Producing Atomically Abrupt Axial Heterojunctions in Silicon–Germanium Nanowires by Thermal Oxidation

Abstract

Compositional abruptness of the interfaces is one of the important factors to determine the performance of Group IV semiconductor heterojunction (Si/Ge or Si/SiGe) nanowire devices. However, forming abrupt interfaces in the nanowires using the common vapor-liquid-solid (VLS) method is restricted because large solubility of Si and Ge in the Au eutectic liquid catalyst makes gradual composition change at the heterojunction after switching the gas phase components. According to the VLS growth mechanism, another possible approach to form an abrupt interface is making a change of the semiconductor concentration in the eutectic liquid before precipitation of the second phase. Here we show that the composition in AuSiGe eutectic liquid on SiGe nanowires of low Ge concentration (≤6%) can be altered by thermal oxidation at 700 °C. During the oxidation process, only Si is oxidized on the surface of the eutectic liquid, and the Ge/Si ratio in the eutectic liquid is increased. The subsequently precipitated SiGe step at the liquid/solid interface has a higher Ge concentration (∼20%), and a compositionally abrupt interface is produced in the nanowires. The growth mechanism of the heterojunction includes diffusion of Si and Ge atoms on nanowire surface into the AuSiGe eutectic liquid and step nucleation at the liquid/nanowire interface.

Recent Advances in Vertically Aligned Nanowires for Photonics Applications

“…Hence, the study of spectral resonances in nanowires based upon cylindrical optical waveguide theory paved the route to utilize vertical nanowires as a building block for spectral filters which not only can sensitively respond specific photon wavelength but also can convert light to photocurrent, expanding color gamut representation and photodetector applications (e.g., artificial retina and flexible imagers) [27][28][29][30]. Structure engineering of vertical nanowires can be conducted by additional shaping steps such as reactive ion etching (RIE), thermal oxidation and potassium hydroxide (KOH) etching, while precisely controlling the nanowire diameters, height and morphology [31][32][33].…”

Growth of heterojunctions in Si–Ge alloy nanowires by altering AuGeSi eutectic composition using an approach based on thermal oxidation

“…The oxidation process forms a uniform oxide shell surrounding the nanowire core. Energy dispersive spectroscopy (EDS) analysis shows that this oxide shell is composed of only Si and O [16], because Si has a higher reactivity with O than Ge does. The nanowire core exhibits a heterojunction interface between the original Si-Ge nanowire and a section of a higher Ge concentration (see figure 2(b)), as revealed from the STEM intensity.…”

“…Such a sharp interface distinctly defines two regions of different physical properties. The interfacial abruptness of this heterojunction structure and the thermal budget for maintaining the abruptness have been studied by Lee et al [16]. During VLS nanowire growth, Au atoms migrate on the sidewalls of nanowires and substrate surface [20].…”

“…Lee et al found that when silicon-germanium alloy (Si-Ge) nanowires with a low Ge concentration are oxidized in air at high temperature (e.g. 700 °C), an atomically abrupt heterojunction interface is formed in the nanowire between two Si-Ge sections of different compositions [16]. Because the heterojunction growth is inside an oxide shell, the nanowires still maintain the one-dimensional geometry.…”

Axially Segmented Semiconductor Heteronanowires

“…Strategies to kinetically increase the interfacial abruptness include exhausting the former precursor before growing the second one, reducing the solubility of foreign atoms in the catalysts by solidifying the catalysts, or accelerating consumption of the residual in catalysts . Therefore, largely reduced solubility of semiconductor atoms in catalysts, combining with relatively low growth rate, in principle enables atomically abrupt interfaces.…”