Degenerate interface layers in epitaxial scandium-doped ZnO thin films

Scandium is a donor in ZnO, in contradiction to all heavier 3d-elements. Although the Sc3+ ion fits exactly the size of Zn2+, the conductivity of epitaxial Sc-doped ZnO thin films is dominated by a degenerated interface layer with pronounced Sc accumulation in the concentration depth profile. Electron paramagnetic resonance clearly excludes the isovalent oxidation state Sc2+ in ZnO, in agreement with the low resistivity of homoepitaxial ZnO : Sc of 1.3 x 10(-2) Omega cm and high Hall mobility of 139 cm(2) V-1 s(-1) at 300 K. Employing the two-layer Hall model of Look and Molnar, a maximum Hall mobility of the volume part of the ZnO : Sc layer of 650 cm(2) V-1 s(-1) at 95 K was extracted. Thermal activation energies of the donors are 28 and 43 meV for a homoepitaxial ZnO : 0.1%Sc film. Homoepitaxial ZnO films with 0.1 and 1 at% Sc on c-plane ZnO are almost phase pure and in-plane lattice matched with perpendicular compressive strain up to 1%. Heteroepitaxial ZnO : Sc on a-plane sapphire shows secondary XRD peaks, pointing to a limited solubility of Sc in ZnO.