摘要：
The interracial structure plays a crucial role in controlling the growth of epitaxial film.However,it is still a formidable task to observe the interface structure at the atomic scale,because the detected signals from the interface become very weak due to the strong attenuation and decoherence when the incident wave passes through the films.With the advent of scanning tunneling microscopy (STM),significant progress has been made in observing the interface structure of thin films.Many studies have demonstrated the typical Si(111)-7×7 superstructure below the metal films in the past.Although the 7×7 superstructures are visible,the 12 Si adatoms in the unit cells can not be resolved.Hence,imaging the 7×7 superstructures below thin films at the atomic scale has not been achieved.In this work,we have grown the Cd epitaxial films with high quality on Si(111)-7×7 substrate,and realized the imaging of interface structure of Cd(0001) at atomic scale by using low temperature scanning tunneling microscopy.Under low bias voltage,we can clearly observe the atomic image of Si(111)-7×7,indicating the Si (111)-7×7 substrate remains intact during the growth process of Cd films.Moreover,due to the quantum size effect,the Cd films with even or odd monolayers exhibit distinct lateral resolution as well as surface roughness,even at the same bias voltage.With the bias variation,such kind of diversities can be reversed.We attribute the highly transparency of Cd(0001) films to the anisotropic effective mass of the electrons in Cd films,i.e.the transverse motion is much faster than the in-plane motion.%界面结构对外延薄膜的生长和控制起着非常重要的作用.但是在原子尺度上观察薄膜的界面结构一直是一个挑战性的课题.原因是入射波穿越薄膜时会发生强烈的衰减和退相干,使得检测的界面信号非常微弱.扫描隧道显微镜(Scanning Tunneling Microscopy,STM)的出现极大地促进了人们对薄膜界面结构方面的研究.早期的研究报道观察到了金属薄膜下的典型的Si(111)-7×7超结构.虽然7×7超结构清晰可见,但从中不能分辨出元胞中的12个Si原子,因此仍未实现原子尺度上的界面结构成像.本文在Si(111)-7×7表面上生长出高质量的Cd(0001)外延薄膜,并利用低温扫描隧道显微镜对Cd(0001)薄膜的界面结构实现了原子尺度成像.在较低的偏压下,清晰地观测到Si(111)-7×7结构的原子分辨像,这也说明了在Cd薄膜生长过程中,Si(111)-7×7的衬底结构得以完整保存.此外还发现,由于量子尺寸效应,偶数层薄膜和奇数层薄膜表现出明显不同的横向分辨率和表面粗糙度,而且这些性质的差异随扫描偏压的变化会发生逆转.把Cd(0001)薄膜的这种优异透明性归因于电子的垂直运动速度远大于面内运动速度,即电子具有高度各向异性的有效质量.
