Ba deposition and oxidation on theta-Al2O3/NiAl(100) ultrathin films. Part II: O2(g) assisted Ba oxidation.

Ba deposition on a theta-Al(2)O(3)/NiAl(100) substrate and its oxidation with gas-phase O(2) at various surface temperatures are investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD) techniques. Oxidation of metallic Ba by gas-phase O(2) at 800 K results in the growth of 2D and 3D BaO surface domains. Saturation of a metallic Ba layer deposited on theta-Al(2)O(3)/NiAl(100) with O(2)(g) at 300 K reveals the formation of BaO(2)-like surface states. These metastable peroxide (O(2)(2-)) states are converted to regular oxide (O(2-)) states at higher temperatures (800 K). In terms of thermal stability, BaO surface layers (theta(Ba) < 5 ML) that are formed by O(2)(g) assisted oxidation on the theta-Al(2)O(3)/NiAl(100) substrate are significantly more stable (with a desorption/decomposition temperature of c.a. 1050 K) than the thick (2 < theta(Ba) < 10 ML) metallic/partially oxidized Ba layers prepared in the absence of gas-phase O(2), whose multilayer desorption features appear as low as 700 K.