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Abstract: In this paper, we reviewed our recent study on ultraviolet stimulated emission and lasing observed at room temperature from nano-structured ZnO thin films. The nano-structured ZnO thin films were grown on sapphire substrates using Laser-Molecular-Beam-Epitaxy (L-MBE). The thin film was consisted of regularly arrayed hexagonal nano-crystallite columns, whose facets form natural micro-cavities. These nano-crystallites confine the centre-of-mass motion of excitons. As a result of the quantum size effect, the oscillation strength of the excitons is largely enhanced, which is favored to the radiate recombination of exciton at room temperature. Excited using the frequency-tripled output of a YAG laser, the nano-structured ZnO thin film showed strong ultraviolet lasing at room temperature with a threshold as low as 24kW/cm2. At a moderate pumping intensity, the room temperature stimulated emission is associated with an exciton-exciton collision process. At higher pumping density, the excitons are dissociated, and the ultraviolet stimulated emission is dominated by an electron-hole plasma recombination process. Because of the large enhancement of oscillator strength of the excitons, the optical gain of the stimulated emission measured at room temperature reaches as high as 320 cm-1, which is an order higher than that observed in bulk ZnO crystals. In comparison with the electron-hole plasma stimulated emission in most of commercial semiconductor lasers, the excitonic stimulated emission can be realized at relatively low external pumping density. The observation of excitonic lasing effect at room temperature might be valuable in realization of practical ultraviolet semiconductor laser devices. [Copyright &y& Elsevier]

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]

Reports on the measurement of third-order optical nonlinearity of ZnO microcrystallite thin films near the excitonic resonance at various temperatures. Use of the femtosecond degenerate four-wave-mixing technique in the study; Response times measured.

The mechanism of ultraviolet stimulated emission was investigated in ZnO/ZnMgO multiquantum wells. Stimulated emission induced by exciton-exciton scattering occurred throughout a range of temperatures from 5 K to room temperature. At temperatures higher than 160 K, stimulated emission due to electron-hole plasma recombination was also observed with a higher excitation threshold than that of exciton-exciton scattering. The exciton binding energies of multiquantum wells were larger than that of bulk ZnO and increased with a decrease in the well widths. This enhancement of exciton binding energy is due to the quantum-confinement effect and is favorable for the stability of exciton states. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Highly (1000) oriented ZnO films were successfully grown onto silicon substrates using a dual plasma deposition process incorporating a metal cathodic arc source and oxygen ambient. The optical properties of a series of ZnO films deposited on quartz using similar conditions were investigated using UV–visible spectrophotometry, and the effects on the band gap were investigated. The absorption of C-excitons resulting in a blue-shift of the band gap at room temperature is accentuated due to strong absorption at higher energy for light polarized ( E // c) in the highly (1000) oriented ZnO film. [Copyright &y& Elsevier]

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]