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Effect of off stoichiometry on Raman scattering from epitaxial and polycrystalline HfNx (0.85≤x≤1.50) grown on MgO(001)

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4 Author(s)
Stoehr, M. ; Department of Materials Science and the Frederick Seitz Materials Research Laboratory, University of Illinois, 104 South Goodwin Avenue, Urbana, Illinois 61801, USA ; Seo, H.-S. ; Petrov, I. ; Greene, J.E.

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Group-IV HfNx transition-metal nitride layers, with 0.85≤x≤1.50, are grown on MgO(001) and analyzed by Raman spectroscopy. Samples with 0.85≤x≤1.20 are single-phase epitaxial NaCl-structure HfNx(001), while layers with higher N concentrations are two-phase mixtures containing N-rich inclusions. All samples exhibit both first- and second-order Raman scattering. The presence of first-order scattering indicates that Oh symmetry, a characteristic of the NaCl crystal structure, is broken even in stoichiometric HfN. As x decreases in understoichiometric HfNx samples, corresponding to a decrease in the number of valence electrons per unit cell from nine (x=1) to eight (x=0.75), the positions of the acoustic lines shift to higher frequencies. This provides an indirect probe of phonon anomalies in acoustic mode dispersion curves and signifies an increase in N vacancy concentration. The persistence of strong first-order acoustic scattering from overstoichiometric (x≫1) samples indicates that adding more N does not substantially reduce the average defect density around Hf sites. The absence of a frequency shift in the acoustic modes of overstoichiometric HfNx, with 1.17≤x≤1.27, also shows that the local order around Hf sites does not change significantly, while the observed shift of the first-order optical modes to higher frequency with increasing x is characteristic of increasing lattice disorder adjacent to N sites. The Raman results, together with the nature of the variation in the HfNx lattice parameter,- the electron carrier concentration, and the superconducting transition temperature as a function of x, can be explained by the presence of N vacancies for x≪1, both N and Hf vacancies in stoichiometric HfN(001), and the combination of N vacancies and NHf antisite defects for x≫1.

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Journal of Applied Physics  (Volume:104 ,  Issue: 3 )