Recrystallization of highly-mismatched Be_xZn_1-xO alloys: Formation of a degenerate interface

We investigate the effect of thermally induced phase transformations on a metastable oxide alloy film, a multiphase Be_xZn_1-xO (BZO), grown on Al₂O₃(0001) substrate for annealing temperatures in the range of 600-950 °C. A pronounced structural transition is shown together with strain relaxation and atomic redistribution in the annealed films. Increasing annealing temperature initiates out-diffusion and segregation of Be and subsequent nucleation of nanoparticles at the surface, corresponding to a monotonic decrease in the lattice phonon energies and band gap energy of the films. Infrared reflectance simulations identify a highly conductive ZnO interface layer (thicknesses in the range of ≈10-29 nm for annealing temperatures ≥800 °C). The highly degenerate interface layers with temperature-independent carrier concentration and mobility significantly influence the electronic and optical properties of the BZO films. A parallel conduction model is employed to determine the carrier concentration and conductivity of the bulk and interface regions. The density-of-states-averaged effective mass of the conduction electrons for the interfaces is calculated to be in the range of 0.31m₀ and 0.67m₀. A conductivity as high as 1.4 × 10³ S·cm^-1 is attained, corresponding to the carrier concentration n_Int = 2.16 × 10²⁰ cm^-3 at the interface layers, and comparable to the highest conductivities achieved in highly doped ZnO. The origin of such a nanoscale degenerate interface layer is attributed to the counter-diffusion of Be and Zn, rendering a high accumulation of Zn interstitials and a giant reduction of charge-compensating defects. These observations provide a broad understanding of the thermodynamics and phase transformations in Be_xZn_1-xO alloys for the application of highly conductive and transparent oxide-based devices and fabrication of their alloy nanostructures.