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.
High mobility in ZnO thin films deposited on perovskite substrates with a low temperature nucleation layer Appl. Phys. Lett. 86, 012109 (2005); 10.1063/1.1844034Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxy
We report on the observation of stimulated emission in ZnO/Mg x Zn 1Ϫx O superlattices well above room temperature. Two kinds of superlattices grown by laser molecular-beam epitaxy showed clear systematics on the quantum subband levels in absorption and spontaneous emission spectra. Stimulated emission with excitonic origin could be observed at very low optical pumping levels. The threshold excitation intensity changed from 11 to 40 kW/cm 2 , and the emission energy could be tuned between 3.2 and 3.4 eV, depending on the well thickness and/or the Mg content in the barrier layers. The excitonic stimulated emission could be observed up to 373 K and the characteristic temperature was as high as 87 K.
We report on the optical properties of ZnO/͑Mg, Zn͒O multiple quantum wells ͑MQWs͒ on lattice-matched ScAlMgO 4 substrates fabricated by laser molecular-beam epitaxy. As the well layer thickness decreased down to 7 Å, the photoluminescence ͑PL͒ and absorption peaks showed a systematic blueshift, consistent with the quantum-size effect. Moreover, a bright PL of free excitons could be observed even at room temperature. As a result, the PL could be tuned in the energy range of 3.3-3.6 eV by choosing the appropriate barrier height and well layer thickness. The widest tunability on the room-temperature luminescence of the excitons could be attained on the basis of the ZnO quantum structure. These favorable properties could not be attained in the MQWs on lattice-mismatched sapphire substrates.
Optical properties in undoped-ZnO epilayers grown by the laser-molecular-beam epitaxy method on lattice-matched ScAlMgO4 substrates were investigated. The absorption spectrum at 5 K has two sharp peaks, both of which are attributed to resonances of A and B excitons, which reflect a small nonradiative damping constant of excitons as well as high film crystallinity accomplished by the virtue of lattice matching. The coupling strengths of exciton-acoustic phonon and of exciton–longitudinal-optical phonon were directly determined from the temperature dependence of exciton absorption spectra independently for A and B excitons, which are close in energy and obey the same selection rule for each other.
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