Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Thermoelectric Enhancement of Silicon Membranes by Ultrathin Amorphous Films Anthony George,† Ryoto Yanagisawa,† Roman Anufriev,† Jinghan He,† Naoko Yoshie,† Naohito Tsujii,§ Quansheng Guo,∥ Takao Mori,§,∥ Sebastian Volz,† and Masahiro Nomura*,†,‡ †
Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan § Center for Functional Sensor & Actuator (CFSN) and International Center for Materials Nanoarchitectonics (WPI-MANA) and ∥ International Center for Materials Nanoarchitectonics, National Institute for Material Science, Tsukuba 305-0044, Japan Downloaded via UNIV OF CAMBRIDGE on March 17, 2019 at 11:45:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: We propose a simple, low-cost, and large-area method to increase the thermoelectric figure of merit (ZT) in silicon membranes by the deposition of an ultrathin aluminum layer. Transmission electron microscopy showed that short deposition of aluminum on a silicon substrate covers the surface with an ultrathin amorphous film, which, according to recent theoretical works, efficiently destroys phonon wave packets. As a result, we measured 30−40% lower thermal conductivity in silicon membranes covered with aluminum films while the electrical conductivity was not affected. Thus, we have achieved 40−45% higher ZT values in membranes covered with aluminum films. To demonstrate a practical application, we applied this method to enhance the performance of a silicon membrane-based thermoelectric device and measured 42% higher power generation. KEYWORDS: thermoelectrics, phonon engineering, silicon, thermal conductivity, figure of merit
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beam lithography, 18 or block copolymer masking for etching,12,19 which would ultimately prevent efficient mass production. Alternatively, the thermal conductivity of nanostructures can be further decreased by roughening20−22 or oxidization23,24 of the surface. Such a reduction in thermal conductivity of silicon nanobeams has recently been achieved through the addition of aluminum nanopillars25 and explained by the increased phonon scattering at the amorphous interface between aluminum and silicon. However, recent simulations26,27 suggested that the amorphous films at the surface may not just scatter phonons diffusely but absorb the energy of a wave packet and either trap it or return it in the form of a highfrequency lattice ripple. Thus, amorphous layers may scatter phonons more efficiently than just the rough surfaces or boundaries. In this work, we further explore this idea and experimentally study the thermoelectric enhancement of silicon membranes through formation of amorphous films on the surface using a simple one-step aluminum deposition technique. First, we experimentally demonstrate that the deposition of aluminum creates an ultrathin amorphous surface layer, which decreases the thermal conductivity and increases the ZT value of the structure. Next, we apply this surface amorphization technique
he emerging market of sensors and devices for the Internet of things drives the development of low-cost energy-harvesting materials and devices. In the field of thermoelectric materials,1 researchers continue to seek methods to enhance the thermoelectric capabilities of silicon.2,3 Unmodified bulk monocrystalline silicon exhibits a relatively low thermoelectric figure of merit (ZT of ∼0.001) in comparison to that of popular thermoelectric materials found in consumer-grade thermoelectric coolers and energy harvesters.2,4 Yet, the abundance, non-toxic nature, and most importantly, compatibility with CMOS technology makes silicon an attractive alternative to the conventional materials.5 The thermoelectric figure of merit is given by ZT = S2σT/κ, where T is the temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. Thus, to obtain large values of ZT, the σ/κ ratio must be increased.6,7 To increase the σ/κ of silicon, researchers tried to decrease the thermal conductivity by structure miniaturization and achieved relatively low values of thermal conductivity by thinning8,9 or narrowing10,11 of silicon membranes. However, this reduction is still insufficient for thermoelectric applications. To further reduce the in-plane thermal conductivity, researchers have explored a wide variety of nanofabrication methods, which typically reduce the thermal conductivity at the cost of fabrication complexity.3,12,13 For example, many studies showed a reduction in thermal conductivity of silicon membranes by etching arrays of holes.12,14−16 In general, this approach requires expensive or unconventional fabrication steps, such as electron-beam lithography,14,15,17 focused ion © XXXX American Chemical Society
Received: November 29, 2018 Accepted: February 7, 2019
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DOI: 10.1021/acsami.8b21003 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces to membrane-based thermoelectric generators and demonstrate an enhancement of the practical output power. First, we studied how the surface of monocrystalline silicon is affected by short sputtering of aluminum using electron beam-assisted physical vapor deposition (EBPVD). Figure 1a,b
Figure 1. Cross-sectional TEM images of the silicon surface (a) before and (b) after aluminum deposition. SEM images of (c) a sample for μTDTR measurements and (d) a sample for electrical measurements.
shows the transmission electron microscopy (TEM) images of a silicon surface before and after the deposition. Before the deposition, the silicon surface is atomically flat with rms surface roughness
