| 摘要: | 在超高真空的環境下(約1×10-10 torr),我們利用可程式控溫歐傑能譜(Temperature Programmed Auger, TPA)，低能量電子繞射(LEED)和可程式控溫熱脫附，來研究鎳鋁合金(100)表面上氧化鋁(Al2O3)的結構以及金與鎳鋁合金表面的交互作用。氧化鋁有序結構的成長，是經由氣相氧化於鎳鋁表面上所形成具有週期性排列的結構。實驗中，我們分別在不同的溫度曝氧(100K~1400K)，並藉由LEED的影像發現在1000K與1100K的條件下曝氧，可形成最佳之一維有序結構。另外，比較特別的是在1400K曝氧後，可以觀察到c(√2×3√2)R45º的結構，意謂著除了在此溫度下會形成更厚的氧化層外，表面上還會曝露出一部份乾淨的鎳鋁面。當加熱溫度超過1400K時，表面的氧化物會以Al2O的形式脫附，以前從尚未有人報導過。溫度1000K以上飽和曝氧樣品的脫附曲線皆有相同前緣及暴落之後緣，這符合了零階脫附(zero-order desorption)的形式，代表已形成多層氧化物。在鋁/鎳鋁(100)系統中，溫度低於550K時表面形成面心結構鋁(111)的面，而加熱至600K~800K後，從TPA與LEED的對照中，發現了多種鎳與鋁有特定比例的結構。至於在金/氧/鎳鋁(100)系統中，我們分別鍍了不同量的金(30sec，60sec，90sec，120sec)在氧化層上，然後以2k/s的加熱速率熱至1100K，並監測金，鎳，鋁及氧的歐傑訊號隨溫度的變化。實驗中發現在600K~760K範圍，金的歐傑訊號會衰減，而氧，鋁，鎳的訊號則增加。應是金在氧化物上聚成較大的三維島。進一步的加熱 (760K~950K)，氧的訊號降低而較厚金膜(90sec，120sec)的金訊號增加，應是氧化物覆蓋面積減少導致金至三維島釋出取代氧化物的結果。再升溫至950K~1100K，由於鎳的歐傑訊號增加且金的訊號減小，我們認為金與鎳鋁形成合金。之後，我們將樣品在900K~1100K之間反復升降溫，發現了金訊號會隨著升溫而減少，降溫而增加。應該是金傾向沾覆表面，金在高溫會擴散至鎳鋁塊材中，金會自三維島釋出以補滿沾覆層，及在低溫時金可能自塊材中再析出等幾種趨勢交互作用之結果。 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. |
