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

Abstract

of trillions of delocalized sensors will require the development of robust micropower sources. [1] While primary batteries are nowadays the preferred solution, the current scenario cannot be extended much longer, as the exponentially growing number of connected devices require too complex logistics, both for their periodic replacement and disposal of such batteries. In this context, energy harvesting combined with small energy buffers (rechargeable batteries or capacitors) represents a more sustainable alternative to single-use batteries. [2][3][4] In particular, thermoelectric harvesters are solid-state devices capable of directly converting waste heat into electricity, with efficiencies in the range of 1%-10%. Among their main advantages, an absence of movable parts, high reliability, and the recently explored possibility of miniaturization make them promising candidates to power the IoT revolution. [5][6][7] The efficiency of a ThermoElectric (TE) system is measured using the so-called zT Figure of Merit, a dimensionless parameter that relates the main TE properties of the materials as zT = S 2 σT/κ, where S is the Seebeck coefficient, σ electrical conductivity, and κ its thermal conductivity. Although the thermoelectric effect is known since the 19 th century, currently, best-performing TE materials are still based on scarce, expensive, environmentally 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-liquidsolid 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.

TEM-compatible microdevice for the complete thermoelectric characterization of epitaxially integrated Si-based nanowires

“…This result agrees with the estimations of Gadea et al 21 using the transmission line method and our independent verification using Scanning Thermal Microscope experiments (SthM). 45 …”

“…2a). The fabrication at the component side (top) follows a similar approach as for the thermoelectric microgenerator devices described in our prior works, 22,30 that is, the patterning of a 300 nm silicon nitride layer for electrical insulation of the heater/thermometer elements (Fig. 2c and d) and the subsequent 200 nm metal deposition (W/Ti) of the electrical paths via a lift-off process (Fig.…”

“…This result agrees with the estimations of Gadea et al 21 using the transmission line method and our independent verification using Scanning Thermal Microscope experiments (SthM). 45 It is also worth noticing that no error in the measurement of k is derived from the estimation of the laser power absorption coefficient c -a very challenging parameter to measure experimentally -, 17 as the heating is done purely electrically. Indeed, c can be here estimated by comparing the DT profile expected when using electrical heating (eqn (1)) with the behaviour when using laser heating as a (quasi) punctual heat source as proposed by Soini et al: 27 DT Laser ðyÞ ¼ P Laser Zc kAL…”

“… 21 This integration approach – based on the epitaxial growth of the nanostructures directly on the test device – enables the electrical probing of the NWs while, at the same time, overcomes the problem of the thermal contact resistance. 22 However, most of these microfabricated test platforms are unsuitable for transmission experiments such as electron microscopy techniques (TEM) or other measurements affected by the substrate like Raman spectroscopy – where the underlying silicon substrate masks the faint signal of the nanowires. Additionally, the efficient thermal dissipation of bulk silicon drastically reduces the accuracy of Seebeck coefficient measurements using microheaters for driving controlled thermal gradients.…”

What Makes On‐Chip Microdevices Stand Out in Electrocatalysis?

“…A thermoelectric device refers to a device that uses the thermoelectric effect to achieve energy conversion when there is a temperature difference between two different conductor materials. Micro/nano thermoelectric devices have a wide range of applications, including energy conversion and storage, [ 79 ] temperature measurement and control, biosensing, temperature sensors, [ 80 ] thermoelectric generator, [ 81 ] etc. Nano thermoelectric refrigerators [ 82 ] have high cooling efficiency, low cooling temperature, fast response, small size, light weight, and low power consumption, and can be used in micro‐refrigeration, electronic component heat dissipation, and optical device cooling.…”