氫分子在矽鍺合金表面熱脫附現象之研究

Abstract

本論文在研究氫分子在鍺/矽(100)表面的熱脫附現象，所使用的樣品為覆蓋率0.4 ML和0.8 ML鍺的矽(100)表面。實驗方法是利用掃瞄穿遂顯微鏡 (STM) 以及核心層光電子激發術 (core-level photoemission) 觀測氫脫附過後的樣品表面，以探討氫分子從表面上的Ge-Ge、Ge-Si以及Si-Si雙原子單體 (dimer) 結構而進行熱脫附的機制。氫分子熱脫附後，會在鍺矽表面留下懸結鍵對 (dangling bond pairs, DB pairs），因此氫分子熱脫附的數量，即是在STM觀測影像上所直接計數的懸結鍵對數目。而在核心層光電子激發術的實驗中，我們將氯原子連結到氫熱脫附後的懸結鍵上。然後由Cl 2p的核心層光電子能譜的Cl-Ge分量與Cl-Si分量，可推得懸結鍵在鍺或矽原子上的分佈情形。結合掃瞄STM與核心層光電子激發術這兩種互補的顯微與光電子能譜技術，可以對氫分子熱脫附的機制進行更詳細的分析。 Ge-H鍵的鍵結能比Si-H鍵要弱。在理想狀態下，在較低的加熱溫時氫分子應從Ge-Ge雙原子單體開始脫附，在稍高的溫度由Ge-Si脫附，在更高的溫度才會由Si-Si脫附。假設氫分子只是單純的由鍺/矽(100)表面的雙原子單體結構進行熱脫附，那麼在較低的溫度時，氫分子大部分是由Ge-Ge與Ge-Si雙原子單體上脫附。因此在Cl 2p核心層光譜中，Cl-Ge分量應大於Cl-Si分量。但是在實驗結果的分析，卻是Cl-Si分量大於Cl-Ge分量。而最近有文獻指出，氫分子在熱脫附的過程中表面的鍺矽原子可能會互相交換，使得大部分的Ge-Ge雙原子單體轉換成Ge-Si雙原子單體。根據這個論點來進行分析，所推得的結果與我們的實驗大致符合，因此我們判斷在氫分子熱脫附的過程中，表面的鍺原子的確會與矽原子進行交換。

We have studied the hydrogen molecular desorption from the 0.4-ML- and 0.8-ML-Ge/Si(100) surfaces. The scanning tunneling microscopy (STM) and core-level photoemission are employed to observe the surfaces after H2 desorption and study the mechanism of the H2 desorption from the Ge-Ge, Ge-Si, and Si-Si dimers. The dangling bond (DB) pairs will reappear on the Ge/Si surface after H2 desorption, so the number of DB pairs in the STM images equals the coverage of H2 desorbed from the surface. In the core-level-photoemission experiment, we terminated the DB pairs by the chlorine atoms before taking spectra. The analysis of the Cl-Ge and Cl-Si components in the Cl 2p core-level spectra could figure out the DBs upon Ge atoms and Si atoms. Combining these two powerful microscopic and spectroscopic techniques, the detailed information about the mechanism of H2 desorption can be obtained. The energy of a Ge-H bond is lower than that of a Si-H bond. Therefore in ideal case, the hydrogen molecules will start to desorb from Ge-Ge dimers at a lower temperature, from Ge-Si dimers at a middle-high temperature, and from Si-Si dimers at a high temperature. Assuming that hydrogen molecules simply desorb from the dimer structure, the most hydrogen molecules will desorb from the Ge-Ge and Ge-Si dimers at lower temperature. Consequently, the intensity of the Cl-Ge component will be larger than that of the Cl-Si component in Cl 2p core-level spectra. However, the experimental result indicated that the intensity of the Cl-Si component was larger than that of the Cl-Ge component. The recent reports suggested that the exchange of Ge and Si atoms will occur during the H2 desorption from Ge/Si surface making the most Ge-Ge dimers transform into the Ge-Si dimers. According to this argument, the result is roughly in agreement with our experiment. Therefore, the replacement of Ge/Si atoms likely occurs during H2 desorbing from the Ge/Si(100) surface.