Microstructure and thermoelectric properties of ScN thin films and metal/ScN superlattices for high-temperature energy conversion

Polina V Burmistrova, Purdue University

Abstract

Despite remarkable efforts to diminish our dependency on foreign oil by developing alternative sources of energy that also reduce the environmental impact from the combustion of fossil fuels, more than 60% of energy generated in the United States is lost in the form of heat. Thermoelectrics can directly convert thermal energy into electricity; yet, their widespread application is limited to niche areas mainly due to low efficiency and high cost of component materials. For years, the thermoelectric figure-of-merit (ZT) representing the efficiency was close to unity. If the average ZT reaches 2, however, thermoelectric solid-state generators could become economically competitive. ScN possesses remarkable physical properties that make it ideal for a variety of applications. Recent studies showed that it has potential as a high temperature thermoelectric material. In this work, epitaxial ScN thin films were deposited by dc reactive magnetron sputtering in Ar/N2 ambient at 2-20 mTorr at 650-850°C. Sputtered under optimal deposition conditions, these ScN thin films show high values of electron mobility, up to 106 cm2/Vs. Thermoelectric measurements yield an in-plane Seebeck coefficient of -156µV/K at 800K and a power factor of 3.3-3.5x10 -3 W/mK2 in the 600-800K range, values that are the highest ever reported for ScN thin films. In addition, the measured power factor is comparable to or greater than the compounds and alloys that form the basis for established thermoelectric materials (e.g.,undopped SiGe and La3Te4). The room temperature thermal conductivity of the ScN thin films is also high (19.8 W/mK); however, it drops to 8.34 W/mK at 800 K, yielding an estimated ZT value of 0.3. Therefore, with further controlled doping, alloying or nanostructuring, ScN may represent a promising parent compound for a new class of high-temperature thermoelectric materials for power generation from the waste heat. Superior thermoelectric properties of nitride metal/semiconductor superlattices are expected through enhancing the power factor (S2σ) by electron energy filtering with the potential barriers and, simultaneously, suppressing the lattice thermal conductivity via phonon scattering at the interfaces. The Schottky barrier height and the concept of electron lateral momentum non-conservation are the key factors determining the thermoelectric properties of such metamaterials. A theoretical study showed that lateral momentum non-conservation during thermionic emission that is achieved by introducing nonplanar interfaces may be the key factor in allowing a larger number of "hot" electrons to participate in conduction. A scanning transmission electron microscopy study of metal/ScN interfaces showed that the as-grown interfaces are not atomically flat. Elemental diffusion and atomic mixing occur at the interfaces during the deposition. Depending on the deposition pressure and the metal layer material, the quality of the ScN/metal and metal/ScN interfaces greatly differs. Thus, the interface width of HfN/ScN and ScN/HfN interfaces is 9.8±0.2 nm and 4.2±0.2nm, respectively, for HfN/ScN superlattices deposited at 2 mTorr. When the deposition pressure increases to 5 mTorr the mixing is much less pronounced, resulting in 4.8±0.2 nm and 3.8 ±0.2nm interface widths for HfN/ScN and ScN/HfN interfaces, respectively. The room temperature thermal conductivity also decreases with increasing deposition pressure from 5.7 to 2.7 W/mK for the HfN/ScN superlattices resulting from more efficient phonon filtering through more abrupt interfaces. Thus, introduction of HfN layers in the ScN matrix in a form of superlattice dramatically reduced the room temperature cross-plane thermal conductivity

Degree

Ph.D.

Advisors

Sands, Purdue University.

Subject Area

Electrical engineering|Materials science

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