Growth and physical properties of epitaxial CeN and polycrystalline Ti_1-xCe_xN layers

T.-Y. Lee, D. Gall, C.-S. Shin, N. Hellgren, J. G. Wen, R. Twesten, J. E. Greene, and I. Petrov

Department of Materials Science, the Materials Research Laboratory, and the Coordinated Science Laboratory, University of Illinois, 104 South Goodwin, Urbana, IL 61801

Abstract

While NaCl-structure transition-metal (TM) nitrides have been widely studied over the past two decades, little is known about the corresponding NaCl-structure rare-earth nitrides. Polycrystalline CeN, for example, has been reported by different groups to be both a wide bandgap semiconductor and a metal. To address this controversy, we have grown epitaxial CeN layers on MgO(001) and measured their physical properties. CeN is metallic with a positive temperature coefficient of resistivity and a temperature-independent carrier concentration of 2.8±0.2×10²²cm^-3 with a room temperature mobility of 0.31 cm² V^-1-s^-1. At temperatures between 2 and 50 K, the resistivity remains constant at 29 mW-cm, while at higher temperatures it increases linearly to reach a room-temperature value of 68.5 mW-cm. The hardness and elastic modulus of CeN(001) were determined from nanoindentation measurements to be 15.0±0.9 and 330±16 GPa.

In an attempt to improve properties of titanium nitride, quasi-binary titanium-nitride-based alloys have received considerable attention. It is reasonable to expect that alloying CeN, which has different lattice constant (a^o_CeN = 0.504 nm) and elastic modulus, with TiN (a^o_TiN = 0.424 nm) will affect significantly microstructure and physical properties of TM nitride polycrystalline layers. In deed, during reactive sputter-deposition of metastable Ti_1-xCe_xN alloys, we observed nanophase films with x > 0.1. Under conditions of low ion-irradiation, i.e. grounded or floating substrates, the nanostructure consists of equiaxed grains which forms due to continuous renucleation induced by CeN segregation. This is analogous to the nanostructure to the one observed in crystalline/amorphous nanocomposites, e.g. TiN/Si₃N₄. In contradistinction, a novel nanocolumnar structure forms when the alloys are grown under intense ion-irradiation with J_i/J_Me > 15 and E_i = 45 eV. The intense ion mixing in the near surface area allows sufficient adatom mobility to form local TiN- and CeN-rich areas that propagate along the growth direction.