Growth of Oxide on NiAl(100) and its Interaction with Au

• Main Authors: Shih-Hsun Chang, 張世勳
• Other Authors: Ker-Jar Song
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/de4x9e

碩士 === 國立中央大學 === 物理研究所 === 94 === We have used Temperature Programmed Auger (TPA), Low Energy Electron Diffraction (LEED) and Temperature Progammed Desorption (TPD) to study the growth of oxide on NiAl(100) surface and the interaction between Au and NiAl surface in an ultrahigh vacuum (UHV) chamber with a base pressure of 1×10-10 torr. The ordered Al2O3 grows through gas-phase oxidation on the NiAl(100) substrate with a periodic structure. We dosed oxygen onto surfaces held at temperatures ranging from 100K to 1400K. From LEED images, we found the optimum 1-dimensional oxide structure is obtained by dosing the surface at 1000K and 1100K. Of special interest is the observation of the c(√2×3√2)R45º structure after the surface is oxidized at 1400K. This means that at 1400K while the surface oxides layer become thicker under dosing, the surface undergoes phase separation such that a significant part of the surface become clean NiAl. When the annealing temperature is over 1400K, the surface oxides will desorb in Al2O form. This finding has never been reported previously. For samples “saturately” dosed with oxygen over 1000K, the desorption curves of Al2O all have the same leading edge and their trailing end all drops abruptly, indicating zero-order desorption kinetics. This simply indicates that multilayers of oxide formed. For Al/NiAl(100) system, the Al film exposed its fcc(111) structure below 550K. While the sample is heated to 600K~800K, LEED and TPA study reveal several ordered structures of alloys of Ni and Al which have specific composition ratios. In the case of Au/O/NiAl(100) system, we have deposited various coverage of Au (30sec, 60sec, 90sec, 120sec) onto the oxide film, and then observed the variation of Auger signals of Au, Ni, Al, O while annealing up to 1100K with heating rate of 2K/s. Below 600K, the Au film is stable. In between 600K~760K, Au signal drops while O, Al, Ni signals increase. We believe this is due to formation of Au 3-d islands on the oxide. At higher temperature of 760K~950K, oxygen signal drops and Au signals increase for thicker (90sec, 120sec) Au films. We suggest that the oxide covered area is reducing, and Au released from the 3-d islands are rewetting the exposed surface. When the temperature is raised to 950K~1100K, Au signal drops very quickly while that of Ni increasing quickly. As TPD shows no Au desorption whatsoever, these suggest that Au form alloy with NiAl. Later, when we subject the sample to repeated heating/cooling cycles in between 900 and 1100K, we found Au signal to drop when the temperature is raised and Au signal increases when the temperature is lowered. These complicated ups and downs should be due to competition among several tendencies: (1) there tend to be one Au layers wetting the surface. (2) Au tend to diffuse into the bulk at high temperature. (3) Au in the 3-d islands can replenish the loss of the wetting layer, and maybe (4) those Au that diffuse into the bulk at high temperature tend to segregate to the surface again at low temperature.