Analysis of surface electronic states based on the characteristics of surface interaction forces by atomic force microscopy

本研究的目的是建立一种利用超高真空非接触原子力显微镜(NC-AFM)表征表面电子态的新方法。在NC-AFM中，电子态是针尖与样品之间相互作用力的来源。相互作用主要由van der Waiils力、化学键合力和交换排斥力组成，对于针尖和样品的分离，它们具有不同的特征。由NC-AFM反馈电路控制的分离以及针尖和样品之间施加的偏置电压决定了产生图像对比度的主力。例如Si(111)7x7和Ge沉积的Si(111)表面。利用自制的NC-AFM，成功地观测到了NC-AFM和阻尼器的原子分辨图像。在特定的成像条件下，衰减图像的原子对比度比NC-AFM图像强得多。基于恒频移和恒幅条件下悬臂梁平均速度的下降，提出了一个解释强吸引相互作用引起的阻尼图像对比度的模型，并研究了对比度与偏置电压的关系。尽管在样品偏置电压小于1V时，缺陷和非缺陷的对比度差较小，但当样品的偏置电压负高于2V时，缺陷一半上的拐角吸附原子被描绘成尖峰状突起，并且这些吸附原子上的衰减大大降低。这表明，偏置电压的变化可以使针尖稳定地靠近样品表面，并在特定的表面原子上产生强烈的吸引相互作用，同时诱导电子共振。频移的微小变化导致了峰值对比的微弱变化。NC-AFM和衰减图像中的突起；可能是Si和Ge的混合对薄膜生长的影响。

The aim of this study is to establish a new method utilizing noncontact atomic force microscopy (nc-AFM) in ultrahigh vacuum for characterization of surface electronic states ; the electronic states are origins of the interaction force between tip and sample in the nc-AFM. The interaction is mainly composed of van der Waiils force, chemical bonding force and exchange repulsive force, which have different characteristics with respect to a separation between tip and sample. The separation controlled by nc-AFM feedback circuits and applied bias voltage between tip and sample determine the dominant force to produce the image contrast.Samples were Si(111)7x7 and Ge-deposited Si(lll) surfaces. Using a home-made nc-AFM, atom-resolved images of nc-AFM and damping were successfully observed. The damping images showed atomic contrast much stronger than that in nc-AFM images under particular imaging conditions. A model explaining the contrast in damping images caused by a strong attractive interaction was proposed, based on the decrease in averaged velocity of a cantilever under conditions of a constant frequency shift and a constant amplitude of the cantilever.The bias voltage dependence of the contrast was also examined. Although at sample bias voltages less than 1 V the contrast difference between faulted and unfaulted half was less, at sample bias voltages negatively higher than 2 V, corner adatoms on the faulted half were depicted as spiky protrusions, and the damping decreased much over these adatoms. This indicates that the bias voltage change can lead to tip stably approaching much closer to the sample surface and a strong attractive interaction over specified surface atoms as the electron resonance is induced. A slight change in frequency shift caused a faint change in contrast of the spiky.protrusions in nc-AFM and damping images ; the intermixing between Si and Ge on the film growth was possibly evidenced.