Xiwu Fan

Low‐Threshold Electrically Pumped Random Lasers

Electrically pumped random lasers are realized in ZnO nanocrystallite films in a simple metal-oxide-semiconductor structure. By introducing an i-ZnO layer, a threshold current of 6.5 mA is obtained. The reported results provide a simple route to electrically pumped random lasing (see figure) with relatively low threshold, a significant step towards the future applications of this kind of laser.

Photoluminescence properties of MgxZn1-xO alloy thin films fabricated by the sol-gel deposition method

The photoluminescence properties of MgxZn1−xO alloy thin films fabricated by the sol-gel deposition method were studied. The Mg2+ content in the films was up to 0.36 and they had the ZnO wurtzite structure. The band gap of the films can be controlled between 3.40 and 3.93 eV by adjusting the Mg2+ proportions. Transmittance spectroscopy was used to characterize the excitonic structure of the alloys, which the excitonic character is clearly visible at room temperature. The intense ultraviolet photoluminescence was observed at room temperature. This emission is indicative of the excitonic nature of the material.

Electrical and structural properties of p-type ZnO:N thin films prepared by plasma enhanced chemical vapour deposition

Thin films of p-type ZnO:N have been obtained by thermally oxidizing zinc oxynitride films prepared by plasma enhanced chemical vapour deposition (PECVD). The p-type ZnO:N thin film with a hole concentration of 2.7 × 1016 cm−3 was obtained after an annealing process was conducted at 600 °C. A conductivity transition from n-type to p-type was observed, which was systematically researched via structural and compositional analyses. In terms of these analyses, it helped to better understand the properties and behaviour of nitrogen in ZnO. First, nitrogen was incorporated into ZnO films during the growth process to occupy oxygen positions, and also partly compensated some donors induced from non-stoichiometric (ZnO1–x) composition. Second, the amount of activated nitrogen gradually increased in an oxidizing atmosphere and exceeded those donor states to realize an effective compensation, yielding p-type conductivity during the course of thermal oxidation.

Optical and electrical properties of highly nitrogen-doped ZnO thin films grown by plasma-assisted molecular beam epitaxy

Nitrogen-doped ZnO thin films with different nitrogen concentrations were fabricated by plasma-assisted molecular beam epitaxy. Hall effect measurements show p-type conduction for samples with low doping concentration. In highly doped ZnO, the p-type conduction converted to high resistance or unstable p-type behavior. This result indicates that highly doped samples are heavily compensated. In the low temperature photoluminescence spectrum, a donor-acceptor pair (DAP) emission band shows a strong redshift and broadening with increasing nitrogen doping concentration. The large shift of the DAP emission is explained by the Coulomb-potential fluctuation model related to compensated semiconductors.

Structure and photoluminescence properties of ZnO microrods

ZnO microrods (whiskers) were fabricated by a simple thermal oxidation method. The morphologies of the samples were detected by the scanning electron microscope, which showed the ZnO microrods were about 30 μm long with a diameter of 1–2 μm. In the photoluminescence (PL) spectra, only the exciton related emission bands in the ultraviolet region could be obtained at room temperature. And, a new emission band due to the exciton–exciton collision process was observed at the low energy side of the free exciton emission under the excitation intensity of 2.1 kW/cm2. This emission band increased nonlinearly. When the excitation intensity was increased above 16 kW/cm2 some fine structures could also be seen clearly in the PL spectra. These fine structures originated from the cavity modes of the Fabry–Perot etalon.

The dependence of emission spectra of rare earth ion on the band-gap energy of MgxZn1-xO alloy

Abstract Rare-earth (RE) Tb3+ ion doped ZnO and Mg0.15Zn0.85O thin films were successfully fabricated by the sol–gel deposition method. The Tb3+ ion was substituted for the Zn2+ ion in the host material, as revealed by X-ray diffraction and optical absorption spectra. The cathodoluminescence properties of the doped samples were also studied. The RE3+ luminescence mechanism is discussed.

Optical properties of MgxZn1-xO polycrystalline thin films prepared by sol-gel deposition method

The MgxZn1−xO alloy thin films were synthesized on Si and quartz substrates by the sol-gel deposition method. The transmittance and cathodoluminescence spectra of the Mg0.05Zn0.95O and Mg0.15Zn0.85O nanoparticle films were obtained at room temperature. It was found that the bandgap of Mg0.05Zn0.95O and Mg0.15Zn0.85O films is as large as 3.72 eV and 3.79 eV, respectively. The ultraviolet emission peaks are located at 376 nm and 370 nm, respectively, for the samples annealed at 600°C. When the annealing temperature is elevated to 1000°C, the band-gap decreases to 3.42 eV and an emission line related to the deep-level defect appears at 500 nm. The mechanism behind these phenomena is discussed.

Photoluminescence properties of ZnO films grown on InP by thermally oxidizing metallic Zn films

Photoluminescence (PL) properties of ZnO films grown on (001) InP substrates by thermal oxidization of metallic Zn films, in which oxygen vacancies and interstitial Zn ions are compensated by P ions diffusing from (001) InP substrates, are investigated. X-ray diffraction spectra indicate that P ions have diffused into the Zn films and chemically combined with Zn ions to form Zn3P2. Intense free exciton emission dominates the PL spectra of ZnO films with very weak deep-level emission. Low-temperature PL spectra at 79 K are dominated by neutral-donor bound exciton emission at 3.299 eV (I4) with a linewidth of 17.3 meV and neutral-acceptor bound exciton emission at 3.264 eV. The free exciton emission increases with increasing temperature and eventually dominates the emission spectrum for temperature higher than 170 K. Furthermore, the visible emission around 2.3 eV correlated with oxygen deficiencies and interstitial Zn defects was quenched to a remarkable degree by P diffusing from InP substrates.

Growth of zinc oxide thin films on Si (400) by plasma-assisted molecular beam epitaxy

ZnO thin films have been grown on a (400) Si substrate by plasma-molecular beam epitaxy (P-MBE). The sample was characterized by X-ray diffraction (XRD) and photoluminescence (PL). X-ray diffraction result exhibits a strong (002) diffraction peak of ZnO thin film. In PL spectra, a dominant ultraviolet light (UL) emission at 3.265eV is observed at room temperature (RT). According to the energy position of the UL emission, this luminescence at RT was considered to be related to exciton recombination. The samples were annealed in oxygen for two hours at different temperatures, XRD shows the improvement of crystal quality with increasing annealing temperature.

Spontaneous and stimulated emission in ZnCdSe/ZnSe asymmetrical double quantum wells

The spontaneous and stimulated emission in ZnCdSe/ZnSe asymmetric double quantum wells have been studied. The exciton photoluminescence both in the narrow well and in the wide well is influenced by two factors, the exciton tunneling and the thermal dissociation processes. The change of the emission intensity is determined by the stronger one. The carrier tunneling through the thin barrier is conductive to the stimulated emission from the wide well, and the threshold can be lowered by optimizing the structure.