Our group has developed a unique approach to synthesizing, at low temperatures (Ts/Tm [special characters omitted] 0.25), a wide-range of single-crystal TM nitrides by employing high-fluxes (Ji/Jme [special characters omitted] 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  HfAlN||MgO. The relaxed lattice parameter of Hf 1-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-xAl xN 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 Hf0.7Al 0.3 alloy target vary from x = 0.3 with E i = 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 Hf 1-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. (Abstract shortened by UMI.)
|Advisor:||Greene, Joseph E., Petrov, Ivan G.|
|School:||University of Illinois at Urbana-Champaign|
|School Location:||United States -- Illinois|
|Source:||DAI-B 72/07, Dissertation Abstracts International|
|Subjects:||Condensed matter physics, Nanotechnology, Materials science|
|Keywords:||Aluminum nitride, Hafnium, Hard coatings, Sputtering, Thin films, Transition metal nitride|
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