title: Recrystallization of highly-mismatched Be(x)Zn(1-x)O alloys: formation of a degenerate interface.
abstract: We investigate the effect of thermally induced phase transformations on a metastable oxide alloy film, a multiphase Be(x)Zn(1-x)O (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 0.31 m0 and 0.67 m0. A conductivity as high as 1.4 × 10(3) S · cm(-1) is attained, corresponding to the carrier concentration n(Int) = 2.16 × 10(20) 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(x)Zn(1-x)O alloys for the application of highly conductive and transparent oxide-based devices and fabrication of their alloy nanostructures.
authors.affiliation: Department of Physics, University of Warwick , Coventry CV4 7AL, United Kingdom.
authors.forename: Dae-Sung
authors.initials: DS
authors.lastname: Park
authors.forename: Sepehr K
authors.initials: SK
authors.lastname: Vasheghani Farahani
authors.forename: Marc
authors.initials: M
authors.lastname: Walker
authors.forename: James J
authors.initials: JJ
authors.lastname: Mudd
authors.forename: Haiyuan
authors.initials: H
authors.lastname: Wang
authors.forename: Aleksander
authors.initials: A
authors.lastname: Krupski
authors.forename: Einar B
authors.initials: EB
authors.lastname: Thorsteinsson
authors.forename: Djelloul
authors.initials: D
authors.lastname: Seghier
authors.forename: Chel-Jong
authors.initials: CJ
authors.lastname: Choi
authors.forename: Chang-Ju
authors.initials: CJ
authors.lastname: Youn
authors.forename: Chris F
authors.initials: CF
authors.lastname: McConville
keywords: BeZnO; atomic diffusion; defect engineering; degenerate interface; oxide alloys; phase transformation