Influence of Different Carrier Gases, Temperature, and Partial Pressure on Growth Dynamics of Ge and Si Nanowires

The broad and fascinating properties of nanowires and their synthesis have attracted great attention as building blocks for functional devices at the nanoscale. Silicon and germanium are highly interesting materials due to their compatibility with standard CMOS technology. Their combination provides optimal templates for quantum applications, for which nanowires need to be of high quality, with carefully designed dimensions, crystal phase, and orientation. In this work, we present a detailed study on the growth kinetics of silicon (length 0.1-1 μm, diameter 10-60 nm) and germanium (length 0.06-1 μm, diameter 10-500 nm) nanowires grown by chemical vapor deposition applying the vapour-liquid-solid growth method catalysed by gold. The effects of temperature, partial pressure of the precursor gas, and different carrier gases are analysed via scanning electron microscopy. Argon as carrier gas enhances the growth rate at higher temperatures (120 nm/min for Ar and 48 nm/min H2), while hydrogen enhances it at lower temperatures (35 nm/min for H2 and 22 nm/min for Ar) due to lower heat capacity. Both materials exhibit two growth regimes as a function of the temperature. The tapering rate is about ten times lower for silicon nanowires than for germanium ones. Finally, we identify the optimal conditions for nucleation in the nanowire growth process.

Transitioning from Si to SiGe Nanowires as Thermoelectric Material in Silicon-Based Microgenerators

The thermoelectric performance of nanostructured low dimensional silicon and silicon-germanium has been functionally compared device-wise. The arrays of nanowires of both materials, grown by a VLS-CVD (Vapor-Liquid-Solid Chemical Vapor Deposition) method, have been monolithically integrated in a silicon micromachined structure in order to exploit the improved thermoelectric properties of nanostructured silicon-based materials. The device architecture helps to translate a vertically occurring temperature gradient into a lateral temperature difference across the nanowires. Such thermocouple is completed with a thin film metal leg in a unileg configuration. The device is operative on its own and can be largely replicated (and interconnected) using standard IC (Integrated Circuits) and MEMS (Micro-ElectroMechanical Systems) technologies. Despite SiGe nanowires devices show a lower Seebeck coefficient and a higher electrical resistance, they exhibit a much better performance leading to larger open circuit voltages and a larger overall power supply. This is possible due to the lower thermal conductance of the nanostructured SiGe ensemble that enables a much larger internal temperature difference for the same external thermal gradient. Indeed, power densities in the μW/cm² could be obtained for such devices when resting on hot surfaces in the 50–200 °C range under natural convection even without the presence of a heat exchanger.

Large-area and adaptable electrospun silicon-based thermoelectric nanomaterials with high energy conversion efficiencies

Large amounts of waste heat generated in our fossil-fuel based economy can be converted into useful electric power by using thermoelectric generators. However, the low-efficiency, scarcity, high-cost and poor production scalability of conventional thermoelectric materials are hindering their mass deployment. Nanoengineering has proven to be an excellent approach for enhancing thermoelectric properties of abundant and cheap materials such as silicon. Nevertheless, the implementation of these nanostructures is still a major challenge especially for covering the large areas required for massive waste heat recovery. Here we present a family of nano-enabled materials in the form of large-area paper-like fabrics made of nanotubes as a cost-effective and scalable solution for thermoelectric generation. A case study of a fabric of p-type silicon nanotubes was developed showing a five-fold improvement of the thermoelectric figure of merit. Outstanding power densities above 100 W/m² at 700 °C are therefore demonstrated opening a market for waste heat recovery.

Power Response of a Planar Thermoelectric Microgenerator Based on Silicon Nanowires at Different Convection Regimes

A thermoelectric microgenerator based on multiple silicon nanowire arrays is fabricated and its performance evaluated for different convection regimes. Mature silicon microfabrication technology is used to fabricate the device structure. As a post-process, a bottom-up approach is used to grow silicon nanowires by a VLS-CVD mechanism. The thermal design of the microgenerator features a thermally isolated silicon platform which is connected to the bulk silicon rim through several arrays of silicon nanowires. Simulations are carried out to evaluate the need of an external heat sink to improve the thermal gradient seen by the nanowires and the power output of the microgenerator. Results show a significant improvement with a heat sink raising the thermal gradient from 3 K to approximately 100 K when the external temperature gradient is 300 K. Experimental measurements with different convection regimes also show a radical improvement on the power output comparing natural convection and two different forced convection regimes. The first forced convection regime is a broad airflow from a commercial CPU fan placed on top of the device, while the second (air jet forced convection) uses a syringe to focus the airflow from the compressed air line to the platform. The maximum output power for a natural convection regime is 2.2 nW for a hotplate temperature of 200 °C, while the air jet forced convection regime generates up to 700 nW, which correspond to 35 µW/cm2 considering a device footprint of 2 mm2.

Towards a full integration of vertically aligned silicon nanowires in MEMS using silane as a precursor

Silicon nanowires present outstanding properties for electronics, energy, and environmental monitoring applications. However, their integration into microelectromechanical systems (MEMS) is a major issue so far due to low compatibility with mainstream technology, which complicates patterning and controlled morphology. This work addresses the growth of 〈111〉 aligned silicon nanowire arrays fully integrated into standard MEMS processing by means of the chemical vapor deposition-vapor liquid solid method (CVD-VLS) using silane as a precursor. A reinterpretation of the galvanic displacement method is presented for selectively depositing gold nanoparticles of controlled size and shape. Moreover, a comprehensive analysis of the effects of synthesis temperature and pressure on the growth rate and alignment of nanowires is presented for the most common silicon precursor, i.e., silane. Compared with previously reported protocols, the redefined galvanic displacement together with a silane-based CVD-VLS growth methodology provides a more standard and low-temperature (<650 °C) synthesis scheme and a compatible route to reliably grow Si nanowires in MEMS for advanced applications.