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Title:Growth, physical properties, and nanostructuring of epitaxial metastable hafnium aluminum nitride
Author(s):Howe, Brandon M.
Director of Research:Petrov, Ivan G.
Doctoral Committee Chair(s):Greene, Joseph E.
Doctoral Committee Member(s):Petrov, Ivan G.; Rockett, Angus A.; Zuo, Jian-Min
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):hafnium aluminum nitride (HfAlN)
hard coatings
thin films
Abstract:The B1-NaCl structure group IVB transition metal (TM) nitrides are well known to have a remarkable range of unique physical properties and thus find their place in a variety of applications including hard wear-resistant coatings on cutting tools, diffusion barriers in microelectronics, and selectively-transmitting abrasion-resistant optical layers. By employing kinetically-limited growth techniques including low growth temperatures (Ts/Tm≲0.25) and high deposition rates, metastable TM nitride alloys can be synthesized and have shown to exhibit extraordinary physical properties. The most famous example is Ti1-xAlxN; many have reported drastically enhanced hardness, age-hardening behavior, elevated oxidation resistance, and the ability to tune the optical and electronic properties by varying the AlN content. Many of these desirable properties are accompanied by the formation of self-organized nanostructures, due to the onset of spinodal decomposition. However, very little has been reported regarding systematic studies on the effects of self-organized nanostructure formation, as well as the addition of semiconducting AlN into metallic TM nitrides, on fundamental physical properties due to the lack of high-quality single-crystals. HfN is the highest melting point (Tm = 3330 °C), largest negative heat of formation (ΔH° = -88.2 kcal mol-1), and one of the highest hardness (H = 25.3 GPa) TM nitrides. Furthermore, HfN and AlN are immiscible at equilibrium, (a prerequisite for nanostructured composites) and the lattice mismatch (δ = 9.8%) is much larger than that between TiN and AlN (δ = 2.9%), giving rise to much larger thermodynamic driving forces for phase segregation. Our group has developed a unique approach to synthesizing, at low temperatures (Ts/Tm≲0.25), a wide-range of single-crystal TM nitrides by employing high-fluxes (Ji/Jme≳5) of low-energy (~10-50eV) ion bombardment during high-rate reactive magnetron sputter deposition. This growth scenario has the effect of extending the single-phase solid-solubity limits of metastable epitaxial TM nitrides thereby allowing for the exploration of a wide-range of alloy compositions. Thus, I use this approach to synthesize single-crystal Hf1-xAlxN(001), in order to conduct a systematic investigation into the effects of alloy composition x and high-flux low-energy ion irradiation on the nanostructuring and physical properties of metastable TM nitride alloys. Single-crystal metastable Hf1-xAlxN(001) layers with 0 ≤ x ≤ 0.50 grown at 600 °C on MgO(001) crystallize in the B1 NaCl-structure with cube-on-cube epitaxial relationship to MgO(001) substrates: (001)HfAlN||(001)MgO and [100]HfAlN||[100]MgO. The relaxed lattice parameter of Hf1-xAlxN(001) decreases with increasing x, ranging from 0.4519 nm for x = 0 to 0.4438 nm for x = 0.50. In the low AlN-content region, with 0 ≤ x ≤ 0.17, Al is randomly distributed throughout the cation sublattice. This creates small changes in the electronic properties due to increased alloy scattering and enhanced crystalline quality. With x ≥ 0.19, there is a step-wise increase in ρ300K(x) to 75 μΩ-cm and TCR decreases, due to increased carrier scattering resulting from the formation of compositionally-modulated HfN- and AlN-rich nanodomains. The hardness H of Hf1-xAlxN is also dominated by the nanostructure formation, undergoing a ~30% increase to 32.4 ± 0.7 GPa with x = 0.29. As x increases to 0.32, ρ300K(x) increases to 299 μΩ-cm, Neff decreases linearly, and TCR becomes negative. The Bruggeman effective medium approximation is used to interpret the dielectric response of Hf1-xAlxN layers. With 0.21 ≤ x ≤ 0.32, the material can be described as a percolated network of spherical metallic clusters, and thus films are metallic. With x ≥ 0.37, the volume fraction of metallic HfN drops below the percolation limit and induces drastic changes in the electronic properties. Neff becomes temperature-dependent and drops two orders of magnitude, while ρ(T) increases rapidly with decreasing T; both are indicative of carrier localization. The effect of high-flux (Ji/Jme = 8) low-energy (Ei = 10-40 eV) ion irradiation during film growth has a strong effect on the composition of Hf1-xAlxN(001) layers. The composition of Hf1-xAlxN(001) layers grown by reactive sputter deposition from a Hf0.7Al0.3 alloy target vary from x = 0.3 with Ei = 10 eV, to 0.27 with Ei = 20 eV, 0.17 with Ei = 30 eV, and ≤ 0.002 with Ei ≥ 40 eV. This remarkably large change in AlN incorporation probability (> two orders of magnitude!) is due to the efficient resputtering of deposited Al atoms (27 amu) by Ar+ ions (40 amu) backscattered from heavy Hf atoms (178.5 amu) in the film. This provides a novel and robust reaction pathway for synthesizing, at high deposition rates, compositionally-complex heterostructures, multilayers, and superlattices from a single alloy target simply by controllably switching Ei. For multilayer and superlattice structures, the choice of Ei values determines the alloy composition while the period of ion incidence determines individual layer thicknesses. This growth scenario has the effect of superimposing a 2D engineered heterostructure on the 3D self-organized nanostructure within the individual Hf0.7Al0.3N superlattice layers. This allows further optimization of the mechanical properties of Hf1-xAlxN alloys, as the hardness H of Hf0.7Al0.3N/HfN(001) multilayers increases from 32.5±0.9 GPa with bilayer thickness Λ = 6.2±0.2 nm to a maximum of 38±1 GPa with Λ = 2 nm.
Issue Date:2011-01-14
Rights Information:Copyright 2010 Brandon Marcus Howe
Date Available in IDEALS:2011-01-14
Date Deposited:December 2

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