原子島在金屬/半導體介面的成長研究 (以鈷/銀/鍺(111)為例)

Study of atomic island growth on metal/semiconductor interfaces (as example of Co/Ag/Ge(111))

黃筱嵐(Xiao-Lan Huang)

指導教授 ： 傅祖怡

國立臺灣師範大學/理學院/物理學系/博士(2012年)

摘要

The thermal reaction of Co on Ag/Ge(111)-(√3×√3)/(4×4) phases was studied by scanning tunneling microscopy, low energy electron diffraction, and Auger electron spectroscopy. Firstly, we address on the controversies over the chemical composition of Co islands by examining the thermal reaction of Co on "Ag/Ge(111)-" √3×√3 phase, as well as the coexisting Ag/Ge(111)-4×4 phase. From the study, one finds that Ag atoms shift from (4×4) phase to (√3×√3) phase because of the interaction between Co and the surface. The fact suggests that it is on the surface where Ag-less phase (4×4) transforms into Ag-richer phase (√3×√3). Secondly, we proof that (√13×√13) periodicity is composed of Co-Ge alloy, whereas (2×2) periodicity is composed of pure Co. Thirdly, we realize that it is "Ag/Ge(111)-" √3×√3 preventing Co from diffusing into substrate when annealing the surface at the temperature between 320 K and 730 K.
It is known that Co"-" 2×2 islands grown on Ag/Ge(111)-√3×√3 surface are in hcp structure with a (11-20) orientation. The island evolution involves the shape transformation of a unit cell from parallelogram into rectangular. Meanwhile, the shape of the island shifts from hexagonal to stripe. In additions, it is identified that Co-2×2 islands grow along two crystallographic directions: pseudo-[0001] and pseudo-[1-100]. We observe a lateral shift between the topmost and the underlying bilayers for islands which grow along pseudo-[0001] direction. On the other hands, no lateral shift is perceived for those growing along pseudo-[1-100] direction.
In terms of the strain–relaxation of Co-2×2 islands grown on Ag/Ge(111)-√3×√3 surface, we analyze the images taken by scanning tunneling microscopy. From the studies, one realizes a common fact that Co"-" 2×2 islands adopt a more compact arrangement than Ge(111) substrate does, whereas each Co-2×2 island is different in the degree of atomic compactness. Yet, we do not observe any distinct relationship between strain–relaxation and the island height. In addition, we identify three different groups of islands from analyzing the correspondence between the strain–relaxation and the island size: (i) small islands (less than 80 nm2) with fixed inter-row distances in high atomic compactness, (ii) small islands with unfixed inter-row distances, and (iii) big islands (bigger than 80 nm2) with fixed inter-row distances in less compact atomic arrangement, as compared to the first two groups. Based on the obtained information, we propose the model that explains the relationship between the strain–relaxation and the island size.
Regarding electronic structure, we study "Ag/Ge(111)-" 4×4 phase, "Ag/Ge(111)-" √3×√3 phase, Co"-" 2×2 island, and "CoxGey-" √13×√13 island by means of scanning tunneling spectroscopy at room temperature. Similar to the one acquired from "Ge(111)-c" 2×8, the spectrum obtained from Ag/Ge(111)-4×4 structure reveals a shoulder at 0.7 V, which indicates that Ge adatoms were donated to the electronic states of the Ag-driven phase. However, the electronic spectrum taken from the "CoxGey-" √13×√13 island shows a large number of peaks, which indicates the complex bonding between "CoxGey-" √13×√13 island and the substrate. In addition, the spectra obtained from the Co-2×2 island grown on the step demonstrate a number of peaks at negative sample bias, which is different comparing to those taken from the Co-2×2 island located on the terrace. The phenomenon explains the various Co-substrate interactions, which are accompanied with the growth of Co islands at different areas of the stepped surface.

關鍵字

Co ； Ge(111) ； Ag ； STM ； epitaxy ； phase transformation

並列摘要

The thermal reaction of Co on Ag/Ge(111)-(√3×√3)/(4×4) phases was studied by scanning tunneling microscopy, low energy electron diffraction, and Auger electron spectroscopy. Firstly, we address on the controversies over the chemical composition of Co islands by examining the thermal reaction of Co on "Ag/Ge(111)-" √3×√3 phase, as well as the coexisting Ag/Ge(111)-4×4 phase. From the study, one finds that Ag atoms shift from (4×4) phase to (√3×√3) phase because of the interaction between Co and the surface. The fact suggests that it is on the surface where Ag-less phase (4×4) transforms into Ag-richer phase (√3×√3). Secondly, we proof that (√13×√13) periodicity is composed of Co-Ge alloy, whereas (2×2) periodicity is composed of pure Co. Thirdly, we realize that it is "Ag/Ge(111)-" √3×√3 preventing Co from diffusing into substrate when annealing the surface at the temperature between 320 K and 730 K.
It is known that Co"-" 2×2 islands grown on Ag/Ge(111)-√3×√3 surface are in hcp structure with a (11-20) orientation. The island evolution involves the shape transformation of a unit cell from parallelogram into rectangular. Meanwhile, the shape of the island shifts from hexagonal to stripe. In additions, it is identified that Co-2×2 islands grow along two crystallographic directions: pseudo-[0001] and pseudo-[1-100]. We observe a lateral shift between the topmost and the underlying bilayers for islands which grow along pseudo-[0001] direction. On the other hands, no lateral shift is perceived for those growing along pseudo-[1-100] direction.
In terms of the strain–relaxation of Co-2×2 islands grown on Ag/Ge(111)-√3×√3 surface, we analyze the images taken by scanning tunneling microscopy. From the studies, one realizes a common fact that Co"-" 2×2 islands adopt a more compact arrangement than Ge(111) substrate does, whereas each Co-2×2 island is different in the degree of atomic compactness. Yet, we do not observe any distinct relationship between strain–relaxation and the island height. In addition, we identify three different groups of islands from analyzing the correspondence between the strain–relaxation and the island size: (i) small islands (less than 80 nm2) with fixed inter-row distances in high atomic compactness, (ii) small islands with unfixed inter-row distances, and (iii) big islands (bigger than 80 nm2) with fixed inter-row distances in less compact atomic arrangement, as compared to the first two groups. Based on the obtained information, we propose the model that explains the relationship between the strain–relaxation and the island size.
Regarding electronic structure, we study "Ag/Ge(111)-" 4×4 phase, "Ag/Ge(111)-" √3×√3 phase, Co"-" 2×2 island, and "CoxGey-" √13×√13 island by means of scanning tunneling spectroscopy at room temperature. Similar to the one acquired from "Ge(111)-c" 2×8, the spectrum obtained from Ag/Ge(111)-4×4 structure reveals a shoulder at 0.7 V, which indicates that Ge adatoms were donated to the electronic states of the Ag-driven phase. However, the electronic spectrum taken from the "CoxGey-" √13×√13 island shows a large number of peaks, which indicates the complex bonding between "CoxGey-" √13×√13 island and the substrate. In addition, the spectra obtained from the Co-2×2 island grown on the step demonstrate a number of peaks at negative sample bias, which is different comparing to those taken from the Co-2×2 island located on the terrace. The phenomenon explains the various Co-substrate interactions, which are accompanied with the growth of Co islands at different areas of the stepped surface.

並列關鍵字

Co ； Ge(111) ； Ag ； STM ； epitaxy ； phase transformation

參考文獻

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