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Park, Dae-Sung,Vasheghani Farahani, Sepehr K.,Walker, Marc,Mudd, James J.,Wang, Haiyuan,Krupski, Aleksander,Thorsteinsson, Einar B.,Seghier, Djelloul,Choi, Chel-Jong,Youn, Chang-Ju,McConville, Chris F American Chemical Society 2014 ACS APPLIED MATERIALS & INTERFACES Vol.6 No.21
<P>We investigate the effect of thermally induced phase transformations on a metastable oxide alloy film, a multiphase Be<SUB><I>x</I></SUB>Zn<SUB>1–<I>x</I></SUB>O (BZO), grown on Al<SUB>2</SUB>O<SUB>3</SUB>(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<I>m</I><SUB>0</SUB> and 0.67<I>m</I><SUB>0</SUB>. A conductivity as high as 1.4 × 10<SUP>3</SUP> S·cm<SUP>–1</SUP> is attained, corresponding to the carrier concentration <I>n</I><SUB>Int</SUB> = 2.16 × 10<SUP>20</SUP> cm<SUP>–3</SUP> 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<SUB><I>x</I></SUB>Zn<SUB>1–<I>x</I></SUB>O alloys for the application of highly conductive and transparent oxide-based devices and fabrication of their alloy nanostructures.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2014/aamick.2014.6.issue-21/am5043388/production/images/medium/am-2014-043388_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am5043388'>ACS Electronic Supporting Info</A></P>