電腦模擬Si(100)表面的Ge原子成長及排列形式

Abstract

本文主要是研究Ge原子在Si(100)表面上的成長過程及在Ge/Si(100)表面上的Ge原子排列形式。之前的實驗觀測顯示Ge原子是以隨機換置Si原子的方式成長在Si(100)表面上，被換置的Si原子則會沿著雙原子單體列（dimer rows）擴散至表面台階邊緣，使得表面台階漸漸擴大覆蓋住原來的表面，而成長在原本表面的Ge原子則會因此被掩埋成為在第二層原子層的Ge原子，在這個成長過程中與Ge原子的內擴散（interdiffusion）並無關聯。根據模擬的結果發現，當Ge原子的數量為1 ML時，有40 %的Ge原子Ge/Si(100)表面下；若繼續將Ge原子的數量增加，會有更多的Ge原子存在於表面原子層之下；當Ge原子的數量為2 ML時，有1.25 ML的Ge原子在Ge/Si(100)表面下，其中有0.4 ML的Ge原子在第三層原子層這。些模擬的結果與實驗結果有良好的契合度。 有實驗觀測指出當Ge原子的數量為0.28 ML時，Ge原子成長在表面上的位置有某種關聯性，而造成在表面上有很長的zigzag dimer row的情形。在此模擬的結果顯示，當Ge原子在Si(100)表面上的數量極少時（約0.05 ML），表面上的zigzag dimer row的平均長度為22.4單位長度，且此時的zigzag dimer row的長度分佈是相當寬廣的；隨著Ge原子的數量增加，表面上的zigzag dimer row的平均長度會跟著變短；當在表面上的Ge原子數量達到0.5 ML時，zigzag dimer row的平均長度為4.7單位長度，此時的zigzag dimer row的平均長度在Ge原子數量介於0 ~ 1 ML是最短的，且zigzag dimer row的長度分佈變得幾乎集中在1 ~ 6單位長度之間。這些模擬的結果也與實驗結果有良好的關聯性。

This thesis studied the growth process of Ge on the Si(100) surface and the Ge site correlation on the Ge/Si(100) surface by simulation. It is known that deposited Ge atoms randomly replace Si atoms on the Si(100) surface. Si atoms that are replaced diffuse along dimer rows to nearby steps. Therefore, the step edges would gradually advance forwards. A fraction of deposited Ge atoms already on the surface is buried in the second atomic layer by this process. In this growth scenario, the indiffusion of Ge atoms is not involved. At 1-ML Ge coverage, the simulation results also showed that 0.4-ML deposited Ge atoms reside in the second atomic layer. If the Ge coverage is more than 1 ML, the deposited Ge atoms started to occupy at the third atomic layer. At 2-ML Ge coverage, 0.4 ML deposited Ge atoms are in the third atomic layer and 0.85 ML deposited Ge atoms are in the second atomic layer. These simulation results are in good agreement with existing experimental results. Previous reports show that the deposited Ge atoms have a very long Ge site correlation length at coverage of 0.28 ML, leading to the formation of zig-zag trains in dimer rows. At Ge coverage of 0.05 ML, the simulation results showed that the site correlation length (the length of the zig-zag trains in the STM images) of the deposited Ge atoms is found to be 22.4 unit length, and the length distribution is wide. As the Ge coverage increases, the site correlation length decreases. At Ge coverage of 0.5 ML, the site correlation length is found to be 4.7 unit length, which is the shortest for the coverage between 0 ~ 1 ML. These simulation results also correlate well with the experimental results.