Growth, surface morphology, and electrical resistivity of fully strained substoichiometric epitaxial TiNx (0.67⩽x<1.0) layers on MgO(001)

We have grown single-crystal NaCl-structure δ-TiNx layers with x ranging from 0.67 to 1.00 on MgO(001) at 700 °C by ultra-high-vacuum reactive magnetron sputtering of Ti in mixed Ar/N2 discharges in order to investigate microstructural evolution and the physical properties of TiNx as a function of the N vacancy concentration. High-resolution x-ray diffraction and transmission electron microscopy results show that all layers grow with a cube-on-cube epitaxial relationship to the substrate, (001)TiN∥(001)MgO and [100]TiN∥[100]MgO. The relaxed lattice parameter ao(x) decreases linearly from 4.240 Å with x=1.00 to 4.226 Å with x=0.67. Stoichiometric TiN(001) layers are fully relaxed at the growth temperature while layers with 0.67⩽x⩽0.92 are fully coherent with their substrates. Surface morphologies vary dramatically with x. TiNx(001) layers with x=0.67–0.82 have very flat surfaces arising from large cation surface diffusion lengths approaching values corresponding to step flow. However, the surfaces of the TiN0.92(001) and TiN1.00(001) layers, which were grown at higher N2 partial pressures, consist of a periodic two-domain ripple structure along the 〈110〉 directions due to kinetic roughening associated with lower cation surface mobilities resulting from higher steady state N coverages. TiN1.0(001) layers grown in pure N2 exhibit growth mounds that are predominantly square with edges aligned along the 〈110〉 directions. The room-temperature resistivity, 13 μΩ cm with x=1.00, increases from 52 μΩ cm for TiNx(001) layers with x=0.92 to 192 μΩ cm with x=0.67, due primarily to increased carrier scattering from N vacancies.