Yijun Sun

High Anatase-Rutile Transformation Temperature of Anatase Titania Nanoparticles Prepared by Metalorganic Chemical Vapor Deposition.

Anatase titania nanoparticles with a particle size of about 10 nm are prepared by metalorganic chemical vapor deposition (MOCVD) and the anatase-rutile tranformation of the nanoparticles is investigated and discussed. The results show that anatase-rutile transformation takes place in a wide range of temperature. When annealing time is 1 h, the anatase-rutile transformation temperature of the nanoparticles is as high as 900 to 1200°C, which is comparable to that of micrometer-sized titania. Based upon X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) results, a possible transformation mechanism is proposed. The high transformation temperature of the nanoparticles is primarily attributed to the retardation of the initial particle growth of the anatase phase due to the lattice strains in the nanoparticles before transformation proceeds.

Effect of Collection Distance on the Lattice Structure of Anatase Titania Nanoparticles Prepared by Metalorganic Chemical Vapor Deposition

Anatase titania nanoparticles are prepared at 700°C by metalorganic chemical vapor deposition (MOCVD) and the nanoparticles are collected by thermophoretic collection method. The effect of collection distance on the lattice structure of the nanoparticles is investigated, followed by discussion. The results show that the collection distance exerts a significant influence on the lattice structure of the nanoparticles. With decreasing collection distance, the lattice constant a increases while c decreases simultaneously, resulting in more significant lattice distortion. X-ray photoelectron spectroscopy and electron paramagnetic resonance results show that both oxygen and titanium vacancies exist in the nanoparticles. Vacancies in the nanoparticles are responsible for the lattice distortion.

Three growth-temperature-dependent regions for nitrogen incorporation in GaNAs grown by chemical beam epitaxy

The effects of growth temperature on nitrogen incorporation in GaNAs grown by chemical beam epitaxy are studied from 340 to 515°C. Generally speaking, with increasing growth temperature, nitrogen content decreases. However, three distinct growth-temperature-dependent regions for nitrogen incorporation with activation energies of 0.59, 0.05, and 0.95 eV can be identified at low, middle, and high growth temperatures, respectively. At low and high growth temperatures, the growth temperature dependences of nitrogen incorporation are due to triethylgallium (TEG)-pyrolysis- and nitrogen-desorption-controlled processes, respectively, while a TEG-transportation-limited process is observed at middle temperatures. Atomic force microscope (AFM) results also show that there are three different surface morphologies for GaNAs grown at different growth temperatures. Based on X-ray diffraction (XRD) and AFM results, the best growth mechanism is determined, and high quality GaN0.007As0.993/GaAs triple quantum wells are obtained.

Formation Mechanism for High-Surface-Area Anatase Titania Nanoparticles Prepared by Metalorganic Chemical Vapor Deposition

High-surface-area anatase titania nanoparticles are prepared by metalorganic chemical vapor deposition (MOCVD) using thermophoretic collection. Based on the dependence of X-ray linewidth (FWHM) on collection distance, a formation mechanism for the nanoparticles is proposed in which it is assumed that the relative changes in the grain size of and vacancy concentration in the nanoparticles are proportional to the velocity of the gas flow in the reactor. Three nondimensional equations that describe the dependence of the properties of the nanoparticles on collection distance are derived. This model explains well the effect of collection distance on X-ray linewidth, and it is not only in good agreement with XPS results but also consistent with TEM observations.

A quantitative model for the blueshift induced by rapid thermal annealing in GaNAs∕GaAs triple quantum wells

The effects of rapid thermal annealing (RTA) on the optical properties of GaNAs∕GaAs triple quantum wells grown by chemical beam epitaxy are studied in detail by photoluminescence (PL) spectroscopy at 77K. Special emphasis is put on the RTA-induced PL peak blueshift. It is found that the blueshift is neither due to nitrogen diffusion from well layer to barrier layer nor due to homogenization of nitrogen composition fluctuations. The blueshift is due to the coupling between the radiative recombination of PL emission and the nonradiative recombination of nonradiative centers. A quantitative model is proposed in which the blueshift is proportional to the relative change of the concentration of nonradiative centers. This model quantitatively explains not only our present results but also previous observations.

Comparative Study on the Properties of GaNAs/GaAs Triple Quantum Wells Annealed by Different Methods

GaNAs/GaAs triple quantum wells (QWs) grown by chemical beam epitaxy are annealed by two methods, ex situ rapid thermal annealing (RTA) and in situ annealing in a growth chamber followed by ex situ RTA (NRTA). The effects of annealing method on the structural, morphological, and optical properties of the QWs are studied comparatively. The results show that although there is surface desorption for both RTA and NRTA, the holes on the sample surface after desorption are clearly different. RTA is better than NRTA from the viewpoints of both surface morphology and optical properties.

Growth Temperature Dependence of Nitrogen Incorporation in GaNAs Grown by Chemical Beam Epitaxy

Recently, considerable attention has been paid to GaNAs and InGaNAs for long wavelength applications. Lasers based upon InGaNAs/GaAs with the nitrogen content of about 1%, operating at a wavelength of 1.3 μm, have been demonstrated. Many authors have tried to find out the growth mechanism of nitrogen incorporation. Researchers have observed up to two temperature dependent regions for nitrogen incorporation, depending on growth methods and precursors used. Auvray et al. studied the effects of growth temperature on the nitrogen incorporation in GaNxAs1-x grown by MOCVD using TMG, AsH3, and DMHy as precursors and found two obvious temperature dependent regions for nitrogen incorporation with activation energies of 3.7 and 0.6 eV, respectively. The observed transition between the two regions is attributed to an increased desorption of nitrogen at high temperatures. Moto et al. found only one temperature dependent region for nitrogen incorporation in GaNxAs1-x grown by MOCVD. Jin et al. found that there is only one temperature dependent region for nitrogen incorporation in GaNxAs1-x grown by CBE using TEG, DMHy and solid arsenic (As4) as precursors. The activation energy is calculated to be 0.97 eV. In this paper we investigate the effects of growth temperature on the nitrogen incorporation in GaNxAs1-x grown by CBE using AsH3, TEG, and active nitrogen species generated by a radio-frequency coupled plasma source (200W) from N2 as sources. It is found that there are three obvious temperature dependent regions for nitrogen incorporation in GaNxAs1-x. The activation energies are calculated to be 0.95, 0.05, and 0.59 eV, respectively, and the incorporation mechanisms at different temperature regions are proposed.