Thermal oxidation induced formation of heterojunction in Si-Ge nanowires

Yu-Tao Sun¹, Hsin-Yu Lee¹, Cheng-Yen Wen^1*

¹Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan

* Presenter:Cheng-Yen Wen, email:cwen@ntu.edu.tw

Group IV semiconductor heterojunction nanowires are potentially useful in electronic and thermoelectric applications. Growth of axial heterojunctions in nanowires is typically by switching gas precursors during the CVD process, no matter using the vapor-liquid-solid (VLS) or vapor-solid-solid (VSS) method. Here we present a new method to form heterojunctions in SiGe alloy nanowires by thermal oxidation. SiGe nanowires of 6 at.% Ge are firstly fabricated in CVD. The nanowires are then oxidized in air at high temperatures, e.g. 700°C. Transmission electron microscopy analysis shows that not only an oxide layer is formed surrounding the nanowires, there is also an atomically abrupt axial hereojunction formed in nanowires. The concentration of Ge in the nanowires changes abruptly from 6% to 20% across the interface. Compositionally analysis shows that the oxide shell is composed of Si and O. Ge atoms and unoxidized Si atoms are ejected from the oxide shell and diffuse at the interface between the oxide shell and nanowire core. The Au eutectic liquid at the nanowire tip is a reservoir of the diffusing atoms; once the concentration of the solute atoms reaches the solubility limit, a SiGe bilayer of a higher Ge concentration is precipitated at the liquid/solid interface. The two sections of the SiGe heterojunction nanowire have different thermal conductivities, so the such a compositionally abrupt heterojunction nanowire may be used for thermal rectifying applications. We will also discuss the application of this growth mechanism for the formation of SiGe dots.

Keywords: SiGe alloy nanowires, Heterojunction, Thermal oxidation, Transmission electron microscopy, Thermal rectifying