Diamond and Related Materials 8 (1999) 1620–1622www.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 coverageon low index planes. To increase the sensitivity of the measurements, elastic scattering using the 16O(a,a)16O resonance at3.045 MeV has been performed. This resonance can enhance, by up to 30 times, the Rutherford cross-section. The samples arenatural diamonds with either (111) or (100) surfaces. These have been cleaned using aqueous and acidic solvents to study theeffect cleaning has on the oxygen coverage. We have found the coverage to be dependent on the vacuum level of the targetchamber, with no effect being shown by the cleaning methods. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Diamond; Surface; Oxygen

1. Introduction RBS. It is therefore appropriate to use elastic scatteringat an energy where there is a useful resonance to improvethe detection limits.Both PC-CVD and natural diamond have been found

to have hydrogen in the bulk and on the surface usingtechniques such as elastic recoil detection analysis(ERDA) [1]. The role of oxygen in CVD growth of 2. Theorydiamonds is to reduce the concentration of the hydro-carbon nutrients such as CH3 by forming CO [2]. This Rutherford backscattering [5] involves the scatteringis stable and thereby removes excess carbon. Trace of light particles in the MeV range off a target. RBS hasamounts of OH and O present in the plasma react with been used extensively for accurate determination ofsp2 impurities far more readily than does atomic stoichiometry, elemental areal density, and impurityhydrogen. Oxygen therefore helps to remove sp2 defects distributions. Thin film elements may be identified byto give a purer CVD crystal. Natural diamond surfaces insertion of the measured energy Ei

1of the high-energy

have been studied using X-ray photoelectron spectro- side of the peak into ki

to calculate the kinematic factorscopy and Rutherford backscattering spectrometry k

ifor the ith element where k

iis given by

(RBS) and a submonolayer of oxygen has been foundki¬Ei

1/E

0(1)[3,4]. RBS is ideal for this purpose as the substrate

(carbon) is lighter than the contamination. Therefore where E0 is the incident ion laboratory energy andthe contamination peak is separated in the spectrumfrom the bulk as the backscattered energy increases with k

i=C(M2

i−M2

1sin2 h)1/2+M

1cos h

M1+M

iD1/2 (2)

the mass of the particle struck [see Eq. (2)]. With oxygenhaving a mass close to that of carbon and there being

h is the laboratory angle through which the incident ionof the order of a monolayer on the surface of theis scattered, and M1 and M

iare the incident and targetdiamond, the experiment is at the detection limit of

particle masses, respectively.The yield of backscattered particles is dependent on

the geometry of the set-up, the density of the target, the* Corresponding author. Fax: +27-112292144.E-mail address: rebuli@src.wits.ac.za (D.B. Rebuli) fraction of the total number of particles which hit the

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1621D.B. Rebuli et al. / Diamond and Related Materials 8 (1999) 1620–1622

detector and the cross-section of the reaction. For RBS diamonds were then rinsed in de-ionised water, exceptfor those cleaned in methanol which were not rinsed.the cross-section is the Rutherford cross-sectionThey were mounted in the target chamber immediately.

The a beam was focused to a spot size of <1 mmsR(E, h)=AZ1Zie2

4E B2 using the Schonland Microprobe. A smaller beam thanthis was not required as the samples have relatively largesurfaces and a representation of the whole surface was×

4[(M2i−M2

1sin2h)1/2+M

icos h ]1/2

Mi

sin4h(M2i−M2

1sin2h)1/2

. (3)required. A silicon surface barrier detector was placedin the target chamber at a scattering angle of 150°. TheThe 16O(a,a)16O resonance at 3.045 MeV was used as itsamples were normal to the incident beam.gives an enhanced cross-section for the detection of a

particles at backward angles over the Rutherford cross-section. This enhancement can be up to 30 times greater

4. Resultsdepending on the angle of the detector with referenceto the incident beam [6 ]. This enhancement results in a

The data were analysed using the analysis packagehigher yield and therefore a greater peak to backgroundRUMP [7] with data files modified for the oxygenratio, i.e. improved detection limits.resonance and the non-Rutherford carbon cross-sec-tions. The errors in these measurements can come fromthe uncertainty in the cross-section and the statistics of3. The experimentthe measurements. The error in the cross-section canarise from the error in the beam energy, where theA Duoplasmatron ion source was used to give an aSiO2 is used to limit this, and from the knowledge ofbeam. The beam was then accelerated using the 6 MVthe cross-sections at a certain energy. The cross-sectionsEN Tandem Van der Graaff Accelerator at theare well documented in the literature [6 ], so the largestSchonland Research Centre. To calibrate the energy oferror arises from the statistics of the measurements.the a beam a piece of SiO2 was placed in the targetTwo sets of data were taken. The first was with achamber. We were then able to fix the energy such thatvacuum of 4×10−6 Torr. The results for the differentthe resonance corresponded to the surface of the targetcleaning methods and surfaces can be seen in Table 1.(see Fig. 1). A higher beam energy would result in theThe sample which was heated was prepared in theresonance occurring in the bulk of the sample at a depthfollowing way: the sample was heated in an atmospherewhere the energy loss was that of the excess energy ofof argon to a temperature of 850°C for 30 min. It wasthe beam. This also allowed us to calculate the cross-allowed to cool in the argon. The sample was thensection of the resonance allowing quantitative analysisexposed to the atmosphere for 10 min before mountingof the data.in the target chamber. Heating to a temperature ofNatural diamonds polished with either (111) or (100)850°C has been shown to remove surface oxygen [8].surfaces were used. The diamonds were cleaned in one

The second set of data was taken with a vacuum ofof three ways: ultrasonically cleaned using methanol as5×10−7 Torr. The samples were re-cleaned before thea solvent; ultrasonically cleaned using Contrad, an indu-experiment was done. These results can be seen instrial chelating cleaning agent, as a solvent; boiled in aTable 2 (see Fig. 2).mixture of sulphuric, nitric and perchloric acids. The

5. Discussion

For a vacuum of 4×10−6 Torr there is no significantdifference in oxygen coverage for the different surfacesor for different cleaning methods. There is significantly

Table 1Samples measured in a vacuum of 4×10−6 Torr (where ML=mono-layer and measured errors are in parentheses)

Cleaning method (111) surface (ML) (100) surface (ML)

Contrad 1.1 (0.1) 1.1 (0.1)Acid 1.0 (0.1) 1.9 (0.2)Methanol 1.1 (0.1) 1.0 (0.1)

Fig. 1. Backscattered energy spectrum of the SiO2 standard with the Heated 1.3 (0.1) –beam at the resonance energy.

1622 D.B. Rebuli et al. / Diamond and Related Materials 8 (1999) 1620–1622

Table 2 form of water molecules sitting on the surface. For aSamples measured in a vacuum of 5×10−7 Torr (where ML=mono- vacuum of 5×10−7 Torr the results of 0.3 and 0.5 MLlayer and measured errors are in parentheses)

for the (111) and (100) surfaces, respectively, are consis-tent with previous results [3,4]. The fact that there isCleaning method (111) surface (ML) (100) surface (ML)no difference to the oxygen coverage for different clean-

Contrad 0.31 (0.02) 0.47 (0.04) ing methods suggests that the fractional monolayer ofAcid 0.30 (0.02) 0.49 (0.04)

oxygen is bonded to the surface and is the preferredMethanol 0.31 (0.02) 0.51 (0.04)state of the diamond surface. This is also in agreementwith possible OH groups on the (111) surface and C–O–C bridges on the (100) surface. Further work withchannelling is necessary to find the lattice location ofthe oxygen and is presently under way.

References

[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 DiamondFilms, 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,Fig. 2. Backscattered energy spectrum of a diamond with the beam at J. Colloid Interface Sci. 130 (1989) 347.the resonance energy. [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,more oxygen than we would have expected for this levelB. Dezfouly-Arjomandy, Nucl. Instrum. Methods B 44 (1990) 260.

of vacuum. This suggests that most of the oxygen was [7] RUMP, Computer Graphic Service, Ithaca, NY, 1987.adsorbed before insertion into the chamber as opposed [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.to bonded to the surface. This could possibly be in the