Shen De-Zhen

Exciton-Phonon Scattering in CdSe/ZnSe Quantum Dots

A temperature-dependent photoluminescence measurement is performed in CdSe/ZnSe quantum dots with a ZnCdSe quantum well. We deduce the temperature dependence of the exciton linewidth and peak energy of the zero-dimensional exciton in the quantum dots and two-dimensional exciton in the CdSe wetting layer. The experimental data reveal a reduction of homogeneous broadening of the exciton line in the quantum dots in comparison with that in the two-dimensional wetting layer, which indicates the decrease of exciton and optical phonon coupling in the CdSe quantum dots.

Photoluminescence and optically pumped ultraviolet lasing from nanocrystalline ZnO thin films prepared by thermal oxidation of high-quality ZnS thin films

We present a simple and useful method for preparing high-quality nanocrystalline ZnO thin films, i.e. the thermal oxidation of high-quality ZnS films prepared by the low-pressure metal-organic chemical vapour deposition technique. The x-ray diffraction measurements reveal that the nanocrystalline ZnO has a hexagonal wurtzite structure. Raman spectra show that the longitudinal optical phonon with the E1-mode appears at 578 cm-1. The multiple phonon scattering process is also observed, indicating the formation of a high-quality nanocrystalline ZnO thin film. The photoluminescence spectrum has a single emission peak at 3.264 eV from the free-exciton mission, under the condition of low excitation power at room temperature. However, when excitation intensities exceed the threshold of 150 kW/cm2, a new and narrow peak emerges at lower energies, which are attributed to exciton-exciton collisions, and is called the P line. The intensity of this peak increases superlinearly with the pumping power over a threshold value. This supplies strong evidence of stimulated emission. The multiple longitudinal cavity modes observed in the stimulated emission spectrum indicate the successful realization of optically pumped lasing from nanocrystalline ZnO films at room temperature.

The Annealing-Induced Shape Deformation of Hydrothermal-Grown ZnO Nanorods

The shape deformation of hydrothermal-grown ZnO nanorods is observed. After annealing at high temperature, hexagonal ZnO nanorods change to become cylinder-like ones. The adjacent nanorods tend to connect to each other to form one nanostructure. Photoluminescence measurements show that a sample annealed at 600°C has a strong ultraviolet emission with a very weak visible emission, and with increasing annealing temperature the visible emission becomes more intense. It can be concluded from analyses of the morphological changes that the surface reaction between the doped C and ZnO is the main reason for the shape deformation of the ZnO nanorods.

Ultraviolet Luminescence in Mg0.12Zn0.88O Alloy Films

We investigate the origin of ultraviolet (UV) emission from Mg0.12Zn0.88O alloy thin films with a wurtzite structure fabricated on c-plane Al2O3 substrates by plasma assisted molecular beam epitaxy. At room temperature, the absorption edge and UV emission band of the Mg0.12Zn0.88O film shift to high-energy side compared with ZnO films. Temperature dependence of the photoluminescence spectra shows that the UV emission is composed of free exciton and neutral donor bound exciton emissions. Two-step dissociation processes of the UV emission are observed with the increasing temperature. The thermal quenching mechanism is attributed to the dissociation of the free exciton from the neutral donor bound exciton in the low temperature region and the dissociation of free electron and hole from the free exciton in the high temperature region.

Formation Mechanisms of Electrical Conductivity and Optical Properties of ZnO:N Film Produced by Annealing Treatment

The effects of annealing on the chemical states of N dopant, electrical, and optical properties of N-doped ZnO film grown by molecular beam epitaxy (MBE) are investigated. Both the as-grown ZnO:N film and the film annealed in N2 are of n-type conductivity, whereas the conductivity converts into p-type conductivity for the film annealed in O2. We suggest that the transformation of conductivity is ascribed to the change in ratio of the N molecular number on O site (N2)O to the N atom number on O site (NO) in ZnO:N films under the various annealed atmosphere. For the ZnO:N film annealed in N2, the percentage content of (N2)O is larger than that of NO, i.e. the ratio > 1, resulting in the n-type conductivity. However, in the case of the ZnO:N film annealed in O2, the percentage content of (N2)O is fewer than that of NO, i.e., the ratio < 1, giving rise to the p-type conductivity. There is an obvious difference between low-temperature (80 K) PL spectra of ZnO:N film annealed in N2 and that of ZnO:N film annealed in O2. An emission band located at 3.358 eV is observed in the spectra of the ZnO:N film after annealed in N2, this emission band is due to donor-bound exciton (D0X). After annealed in O2, the PL of the donor-bound exciton disappeared, an emission band located at 3.348 eV is observed, this emission band is assigned to acceptor-bound exciton (A0X).

Temperature-Dependent Photoluminescence in Coupling Structures of CdSe Quantum Dots and a ZnCdSe Quantum Well

A coupling structure of CdSe quantum dots (QDs) and a ZnCdSe quantum well (QW) is fabricated by using the molecular-beam epitaxy technique. The effect of temperature on the photoluminescence (PL) of the structure is studied. The results reveal that the activation energy of exciton dissociation in the coupling QDs/QW structure is much higher than that of simple CdSe QDs, which is attributed to the exciton tunnelling from the QW to QDs through a thin ZnSe barrier layer. The results also reveal that the position and width of the emission band of the QDs vary discontinuously at certain temperatures. This phenomenon is explained by the QD ionization and exciton tunnelling from the QW to the QDs. It is demonstrated that the coupling structure significantly improves the PL intensity of CdSe QDs.

Temperature-Dependent Photoluminescence of ZnTe Films Grown on Si Substrates

ZnTe films have been prepared on Si substrates by metal-organic chemical vapour deposition (MOCVD), and the temperature-dependent photoluminescence (PL) properties were investigated. The near-band-edge (NBE) emission of the ZnTe sample at 83 K shows an asymmetry line shape, which can be decomposed into two Gaussian lines labelled by FE and BE. Temperature-dependent PL intensity of the NBE peak shows two variation regions, and an expression with two dissociation channels fits well to the experimental data. The results of the temperature-dependent full width at half maximum (FWHM) and peak energy were well understood under the framework of the two-dissociation-channel model. That is, at low temperature, the emission from bound excitons governs the NBE peak, while above 157 K, the free exciton emission becomes dominant gradually. A simple model with three energy levels was employed to describe the variation in emission intensity of BE and FE with temperature.

Blue cathodoluminescence from highly er-doped ZnO thin films induced by the phonon bottleneck effect

The room-temperature blue cathodoluminescence (CL) from highly Er-doped ZnO thin films has been studied by using different electron beam currents (EBCs). The ZnO:Er thin films used in our experiment were prepared by simultaneous evaporation from two sources. The x-ray diffraction spectra show that the thin films have a strong preferential c-axis (0002) orientation with a hexagonal crystalline structure. The blue emission at 455 nm originating from the intra-4f shell transition (4F5/24I15/2) in Er3+ ions was observed at room temperature. The nonlinear dependence of the CL intensity on the EBC shows a more intense blue emission above the threshold EBC of 0.6 µA, which is attributed to the phonon bottleneck effect.

Time-Resolved Photoluminescence Spectroscopy: A Novel Technique for Determination of Luminescence of Quantum Dots

The time-resolved photoluminescence (PL) spectroscopy measured by the gradually increasing start delay time is utilized as a tool for the determination of the luminescence of quantum dots (QDs). The luminescence evolution of self-assembled CdSe QDs during the luminescence decay is fully revealed in terms of the experiment technique. The characteristic narrow luminescence lines of self-assembled CdSe QDs are obtained with increasing start delay time.

Spectroscopy of the Forming Process of Quantum Dots Accompanied by the Thinning of Wetting Layer

A series of Cd0.44Zn0.56Se/ZnSe sandwich structures with different Cd0.44Zn0.56Se embedded layer thicknesses were fabricated by metal organic chemical vapor deposition. When the embedded layer thickness exceeded 3.0 nm, the photoluminescence spectra of the sample changed into the two-band structure from the one-band structure, and atomic force microscopy images indicate that Cd0.44Zn0.56Se quantum dots were formed. In the two-band photoluminescence spectrum, the band at the low energy side was attributed to be from quantum dots, and the high-energy one arose from the wetting layer. Thinning of the wetting layer with quantum dots forming was confirmed indirectly by the significant blue shift of the wetting-layer photoluminescence band compared to the photoluminescence band of the samples for which the Cd0.44Zn0.56Se layer thickness was less than 3.0 nm. This thinning arose from mass migration during the Stranski–Krastanow growth of Cd0.44Zn0.56Se quantum dots.