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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.
The interface structure plays an important role in the growth and control of epitaxial thin films, but observing the interface structure of the thin films at the atomic scale has been a challenging issue because of the strong attenuation and decoherence of incident waves across the thin film, Which makes the detected interface signal very weak.The appearance of Scanning Tunneling Microscopy (STM) has greatly promoted the research on the interface structure of thin films.It has been reported in the early research that the typical Si (111) - 7 × 7 super structure.Although the 7 × 7 super structure is clearly visible, the 12 Si atoms in the cell can not be distinguished from it, so the interface structure imaging at the atomic scale has not yet been realized.In this paper, the Si (111) -7 × 7, a high quality Cd (0001) epitaxial film was grown on the surface of the Cd (0001) film and the interface structure of Cd (0001) thin film was imaged by low temperature scanning tunneling microscopy.At low bias voltage, Si 111) -7 × 7 structure, which also shows that the substrate structure of Si (111) -7 × 7 can be completely preserved during the growth of Cd film.In addition, it is also found that due to the quantum size effect, the even layer thin And odd-layer films exhibit significantly different lateral resolution and surface roughness, and the differences in these properties are reversed with changes in scan bias. This excellent transparency of Cd (0001) films is attributed to the vertical The speed of motion is much greater than the speed of in-plane motion, ie, the effective mass of highly anisotropic electrons.