plasmon region is observed. With increasing temperature, it is transformed to the characteristic shape of graphite. Conversely, the -7 eV structure does not vary (in the non-derivative spectra, not shown here, there is no noticeable variation of its intensity). Such an evolution strongly suggests a graphitization of the film surface. Then, the constancy of the - 7 eV structure during this process confirms that this graphitic peak is not correlated to a graphitic rate on the surface and that it is an intrinsic character of our diamond phase. Is this graphitic character due to microscopic graphite inclusions unifo~~y distributed on our sample material? We have no definitive experimental proof to support or reject this assumption. However, the coexistence in our spectra both of pure diamond contributions (typical bulk and surface plasmons) and only partial
C
* Fig.
6. UPS
, 15
,
I
-10 -5 BINDING ENERGY &,
energy
distribution
energy for a 212 eV He1 excitation. cleaved natural graphite,
cures
versus
initial
(a), (bf Diamond
(d) Ar ion-bombarded
state
films, (c)
graphite.
graphite contribution (the -7 eV structure only, but not the -26 eV typical plasmon) tends to rule out such a situation. 3.3. UPS results
ENERGY LOSS (eV) Fig. 5. Energy
loss derivative
nealed sample.
(a) Just after an outgassing at 450°C.
after 10 min annealing
spectra
at 610,830
measured
on an an(b)-(d)
and 940°C. respectively.
Figs. 6a and 6b show two spectra measured on two diamond films. The slight differences between them are representative of the dispersion in the measurements from sample to sample. They show a maximum at about -8.5 eV below E,, with a weak shouIder on its low energy side and a wide plateau extending towards the Fermi level where the emission falls to zero. The variation between the samples is essentially concerned with this plateau of emission. As we have verified that our measurements do not depend on the collection angle of the photoelectrons, which indicate absence of any texture effects in the films, our results can be directly compared to those of Pate [3], obtained with integrated photoemission on natural diamond. In the two works, the maximum peak located at -8.5 eV, with its shoulder at lower energy, appears in a similar way. On the
270
R.C. Cinti et at. / AES, EELS
I *-
’ e 1 I -10 -5 15 BINDING ENERGY &,
Fig. 7. UPS spectra measured 45O”C, (b)-(d)
annealed EDC
on a sample. (a) Outgassed
at
10 min at 610, 830 and 94O”C, (e)
from ion-bombarded
graphite.
contrary, natural diamond does not usually show the plateau of emission we observe in films. Emission in this binding energy range exists in graphite (our measurements: figs. 6c and 6d, and ref. f9]) and has been attributed to the rr electronic states of sp2 hybridized orbitals. It is also observed in natural diamond, but after annealing up to about 950°C and has been attributed to r surface states of the reconstructed (111) diamond face as the consequence of the desorption of hydrogen which normally fills the dangling bonds I31. Observation of emission originating in graphitic 5- type electron states in our samples is consistent with the EELS results discussed above. This agreement is also found in the evolution of UPS spectra upon annealing at high temperature. Fig. 7 shows the transformations observed after successive 10 min annealing steps at 610, 830 and 940°C. As for EELS spectra, already at 610°C an evolution is detected as a shift of the maximum towards low energies and an increasing emission in the Z- state region. This emission increases again after heating at 830-940°C and tends to
and UPS of synthetic diamond fiims
iook like that observed in disordered graphite. We interpret this evolution by a progressive graphitization of the surface, as we did for EELS results. To close the discussion, let us come back to the problem raised up above, concerning the dual character of our film material. We have seen by EELS results that two distinct carbon phases could be ruled out with a good confidence. Besides, UPS results show a diamond characteristic with extra x type emission, which can be compared to the surface state emission observed on natural diamond reconstructed surfaces. Considering these behaviors, a coherent situation can be imagined where the superficial zone accessible by our surface sensitive techniques is largely dominated by the perturbed (c~stallographi~ally and electronically) diamond crystallite surfaces. Taking into account our results, these perturbations must be imaginated clearly more active, electronically, than simple surface reconstructions. In this way, they could explain, by a growing effect, the sensibility of our material to graphitization that we observe at a temperature as low as * 600°C well below the temperature needed to transform pure natural diamond.
4. Conclusions In summary, we have shown that surface sensitive electron spectroscopies (AES, UPS and EELS) can be fruitfully used for the analysis of synthetic diamond film surfaces. Our main conclusions are: - AES measurements on our samples always give the specific peak shape of natural diamond. It does not enable the detection of important thermally induced transformations in fitm surfaces seen by our other spectroscopies. - EELS and UPS measurements show that the material has a complex electronic structure coupling specific diamond characteristics and specific graphite characteristics, although the two separate carbon phases are proved improbable. - The film surfaces graphitize at temperatures as low as - 600°C clearly below the limit where natural diamond is transformed.
R. C. Cinti et al. / AES, EELS and UPS of synthetic diamond films
- All terpreted electronic diamond
these behaviors can be consistently inin a hypothetical model where large perturbations must be imagined on the crystallites forming the films.
Acknowledgements
The support of this research by KODAK and DRET is gratefully acknowledged. References [l] [2] [3] [4]
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