Gerrard Alex L, Chen Jau-Jiun, Weaver Jason F
Department of Chemical Engineering, P.O. Box 116005, University of Florida, Gainesville, Florida 32611, USA.
J Phys Chem B. 2005 Apr 28;109(16):8017-28. doi: 10.1021/jp044434i.
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 NH3 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 NH3 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 x 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 Si2=N-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 O2 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 O2 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 O2.
在超高真空（UHV）条件下，使用X射线光电子能谱（XPS）研究了氨对Si(100)的氮化作用以及随后氮化表面被气态原子氧和分子氧氧化的过程。通过NH₃的热分解对Si(100)进行氮化会导致形成亚表面氮化物，并使表面悬空键位点的浓度降低。根据NH₃吸附和分解后获得的N1s光谱变化，我们估计氮化物位于真空 - 固体界面以下约四到五层，并且氮化后表面悬空键的浓度仅为其在Si(100)-(2×1)上的值的59%。发现氮化表面的氧化会产生一种氧化物相，该氧化物相保留在固体的外层，并且对于高达2.5 ML的氧覆盖量，与下面的氮化物仅发生微弱相互作用。在300 K氧化后观察到的N1s光谱的轻微变化主要被认为是由于氮化物内部应变的引入以及在氮化物 - 氧化物界面附近形成少量Si₂=N - O物种所致。在800 K氧化后，氮的键合环境变化可忽略不计，这表明在升高的表面温度下有更大的相分离。还发现氮化会显著降低Si(100)表面对原子氧和分子氧的反应性。对清洁表面和氮化表面上的氧吸收进行比较定量显示，悬空键浓度的降低是氮化表面氧化活性降低的原因，因此悬空键是气态氧原子和分子的初始吸附位点。当使用O₂作为氧化剂时，发现提高表面温度会促进氧的吸收，但对气态O原子的吸收仅带来很小的增强。表面温度对O和O₂吸收的不同影响是根据高温下表面悬空键对再生的效率以及O和O₂吸附的不同位点要求来解释的。
