Local Heat Dissipation and Elasticity of Suspended Silicon Nanowires Revealed by Dual Scanning Electron and Thermal Microscopies.

A novel combined setup, with a scanning thermal microscope (SThM) embedded in a scanning electron microscope (SEM), is used to characterize a suspended silicon rough nanowire (NW), which is epitaxially clamped at both sides and therefore monolithically integrated in a microfabricated device. The rough nature of the NW surface, which prohibits vacuum-SThM due to loose contact for heat dissipation, is circumvented by decorating the NW with periodic platinum dots. Reproducible approaches over these dots, enabled by the live feedback image provided by the SEM, yield a strong improvement in thermal contact resistance and a higher accuracy in its estimation. The results-thermal resistance at the tip-sample contact of 188±3.7K µW-1 and thermal conductivity of the NW of 13.7±1.6W m-1 K-1-are obtained by performing a series of approach curves on the dots. Noteworthy, the technique allows measuring elastic properties at the same time-the moment of inertia of the NW is found to be (6.1±1.0) × 10-30m4-which permits to correlate the respective effects of the rough shell on heat dissipation and on the NW stiffness. The work highlights the capabilities of the dual SThM/SEM instrument, in particular the interest of systematic approach curves with well-positioned and monitored tip motion.

Superior Thermoelectric Performance of SiGe Nanowires Epitaxially Integrated into Thermal Micro-Harvesters.

Semiconductor nanowires have demonstrated fascinating properties with applications in a wide range of fields, including energy and information technologies. Particularly, increasing attention has focused on SiGe nanowires for applications in a thermoelectric generation. In this work, a bottom-up vapour-liquid-solid chemical vapour Deposition methodology is employed to integrate heavily boron-doped SiGe nanowires on thermoelectric generators. Thermoelectrical properties -, i.e., electrical and thermal conductivities and Seebeck coefficient - of grown nanowires are fully characterized at temperatures ranging from 300 to 600 K, allowing the complete determination of the Figure-of-merit, zT, with obtained values of 0.4 at 600 K for optimally doped nanowires. A correlation between doping level, thermoelectric performance, and elemental distribution is established employing advanced elemental mapping (synchrotron-based nano-X-ray fluorescence). Moreover, the operation of p-doped SiGe NWs integrated into silicon micromachined thermoelectrical generators is shown over standalone and series- and parallel-connected arrays. Maximum open circuit voltage of 13.8 mV and power output as high as 15.6 µW cm-2 are reached in series and parallel configurations, respectively, operating upon thermal gradients generated with hot sources at 200 °C and air flows of 1.5 m s-1 . These results pave the way for direct application of SiGe nanowire-based micro-thermoelectric generators in the field of the Internet of Things.

Thermal conductivity of individual Si and SiGe epitaxially integrated NWs by scanning thermal microscopy.

Semiconductor nanowires have demonstrated fascinating properties with application in a wide range of fields including energy and information technologies. In particular, increasing attention has been focused on Si and SiGe nanowires for application in thermoelectric generation after recent successful implementation in miniaturized devices. Despite this interest, an appropriate evaluation of thermal conductivity in such nanostructures still poses a great challenge, especially if the characterization of the device-integrated nanowire is desired. In this work, a spatially resolved technique based on scanning thermal microscopy has been demonstrated for the assessment of the thermal conductivity of Si and SiGe nanowires integrated in thermoelectrical microgenerators. Thermal conductivity values of 15.8 ± 0.8 W m-1 K-1 and 4.2 ± 0.3 W m-1 K-1 were measured for Si and SiGe nanowires, respectively, epitaxially grown on silicon microstructures. Moreover, the range of applicability according to the sample thermal conductance and associated errors are discussed to establish the potential of the novel technique. The results presented here show the remarkable utility of scanning thermal microscopy for the challenging thermal characterization of integrated nanostructures and the development of multiple devices such as thermoelectric generators or photovoltaic cells.