Abstract
Uniform metal contacts are critical for advanced thermoelectric devices. The uniformity of the contact resistance for gold, tungsten, and SrRuO3 electrodes on polycrystalline ternary Bi2Te3-based alloys for different types of surface cleaning procedures was characterized. The presence of a nanometer-thick native oxide layer on the Bi2Te3 surface leads to large and non-uniform contact resistance. Surface treatments included solvent cleans and chemical and dry etching prior to metallization of the Bi2Te3. Only etching the surface led to a significant improvement in contact resistance uniformity. None of the tested contacts reacted with the underlying Bi2Te3 substrate. Etching resulted in the removal of the native oxide on the Bi2Te3 surface, which was characterized using X-ray photoelectron spectroscopy (XPS). The average thickness, chemistry, and dry etch rate of the native oxide was further characterized using XPS. The non-uniformity in contact resistance suggests that the native oxide grows non-uniformly on polycrystalline bismuth telluride surfaces.
Highlights
Contact resistance can make a large contribution to the total resistance of a thermoelectric device, leading to Joule heating and a decrease in thermoelectric efficiency
An approximate expression for the effective figure of merit of a thin conductive metal on a thermoelectric material is Z’/Z ∼ (1 + 2ρC/Lρ), where Z’ is the effective figure of merit with electrical contact resistance, ρC is the specific contact resistance, ρ is volume resistivity of the thermoelectric material, and L is the length of the thermoelectric device
To make this method work, a wide variety of diameters is needed for a high fidelity two parameter fit, where the small contact diameters are more sensitive to changes in contact resistance and larger diameters give a better estimate of the back contact resistance
Summary
Contact resistance can make a large contribution to the total resistance of a thermoelectric device, leading to Joule heating and a decrease in thermoelectric efficiency. Given the present materials limitations to the figure of merit, thermoelectric energy conversion systems operate at the margins of efficiency and even relatively modest increases in contact resistance can unacceptably degrade device performance. For this reason, many previous works have studied the effect of various surface treatments on contact resistance, in particular for binary bismuth telluride in thin film and bulk form, and for ternary materials (Bi, Sb)2Te3 (P-Bi2Te3) and Bi2(Te,Se) (N-Bi2Te3), which have the best thermoelectric properties for most applications below 400 K.
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