| AVS 62nd International Symposium & Exhibition | |
| Thin Film | Thursday Sessions |
| Session TF+AS+NS+SA-ThA |
| Session: | Thin Film: Growth and Characterization, Optical and Synchrotron Characterization II |
| Presenter: | Antonio Mei, University of Illinois at Urbana-Champaign |
| Authors: | A.B. Mei, University of Illinois at Urbana-Champaign O. Hellman, California Institute of Technology N. Wireklint, Chalmers University of Technology, Sweden C.M. Schlepütz, Argonne National Laboratory D.G. Sangiovanni, Linköping University, Sweden B. Alling, Linköping University, Sweden A. Rockett, University of Illinois at Urbana-Champaign L. Hultman, Linköping University, Sweden I. Petrov, University of Illinois at Urbana-Champaign J.E. Greene, University of Illinois at Urbana-Champaign |
| Correspondent: | Click to Email |
Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), and resistivity measurements combined with high-resolution cross-sectional transmission electron microscopy (HR-XTEM) and ab-initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, at temperatures below Tc = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a non-centrosymmetric tetragonal structure. At T ≤ Tc, forbidden reflections intensify continuously following the scaling behavior I α (Τc - T)1/2. This suggests a transition of second-order character, but the incompatible symmetries of the parent cubic VN (space group) and product tetragonal VN () phases ultimately requires that the structural transition be first order. Resistivity ρ(T) plots between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in cubic and tetragonal VN.
The VN transport Eliashberg spectral function α2trF(ℏω), the product of the phonon density-of-states F(ℏω) and the transport electron/phonon coupling strength α2tr(ℏω), is determined and used in combination with room-temperature inelastic neutron scattering results to establish that while entropy favors the higher-temperature cubic phase, as T is lowered below Tc, tetragonal VN becomes the thermodynamic ground state due to its lower enthalpy. AIMD renormalized phonon dispersion relations are in agreement, showing that the energy of (00l) X-point phonons in NaCl-structure VN decreases and eventually becomes imaginary at T ≤ 250 K. We demonstrate that free energy contributions due to vibrational entropy, often-neglected in theoretical modeling, are essential in understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.