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Oxygen and hydrogen on the surface of diamond
Nuclear Instruments and Methods in Physics Research 218 (1983) 559 562 North-Holland, Amsterdam
559
OXYGEN A N D H Y D R O G E N O N THE SURFACE OF D I A M O N D T . E . D E R R Y , C.C.P. M A D I B A and J.P.F. S E L L S C H O P Nuclear Physics Research Unit. University of the Witwatersrand, Johannesburg, South Africa
We have observed that the cleanest diamond surfaces which we have prepared are characterised by overlayers of oxygen or hydrogen or both. It is likely that the adatoms fulfil the function of saturating the "dangling bonds" whose existence on the surface of diamond-structured solids has been the subject of some speculation. An investigation of these layers using Rutherford backscattering and nuclear reaction analysis is presented here.
1. Introduction The nature of the diamond surface has been the subject of continuing if sporadic speculation in the literature during recent years (for reviews see ref. 1). A simple truncation of the bulk covalent structure would result in "dangling bonds" on any surface plane, but the expected evidence for this (e.g. surface reactivity, high sticking coefficient between surfaces, unpaired spins detectable by ESR) is not apparent [1]. The results presented here arose out of two separate lines of research. On the one hand, attempts to produce clean diamond surfaces for ion-channeling experiments [2], in which surface impurities were monitored by Rutherford backscattering (RBS) of 1.0 MeV He + ions. revealed that diamonds cleaned thoroughly in a strong detergent always had a residual layer (about a monolayer) of oxygen atoms [3]. On the other hand, experiments to determine hydrogen as an intrinsic impurity in diamond, using the H(19F, ay)160 reaction with accelerated fluorine ions, always detected a surface hydrogen peak [4]. Several workers [5,6] have deduced that the "dangling bonds" on diamond surfaces may be terminated by hydrogen atoms; furthermore, calculations [7] have demonstrated that this is energetically favourable. There is also the possibility of surface reconstruction [5] to eliminate "dangling bonds". Our proposal [8] has been that reconstruction does not occur as long as adatoms (or monovalent radicals) are available to satisfy the surface bonds. In particular, polished surfaces should be terminated by hydrogen atoms as the diamond polishing process is carried out in the presence of oil; detergentcleaned surfaces should be covered by hydroxyl ( - O H ) groups; and there might be other possibilities determined by the medium in which the surface was prepared, and the geometry of the bonds on particular surface planes. Polished surfaces may be complicated if the polishing proceeds by the process proposed by the Oxford 0167-5087/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
University group, who assume [9] a mechanism of brittle fracture of small chips bounded mainly by the (111) easy-cleavage planes; this would mean that all polished surfaces consist in fact of (111) microfacets. We shall return to this point later. Worth mentioning here is the work of Sappok and Boehm [10], who investigated the adsorption of hydrogen and oxygen by diamond powders having a large surface-to-mass ratio. Their measurements of heats of hydration, electron spin resonance, infrared spectra, and qualitative micro-analysis were supplemented by the use of standard organic functional group analysis, which enabled them to identify the presence of CH, CH 2, C=O, COOH, and C - O - C groups bonded to some of the surface atoms of their particles.
2. Experimental Investigations were carried out on the following diamond surfaces: natural (111), natural and polished (110), and polished (100). (It is very difficult to polish the (111) planes, and natural (100) faces are rare.) Some of the oxygen and hydrogen measurements were carried out successively on the same surface, others were made independently. The oxygen measurements were made by the standard RBS procedure using 1.0 MeV He + ions, and normalised by comparison with the carbon spectrum (the target was rotated continuously to minimise channeling effects). Hydrogen was measured using the H(19F, ay)160 reaction exploiting the 16.44 MeV resonance which has a cross-section of - 5 0 0 mbarn. The specific details of the scattering chamber were presented earlier [4]. Samples were irradiated with 19F ions in the energy range 16.40 to 16.80 MeV in 50 keV steps. By detecting the 6.13, 6.92 and 7.12 MeV 7-rays emitted and using standard reference materials, hydrogen concentrations were quantified. Deconvolution of the
560
7CE. Der O' el aL / 0 a n d H on the surface of d i a m o n d
surface concentrations was done by c o m p u t e r using the c o m p u t e r code U N F O L D [11]. It was observed earlier [4] that the measurement of hydrogen in the bulk of solid samples requires that they be cooled to reduce hydrogen mobility and diffusion. This, however, has the effect of e n h a n c i n g surface concentrations due to adsorption of hydrogen containing volatiles. The a p p r o a c h adopted for measuring surface c o n c e n t r a t i o n s on d i a m o n d was to irradiate the samples at near room temperature. To reduce surface contamination, a copper shroud a r o u n d the specimen was kept at LN 2 temperature. The pressure in the target c h a m b e r was m a i n t a i n e d at < 10 6 Torr using cryotrapping facilities and vacion pumping. The areal densities of atoms thus determined were converted to " n u m b e r of monolayers" by dividing by the calculated n u m b e r of " d a n g l i n g b o n d s " per unit area on each surface. These were: for (111) 1.82, for (110) 2.22, a n d for (100) 3.14, in units of 10 ~5 cm 2
3. Results and discussion It was expected that the quantitative detection of surface monolayers of light elements in an accelerator e n v i r o n m e n t would be extremely sensitive to contamination, a n d this was b o r n e out by the scatter in the results obtained, despite the precautions taken. How-
ever, a broad consideration of some three dozen d e t e r m i n a t i o n s of oxygen or hydrogen confirmed that either or b o t h elements were consistently present at the monolayer level, with no RBS evidence of the significant presence of other elements. (Polished surfaces, for example, might have been expected to have iron atoms picked up from the polishing wheel.) An example of the RBS spectrum for a clean (111) surface is shown in fig. 1, on a logarithmic scale. Oxygen is the only surface impurity (of mass greater than carbon) which is visible in any significant amount. The slight peak at a b o u t 600 keV is silicon due to previous cont a m i n a t i o n with pump-oil. A hydrogen surface peak traced by fluorine-19 b e a m analysis is shown in fig. 2. Our largest body of data is for oxygen analysis of natural (111) faces cleaned in the commercial product "' C o n t r a d " or " D e c o n 90". Of the 8 samples tested, the n u m b e r of monolayers ranged from t7 = 1.2 to 2.3, but with about half the values clustering below 1.5. It was evident that the measured n is increased with beam exposure, and that the zero exposure value must be close to 1.0. The trend in the hydrogen measurements was not as clear: in fact a reduction in analysed hydrogen with time was sometimes seen, due apparently to the evaporation of condensed water layers as the beam heated the sample. Accordingly the average of the hydrogen m e a s u r e m e n t s should be considered: it was about 1 for (111) surfaces. These results are consistent with
Counts so
,¥
,&
7g-
'% "? =,.?.
Id.-~
"i;.." .'. •
• •
•
.•
. " _ " "
.,
I
500
•
" . .
I
1000
r
Enercjy (keV)
Fig. I. Rutherford backscattered spectrum of 1.0 MeV He + ions from a cleaned (111) diamond surface.
561
T.E. Derry et al. / 0 and H on the surface of diamond
2500
&
2000 A
1500
1000
500
o 16,2
16,
i
4
16,
i
6 16. INCIDENT FLUORINE ENERGY (NOV)
i
8
Fig. 2. Nuclear resonance sweep through a hydrogen layer on the surface of diamond.
the occupation of the bonds by hydroxyl groups after cleaning with the strongly alkaline detergent used. Details are presented in table 1. A freshly polished (110) surface, cleaned only in organic solvents, was essentially oxygen-free at the sensitivity level of the RBS technique (n < 0.2), in agreement with the prediction made earlier. After Contrad cleaning, however, the level rose to a mean value of n = 1.1, with one or two outliers at higher values. The hydrogen measurements were uncorrelated with the treatment used and lay in the range 0.5 to 1.4, with a mean n = 1.0. The conclusion is that polishing terminates the bonds with hydrogen atoms, which are replaced by hydroxyl groups during detergent treatment. The polished (100) surface was virtually hydrogenfree in some cases (n < 0.2). An examination of a three-
dimensional d i a m o n d model reveals that the "dangling b o n d s " of adjacent atoms on a (100) surface are tilted towards one another at an angle of 70°32 ' to the surface normal. There is thus the possibility of bridging between such pairs of atoms with a divalent atom such as oxygen, forming an ether linkage. (The covalent radii of oxygen and carbon, which occupies the bridge position in the bulk, are similar.) A surface terminated thus would be hydrogen-free; the oxygen coverage would be n = 0.5. The same would be true if each oxygen were d o u b l y - b o n d e d to a single carbon atom (each of which has two free bonds) to form a carbonyl group. Our oxygen measurements on freshly polished (100) surfaces give a lowest value of n = 0.3, and one of the above possibilities seems likely. But the lowest oxygen values seem to be associated with the higher hydrogen values
Table 1 Oxygen and hydrogen coverage of diamond surfaces Surface
Treatment
(111)
Contrad cleaned polished + Contrad natural + Contrad polished + Contrad natural + Contrad
(110)
(100)
Oxygen
Hydrogen
Monolayers
Experiments
Monolayers
Experiments
1.2-2.1 usually < 1.5 0.1 1.0-1.5
8
0.4-2.3
3
1 3
0.6 0.5-1.4
1 3
1.2-1.6 0.3-1.3 0.3-0.7
5 4 3
0.6 0.2-1.1 0.7
l 3 1
0.8
1
562
T.E. Der(v et a L / 0 and H on the surface of diamond
(about 0.6), indicating that in these cases only a b o u t half the b o n d s are bridged a n d the r e m a i n d e r h y d r o g e n - t e r m i n a t e d . These values are quite stable against detergent treatment. The origin of the oxygen a t o m s during polishing could be either from the polishing oil (olive oil) which is not a pure hydrocarbon, or from the air. In one case merely leaving the d i a m o n d exposed to the atmosphere for a few days resulted in the oxygen coverage rising from 0.5 to 1.0 monolayers. The p h e n o m e n o n is evidently complex and to be investigated further. Because of the specific geometry of the d i a m o n d structure, the areal densities of b o n d s on the (110) and (100) surfaces is unaffected by possible (111) microfacetting during polishing. However, conclusions a b o u t the b o n d structure on (100) surfaces would be affected. Since other atomic groupings have been reported [10] on d i a m o n d powder surfaces, they may be discernable, u n d e r improved resolution, on polished surfaces. Surfaces prepared by other methods used in this laboratory (gas-phase oxidation, heating in hydrogen, annealing in vacuo) may be terminated differently. The RBS experiments are being moved into ultra-high-vacuum to reduce the c o n t a m i n a t i o n problem. We are very grateful to M. R e b a k for preparing the polished surfaces with specified orientations. We wish
to thank the de Beers D i a m o n d Research Laboratory, the Council for Scientific and Industrial Research. the Atomic Energy Board, and the University of the Witwatersrand, for support.
References [1] J.M. Thomas and E.L. Evans, Diamond Research (1975) p.2; D. Tabor, Diamond Research (1975) p. 9 (published by Industr. Diamond Inf. Bureau, London). [2] T.E. Derry, R.W. Fearick and J.P.F. Sellschop, Phys. Rev. B24 (1981) 3675: Phys. Rev. B26 (1982) 17. [3] M. Rebak, J.P.F. Sellschop, T.E. Derry and R.W. Fearick, Nucl. Instr. and Meth. 167 (1979) 115. [4] J.P.F. Sellschop, C.C.P. Madiba and H.J. Annegarn, Nucl. Instr. and Meth. 168 (1980) 529. [5] P.G. Lurie and J.M. Wilson, Surf. Sci. 65 (1977) 453. [61 B.B. Pate, M.H. Hecht, C. Binns, 1. Lindau and W.E. Spicer, J. Vac. Sci. Technol. 21 (1983) 364. [7] W.S. Verwoerd. Surf. Sci. 108 (1981) 153. [8] T.E. Derry and J.P.F. Sellschop, Proc. Symp. on Surface Science and its Industrial Implications, Pretoria 1981 (CS1R, Pretoria, 1982) p. 31. [9] A.G. Thornton and J. Wilks, J. Phys. D: Appl. Phys. 9 (1976) 27 and refs. therein. [101 R. Sappok and H.P. Boehm, Carbon 6 (1968) 283: 573. [11] G.J. Clark, C.W. White, D.D. Alfred and B.R. Appleton, Nucl. Instr. and Meth. 149 (1978) 9.
559
OXYGEN A N D H Y D R O G E N O N THE SURFACE OF D I A M O N D T . E . D E R R Y , C.C.P. M A D I B A and J.P.F. S E L L S C H O P Nuclear Physics Research Unit. University of the Witwatersrand, Johannesburg, South Africa
We have observed that the cleanest diamond surfaces which we have prepared are characterised by overlayers of oxygen or hydrogen or both. It is likely that the adatoms fulfil the function of saturating the "dangling bonds" whose existence on the surface of diamond-structured solids has been the subject of some speculation. An investigation of these layers using Rutherford backscattering and nuclear reaction analysis is presented here.
1. Introduction The nature of the diamond surface has been the subject of continuing if sporadic speculation in the literature during recent years (for reviews see ref. 1). A simple truncation of the bulk covalent structure would result in "dangling bonds" on any surface plane, but the expected evidence for this (e.g. surface reactivity, high sticking coefficient between surfaces, unpaired spins detectable by ESR) is not apparent [1]. The results presented here arose out of two separate lines of research. On the one hand, attempts to produce clean diamond surfaces for ion-channeling experiments [2], in which surface impurities were monitored by Rutherford backscattering (RBS) of 1.0 MeV He + ions. revealed that diamonds cleaned thoroughly in a strong detergent always had a residual layer (about a monolayer) of oxygen atoms [3]. On the other hand, experiments to determine hydrogen as an intrinsic impurity in diamond, using the H(19F, ay)160 reaction with accelerated fluorine ions, always detected a surface hydrogen peak [4]. Several workers [5,6] have deduced that the "dangling bonds" on diamond surfaces may be terminated by hydrogen atoms; furthermore, calculations [7] have demonstrated that this is energetically favourable. There is also the possibility of surface reconstruction [5] to eliminate "dangling bonds". Our proposal [8] has been that reconstruction does not occur as long as adatoms (or monovalent radicals) are available to satisfy the surface bonds. In particular, polished surfaces should be terminated by hydrogen atoms as the diamond polishing process is carried out in the presence of oil; detergentcleaned surfaces should be covered by hydroxyl ( - O H ) groups; and there might be other possibilities determined by the medium in which the surface was prepared, and the geometry of the bonds on particular surface planes. Polished surfaces may be complicated if the polishing proceeds by the process proposed by the Oxford 0167-5087/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
University group, who assume [9] a mechanism of brittle fracture of small chips bounded mainly by the (111) easy-cleavage planes; this would mean that all polished surfaces consist in fact of (111) microfacets. We shall return to this point later. Worth mentioning here is the work of Sappok and Boehm [10], who investigated the adsorption of hydrogen and oxygen by diamond powders having a large surface-to-mass ratio. Their measurements of heats of hydration, electron spin resonance, infrared spectra, and qualitative micro-analysis were supplemented by the use of standard organic functional group analysis, which enabled them to identify the presence of CH, CH 2, C=O, COOH, and C - O - C groups bonded to some of the surface atoms of their particles.
2. Experimental Investigations were carried out on the following diamond surfaces: natural (111), natural and polished (110), and polished (100). (It is very difficult to polish the (111) planes, and natural (100) faces are rare.) Some of the oxygen and hydrogen measurements were carried out successively on the same surface, others were made independently. The oxygen measurements were made by the standard RBS procedure using 1.0 MeV He + ions, and normalised by comparison with the carbon spectrum (the target was rotated continuously to minimise channeling effects). Hydrogen was measured using the H(19F, ay)160 reaction exploiting the 16.44 MeV resonance which has a cross-section of - 5 0 0 mbarn. The specific details of the scattering chamber were presented earlier [4]. Samples were irradiated with 19F ions in the energy range 16.40 to 16.80 MeV in 50 keV steps. By detecting the 6.13, 6.92 and 7.12 MeV 7-rays emitted and using standard reference materials, hydrogen concentrations were quantified. Deconvolution of the
560
7CE. Der O' el aL / 0 a n d H on the surface of d i a m o n d
surface concentrations was done by c o m p u t e r using the c o m p u t e r code U N F O L D [11]. It was observed earlier [4] that the measurement of hydrogen in the bulk of solid samples requires that they be cooled to reduce hydrogen mobility and diffusion. This, however, has the effect of e n h a n c i n g surface concentrations due to adsorption of hydrogen containing volatiles. The a p p r o a c h adopted for measuring surface c o n c e n t r a t i o n s on d i a m o n d was to irradiate the samples at near room temperature. To reduce surface contamination, a copper shroud a r o u n d the specimen was kept at LN 2 temperature. The pressure in the target c h a m b e r was m a i n t a i n e d at < 10 6 Torr using cryotrapping facilities and vacion pumping. The areal densities of atoms thus determined were converted to " n u m b e r of monolayers" by dividing by the calculated n u m b e r of " d a n g l i n g b o n d s " per unit area on each surface. These were: for (111) 1.82, for (110) 2.22, a n d for (100) 3.14, in units of 10 ~5 cm 2
3. Results and discussion It was expected that the quantitative detection of surface monolayers of light elements in an accelerator e n v i r o n m e n t would be extremely sensitive to contamination, a n d this was b o r n e out by the scatter in the results obtained, despite the precautions taken. How-
ever, a broad consideration of some three dozen d e t e r m i n a t i o n s of oxygen or hydrogen confirmed that either or b o t h elements were consistently present at the monolayer level, with no RBS evidence of the significant presence of other elements. (Polished surfaces, for example, might have been expected to have iron atoms picked up from the polishing wheel.) An example of the RBS spectrum for a clean (111) surface is shown in fig. 1, on a logarithmic scale. Oxygen is the only surface impurity (of mass greater than carbon) which is visible in any significant amount. The slight peak at a b o u t 600 keV is silicon due to previous cont a m i n a t i o n with pump-oil. A hydrogen surface peak traced by fluorine-19 b e a m analysis is shown in fig. 2. Our largest body of data is for oxygen analysis of natural (111) faces cleaned in the commercial product "' C o n t r a d " or " D e c o n 90". Of the 8 samples tested, the n u m b e r of monolayers ranged from t7 = 1.2 to 2.3, but with about half the values clustering below 1.5. It was evident that the measured n is increased with beam exposure, and that the zero exposure value must be close to 1.0. The trend in the hydrogen measurements was not as clear: in fact a reduction in analysed hydrogen with time was sometimes seen, due apparently to the evaporation of condensed water layers as the beam heated the sample. Accordingly the average of the hydrogen m e a s u r e m e n t s should be considered: it was about 1 for (111) surfaces. These results are consistent with
Counts so
,¥
,&
7g-
'% "? =,.?.
Id.-~
"i;.." .'. •
• •
•
.•
. " _ " "
.,
I
500
•
" . .
I
1000
r
Enercjy (keV)
Fig. I. Rutherford backscattered spectrum of 1.0 MeV He + ions from a cleaned (111) diamond surface.
561
T.E. Derry et al. / 0 and H on the surface of diamond
2500
&
2000 A
1500
1000
500
o 16,2
16,
i
4
16,
i
6 16. INCIDENT FLUORINE ENERGY (NOV)
i
8
Fig. 2. Nuclear resonance sweep through a hydrogen layer on the surface of diamond.
the occupation of the bonds by hydroxyl groups after cleaning with the strongly alkaline detergent used. Details are presented in table 1. A freshly polished (110) surface, cleaned only in organic solvents, was essentially oxygen-free at the sensitivity level of the RBS technique (n < 0.2), in agreement with the prediction made earlier. After Contrad cleaning, however, the level rose to a mean value of n = 1.1, with one or two outliers at higher values. The hydrogen measurements were uncorrelated with the treatment used and lay in the range 0.5 to 1.4, with a mean n = 1.0. The conclusion is that polishing terminates the bonds with hydrogen atoms, which are replaced by hydroxyl groups during detergent treatment. The polished (100) surface was virtually hydrogenfree in some cases (n < 0.2). An examination of a three-
dimensional d i a m o n d model reveals that the "dangling b o n d s " of adjacent atoms on a (100) surface are tilted towards one another at an angle of 70°32 ' to the surface normal. There is thus the possibility of bridging between such pairs of atoms with a divalent atom such as oxygen, forming an ether linkage. (The covalent radii of oxygen and carbon, which occupies the bridge position in the bulk, are similar.) A surface terminated thus would be hydrogen-free; the oxygen coverage would be n = 0.5. The same would be true if each oxygen were d o u b l y - b o n d e d to a single carbon atom (each of which has two free bonds) to form a carbonyl group. Our oxygen measurements on freshly polished (100) surfaces give a lowest value of n = 0.3, and one of the above possibilities seems likely. But the lowest oxygen values seem to be associated with the higher hydrogen values
Table 1 Oxygen and hydrogen coverage of diamond surfaces Surface
Treatment
(111)
Contrad cleaned polished + Contrad natural + Contrad polished + Contrad natural + Contrad
(110)
(100)
Oxygen
Hydrogen
Monolayers
Experiments
Monolayers
Experiments
1.2-2.1 usually < 1.5 0.1 1.0-1.5
8
0.4-2.3
3
1 3
0.6 0.5-1.4
1 3
1.2-1.6 0.3-1.3 0.3-0.7
5 4 3
0.6 0.2-1.1 0.7
l 3 1
0.8
1
562
T.E. Der(v et a L / 0 and H on the surface of diamond
(about 0.6), indicating that in these cases only a b o u t half the b o n d s are bridged a n d the r e m a i n d e r h y d r o g e n - t e r m i n a t e d . These values are quite stable against detergent treatment. The origin of the oxygen a t o m s during polishing could be either from the polishing oil (olive oil) which is not a pure hydrocarbon, or from the air. In one case merely leaving the d i a m o n d exposed to the atmosphere for a few days resulted in the oxygen coverage rising from 0.5 to 1.0 monolayers. The p h e n o m e n o n is evidently complex and to be investigated further. Because of the specific geometry of the d i a m o n d structure, the areal densities of b o n d s on the (110) and (100) surfaces is unaffected by possible (111) microfacetting during polishing. However, conclusions a b o u t the b o n d structure on (100) surfaces would be affected. Since other atomic groupings have been reported [10] on d i a m o n d powder surfaces, they may be discernable, u n d e r improved resolution, on polished surfaces. Surfaces prepared by other methods used in this laboratory (gas-phase oxidation, heating in hydrogen, annealing in vacuo) may be terminated differently. The RBS experiments are being moved into ultra-high-vacuum to reduce the c o n t a m i n a t i o n problem. We are very grateful to M. R e b a k for preparing the polished surfaces with specified orientations. We wish
to thank the de Beers D i a m o n d Research Laboratory, the Council for Scientific and Industrial Research. the Atomic Energy Board, and the University of the Witwatersrand, for support.
References [1] J.M. Thomas and E.L. Evans, Diamond Research (1975) p.2; D. Tabor, Diamond Research (1975) p. 9 (published by Industr. Diamond Inf. Bureau, London). [2] T.E. Derry, R.W. Fearick and J.P.F. Sellschop, Phys. Rev. B24 (1981) 3675: Phys. Rev. B26 (1982) 17. [3] M. Rebak, J.P.F. Sellschop, T.E. Derry and R.W. Fearick, Nucl. Instr. and Meth. 167 (1979) 115. [4] J.P.F. Sellschop, C.C.P. Madiba and H.J. Annegarn, Nucl. Instr. and Meth. 168 (1980) 529. [5] P.G. Lurie and J.M. Wilson, Surf. Sci. 65 (1977) 453. [61 B.B. Pate, M.H. Hecht, C. Binns, 1. Lindau and W.E. Spicer, J. Vac. Sci. Technol. 21 (1983) 364. [7] W.S. Verwoerd. Surf. Sci. 108 (1981) 153. [8] T.E. Derry and J.P.F. Sellschop, Proc. Symp. on Surface Science and its Industrial Implications, Pretoria 1981 (CS1R, Pretoria, 1982) p. 31. [9] A.G. Thornton and J. Wilks, J. Phys. D: Appl. Phys. 9 (1976) 27 and refs. therein. [101 R. Sappok and H.P. Boehm, Carbon 6 (1968) 283: 573. [11] G.J. Clark, C.W. White, D.D. Alfred and B.R. Appleton, Nucl. Instr. and Meth. 149 (1978) 9.