Oxidation of Nitrided Si(100) by Gaseous Atomic and Molecular Oxygen

Abstract

The nitridation of Si(100) by ammonia and the subsequent oxidation of the nitrided surface by both gaseous atomic and molecular oxygen was investigated under ultrahigh vacuum (UHV) conditions using X-ray photoelectron spectroscopy (XPS). Nitridation of Si(100) by the thermal decomposition of NH 3 results in the formation of a subsurface nitride and a decrease in the concentration of surface dangling bond sites. On the basis of changes in the N1s spectra obtained after NH 3 adsorption and decomposition, we estimate that the nitride resides about four to five layers below the vacuum-solid interface and that the concentration of surface dangling bonds after nitridation is only 59% of its value on Si(100)-(2 × 1). Oxidation of the nitrided surface is found to produce an oxide phase that remains in the outer layers of the solid and interacts only weakly with the underlying nitride for oxygen coverages up to 2.5 ML. Slight changes in the N1s spectra observed after oxidizing at 300 K are suggested to arise primarily from the introduction of strain within the nitride, and by the formation of a small amount of Si 2 dN-O species near the nitride-oxide interface. The nitrogen bonding environment changes negligibly after oxidizing at 800 K, which is indicative of greater phase separation at elevated surface temperature. Nitridation is also found to significantly reduce the reactivity of the Si(100) surface toward both atomic and molecular oxygen. A comparison of the oxygen uptake on the clean and nitrided surfaces shows quantitatively that the decrease in dangling bond concentration is responsible for the reduced activity of the nitrided surface toward oxidation, and therefore that dangling bonds are the initial adsorption site for both gaseous oxygen atoms and molecules. Increasing the surface temperature is found to promote the uptake of oxygen when O 2 is used as the oxidant, but brings about only a small enhancement in the uptake of gaseous O-atoms. The different effects of surface temperature on the uptake of O versus O 2 are interpreted in terms of the efficiency at which dangling bond pairs are regenerated on the surface at elevated temperature and the different site requirements for the adsorption of O and O 2 .

Oxidation of Pt(100)-hex-R0.7° by gas-phase oxygen atoms

“…Previous studies [18,21,22] provide convincing evidence that these oxygen atom beams primarily contain ground-state, neutral species. Furthermore, we estimate that the sample surface experiences a nearly uniform atomic oxygen flux with a lower bound of $4 · 10 12 cm À2 s À1 ($3 · 10 À3 ML s À1 ).…”

“…Previous studies [18,21,22] provide details of the threelevel UHV chamber utilized for the present experiments. The Pt(1 0 0) crystal employed in this study is a circular disk (8 mm · $1 mm) spot-welded to W wires and attached to a copper sample holder in thermal contact with a liquid nitrogen cooled reservoir.…”

Oxidation of Pt(111) by gas-phase oxygen atoms

“…The experiments were conducted in a three-level UHV chamber that reaches a base pressure less than 2 · 10 À10 Torr, as described previously [17]. Briefly, the chamber is evacuated by an ion pump (400 l/s), a turbomolecular pump (210 l/s) and a titanium sublimation pump that is inserted into a liquid-nitrogen cooled cryoshield.…”

Role of surface strain in the subsurface migration of adsorbates on silicon

“…[27][28][29][30] Dissociated species may occupy the same dimer or neighboring dimers, known respectively as intradimer and interdimer dissociations. 26,31 Adsorption of both NH 3 and H 2 O has been studied not only to functionalize a surface but also to unveil the initial stages of surface nitridation 18,19,[32][33][34][35][36] and oxidation. [27][28][29][37][38][39] For the NH 3 / Si͑100͒ system, in particular, a previous publication extensively reviewed the literature available.…”

“…Supporting this view, it has been shown that the thermal decomposition of H 2 O on Si͑100͒-2 ϫ 1 starts with the insertion of O atoms into the Si-Si dimer bond. 28 Although additional oxygen atoms can insert into the Si-Si backbond at higher temperatures, there is no evidence of oxygen migration beyond the second layer in the temperature range of 300-900 K. [27][28][29][37][38][39] On the other hand, complete decomposition of NH 3 on Si͑100͒-2 ϫ 1 results in N atoms residing between the third and fifth layers, [32][33][34] indicating that in this case subsurface migration is dominant. Moreover, other investigations have suggested that ͑Si͒NH 2 species undergo subsurface insertion, either preferentially or competing with surface insertion.…”