Aluminum silicide microparticles transformed from aluminum thin films by hypoeutectic interdiffusion
© Noh; licensee Springer. 2014
Received: 14 May 2014
Accepted: 14 June 2014
Published: 21 June 2014
Aluminum silicide microparticles with oxidized rough surfaces were formed on Si substrates through a spontaneous granulation process of Al films. This microparticle formation was caused by interdiffusion of Al and Si atoms at hypoeutectic temperatures of Al-Si systems, which was driven by compressive stress stored in Al films. The size, density, and the composition of the microparticles could be controlled by adjusting the annealing temperature, time, and the film thickness. High-density microparticles of a size around 10 μm and with an atomic ratio of Si/Al of approximately 0.8 were obtained when a 90-nm-thick Al film on Si substrate was annealed for 9 h at 550°C. The microparticle formation resulted in a rapid increase of the sheet resistance, which is a consequence of substantial consumption of Al film. This simple route to size- and composition-controllable microparticle formation may lay a foundation stone for the thermoelectric study on Al-Si alloy-based heterogeneous systems.
Thermoelectric energy generation, by which waste heat is converted into electricity, has emerged as one of key energy renewal technologies since a huge amount of world energy is wasted in the form of heat [1–3]. The performance of a thermoelectric material is determined cooperatively by the Seebeck coefficient (S), thermal conductivity (κ), and the electrical conductivity (σ) of the material . Unfortunately, these three parameters have some intercorrelations in bulk, limiting the thermoelectric performance of a bulk material . In this regard, one-dimensional (1D) nanowires have been highlighted, where a combination of quantum confinement effect and phonon boundary scattering drastically enhances the thermoelectric performance [6–8]. However, the controlled growth of thermoelectric nanowires and the reproducible fabrication of energy conversion modules based on them should be further demonstrated. Two-dimensional (2D) thin films have the superiority in terms of the ease of material and module fabrication and the reproducibility of the thermoelectric performance.
The best thermoelectric materials reported to date include Bi2Te3, AgPbmSbTe2+m, and In4Se3−δ. These materials, however, contain chalcogens (Se, Te), heavy metals (Pb, Sb), and rare metals (Bi, In), all of which are expected to restrict the widespread use of these materials. Recently, it has been demonstrated that even a conventional semiconductor, silicon (Si), can exhibit thermoelectric performance by adopting nanostructures such as nanowires , nanomeshes , and holey thin films . Although Si has a high S of 440 μV/K, its electrical conductivity is poor (0.01 ~ 0.1 S/cm) . Thus, alloying Si with a good metal could lead to the improved thermoelectric performance. Aluminum (Al) is a typical good metal that has the advantages of high electrical conductivity (approximately 3.5 × 105 S/cm) , light weight, and low cost. Despite the expected high electrical conductivity, the thermal conductivity of Si-Al alloys may be still high due to the large thermal conductivities of the constituents: κAl = 210 ~ 250 W/m K and κSi = 149 W/m K at room temperature . The thermal conductivity of the alloy can be reduced by introducing nano- or microstructures on the alloy film. For this reason, embodying nano- or microstructures on Al-Si alloy films is a critical prerequisite for the study of thermoelectric performance of heterostructures made of Al-Si alloys.
In this work, aluminum silicide microparticles were formed from Al thin films on Si substrates through self-granulation. This process resulted from solid-state interdiffusion of Al and Si at hypoeutectic temperatures, which was activated by compressive stress stored in the films. This stress-induced granulation technique is a facile route to the composition-controlled microparticle formation with no need of lithography, template, and chemical precursor.
The surface morphology of Al films on Si substrates was examined first at micrometer scale using a laser-scanning microscope (LSM, Olympus CLS 4000; Olympus Corporation, Tokyo, Japan). This was conducted on both as-deposited films and heat-treated films. More in-depth morphology study was performed employing a field emission scanning electron microscope (FE-SEM, Hitachi S4300; Hitachi High-Tech, Tokyo, Japan) equipped with energy-dispersive x-ray spectrometer (EDX). The electron acceleration voltage was set at 15 kV. Atomic force microscopy (AFM, Veeco Metrology, Santa Barbara, CA, USA) was also utilized for nanoscale analysis and step height measurement. The structure and the composition of heat-treated samples were analyzed using x-ray diffraction (XRD, Philips X’Pert PW3040; Koninklijke Philips N.V., Amsterdam, Netherlands). In addition, sheet resistances of untreated and heat-treated samples were measured employing a standard four-probe method and their correlation with structural transformation was studied.
Results and discussion
Al thin films of 20 to 90 nm in thickness were deposited on Si (100) substrates by RF sputtering. Al films on Si were vacuum-annealed for 3 to 9 h at 400°C and 550°C, which are lower than the eutectic temperature of Al-Si systems. At hypoeutectic temperatures, compressive stress is developed in the films due to the larger thermal expansion of Al film than Si substrate, and this stress facilitates diffusional flow of Al atoms followed by outward diffusion of Si atoms. This interdiffusion of Al and Si atoms resulted in Al-Si alloy microparticles with rough surfaces, which were spontaneously granulated at the cost of the initial Al film. The density, average size, and the composition of the microparticles could be controlled by adjusting several parameters such as the film thickness, annealing temperature, and time. The surfaces of the microparticles and the residual Al film turned out to be oxidized, presumably during cooling and at ambient condition. As a consequence of the microparticle formation, the sheet resistance of Al film on Si substrate increased 27-fold after 9 h annealing at 550°C. This simple technique for the formation of Al-Si microparticles on Si substrate would be a stepping stone for the systematic study of the thermoelectric performance of heterogeneous systems based on Al-Si alloys.
This research was supported by the Gachon University. The author thanks Professor Kwang S. Suh of Korea University for his assistance.
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