Mo(110)之碳化與Si(111)-7×7之化學反應

差排(dislocation)理論是研究晶體缺陷很有用的工具。過去多半的掃描穿隧電子顯微術(STM)實驗是以差排理論研究結構上較為簡單的面心立方晶體。這裡我們使用差排理論研究以STM在體心立方的過渡金屬Mo(110)表面上看到的表面缺陷，並討論它們合理的產生機制。 STM可以用來觀察導電樣品表面的原子結構，而過渡金屬的表面反應性可以藉由製作碳化層來改變。搭配低能量電子繞射儀(LEED)我們成功地解出Mo(110)表面在真實空間中的不同碳化結構。隨著碳化條件的不同，表面分別會有碳原子覆蓋度為1/30、1/24和1/16 ML的(50-26)、(12×4)–2C和(4×4)的碳化結構。此外，若C2H4曝量相同但退火時間增加，表面則會隨退火時間的增加由高覆蓋度的結構轉變成低覆蓋度的結構。此現象主要是碳原子在高溫時向樣品內擴散所造成。 STM也可以用來解析樣品對不同吸附氣體分子的反應性。甲醇和氨氣分子在與Si(111)-7×7表面進行吸附反應後，會在表面上形成兩個反應原子：一個矽adatom及其最近鄰的矽rest atom。由STM影像統計出氣體吸附前後表面影像對比有明顯變化位置的數量後，可以得到不同原子位置的氣體吸附比例，進而研究不同位置的矽原子跟這些氣體分子的反應活性。STM可以掃描模式使吸附在樣品表面的分子產生分解。當樣品偏壓低於-1.5 V時，吸附在Si(111)-7×7上的氨氣分子會被非彈性穿隧電子分解，計算氨氣分子在非彈性電子穿隧效應下的產率或是分解速率可以研究氨氣分子在矽表面的分解機制，有助於探討氮化矽薄膜的生成機制。

關鍵字

碳化鉬；插排；非彈性穿襚電子；低能量電子繞射儀；掃描探針顯微術

並列摘要

Dislocation theory is a powerful tool in the investigation of crystal defects. In the past dislocation theory was adopted to explain the results of scanning tunneling microscopy (STM) experiments on face-centered crystal surfaces. Here, we used the STM observations and dislocation theory to investigate the surface defects on body-centered cubic transition metal Mo(110) surface, and discussed the reasonable formation mechanism of them. STM can be used to resolve the surface structures of conductive samples. On the other hand, the surface reactivity of transition metal can be modified by forming carbide structures. Combining STM with low energy electron diffraction (LEED), we successfully determined three carbide structures on the Mo(110) surface. By different carburization conditions, the coverages of carbon of structures (50-26), (12×4)–2C and (4×4) formed on the Mo(110) surface are 1/30, 1/24 and 1/16 ML, respectively. Besides, increasing the annealing time will change a high-coverage structure to a low-coverage one. This is caused by the inward diffusion of carbon atoms at high temperatures. STM can also be used to observe the reactivity of the surface toward different adsorption molecules. When a methanol or an ammonia molecule interacts with the Si(111)-7×7 surface, two reacted sites will be produced: a Si adatom and its nearest neighboring rest atom. Comparing the STM images, prior to and after the exposure of gas molecule, and counting the reacted sites, we obtained the reacted ratio of different adsorption sites. Thus, the reactivities toward gaseous molecule between different Si atom sites can be studied. Furthermore, the dissociation of adsorbed molecules can be stimulated by a scanning STM tip. At sample voltages lower than □1.5 V, the absorbed ammonia molecules on the Si(111)-7×7 surface can be dissociated by the inelastic tunneling electrons. By counting the dissociation yield or rate of NH3 molecules by inelastic tunneling electrons, the dissociation mechanism of NH3 on the surface can be investigated, and this investigation is useful in interpretation of the growth mechanism of silicon nitride thin film on Si(111)-7×7 surface.

並列關鍵字

carburized Mo ； dislocation ； inelastic tunneling electron ； low energy electron diffraction ； scanning tunneling microscopy

參考文獻

[1] P. Avouris, and R. Wolkow, Phys. Rev. B 39, 5091 (1989). “Atom-resolved surface chemistry studied by scanning tunneling microscopy and spectroscopy”.

[2] F. Calleja, A. Arnau, J. J. Hinarejos, A. L. V. de Parga, W. A. Hofer, P. M. Echenique, and R. Miranda, Phys. Rev. Lett. 92, 206101 (2004). “Contrast reversal and shape changes of atomic adsorbates measured with scanning tunneling microscopy”.

[3] G. Dujardin, F. Rose, J. Tribollet, and A. J. Mayne, Phys. Rev. B 63, 081305 (2001). “Inelastic transport of tunnel and field-emitted electrons through a single atom”.

[4] J. Engbæk, J. Schiøtz, B. Dahl-Madsen, and S. Horch, Phys. Rev. B 74, 195434 (2006). “Atomic structure of screw dislocations intersecting the Au(111) surface: A combined scanning tunneling microscopy and molecular dynamics study”.

[5] K. Fukui, R.-L. Lo, S. Otani, and Y. Iwasawa, Chem. Phys. Lett. 325, 275 (2000). “Novel selective etching reaction of carbon atoms on molybdenum carbide by oxygen at room temperature visualized by scanning tunneling microscopy”.

國際替代計量

Mo(110)之碳化與Si(111)-7×7之化學反應

全文下載

主題瀏覽