Growth of poly- and single-crystal ScN on MgO (001):: Role of low-energy N+2 irradiation in determining texture, microstructure evolution, and mechanical properties

ScN layers, 345 nm thick, were grown on MgO(001) substrates at 750 degrees C by ultrahigh-vacuum reactive magnetron sputter deposition in pure N-2 discharges at 5 mTorr. The N-2(+) to Sc ratio incident at the substrate and growing film was maintained constant at 14, while the ion energy EN+2 was varied from 13 to 50 eV. All films were stoichiometric with N/ Sc ratios of 1.00+/-0.02. However, microstructural and surface morphological evolution were found to depend strongly on EN2+. The nucleation and initial growth stages of ScN films deposited with EN2+ = 13 eV are dominated by the formation of 111- and 002-oriented islands, but preferred orientation rapidly evolves toward a purely 111 texture by a film thickness of similar or equal to 50 nm as 002 grains grow out of existence in a kinetically limited competitive growth mode. In distinct contrast, films deposited with EN2+ = 20 eV grow in a cube-on-cube epitaxial relationship with the substrate and exhibit no indication of 111-oriented grains, even in the earliest stages. Increasing EN2+ to 50 eV still results in epitaxial layers, but with high in-plane compressive stress and the presence of N-2 gas bubbles. All epitaxial layers contain rectangular nanopipes similar or equal to 1 nm wide and aligned along the growth direction. The nanopipes result from atomic shadowing near the bottom of a periodic array of surface cusps which form along orthogonal [100] directions due to kinetic roughening during growth. The hardness H and elastic modulus E of the epitaxial ScN(001) layer grown with EN2+ = 20 eV are 21.1+/-1.1 and 356+/-18 GPa, respectively. H and E increase (decrease) with increasing (decreasing) EN2+. (C) 1998 American Institute of Physics. [S0021- 8979(98)01623- 5].