Process development and material characterization of polycrystalline Bi2Te3, PbTe, and PbSnSeTe thin films on silicon for millimeter-scale thermoelectric generators

Abstract

In this work, deposition, patterning, and metallization of vapor-deposited polycrystalline thermoelectric (TE) thin films of Bi2Te3, PbTe, and PbSnSeTe on silicon (Si) substrates are investigated. These fundamental microfabrication methods are intended for use in integrating TE films into thermally powered micro-electro-mechanical systems (MEMS)-based power generators. P-type polycrystalline Bi2Te3, PbTe, and PbSnSeTe films were successfully deposited on thermally oxidized (100) Si substrates to thicknesses ranging from 0.4 to 9 μm. Dry etch rates of about 0.6−0.7 μm/min were achieved for Bi2Te3 and PbTe/PbSnSeTe using methane-based gases. Wet etch rates of ∼3 μm/min were achieved using various acid wet chemistries. Films were electrically characterized using van der Pauw and transfer length method (TLM) test structures. Postdeposition resistivities were measured as low as 23 mΩ cm for Bi2Te3, 134 mΩ cm for PbTe, and 52 mΩ cm for PbSnSeTe. The Seebeck coefficients were measured at up to 94 μV/K for undoped Bi2Te3, and 160 and 42 μV/K for doped PbTe and PbSnSeTe, respectively. Metal contact resistivities (0.18−42 mΩ cm2) were also extracted for a variety of thin film metals (Pt, Au, Cu, Ni, Cr/Pt/Au, Ti/Pt). Various postdeposition annealing treatments were explored for reducing film resistivity that would enable higher power delivery for TE generator applications. Rapid thermal annealing in nitrogen at 400 °C was shown to reduce the resistivity of PbTe and improve film adhesion to oxidized silicon substrates. Also, after successive heatings in air at 200 °C, the resistivity of the PbTe films remained stable while that of the PbSnSeTe increased up to 10×.