Formation of a Heterojunction in SiGe Nanowires by Thermal Oxidation
Hsin-Yu Lee1*, Tzu-Hsien Shen1, Cheng-Yen Wen1
1Department of Materials Science and Engineering, National Taiwan University, Taiwan
* presenting author:Hsin-Yu Lee,
Group IV semiconductor heterojunction nanowires have a wide range of potential applications in transistors and thermoelectric devices. For theses applications, perfection of the interfacial structure at the heterojunctions is essential for the ideal device performance. Therefore, a reliable process to fabricate defect-free and compositionally abrupt heterojunction nanowires will be useful. Heterojunction nanowires are usually fabricated by switching the gas precursors during the vapor-liquid-solid (VLS) or the vapor-solid-solid (VSS) growth. Here we show that heterojunctions between two SiGe alloys of different Si/Ge ratios can be formed by oxidizing SiGe alloy nanowires. The SiGe alloy nanowires are epitaxially grown on (111) Si substrates via the VLS method using AuGeSi eutectic liquid as the catalysts. The Ge concentration in the SiGe nanowires is about 6%. After oxidizing the SiGe nanowires in air at 700°C for 6 hours, a new SiGe segment with a higher Ge content is formed on top of the SiGe nanowire, accompanied by the formation of surface oxide, which is composed of only silicon and oxygen, on the sidewalls of the SiGe nanowires. The width of the interface between the two SiGe alloys is about 1.6 nm, as observed in the high-angle annular dark-field scanning transmission electron microscopy image, and there is no misfit dislocation at the interface. We propose that the formation of the heterojunction in the SiGe nanowire is via the solid-liquid-solid (SLS) mechanism. During oxidation of the SiGe nanowires, silicon oxide is formed on the sidewalls. The unreacted Si and Ge atoms diffuse to the liquid catalysts on the nanowire tip, where Si atoms are further oxidized to form an oxide layer on the surround of the catalyst. The remaining Ge atoms are accumulated, and precipitation of a SiGe layer occurs once the solute concentration in the liquid catalyst reaches saturation limit. The oxidation-induced SiGe segment has an increasing Ge concentration up to 80% along with its growth. If a system meets the condition such as selective oxidation, we believe such a mechanism can possibly be applied to other alloy semiconductor nanowires.

Keywords: heterojunctions, SiGe nanowires, oxidation