An Auger and XPS Study on CVD and Natural Diamonds
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MATERIALS AND EXPERIMENTAL CVD diamond film ET100 manufactured by Norton Diamond Film, and type 2A natural diamonds with specific cuts, i.e., {11 }, { 110}, and {100}, by Drukker International, The Netherlands, were the subjects of the research effort. Highly oriented pyrolytic graphite (HOPG), grade ZYH, manufactured by Union Carbide, and the glassy carbon, manufactured by SGL, were the reference. HOPG and glassy carbon were chosen simply because they are the crystalline and amorphous carbon materials with sp2 bond character. Although the CVD diamond to be investigated was of high quality approaching that of bulk diamond in crystallite size (60 .tm) and well defined crystallite orientation (growth orientation is while { 111) and {100) are the surface facets), impurities such as Si, Al, and hydrogen were found enriched at the grain boundary [9]. Therefore, investigation on high purity natural diamonds with specific cuts lead to understand how impurity and heterogeneity influence the surface chemistry, a fundamental aspect of oxidation behavior. Oxygen treatment was carried out using an isothermal gravimetric analysis on a Setaram TAG24 Thermoanalyzer, in 95 kPa ultrahigh purity 02 at 420, 500, 550, and 575 'C for 170 min with a flow rate of 80 sccm. The natural diamonds and glassy carbon were examined asreceived (A.R.) after an ultrasonicate-rinse in methanol or ethanol. Auger and XPS of HOPG were obtained from the freshly cleaved {0002} using the conventional "Scotch Tape" method. Auger spectra were obtained using PHI 680 Auger Nanoprobe. The accelerating potential was 2 kV and the beam current was 1 nA. Auger spectra of clean CVD diamond surface was obtained from surface fractured in vacuo. XPS study was carried out using a PHI 5600 X-Ray Photoelectron Spectrometer with Al kcu at 15 keV. XP spectra were collected with the neutralizer on. Ion sputtering was conducted using Ar gas at 3.5 kV rastered an area of 2x2 mm 2 which produced a sputtering rate of 200 A/min. RESULTS Figure 1 presents the Auger spectra of natural diamonds, CVD diamonds, HOPG and glassy carbon. Spectra of CVD diamonds were collected from atomically clean surface and specimens were treated in oxygen at temperatures of 420, 550, and 575 TC. Although it is difficult to give a full and unambiguous interpretation of the Auger transition in solids, a comparison between the Auger fine structure and predominant features in the band structure is possible. Ramaker showed that theoretically, carbon KVV Auger lines shapes are the convolution of the main peak generated by the normal KVV process as well as up to four satellite peaks due to the resonant electron excitations [13]. Peak shifts observed in the diamond spectra were mainly due to charging. Charging masked the subtle change of peak position caused by the change of valence band structure [11,12]. We thus limited our discussion to the change of line shape changes of A0 at 275 eV and the major satellite peak At at - 262 eV. The three natural diamond spectra, collected from as-received surfaces of {II
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