X.W. Fan

Optical properties of ZnO and ZnO:In nanorods assembled by sol-gel method

Self-assembled zinc oxide (ZnO) and indium-doping zinc oxide (ZnO:In) nanorod thin films were synthesized on quartz substrates without catalyst in aqueous solution by sol-gel method. The samples were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), Raman-scattering spectroscopy, room-temperature photoluminescence (PL) spectra, and temperature-dependent PL spectra measurements. XRD and Raman spectra illustrated that there were no single In2O3 phase in ZnO lattice after indium doping. The PL spectra of ZnO showed a strong UV emission band located at 394 nm and a very weak visible emission associated with deep-level defects. Indium incorporation induced the shift of optical band gap, quenching of the near-band-edge photoluminescence and enhanced LO mode multiphonon resonant Raman scattering in ZnO crystals at different temperatures. Abnormal temperature dependence of UV emission integrated intensity of ZnO and ZnO:In samples is observed. The local state emission peak of ZnO:In samples at 3.37 eV is observed in low-temperature PL spectra. The near-band-edge emission peak at room temperature was a mixture of excitons and impurity-related transitions for both of two samples.

MgxZn1−xO-based photodetectors covering the whole solar-blind spectrum range

A series of MgxZn1−xO thin films has been prepared by metalorganic chemical vapor deposition and metal-semiconductor-metal structured ultraviolet photodetectors are fabricated from these films. The cutoff wavelengths of the photodetectors can cover the whole solar-blind spectrum range (220–280 nm) by varying Mg content in the MgxZn1−xO thin films. As a representative, the photodetector fabricated from Mg0.52Zn0.48O shows an ultraviolet/visible rejection ratio of about four orders of magnitude, and the dark current is 15 pA at 10 V bias. These results demonstrate that high-performance photodetectors operating in the whole solar-blind spectrum range can be realized in MgxZn1−xO films.

F-doping effects on electrical and optical properties of ZnO nanocrystalline films

F-doped and undoped ZnO nanocrystalline films were prepared from thermal oxidation of ZnF2 films deposited on a silica substrate by electron beam evaporation. The F-doped ZnO film has very low electrical resistivity of 7.95×10−4Ωcm and a high optical transmittance. The study also indicated that (1) the substitutional F atoms in the film serve as donors to increase the carrier concentration and the optical band gap with respect to undoped ZnO film, and (2) F passivation reduces the known number of Os2−/Os− surface states and increases carrier mobility.

Structure and photoluminescence of Mn-passivated nanocrystalline ZnO thin films

We have studied the structure and the photoluminescence of Mn-passivated nanocrystalline ZnO thin films. The ZnO thin films were prepared by thermally oxidizing ZnS:Mn films grown by low-pressure metalorganic chemical vapor deposition. The structural properties of the ZnO films were examined by X-ray diffraction and X-ray photoelectron spectroscopy. It was demonstrated that the Mn passivation could dramatically change the emission characteristics of nanocrystalline ZnO thin films. The photoluminescence spectra of the films with an optimized Mn-doped concentration showed only ultraviolet emission, while the green emission was fully quenched due to the Mn passivation of the ZnO nanocrystallite surface. A core-shell structure model of the surface passivation is presented. The note of the oxygen vacancy (Vo**) as the dominant recombination center for green emission is discussed.

High quality ZnO thin films grown by plasma enhanced chemical vapor deposition

High quality ZnO thin films have been grown on a Si(100) substrate by plasma enhanced chemical vapor deposition using a zinc organic source [Zn(C2H5)2] and carbon dioxide (CO2) gas mixtures at the low temperature of 180 °C. The dependence of ZnO thin film quality on the gas flow rate ratio of Zn(C2H5)2 to CO2 (GFRRZC) is studied by using x-ray diffraction (XRD), optical absorption (OA) spectra, and cathodoluminescence (CL) spectra. High quality ZnO thin films with a c-axis-oriented wurtzite structure are obtained when the GFRRZC is 0.33. XRD shows that the full width at half maximum of (0002) ZnO located at 34.42° is about 0.2°. At room temperature, a pronounced free exciton absorption peak around 365 nm is clearly observed. Also, a strong free exciton emission without deep level defect emission is observed around 385 nm, and its temperature dependence is studied from the photoluminescence spectra. These observations indicate the formation of a high quality ZnO film. Additionally, nitridation of the Si su...

Structure and optically pumped lasing from nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films

In this article, we observe the optically pumped lasing from the high-quality nanocrystalline ZnO thin films obtained by thermal oxidation of ZnS thin films, which were grown on SiO2 substrates by low-pressure-metalorganic chemical vapor deposition technique. The x-ray diffraction (XRD) patterns indicate that high-quality ZnS films possess a preferred (111) orientation. ZnS has a transformation to ZnO at an annealing temperature (Ta) of 500 °C, and fully transforms into ZnO at Ta⩾700 °C from the XRD patterns. The obtained ZnO films possess a polycrystalline hexagonal wurtzite structure. The fifth-order Raman scattering is observed in the films, which indicates that a large deformation energy exists in the lattice. In photoluminescence (PL), spectra, for all the samples with different annealing temperatures, the near-band-edge (NBE) PL peak has a pronounced blueshift with increasing annealing temperature, while the full width at half maximum (FWHM) decreases gradually. We think that emissions of the bound ...

High responsivity ultraviolet photodetector realized via a carrier-trapping process

Metal-semiconductor-metal structured ultraviolet (UV) photodetector has been fabricated from zinc oxide films. The responsivity of the photodetector can reach 26 000 A/W at 8 V bias, which is the highest value ever reported for a semiconductor ultraviolet photodetector. The origin of the high responsivity has been attributed to the carrier-trapping process occurred in the metal-semiconductor interface, which has been confirmed by the asymmetric barrier height at the two sides of the metal-semiconductor interdigital electrodes. The results reported in this paper provide a way to high responsivity UV photodetectors, which thus may address a step toward future applications of UV photodetectors.

The structural and optical properties of Cu2O films electrodeposited on different substrates

Cuprous oxide films were successfully electrodeposited onto three different substrates through the reduction of copper lactate in alkaline solution at pH = 10. The substrates include indium tin oxide film coated glass, n-Si wafer with (001) orientation and Au film evaporated onto Si substrate. The substrate effects on the structural and optical properties of the electrodeposited films are investigated by in situ voltammetry, current versus time transient measurement, ex situ x-ray diffraction, scanning electron microscopy, UV–vis transmittance and reflectance and photoluminescence techniques. The results indicate that the choice of substrate can strongly affect the film morphology, structure and optical properties.

Effects of nitrogen doping and illumination on lattice constants and conductivity behavior of zinc oxide grown by magnetron sputtering

A yellow-orange nitrogen-doped zinc oxide (ZnO:N) film was deposited on a quartz glass substrate at 510K by reactive radio-frequency magnetron sputtering of a ZnO target with sputtering gas of nitrogen. The lattice constants of the as-grown ZnO:N are much larger than those of undoped ZnO, and decrease with increasing annealing temperature due to escape of the nitrogen from the ZnO:N and decrease of tensile stress, accompanied with color change from yellow-orange to pale yellow. The nitrogen occupies two chemical environments in the ZnO:N based on x-ray photoelectron spectroscopy measurement. One is NO acceptor formed by substitution of N atom for O sublattice, and another is (N2)O double donors produced by substitution of N molecular for O site, which make the lattice constants expanded. The as-grown ZnO:N film shows insulating, but behaves p-type conduction in the dark after annealed at 863K for 1h under 10−3Pa. Unfortunately, the p-type conduction is not stable and reverts to n type soon. However, after...

X-ray photoelectron spectroscopy measurement of n-ZnO/p-NiO heterostructure valence-band offset

Valence-band offset (VBO) of n-ZnO/p-NiO heterojunction has been investigated by x-ray photoelectron spectroscopy. Core levels of Zn 2p and Ni 2p were used to align the VBO of n-ZnO/p-NiO heterojunction. It was found that n-ZnO/p-NiO heterojunction has a type-II band alignment and its VBO is determined to be 2.60±0.20 eV, and conduction-band offset is deduced to be 2.93±0.20 eV. The experimental VBO value is in good agreement with the calculated value based on the electron affinity of ZnO and NiO.