The Evolution of 3-Fold Symmetry Hexagon in Si Nanowire As a Function of Oxidation Time

The advantage of Si over other semiconductor materials is that its thermally grown oxide, SiO₂, provides a nearly ideal surface passivation layer.  Recently, Si nanowires (NWs) and oxidized Si NWs have also received considerable attention as building blocks for nanoelectronic devices, and chemical and biological sensors.  Numerous researches on electrical properties of the Si/SiO₂ core/shell structure have been studied to utilize the Si NW into the Si based devices for past decades.  Herein, we investigated the morphological characteristics of cross-sectional shape of Si NW core as a function of oxidation time, which is closely related in SiO₂ shell thickness.  In as-grown Si NW, cross sectional shape of Si core revealed 3-fold symmetry hexagon, not 6-fold symmetry hexagon.  While thermal oxidation continued, 3-fold symmetry hexagon was transformed into triangular shape with roundish edge up to thermal oxidation of 2 hrs.  The changing in the shape of the Si core during oxidation process can be understood by equilibrium crystal shapes lowering the surface free energy.  In our oxidation system, we suggested the schematic modeling to calculate the initial diameter of the Si core NW before oxidation process.  This is in good agreement with our experimental results.  Furthermore, we found the distinguishable defect states formed into native oxide (broad peak near the 522 nm) and thermal oxide (sharp peak around 400 nm) from the low-PL spectra.  The defects state of native oxide is attributed to neutral oxygen vacancy and that of thermal oxide is ascribed to the emission band of defects near Si direct band gap.