Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion

Epitaxial nitride thin films and heterostructures are one of the most celebrated class of materials not only due to their utility in fundamental materials science and device physics studies, but also for their numerous industrial applications from hard coating technology to solid-state lighting. Transition metal nitrides such as TiN and others have been utilized for decades in hard coating and tribology applications. The last two decades have also seen the emergence and dominance of GaN for solid-state lighting and power electronic applications. Though TiN, and other wurtzite III-nitride semiconductor such as GaN remain the most important nitride coating materials for a range of applications, several other rocksalt nitride thin film and superlattice heterostructures such as ScN, CrN, and TiN/(Al,Sc)N metal/semiconductor superlattices have attracted significant interests in recent years for applications in thermoelectricity, plasmonics, solar energy conversion, and in high temperature electronic, optoelectronic, and plasmonic devices. In this chapter, we present an up-to-date summary of rocksalt nitride thin film and heterostructure coating materials for their applications in energy transport and conversion research fields. The suitability and usefulness of such nitride coating materials in the most recent scientific and engineering advances related to the energy transport and conversion research fields are highlighted

Metal/Semiconductor Superlattice Heterostructures: A New Paradigm in Solid-State Energy Conversion

Since the 1960s, researchers exploring the potential of artificially-structured materials for applications in quantum electronic devices have sought combinations of metals and semiconductors that could be combined on the nano-scale with atomically-sharp interfaces. Early work with multilayers of polycrystalline elemental metals and amorphous semiconductors showed promise in tunneling devices. More recently, similar metal/semiconductor multilayers have been utilized to demonstrate novel optical metamaterials. These metal/semiconductor multilayers, however, are not amenable to atomic-scale control of interfaces. We developed the first epitaxial metal/semiconductor multilayer and superlattice heterostructures that are free of extended defects. These rocksalt nitride superlattices have atomically sharp interfaces and properties that are tunable by alloying, doping and quantum size effects. Furthermore, these nitride superlattices exhibit exceptional mechanical hardness, chemical stability and thermal stability up to ~1000֩C

Thermal and thermoelectric properties of nitride metal/semiconductor superlattices

Since the 1960s, researchers exploring the potential of artificially-structured materials for applications in quantum electronic devices have sought combinations of metals and dielectrics that could be combined on the nanoscale with atomically-sharp interfaces. Early work with multilayers of polycrystalline elemental metals and amorphous dielectrics showed promise in tunneling devices. More recently, similar metal/dielectric multilayers have been utilized to demonstrate novel optical metamaterials. These metal/dielectric multilayers, however, are not amenable to atomic-scale control of interfaces. We developed the first epitaxial metal/semiconductor multilayers that are free of extended defects. These rocksalt nitride superlattices have atomically sharp interfaces and properties that are tunable by alloying, doping and quantum size effects. Furthermore, these nitride superlattices exhibit exceptional mechanical hardness, chemical stability and thermal stability up to ~1000°C. In this thesis, I have described the growth and transport properties of nitride metal/semiconductor superlattices including (Ti,W)N/(Al,Sc)N and (Hf, Zr)N/ScN). Potential applications in thermoelectric devices and plasmonic metamaterials are outlined. Futhermore, I have described recent experiments that employ these superlattices as model materials for investigating the fundamentals of heat transport in nanostructured materials

First-principles analysis of ZrN/ScN metal/semiconductor superlattices for thermoelectric energy conversion

We present a first-principles density functional theory-based analysis of the electronic structure, vibrational spectra, and transport properties of ZrN/ScN metal/semiconductor superlattices aiming to understand its potential and suitability for thermoelectric applications. We demonstrate (a) the presence of Schottky barriers of 0.34 eV at the metal/semiconductor interface and (b) a large asymmetry in the electronic densities of states and flattening of electronic bands along the cross-plane directions near the Fermi energy of these superlattices, desirable for high Seebeck coefficient. The vibrational spectra of these superlattices show softening of transverse acoustic phonon modes along the growth direction and localization of ScN phonons in the vibrational energy gap between metal and semiconductor layers. Boltzmann transport theory-based analysis suggests a reduction of lattice thermal conductivity by an order of magnitude compared to its individual bulk components, which makes these materials suitable for thermoelectric applications. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3569734

Electronic structure, vibrational spectrum, and thermal properties of yttrium nitride: A first-principles study

Yttrium nitride (YN) is a promising semiconductor for use in metal/semiconductor superlattices for thermoelectric applications. We determine its electronic structure, vibrational spectrum, and thermal properties using first-principles density functional theory (DFT) based simulations with a generalized gradient approximation (GGA) of the exchange correlation energy. We employ GGA+U and GW approximations in our calculations to (a) improve the accuracy of the calculation of bandgaps and (b) determine specific features of its electronic structure relevant to transport properties, such as transverse (m(t)*) and longitudinal (m(1)*) conduction band effective mass. To evaluate consequences of forming alloys of YN with other materials to its electronic properties, we have determined the volume deformation potentials. Our results for phonons show a large longitudinal optical (LO) and transverse optical (TO) splitting at the Gamma point in the vibrational spectrum with a gap of 325 cm(-1) arising from long-range dipole-dipole interactions. We estimate temperature dependent lattice specific heat and lattice thermal conductivity based on Boltzmann transport theory to assess YN\u27s potential for thermoelectric applications. (C) 2011 American Institute of Physics. [doi:10.1063/1.3561499

Electronic structure, phonons, and thermal properties of ScN, ZrN, and HfN: A first-principles study

With a motivation to understand microscopic aspects of ScN, ZrN, and HfN relevant to the thermoelectric properties of nitride metal/semiconductor superlattices, we determine their electronic structure, vibrational spectra and thermal properties using first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy. We find a large energy gap in the phonon dispersions of metallic ZrN and HfN, but a gapless phonon spectrum for ScN spanning the same energy range, this suggests that a reduced thermal conductivity, suitable for thermoelectric applications, should arise in superlattices made with ScN and ZrN or ScN and HfN. To obtain an electronic energy band gap of ScN comparable to experiment, we use a Hubbard correction with a parameter U (=3.5 eV). Anomalies in the acoustic branches of the phonon dispersion of ZrN and HfN, manifested as dips in the bands, can be understood through the nesting of Fermi surface determined from our calculations. To connect with transport properties, we have determined effective masses of ScN and determined their dependence on the U parameter. Using the relaxation time approximation in the Boltzmann transport theory, we estimate the temperature dependence of the lattice thermal conductivity and discuss the chemical trends among these nitrides