We describe the structural and optical properties of II-VI oxide alloys, Mg x Zn 1Ϫx O and Cd y Zn 1Ϫy O, grown by pulsed-laser deposition. Single-phase alloyed films of ͑Mg,Zn͒O and ͑Cd,Zn͒O with c-axis orientations were epitaxially grown on sapphire ͑0001͒ substrates. The maximum magnesium and cadmium concentrations ͑xϭ0.33 and yϭ0.07, respectively͒ were significantly larger than the thermodynamic solubility limits. The band gap energies systematically changed from 3.0 (yϭ0.07) to 4.0 eV (xϭ0.33) at room temperature. The photoluminescence peak energy deduced at 4.2 K could be tuned from 3.19 to 3.87 eV by using Cd 0.07 Zn 0.93 O and Mg 0.33 Zn 0.67 O at both ends, respectively. The lattice constants of the a axis were monotonically increasing functions of the concentrations of both alloys. The exciton-phonon coupling strength was determined in Cd 0.01 Zn 0.99 O grown on a lattice-matched ScAlMgO 4 substrate.
We have examined the thermal stability of wurtzite-phase Mg x Zn 1Ϫx O alloy films and ZnO/Mg x Zn 1Ϫx O bilayer films with x exceeding the reported solubility limit of 0.04. When a Mg 0.23 Zn 0.78 O film was annealed, the segregation of MgO started at 850°C and the band gap was reduced to the value of that for an xϭ0.15 film after annealing at 1000°C. Mg 0.15 Zn 0.85 O films showed no change of the band gap even after annealing at 1000°C. Therefore, we conclude that the thermodynamic solubility limit of MgO in Mg x Zn 1Ϫx O epitaxial film is about xϭ0.15. The thermal diffusion of Mg across the Mg x Zn 1Ϫx O/ZnO interface was observed only after annealing above 700°C. Unlike other II-VI semiconductors, ZnO-based alloy films and heterointerfaces are stable enough for the fabrication of high-crystallinity heterostructures.
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