Sol-gel combustion synthesis and luminescence Of Y4Al2O9 : Eu3+ nanocrystal

Y4Al2O9:EU3+ phosphor was synthesized through a sol-gel combustion method. The Y4Al2O9 phase can form through sintering at 800 degrees C. This temperature is much lower than that required via the solid state reaction method. The average grain size of the phosphor was about 30 run. Compared with the amorphous phosphor, the charge transfer band of crystalline phosphor shows a blue shift. The emission Of Y4Al2O9:Eu3+ indicates the existence of two luminescent centers, in agreement with the crystal structure of Y4Al2O9. Higher doping concentration could be realized in Y4Al2O9 nanocrystal host lattice. This indicates that the sol-gel combustion synthesis method can increase emission intensity and quenching concentration due to a good distribution of EU3+ activators in Y4Al2O9 host. (c) 2005 Elsevier B.V. All rights reserved.

Preparation of crystalline beta barium borate thin films on Sr2+-doped alpha barium borate substrates by liquid phase epitaxy

Thin films of beta barium borate have been prepared by liquid phase epitaxy on Si2+-doped alpha-BaB2O4 (alpha-BBO, the high temperature phase of barium berate) (001) and (110) substrates. The results of X-ray diffraction indicate that the films show highly (001) preferred orientation on (001)-oriented substrates while the films grown on (110) substrates are textured with (140) orientation. The crystallinity of these films was found to depend on growth temperature, rotation rate, dip time and orientation of substrate. Growth conditions were optimized to grow films with (001) orientation on (001) substrates reproducibly. The films show second harmonic generation of 400 nm light upon irradiation with 800 nm Ti: Sapphire femtosecond laser light. (c) 2005 Elsevier B.V. All rights reserved.

ADSORBABILITY AND SEDIMENTATION EFFECT OF SUBMERGED MACROPHYTES ON SUSPENDED SOLIDS

To select better plant species for adsorption and deposition of suspended solids in water, effectively, eight species of submerged macrophytes, which are popular in the middle or downstream area of Yangtze River, were studied. The effects of their adsorbability and sedimentation on suspended solids were monitored in the microcosm systems with natural lake's sediment and water. Within one week's regular disturbance, according to their adsorbability of the macrophytes on suspended solids, the macrophytes were divided into two groups. Elodea nuttallii, Potamogeton crispus, Hydrilla verticillata, Myriophyllum spicatum, Potamogeton malaianus and Najas graminea were the macrophytes with stronger adsorbability. Vallisneria natans and Ceratoplyllum demersum were the macrophytes with weaker adsorbability. The average adsorbabilities of the two groups were 28.0 and 14.5 mg g(-1) FW-1, respectively. According to the sedimentation rate, the macrophytes were divided into three groups. P. crispus and H. verticillata were in the highest group. V.natans was in higher group. E nuttallii, M. spicatum, N. graminea, C demersum and P. malaianus were in the lowest group. The average sedimentation rates of the three groups were 3.42, 2.11 and 0.69 mg l(-1) d(-1). respectively. Therefore, P. crispus and H. verticillata were excellent species. C demersum was a poor species to improve transparency of water body.

Dynamics of photo-enhanced magneto-crystalline anisotropy in diluted ferromagnetic GaMnAs

By chopping a pump beam in conventional time-resolved Kerr rotation (TRKR) experiments and measuring the time evolution of M-shaped "major" hysteresis loops of magnetic linear dichroism (Delta MLD = MLDpump-on MLDpump-off), the differential MLD signal in the presence and the absence of the pump beam, we studied the dynamics of photo-enhanced magneto-crystalline anisotropy, and found that its very long recovering time (much longer than 13 ns) might reflect the nature of the coherent coupling between photo-excited holes and localized spins in the d shell of manganese.

Easy axis reorientation and magneto-crystalline anisotropic resistance of tensile strained (Ga,Mn)As films

We present a study of magnetic anisotropy by using magneto-transport and direct magnetization measurements on tensile strained (Ga,Mn)As films. The magnetic easy axis of the films is in-plane at low temperatures, while the easy axis flips to out-of-plane when temperature is raised or hole concentration is increased. This easy axis reorientation is explained qualitatively in a simple physical picture by Zeners pd model. In addition, the magneto-crystalline anisotropic resistance was also investigated experimentally and theoretically based on the single magnetic domain model. The dependence of sheet resistance on the angle between the magnetic field and [1 0 0] direction was measured. It is found that the magnetization vector M in the single-domain state deviates from the external magnetic field H direction at low magnetic field, while for high magnetic field, M continuously moves following the field direction, which leads to different resistivity function behaviors.

Electroluminescence from nano-crystalline Si/SiO2 structures embedded in pn junctions

Phosphorous-doped and boron-doped amorphous Si thin films as well as amorphous SiO2/Si/SiO2 sandwiched structures were prepared in a plasma enhanced chemical vapor deposition system. Then, the p-i-n structures containing nano-crystalline Si/SiO2 sandwiched structures as the intrinsic layer were prepared in situ followed by thermal annealing. Electroluminescence spectra were measured at room temperature under forward bias, and it is found that the electroluminescence intensity is strongly influenced by the types of substrate. The turn-on voltages can be reduced to 3 V for samples prepared on heavily doped p-type Si (p(+)-Si) substrates and the corresponding electroluminescence intensity is more than two orders of magnitude stronger than that on lightly doped p-type Si (p-Si) and ITO glass substrates. The improvements of light emission can be ascribed to enhanced hole injection and the consequent recombination of electron-hole pairs in the luminescent nanocrystalline Si/SiO2 system. (C) 2008 Elsevier Ltd. All rights reserved.

Enhancement of electroluminescence in p-i-n structures with nano-crystalline Si/SiO2 multilayers

Nano-crystalline Si/SiO2 multilayers were prepared by alternately changing the ultra-thin amorphous Si film deposition and the in situ plasma oxidation process followed by the post-annealing treatments. Well-defined periodic structures can be achieved with 2.5 nm thick SiO2 sublayers. It is shown that the size of formed nano-crystalline Si is about 3 nm. Room temperature electroluminescence can be observed and the spectrum contains two luminescence bands located at 650 nm and 520 nm. In order to improve the hole injection probability, p-i-n structures containing a nanocrystalline Si/SiO2 luminescent layer were designed and fabricated on different p-type substrates. It is found that the turn-on voltage of p-i-n structures is obviously reduced and the luminescence intensity increases by 50 times. It is demonstrated that the use of a heavy-doped p-type substrate can increase the luminescence intensity more efficiently compared with the light-doped p-type substrate due to the enhanced hole injection.

Effects of crystalline quality on the ultraviolet emission and electrical properties of the ZnO films deposited by magnetron sputtering

The ZnO films were deposited on c-plane sapphire, Si (0 0 1) and MgAl2O4 (1 1 1) substrates in pure Ar ambient at different substrate temperatures ranging from 400 to 750 degrees C by radio frequency magnetron sputtering. X-ray diffraction, photoluminescence and Hall measurements were used to evaluate the growth temperature and the substrate effects on the properties of ZnO films. The results show that the crystalline quality of the ZnO films improves with increasing the temperature up to 600 degrees C, the crystallinity of the films is degraded as the growth temperature increasing further, and the ZnO film with the best crystalline quality is obtained on sapphire at 600 degrees C. The intensity of the photoluminescence and the electrical properties strongly depend on the crystalline quality of the ZnO films. The ZnO films with the better crystallinity have the stronger ultraviolet emission, the higher mobility and the lower residual carrier concentration. The effects of crystallinity on light emission and electrical properties, and the possible origin of the n-type conductivity of the undoped ZnO films are also discussed. (C) 2009 Elsevier B. V. All rights reserved.

Effects of In surfactant on the crystalline and photoluminescence properties of GaN nanowiires

GaN nanowires have been grown with and without In as an additional source. The effects of In surfactant on the crystal quality and photoluminescence property of GaN nanowires are reported for the first time. X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, and photoluminescence measurements are employed to analyse the products. The results show that introducing a certain amount of In surfactant during the growth process can improve the crystal quality of the GaN nanowires, and enhance the photolurainescence of them. In addition, the as-prepared GaN nanowires have the advantage of being easy to be separated, which will benefit the subsequent nanodevice fabrication.

Effects of crystalline quality on the phase stability of cubic boron nitride thin films under medium-energy ion irradiation

To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) thin films, c-BN films with various crystalline qualities prepared by dual beam ion assisted deposition were irradiated at room temperature with 300 keV Ar+ ions over a large fluence range up to 2 x 10(16) cm(-2). Fourier transform infrared spectroscopy (FTIR) data were taken before and after each irradiation step. The results show that the c-BN films with high crystallinity are significantly more resistant against medium-energy bombardment than those of lower crystalline quality. However, even for pure c-BN films without any sp(2)-bonded BN, there is a mechanism present, which causes the transformation from pure c-BN to h-BN or to an amorphous BN phase. Additional high resolution transmission electron microscopy (HRTEM) results support the conclusion from the FTIR data. For c-BN films with thickness smaller than the projected range of the bombarding Ar ions, complete stress relaxation was found for ion fluences approaching 4 x 10(15) cm(-2). This relaxation is accompanied, however, by a significant increase of the width of c-BN FTIR TO-line. This observation points to a build-up of disorder and/or a decreasing average grain size due to the bombardment. (c) 2005 Elsevier B.V. All rights reserved.

Thermally tunable optical filter with crystalline silicon as cavity

A novel method to fabricate a thermally tunable filter with a tuning range of 26 nm from 1.504 to 1.530 mum is reported. The high-reffectivity bottom mirror is deposited in the hole formed by anisotropically etching in the basic solution from the backside of the slice with the buried SiO2 layer in silicon-on-insulator substrate as the etching-stop layer. Because of the formation of the mesa and the removing of the substrate of the hole, the power from the metal heater can be more effectively consumed in the crystalline silicon cavity. So it lowers the power consumption and the filter has a higher tuning range. (C) 2004 Elsevier B.V. All rights reserved.

Effects of doping on the crystalline quality and composition distribution in InGaN/GaN structure grown by MOCVD

The effects of Si and Mg doping on the crystalline quality and In distribution in the InGaN films were studied by atomic force microscope (AFM), triple crystal X-ray diffraction (TCXRD) and Rutherford backscattering spectrometry (RBS). The undoped, Si-doped and Mg-doped InGaN films were grown by metalorganic chemical vapor deposition (MOCVD) on (0 0 0 1) sapphire substrates. The electronic concentration in the Si-doped InGaN is about 2 x 10(19) cm(-3). It is found that the crystalline quality and In distribution in InGaN is slightly affected by the Si doping. In the Mg doped-case, the hole concentration is about 4 x 10(18) cm(-3) after annealing treatment. The surface morphology and crystalline quality of the Mg-doped InGaN are deteriorated significantly compared with the undoped InGaN. The growth rate of Mg-doped InGaN is higher than the undoped InGaN. Mg doping enhances the In incorporation in the InGaN alloy. The increase in In composition in the growth direction is more severe than the undoped InGaN. (c) 2006 Elsevier B.V. All rights reserved.