Oxidation of heated diamond C(100): H surfaces

This paper extends a previous study (Pehrsson and Mercer, submitted to Surf . Sci.) on unheated, hydrogenated, natural diamond (100) surfaces oxidized with thermally activated oxygen (O-2(.)). In this paper, the oxidation is p erformed at substrate temperatures from T-sub = 24 to 670 degrees C. The di amond surface composition and structure were then investigated with high re solution electron energy loss spectroscopy (HREELS), Auger electron spectro scopy (AES), electron loss spectroscopy (ELS) and low energy electron diffr action (LEED). The oxygen coverage (theta) increased in two stages, as it did during oxida tion at T < 80 degrees C. However, there are fundamental differences betwee n the oxidation of nominally unheated and heated diamond surfaces. This dif ference is attributed to simultaneous adsorption and rapid desorption of ox ygen species at higher temperatures; the desorption step is much slower wit hout heating. The initial oxidation rates were similar regardless of the su bstrate temperatures, but the peak coverage (theta) was lower at higher tem peratures. For example, a plateaued at 0.4 +/- 0.1 ML at 600 degrees C. The lower saturation coverage is again attributed to oxygen desorption during oxidation. Consistent results were obtained on fully oxidized surfaces, whi ch when heated in vacuum to T-sub = 600 degrees C, lost similar to 60% of t heir adsorbed oxygen. ELS revealed few C=C dimers on the oxidized surfaces, and more graphitization than on unheated surfaces. Oxidation at elevated t emperatures also increased the carbonyl to ether ratio, reflecting etching- induced changes in the types of surface sites. The carbonyl and C-H stretch frequencies increased with oxygen dose due to fonnation of higher oxidatio n states and/or hydrogen bonding between adjacent groups. The oxygen types did not interconvert when the oxidized surfaces were heated in vacuum. Oxyg en desorption generated a much more reactive surface than heating-induced d ehydrogenation of the smooth, hydrogenated surface. (C) 2000 Elsevier Scien ce B.V. All rights reserved.