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Oxygen on diamond surfaces
Diamond and Related Materials 8 (1999) 1620–1622 www.elsevier.com/locate/diamond
Oxygen on diamond surfaces D.B. Rebuli *, T.E. Derry, E. Sideras-Haddad, B.P. Doyle, R.D. Maclear, S.H. Connell, J.P.F. Sellschop Schonland Research Centre for Nuclear Sciences, University of the Witwatersrand, Private Bag 3, Johannesburg 2050, South Africa Accepted 14 December 1998
Abstract Studies into the effect of hydrogen and oxygen on the growth of CVD diamonds have been in progress for a number of years. Surface oxygen studies on natural diamond using Rutherford backscattering spectrometry have yielded partial monolayer coverage on low index planes. To increase the sensitivity of the measurements, elastic scattering using the 16O(a,a)16O resonance at 3.045 MeV has been performed. This resonance can enhance, by up to 30 times, the Rutherford cross-section. The samples are natural diamonds with either (111) or (100) surfaces. These have been cleaned using aqueous and acidic solvents to study the effect cleaning has on the oxygen coverage. We have found the coverage to be dependent on the vacuum level of the target chamber, with no effect being shown by the cleaning methods. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Surface; Oxygen
1. Introduction Both PC-CVD and natural diamond have been found to have hydrogen in the bulk and on the surface using techniques such as elastic recoil detection analysis ( ERDA) [1]. The role of oxygen in CVD growth of diamonds is to reduce the concentration of the hydrocarbon nutrients such as CH by forming CO [2]. This 3 is stable and thereby removes excess carbon. Trace amounts of OH and O present in the plasma react with sp2 impurities far more readily than does atomic hydrogen. Oxygen therefore helps to remove sp2 defects to give a purer CVD crystal. Natural diamond surfaces have been studied using X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry (RBS) and a submonolayer of oxygen has been found [3,4]. RBS is ideal for this purpose as the substrate (carbon) is lighter than the contamination. Therefore the contamination peak is separated in the spectrum from the bulk as the backscattered energy increases with the mass of the particle struck [see Eq. (2)]. With oxygen having a mass close to that of carbon and there being of the order of a monolayer on the surface of the diamond, the experiment is at the detection limit of * Corresponding author. Fax: +27-112292144. E-mail address: [email protected] (D.B. Rebuli)
RBS. It is therefore appropriate to use elastic scattering at an energy where there is a useful resonance to improve the detection limits.
2. Theory Rutherford backscattering [5] involves the scattering of light particles in the MeV range off a target. RBS has been used extensively for accurate determination of stoichiometry, elemental areal density, and impurity distributions. Thin film elements may be identified by insertion of the measured energy Ei of the high-energy 1 side of the peak into k to calculate the kinematic factor i k for the ith element where k is given by i i k ¬Ei /E (1) i 1 0 where E is the incident ion laboratory energy and 0 (M2 −M2 sin2 h)1/2+M cos h 1/2 i 1 1 k= (2) i M +M 1 i h is the laboratory angle through which the incident ion is scattered, and M and M are the incident and target 1 i particle masses, respectively. The yield of backscattered particles is dependent on the geometry of the set-up, the density of the target, the fraction of the total number of particles which hit the
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detector and the cross-section of the reaction. For RBS the cross-section is the Rutherford cross-section s (E, h)= R
A
B
Z Z e2 2 1 i 4E
4[(M2 −M2 sin2h)1/2+M cos h]1/2 i 1 i . (3) M sin4h(M2 −M2 sin2h)1/2 i i 1 The 16O(a,a)16O resonance at 3.045 MeV was used as it gives an enhanced cross-section for the detection of a particles at backward angles over the Rutherford crosssection. This enhancement can be up to 30 times greater depending on the angle of the detector with reference to the incident beam [6 ]. This enhancement results in a higher yield and therefore a greater peak to background ratio, i.e. improved detection limits. ×
3. The experiment A Duoplasmatron ion source was used to give an a beam. The beam was then accelerated using the 6 MV EN Tandem Van der Graaff Accelerator at the Schonland Research Centre. To calibrate the energy of the a beam a piece of SiO was placed in the target 2 chamber. We were then able to fix the energy such that the resonance corresponded to the surface of the target (see Fig. 1). A higher beam energy would result in the resonance occurring in the bulk of the sample at a depth where the energy loss was that of the excess energy of the beam. This also allowed us to calculate the crosssection of the resonance allowing quantitative analysis of the data. Natural diamonds polished with either (111) or (100) surfaces were used. The diamonds were cleaned in one of three ways: ultrasonically cleaned using methanol as a solvent; ultrasonically cleaned using Contrad, an industrial chelating cleaning agent, as a solvent; boiled in a mixture of sulphuric, nitric and perchloric acids. The
diamonds were then rinsed in de-ionised water, except for those cleaned in methanol which were not rinsed. They were mounted in the target chamber immediately. The a beam was focused to a spot size of <1 mm using the Schonland Microprobe. A smaller beam than this was not required as the samples have relatively large surfaces and a representation of the whole surface was required. A silicon surface barrier detector was placed in the target chamber at a scattering angle of 150°. The samples were normal to the incident beam.
4. Results The data were analysed using the analysis package RUMP [7] with data files modified for the oxygen resonance and the non-Rutherford carbon cross-sections. The errors in these measurements can come from the uncertainty in the cross-section and the statistics of the measurements. The error in the cross-section can arise from the error in the beam energy, where the SiO is used to limit this, and from the knowledge of 2 the cross-sections at a certain energy. The cross-sections are well documented in the literature [6 ], so the largest error arises from the statistics of the measurements. Two sets of data were taken. The first was with a vacuum of 4×10−6 Torr. The results for the different cleaning methods and surfaces can be seen in Table 1. The sample which was heated was prepared in the following way: the sample was heated in an atmosphere of argon to a temperature of 850°C for 30 min. It was allowed to cool in the argon. The sample was then exposed to the atmosphere for 10 min before mounting in the target chamber. Heating to a temperature of 850°C has been shown to remove surface oxygen [8]. The second set of data was taken with a vacuum of 5×10−7 Torr. The samples were re-cleaned before the experiment was done. These results can be seen in Table 2 (see Fig. 2).
5. Discussion For a vacuum of 4×10−6 Torr there is no significant difference in oxygen coverage for the different surfaces or for different cleaning methods. There is significantly Table 1 Samples measured in a vacuum of 4×10−6 Torr (where ML=monolayer and measured errors are in parentheses)
Fig. 1. Backscattered energy spectrum of the SiO standard with the 2 beam at the resonance energy.
Cleaning method
(111) surface (ML)
(100) surface (ML)
Contrad Acid Methanol Heated
1.1 1.0 1.1 1.3
1.1 (0.1) 1.9 (0.2) 1.0 (0.1) –
(0.1) (0.1) (0.1) (0.1)
1622
D.B. Rebuli et al. / Diamond and Related Materials 8 (1999) 1620–1622
Table 2 Samples measured in a vacuum of 5×10−7 Torr (where ML=monolayer and measured errors are in parentheses) Cleaning method
(111) surface (ML)
(100) surface (ML)
Contrad Acid Methanol
0.31 (0.02) 0.30 (0.02) 0.31 (0.02)
0.47 (0.04) 0.49 (0.04) 0.51 (0.04)
form of water molecules sitting on the surface. For a vacuum of 5×10−7 Torr the results of 0.3 and 0.5 ML for the (111) and (100) surfaces, respectively, are consistent with previous results [3,4]. The fact that there is no difference to the oxygen coverage for different cleaning methods suggests that the fractional monolayer of oxygen is bonded to the surface and is the preferred state of the diamond surface. This is also in agreement with possible OH groups on the (111) surface and C– O–C bridges on the (100) surface. Further work with channelling is necessary to find the lattice location of the oxygen and is presently under way.
References
Fig. 2. Backscattered energy spectrum of a diamond with the beam at the resonance energy.
more oxygen than we would have expected for this level of vacuum. This suggests that most of the oxygen was adsorbed before insertion into the chamber as opposed to bonded to the surface. This could possibly be in the
[1] S.H. Connell, J.P.F. Sellschop, R.D. Maclear, B.P. Doyle, I.Z. Machi, R.W.N. Nilen, J.E. Butler, K. Bharuth-Ram, Mater. Sci. Forum 258–263 (1997) 751. [2] D.G. Goodwin, J.E. Butler, in: M.A. Prelas, G. Popovici, L.K. Bigelow (Eds.), Handbook of Industrial Diamonds and Diamond Films, Dekker, New York, 1997, p. 527. [3] J.O. Hansen, T.E. Derry, P.E. Harris, R.G. Copperthwaite, J.P.F. Sellschop, Adv. Ultrahard Mater. Appl. Technol. 4 (1988) 76. [4] J.O. Hansen, R.G. Copperthwaite, T.E. Derry, J.M. Pratt, J. Colloid Interface Sci. 130 (1989) 347. [5] P.D. Townsend, J.C. Kelly, N.E.W. Hartley, in: Ion Implantation, Sputtering and Their Applications, Academic Press, New York, 1976, p. 181. [6 ] J.A. Leavitt, L.C. McIntyre, M.D. Ashbaugh, J.G. Oder, Z. Lin, B. Dezfouly-Arjomandy, Nucl. Instrum. Methods B 44 (1990) 260. [7] RUMP, Computer Graphic Service, Ithaca, NY, 1987. [8] W.J. Huisman, J.F. Peters, S.A. de Vries, E. Vlieg, W.S. Yang, T.E. Derry, J.F. van der Veen, Surf. Sci. 387 (1997) 342.
Oxygen on diamond surfaces D.B. Rebuli *, T.E. Derry, E. Sideras-Haddad, B.P. Doyle, R.D. Maclear, S.H. Connell, J.P.F. Sellschop Schonland Research Centre for Nuclear Sciences, University of the Witwatersrand, Private Bag 3, Johannesburg 2050, South Africa Accepted 14 December 1998
Abstract Studies into the effect of hydrogen and oxygen on the growth of CVD diamonds have been in progress for a number of years. Surface oxygen studies on natural diamond using Rutherford backscattering spectrometry have yielded partial monolayer coverage on low index planes. To increase the sensitivity of the measurements, elastic scattering using the 16O(a,a)16O resonance at 3.045 MeV has been performed. This resonance can enhance, by up to 30 times, the Rutherford cross-section. The samples are natural diamonds with either (111) or (100) surfaces. These have been cleaned using aqueous and acidic solvents to study the effect cleaning has on the oxygen coverage. We have found the coverage to be dependent on the vacuum level of the target chamber, with no effect being shown by the cleaning methods. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Surface; Oxygen
1. Introduction Both PC-CVD and natural diamond have been found to have hydrogen in the bulk and on the surface using techniques such as elastic recoil detection analysis ( ERDA) [1]. The role of oxygen in CVD growth of diamonds is to reduce the concentration of the hydrocarbon nutrients such as CH by forming CO [2]. This 3 is stable and thereby removes excess carbon. Trace amounts of OH and O present in the plasma react with sp2 impurities far more readily than does atomic hydrogen. Oxygen therefore helps to remove sp2 defects to give a purer CVD crystal. Natural diamond surfaces have been studied using X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry (RBS) and a submonolayer of oxygen has been found [3,4]. RBS is ideal for this purpose as the substrate (carbon) is lighter than the contamination. Therefore the contamination peak is separated in the spectrum from the bulk as the backscattered energy increases with the mass of the particle struck [see Eq. (2)]. With oxygen having a mass close to that of carbon and there being of the order of a monolayer on the surface of the diamond, the experiment is at the detection limit of * Corresponding author. Fax: +27-112292144. E-mail address: [email protected] (D.B. Rebuli)
RBS. It is therefore appropriate to use elastic scattering at an energy where there is a useful resonance to improve the detection limits.
2. Theory Rutherford backscattering [5] involves the scattering of light particles in the MeV range off a target. RBS has been used extensively for accurate determination of stoichiometry, elemental areal density, and impurity distributions. Thin film elements may be identified by insertion of the measured energy Ei of the high-energy 1 side of the peak into k to calculate the kinematic factor i k for the ith element where k is given by i i k ¬Ei /E (1) i 1 0 where E is the incident ion laboratory energy and 0 (M2 −M2 sin2 h)1/2+M cos h 1/2 i 1 1 k= (2) i M +M 1 i h is the laboratory angle through which the incident ion is scattered, and M and M are the incident and target 1 i particle masses, respectively. The yield of backscattered particles is dependent on the geometry of the set-up, the density of the target, the fraction of the total number of particles which hit the
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 9 ) 0 0 02 6 - 6
C
D
1621
D.B. Rebuli et al. / Diamond and Related Materials 8 (1999) 1620–1622
detector and the cross-section of the reaction. For RBS the cross-section is the Rutherford cross-section s (E, h)= R
A
B
Z Z e2 2 1 i 4E
4[(M2 −M2 sin2h)1/2+M cos h]1/2 i 1 i . (3) M sin4h(M2 −M2 sin2h)1/2 i i 1 The 16O(a,a)16O resonance at 3.045 MeV was used as it gives an enhanced cross-section for the detection of a particles at backward angles over the Rutherford crosssection. This enhancement can be up to 30 times greater depending on the angle of the detector with reference to the incident beam [6 ]. This enhancement results in a higher yield and therefore a greater peak to background ratio, i.e. improved detection limits. ×
3. The experiment A Duoplasmatron ion source was used to give an a beam. The beam was then accelerated using the 6 MV EN Tandem Van der Graaff Accelerator at the Schonland Research Centre. To calibrate the energy of the a beam a piece of SiO was placed in the target 2 chamber. We were then able to fix the energy such that the resonance corresponded to the surface of the target (see Fig. 1). A higher beam energy would result in the resonance occurring in the bulk of the sample at a depth where the energy loss was that of the excess energy of the beam. This also allowed us to calculate the crosssection of the resonance allowing quantitative analysis of the data. Natural diamonds polished with either (111) or (100) surfaces were used. The diamonds were cleaned in one of three ways: ultrasonically cleaned using methanol as a solvent; ultrasonically cleaned using Contrad, an industrial chelating cleaning agent, as a solvent; boiled in a mixture of sulphuric, nitric and perchloric acids. The
diamonds were then rinsed in de-ionised water, except for those cleaned in methanol which were not rinsed. They were mounted in the target chamber immediately. The a beam was focused to a spot size of <1 mm using the Schonland Microprobe. A smaller beam than this was not required as the samples have relatively large surfaces and a representation of the whole surface was required. A silicon surface barrier detector was placed in the target chamber at a scattering angle of 150°. The samples were normal to the incident beam.
4. Results The data were analysed using the analysis package RUMP [7] with data files modified for the oxygen resonance and the non-Rutherford carbon cross-sections. The errors in these measurements can come from the uncertainty in the cross-section and the statistics of the measurements. The error in the cross-section can arise from the error in the beam energy, where the SiO is used to limit this, and from the knowledge of 2 the cross-sections at a certain energy. The cross-sections are well documented in the literature [6 ], so the largest error arises from the statistics of the measurements. Two sets of data were taken. The first was with a vacuum of 4×10−6 Torr. The results for the different cleaning methods and surfaces can be seen in Table 1. The sample which was heated was prepared in the following way: the sample was heated in an atmosphere of argon to a temperature of 850°C for 30 min. It was allowed to cool in the argon. The sample was then exposed to the atmosphere for 10 min before mounting in the target chamber. Heating to a temperature of 850°C has been shown to remove surface oxygen [8]. The second set of data was taken with a vacuum of 5×10−7 Torr. The samples were re-cleaned before the experiment was done. These results can be seen in Table 2 (see Fig. 2).
5. Discussion For a vacuum of 4×10−6 Torr there is no significant difference in oxygen coverage for the different surfaces or for different cleaning methods. There is significantly Table 1 Samples measured in a vacuum of 4×10−6 Torr (where ML=monolayer and measured errors are in parentheses)
Fig. 1. Backscattered energy spectrum of the SiO standard with the 2 beam at the resonance energy.
Cleaning method
(111) surface (ML)
(100) surface (ML)
Contrad Acid Methanol Heated
1.1 1.0 1.1 1.3
1.1 (0.1) 1.9 (0.2) 1.0 (0.1) –
(0.1) (0.1) (0.1) (0.1)
1622
D.B. Rebuli et al. / Diamond and Related Materials 8 (1999) 1620–1622
Table 2 Samples measured in a vacuum of 5×10−7 Torr (where ML=monolayer and measured errors are in parentheses) Cleaning method
(111) surface (ML)
(100) surface (ML)
Contrad Acid Methanol
0.31 (0.02) 0.30 (0.02) 0.31 (0.02)
0.47 (0.04) 0.49 (0.04) 0.51 (0.04)
form of water molecules sitting on the surface. For a vacuum of 5×10−7 Torr the results of 0.3 and 0.5 ML for the (111) and (100) surfaces, respectively, are consistent with previous results [3,4]. The fact that there is no difference to the oxygen coverage for different cleaning methods suggests that the fractional monolayer of oxygen is bonded to the surface and is the preferred state of the diamond surface. This is also in agreement with possible OH groups on the (111) surface and C– O–C bridges on the (100) surface. Further work with channelling is necessary to find the lattice location of the oxygen and is presently under way.
References
Fig. 2. Backscattered energy spectrum of a diamond with the beam at the resonance energy.
more oxygen than we would have expected for this level of vacuum. This suggests that most of the oxygen was adsorbed before insertion into the chamber as opposed to bonded to the surface. This could possibly be in the
[1] S.H. Connell, J.P.F. Sellschop, R.D. Maclear, B.P. Doyle, I.Z. Machi, R.W.N. Nilen, J.E. Butler, K. Bharuth-Ram, Mater. Sci. Forum 258–263 (1997) 751. [2] D.G. Goodwin, J.E. Butler, in: M.A. Prelas, G. Popovici, L.K. Bigelow (Eds.), Handbook of Industrial Diamonds and Diamond Films, Dekker, New York, 1997, p. 527. [3] J.O. Hansen, T.E. Derry, P.E. Harris, R.G. Copperthwaite, J.P.F. Sellschop, Adv. Ultrahard Mater. Appl. Technol. 4 (1988) 76. [4] J.O. Hansen, R.G. Copperthwaite, T.E. Derry, J.M. Pratt, J. Colloid Interface Sci. 130 (1989) 347. [5] P.D. Townsend, J.C. Kelly, N.E.W. Hartley, in: Ion Implantation, Sputtering and Their Applications, Academic Press, New York, 1976, p. 181. [6 ] J.A. Leavitt, L.C. McIntyre, M.D. Ashbaugh, J.G. Oder, Z. Lin, B. Dezfouly-Arjomandy, Nucl. Instrum. Methods B 44 (1990) 260. [7] RUMP, Computer Graphic Service, Ithaca, NY, 1987. [8] W.J. Huisman, J.F. Peters, S.A. de Vries, E. Vlieg, W.S. Yang, T.E. Derry, J.F. van der Veen, Surf. Sci. 387 (1997) 342.