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Title:  Lattice dynamics and electron/phonon interactions in epitaxial transitionmetal nitrides 
Author(s):  Mei, Antonio Rodolph Bighetti 
Director of Research:  Petrov, Ivan 
Doctoral Committee Chair(s):  Rockett, Angus A. 
Doctoral Committee Member(s):  Greene, Joseph E; Allen, Lesli; Zuo, JianMin 
Department / Program:  Materials Science & Engineerng 
Discipline:  Materials Science & Engr 
Degree Granting Institution:  University of Illinois at UrbanaChampaign 
Degree:  Ph.D. 
Genre:  Dissertation 
Subject(s):  nitrides
transitionmetal nitrides titanium nitride vanadium nitride zirconium nitride hafnium nitride magnetron sputtering unbalanced magnetron sputter deposition anharmonic phonons transport resistivity elastic constants eliashberg electronphonon coupling ellipsometry dielectric function hardness modulus thin film epitaxial epitaxy growth raman 
Abstract:  Transition metal (TM) nitrides, due to their unique combination of remarkable physical properties and simple NaCl structure, are presently utilized in a broad range of applications and as model systems in the investigation of complex phenomena. GroupIVB nitrides TiN, ZrN, and HfN have transport properties which include superconductivity and high electrical conductivity; consequentially, they have become technologically important as electrodes and contacts in the semiconducting and superconducting industries. The GroupVB nitride VN, which exhibits enhanced ductility, is a fundamental component in superhard and tough nanostructured hard coatings. In this thesis, I investigate the lattice dynamics responsible for controlling superconductivity and electrical conductivities in GroupIVB nitrides and elasticity and structural stability of the NaClstructure GroupVB nitride VN. Our group has already synthesized highquality epitaxial TiN, HfN, and CeN layers on MgO(001) substrates. By irradiating the growth surface with high ion fluxes at energies below the bulk latticeatom displacement threshold, dense epitaxial single crystal TM nitride films with extremely smooth surfaces have been grown using ultrahigh vacuum magneticallyunbalanced magnetron sputter deposition. Using this approach, I completed the GroupIVB nitride series by growing epitaxial ZrN/MgO(001) films and then grew GroupVB nitride VN films epitaxially on MgO(001), MgO(011), and MgO(111). The combination of highresolution xray diffraction (XRD) reciprocal lattice maps (RLMs), highresolution crosssectional transmission electron microscopy (HRXTEM), and selectedarea electron diffraction (SAED) show that singlecrystal stoichiometric ZrN films grown at 450 °C are epitaxially oriented cubeoncube with respect to their MgO(001) substrates, (001)ZrN(001)MgO and [100]ZrN[100]MgO. The layers are essentially fully relaxed with a lattice parameter of 0.4575 nm. Xray reflectivity results reveal that the films are completely dense with smooth surfaces (roughness = 1.3 nm, consistent with atomicforce microscopy analyses). Based upon temperaturedependent electronic transport measurements, epitaxial ZrN/MgO(001) layers have a roomtemperature resistivity ρ300K of 12.0 μΩcm, a temperature coefficient of resistivity between 100 and 300 K of 5.6×108 Ωcm K 1, a residual resistivity ρo below 30 K of 0.78 μΩcm (corresponding to a residual resistivity ratio ρ300Κ/ρ15K = 15), and the layers exhibit a superconducting transition temperature Tc = 10.4 K. The relatively high residual resistivity ratio, combined with long inplane and outofplane xray coherence lengths, ξ = 18 nm and ξ⊥ = 161 nm, indicates high crystalline quality with low mosaicity. The reflectance of ZrN(001), as determined by variableangle spectroscopic ellipsometry, decreases slowly from 95% at 1 eV to 90% at 2 eV with a reflectance edge at 3.04 eV. Interband transitions dominate the dielectric response above 2 eV. The ZrN(001) nanoindentation hardness and modulus are 22.7±1.7 and 450±25 GPa. Transport electron/phonon coupling parameters and Eliashberg spectral functions αtr2F(ℏω) are determined for GroupIV TM nitrides TiN, ZrN, and HfN, and the rareearth (RE) nitride CeN using an inversion procedure based upon temperaturedependent (4 < T < 300 K) resistivity measurements. Transport electron/phonon coupling parameters λtr vary from 1.11 for ZrN to 0.82 for HfN, 0.73 for TiN, and 0.44 for CeN. The small variation in λtr among the TM nitrides and the weak coupling in CeN are consistent with measured Tc values: 10.4 (ZrN), 9.18 (HfN), 5.35 (TiN), and < 4 K for CeN. The Eliashberg spectral function describes the strength and energy spectrum of electron/phonon coupling in conventional superconductors. Spectral peaks in α2F(ℏω), corresponding to regions in energyspace for which electrons couple to acoustic ℏωac and optical ℏωop phonon modes, are centered at ℏωac = 33 and ℏωop = 57 meV for TiN, 25 and 60 meV for ZrN, 18 and 64 meV for HfN, and 21 and 39 meV for CeN. The acoustic modes soften with increasing cation mass; optical mode energies remain approximately constant for the TM nitrides, but are significantly lower for the RE nitride due to a lower interatomic force constant. Optical/acoustic peakintensity ratios are 1.15±0.1 for all four nitrides, indicating similar electron/phonon coupling strengths αtr(ℏω) for both modes. Elastic constants are determined for singlecrystal stoichiometric NaClstructure VN(001), VN(011), and VN(111) epitaxial layers grown by magneticallyunbalanced reactive magnetron sputter deposition on 001, 011, and 111oriented MgO substrates at 430 oC. The relaxed lattice parameter ao = 0.4134±0.0004 nm, obtained from highresolution reciprocal space maps, and the mass density ρ = 6.1 g/cm3, determined from the combination of Rutherford backscattering spectroscopy and film thickness measurements, of the VN layers are both in good agreement with reported values for bulk crystals. Subpicosecond ultrasonic optical pump/probe techniques are used to generate and detect VN longitudinal sound waves with measured velocities v001 = 9.8±0.3, v011 = 9.1±0.3, and v111 = 9.1±0.3 km/s. The VN c11 elastic constant is determined from the sound wave velocity measurements as 585±30 GPa; the c44 elastic constant, 126±3 GPa, is obtained from surface acoustic wave measurements. From the combination of c11, c44, vhkl, and ρ, the VN c12 elastic constant is 178±33 GPa, the VN elastic anisotropy A = 0.62, the isotropic Poisson ratio ν = 0.29, and the anisotropic Poisson ratios ν001 = 0.23, ν011 = 0.30, and ν111 = 0.29. The elastic stability criteria requires cubic crystals to resist [001] and [011] shears as well as isotropic compression or, equivalently, for G001 = (c11 – c12)/2 > 0, G011 = c44 > 0, and B = (c11 + 2c12)/3 > 0, in which G001 and G011 are directional shear moduli and B is the bulk modulus. Thus, NaClstructure VN is elastically stable at room temperature. Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperaturedependent synchrotron XRD, SAED, resistivity measurements, HRXTEM, and abinitio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below Tc = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I ∝ (Τc  T)1/2. Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal VN phases. The VN transport Eliashberg spectral function α2trF(ℏω), the product of the phonon densityofstates F(ℏω) and the transport electron/phonon coupling strength α2tr(ℏω), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T > Tc. Freeenergy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the roomtemperature stability of NaClstructure VN, and of strongly anharmonic systems in general. 
Issue Date:  20150520 
Type:  Thesis 
URI:  http://hdl.handle.net/2142/87950 
Rights Information:  Copyright 2015 Antonio Rodolph Bighetti Mei 
Date Available in IDEALS:  20150929 
Date Deposited:  August 201 
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Dissertations and Theses  Materials Science and Engineering

Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois