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在超高真空条件下利用非接触原子力显微镜（nc-AFM）表征表面电子态的新方法；电子态是nc-AFM中针尖与样品相互作用力的来源。相互作用主要由范德威力、化学键合力和交换排斥力组成，它们在针尖与样品分离时具有不同的特征。由nc-AFM反馈电路控制的分离和针尖与样品之间施加的偏置电压决定了产生图像对比度的主导力。样品为Si(111)7x7和ge沉积Si（ll）表面。利用自制的纳米afm，成功地观察了纳米afm和阻尼的原子分辨图像。在特定的成像条件下，阻尼图像显示出比nc-AFM图像更强的原子对比度。基于恒定频移和恒定幅值条件下悬臂梁平均速度的下降，提出了一个解释由强引力相互作用引起的阻尼图像对比度的模型。对比剂的偏置电压依赖性也被检查。虽然在样品偏置电压小于1 V时，故障和未故障的一半之间的对比度差异较小，但在样品偏置电压负高于2 V时，故障一半的角状附着原子被描绘为尖状突起，并且这些附着原子的阻尼大大降低。这表明，偏置电压的变化可以导致尖端稳定地接近样品表面，并在诱导电子共振时在特定表面原子之间产生强烈的吸引相互作用。频移的轻微变化会引起尖状物对比度的微弱变化。nc-AFM和阻尼图像中的突起；在薄膜生长过程中可能存在Si和Ge的混合作用。

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.