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The second-order non-linear susceptibility components were measured using 1.064 μm incident light for ZnO thin films of various thicknesses from 24.4 to 283 nm self-assembled on sapphire substrates by laser molecular beam epitaxy. It was found that the values of the non-linear susceptibility for the films are almost the same as those of bulk material, except the samples with thicknesses ranging from 35 to 64.8 nm, which show a large enhancement effect. For the sample with a thickness of 44.4 nm, the second-order non-linear susceptibility components were found to be approximately 14.7 pm/v for d31, 15.2 pm/v for d15, and −83.7 pm/v, a value approximately 14 times that of the bulk material, for d33. The second-order non-linear coefficient enhancement in the thin films may be resulted from the microcrystallite structures. [Copyright &y& Elsevier]

The lasing properties in ZnO thin films grown by laser molecular beam epitaxy on sapphire substrates have been studies. The excitation dependence of the optical gain spectra has been obtained at room temperature using a variable stripe length method. At moderate excitation intensities, the optical gain responsible for lasing is shown as a result of an exciton–exciton scattering process. At higher excitation intensities, the exciton–hole plasma processes become more dominant and exhibit the highest gain reaching value 571 cm−1 at an excitation density of 280 kW/cm2. An optical loss in ZnO thin film at room temperature was found to be a low value of 68 cm−1. A lasing threshold of 85 kW/cm2 has been observed at room temperature. [Copyright &y& Elsevier]

We report on the observation of stimulated emission in ZnO/Mg[sub x]Zn[sub 1-x]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[sup 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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The undoped ZnO thin films grown on Si (111) substrates by plasma-assisted molecular beam epitaxy (P-MBE) were reported. The effects of growth temperature, the thickness of low temperature ZnO buffer layer and Zn/O ratio on the quality of high-temperature-overgrown ZnO main layer were studied by atomic force microscopy (AFM), X-ray diffraction (XRD) and room-temperature photoluminescence (PL) spectra. These results showed that it was difficult to directly obtain high quality ZnO films on Si substrate. By introducing a thin ZnO buffer layer at 350°C, c-axis preferred orientation ZnO films with improved optical properties were obtained at 750°C. However, the thickness of ZnO buffer layer and Zn/O ratio in the ZnO main layer greatly influenced the quality of high-temperature-overgrown ZnO main layer. [Copyright &y& Elsevier]

Abstract: A co-activating route was employed to fabricate ZnO light-emitting diode (LED) by using molecular beam epitaxy. N2 was used as the acceptor dopant source and O2 was used as assistant gas for N2 decomposing more than only oxygen source. Emission spectra of the N2+O2 mixture plasma were monitored in situ in order to adjust growth parameters timely. Under the assistance of O2, N atoms content in the plasma of the mixture shows a significant increase compared to the case without O2 assistance. Electrical measurements of the as-grown p-type ZnO on sapphire show a carrier concentration of 1.2×18cm−3 and a mobility approaching 1cm2 V−1 s−1. A ZnO LED was fabricated by depositing undoped n-type ZnO on the p-type layer. The turn-on voltage at 100K is about 3.70V, which approaches the bandgap of ZnO. Electroluminescence spectra show two bands: one is centered at 423 and the other centered at 523nm, respectively. [Copyright &y& Elsevier]