We report on the microstructural evolution and physical properties of epitaxial delta-TaN layers grown on MgO(001) at 600 degreesC by ultrahigh vacuum reactive magnetron sputtering of Ta in mixed Ar/N-2 discharges as a function of the N-2 fraction fN(2) (0.100 less than or equal to f(N2) less than or equal to 0.275) and the incident ion energy E-i (8.4less than or equal toE(i)less than or equal to65 eV). The ratio of the ion-to-Ta fluxes J(i)/J(Ta) incident at the growing film was maintained at 11+/-0.5 with 0.100less than or equal tof(N2)less than or equal to0.200 and 15+/-0.3 with 0.250less than or equal tof(N2)less than or equal to0.275. High-resolution x-ray diffraction, transmission electron microscopy, and Rutherford backscattering spectrometry results show that films grown with E(i)less than or similar to40 eV are single-phase delta-TaNx(001), with x increasing from 0.94 with f(N2)=0.100 to 1.37 with f(N2)=0.275, exhibiting a cube-on-cube epitaxial relationship with the substrate: (001)(delta-TaN)parallel to(001)(MgO) and [100](delta-TaN)[100]Mgo. However, the use of E(i)less than or equal to20 eV leads to delta- TaNx(001) layers which are underdense with a self-organized array of 1-nm-wide nanopipes oriented predominantly along orthogonal (100) directions. The nanopipes, which are first observed at film thicknesses of similar or equal to5 nm and extend to the surface, form due to the combination of low adatom surface diffusivities, leading to kinetic roughening, and atomic shadowing. Fully dense stoichiometric epitaxial delta-TaN(001) films were obtained with E-i=30 eV and f(N2)=0.125. Increasing E-i to 40 eV results in a high density of {111} stacking faults. Films grown with even higher ion energies, 40