Growth mechanism of the ultra-thin oxide films on metal substrates

金属基板上での極薄酸化膜の成長機構

Wilde, M.*; 福谷 克之*; 盛谷 浩右; 吉越 章隆 ; 寺岡 有殿

Wilde, M.*; Fukutani, Katsuyuki*; Moritani, Kosuke; Yoshigoe, Akitaka; Teraoka, Yuden

NiAl(100)表面酸化過程に見られる水素の影響をリアルタイムXPSを用いて調べた。酸素分子線及び水分子線を用いて表面を酸化し、膜厚が飽和した試料について酸化膜由来のAl信号強度と基板由来のAl信号強度の取り出し角依存性を測定した。その結果、水分子線を用いて酸化した試料の方がAl信号強度が強く、飽和酸化膜厚が厚いことがわかった。このことは、表面にOH基が形成されそれに基づく強い電場のために酸化が促進されるというモデルを支持すると考えられる。

Our previous real-time synchrotron X-ray photoelectron spectroscopy (XPS) investigations of the growth kinetics of ultra-thin AlO films on NiAl(100) in O have revealed a strong acceleration of the oxidation reaction by water. To clarify the oxidation mechanism, the present study examines whether the growth rate acceleration by HO correlates with an increased oxide thickness, as would be expected within the Cabrera-Mott model at large surface hydroxyl (OH) coverages. We thus evaluate the saturation thickness of the AlO films from the take-off angle dependence of Al2p XPS emission signals from the oxide overlayer (Al) and the metallic NiAl substrate (Al). The results confirm our expectation that at temperatures where OH is stable on AlO, HO oxidation should yield slightly thicker AlO films than those grown in O. This confirms our hypothesis that the rate acceleration by HO below 500K is due to its ability to provide a higher stationary oxidant (OH) coverage than O. Consistent with the thermal stability of surface OH groups, the HO-related effects decrease at higher oxidation temperatures.