Microstructure and electronic properties of the refractory semiconductor ScN grown on MgO(001) by ultra-high-vacuum reactive magnetron sputter deposition

Abstract

ScN layers, 180 nm thick, were grown on MgO(001) substrates at 750 °C by ultra-high-vacuum reactive magnetron sputter deposition in pure N2 discharges. N/Sc ratios, determined by Rutherford backscattering spectroscopy, were 0.98±0.02. X-ray diffraction θ–2θ scans and pole figures combined with plan-view and cross-sectional transmission electron microscopy showed that the films are strongly textured, both in plane and along the growth direction, and have a columnar microstructure with an average column width near the film surface of 30±5 nm. During nucleation and the early stages of film growth, the layers consist of approximately equal volume fractions of 002- and 111-oriented grains. However, preferred orientation evolves toward a purely 111 texture within ≃40 nm as the 002 grains grow out of existence in a kinetically limited competitive growth mode. 002 grains exhibit local cube-on-cube epitaxy with an orientation relationship (001)ScN∥(001)MgO and [010]ScN∥[010]MgO while 111 grains have a complex fourfold 90°-rotated in-plane preferred orientation in which strained ScN triangular {111} surface unit cells exhibit local epitaxy with square MgO unit cells yielding the orientation relationship (111)ScN∥(001)MgO, [11̄0]ScN∥[11̄0]MgO, and [112̄]ScN∥[110]MgO. Room-temperature electrical resistivity ρ is 1.2×104 μΩ cm while dρ/dT was found to be negative, indicating semiconducting behavior, with ρ varying from 1.6×104μΩ cm at 80 K to 1.1×104 μΩ cm at 400 K. Optical absorption coefficients ranged from 1×104 cm−1 at 1.5 eV to 2.6×105 cm−1 at 3.5 eV with a well-defined edge corresponding to a direct transition at 2.37±0.05 eV.