Germanium Nanostructures on Silicon Observed by Scanning Probe Microscopy
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Germanium
Nanostructures on Silicon Observed by Scanning Probe Microscopy Masahiko Tomitori and Toyoko Arai
Abstract Scanning tunneling microscopy and noncontact atomic force microscopy have been used to observe germanium growth on Si(001) and Si(111). The atomically resolved images provide invaluable information on heteroepitaxial film growth from the viewpoints of both industrial application and basic science. We briefly review the history of characterizing heteroepitaxial elemental semiconductor systems by means of scanning probe microscopy (SPM), where the Stranski–Krastanov growth mode can be observed on the atomic scale: the detailed phase transition from layer-by-layer growth to three-dimensional cluster growth was elucidated by the use of SPM. In addition, we comment on the potential of SPM for examining the spectroscopic aspects of heteroepitaxial film growth, through the use of SPM tips with well-defined facets. Keywords: AFM, atomic force microscopy, crystal growth, elemental semiconductors, germanium, nanostructures, scanning probe microscopy, scanning tunneling microscopy, silicon, SPM, STM.
Introduction It is widely accepted that scanning probe microscopy (SPM)1 has greatly contributed to the development of nanoscale materials science and technology. Using SPM, we can obtain information on surface topography and simultaneously measure the physical properties of the surface with atomic resolution. In the early 1980s, the advent of scanning tunneling microscopy (STM)2 as the first member of the SPM family of techniques was a landmark achievement in the field of high-resolution microscopies: a mechanically sharpened tip was able to precisely depict a sample surface with atomic resolution. Moreover, the more recent technique of noncontact atomic force microscopy (nc-AFM)3,4 can also achieve atomic resolution, even on low-conductivity and insulating materials;5 this noticeable feature has not been achieved by any other microscopic technique to date. With SPM,
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the development of a mechanical scanning method using an atomically sharp tip is leading us into a fascinating “nanoworld.” In materials science and technology on the nanoscale, the fabrication and evaluation of heteroepitaxial nanostructures have attracted much interest for realizing novel optoelectronic devices, in particular, by direct observation and manipulation using SPM. In general, nanostructures fabricated on surfaces are surrounded by characteristic crystallographic small planes (i.e., facets) in such a way that the total energy around the nanostructures decreases under pseudoequilibrium conditions. In principle, the optoelectronic properties concerning quantum states—for example, light emission with a blueshift—are governed by the shape, atomic arrangement, and composition of nanostructures. Thus, three-dimensional (3D) topographic observation is indispen-
sable for characterizing the properties of faceted nanostructures and designing and creating novel functional devices using them. It is not easy to observe nanostructures with c
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