Growth and characterization of metal-semiconductor nitride multilayers and superlattices for solid state thermionic energy conversion

Vijay Rawat, Purdue University

Abstract

The possibility of using metal/semiconductor superlattices for thermionic energy conversion and solid-state refrigeration was proposed in the 1990's. Subsequent theoretical work showed that cross-plane transport through such metal/semiconductor superlattices has the potential to yield values of the figure-of-merit that are much higher than those of currently available thermoelectric materials. The primary challenge involved in realizing a solid-state thermionic energy converter is selecting materials suitable for fabrication of such superlattices. We have identified TiN/GaN and ZrN/ScN as two possible metal/semiconductor combinations for thermionic energy conversion with the hot side temperature in the range of 300–650°C. The first nitride superlattice system consists of TiN as the metal layer and GaN, in its wurtzite phase, as the semiconductor layer, grown on sapphire substrates. Though the multilayers were polycrystalline, the TiN layers were found to grow along <111> direction on <0001> oriented GaN layers, thus enabling localized epitaxy between columnar grains of TiN and GaN within the respective layers. This localized epitaxy assisted the growth of micron scale thick multilayer comprising of materials with such disparate crystal structures. In an effort to grow a pure rocksalt structured superlattice, the metastable rocksalt GaN (rs-GaN) phase was stabilized by pseudomorphic epitaxy on a metallic rocksalt TiN underlayer, and its existence has been verified using high-resolution x-ray diffraction and transmission electron microscopy. The second pure rocksalt-structured superlattice system analyzed consists of alternating layers of metallic ZrN and semiconducting ScN. These epitaxial superlattices were grown on rocksalt MgO substrates using DC magnetron sputtering in a Ar/N2 ambient. The Schottky barrier height of the metal/semiconductor combination, measured using temperature dependent I-V measurement, was found to be 280 meV which is close to the value desired for maximizing effective ZT at operating temperatures in the range of 650°C. In an attempt to reduce the small 1.7% mismatch in lattice parameters between ZrN (a = 0.458 nm) and ScN (a = 0.450 nm), the ZrN layers are alloyed with W2N (a = 0.412 nm). It was observed by high resolution X-ray diffraction and high resolution TEM that by alloying ZrN layers with approximately 15% W, lattice matched superlattices of (Zr,W)N/ScN are obtained. Additionally, incorporation of W which is a heavier element than both Zr and Sc, also reduced the thermal conductivity to below 2 W/m-K. The structural, electrical and thermal properties of these two metal/semiconductor systems are explored and reported in this thesis.

Degree

Ph.D.

Advisors

Sands, Purdue University.

Subject Area

Materials science

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