Raman scattering from epitaxial TaNx (0.94≤x≤1.37) layers grown on MgO(001)

Single-crystal group-V delta-TaNx transition-metal (TM) nitride layers, with x ranging from 0.94 to 1.37, are grown on MgO(001) and analyzed by Raman spectroscopy. In perfect NaCl-structure crystals, the first-order Raman modes are not allowed by the O-h crystal symmetry. Since first-order acoustic and optical bands are observed for all delta-TaNx samples, this indicates the presence of point defects, even for stoichiometric TaN. The large mass difference between Ta and N leads to a correspondingly large separation between the first-order acoustic and optical bands centered at 150 and 550 cm(-1), respectively. A Raman peak at 440 cm(-1), overlapping the first-order optical band, is identified as the difference between the first-order optical and acoustic modes. Contrary to the case for most TM nitrides, there is no peak visible in the second-order high-frequency range (700-1500 cm(-1)). In addition, the first-order transverse (120 cm(-1)) and longitudinal acoustic peaks (175 cm(-1)) do not exhibit the frequency shift with N/Ta ratio x (i.e., with the valence electron density) that has been reported for group-IV TM nitrides. This is consistent with the resistivity of TaNx remaining essentially constant, as well as with the very weak dependence of the lattice parameter as a function of the N/Ta ratio, and suggests that the wide single-phase region in this system is due to the presence of isoelectronic antisite substitutions rather than, as has been proposed previously, cation and anion vacancies. (c) 2007 American Institute of Physics.