Chaotong Lin

Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+ layers for field emission displays

Spherical SiO2 particles have been coated with Y2O3:Eu3+ phosphor layers (SiO2@Y2O3:Eu3+) by a Pechini sol-gel process. X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence, and cathodoluminescence spectra were utilized to characterize the SiO2@Y2O3:Eu3+ core-shell-structured phosphor particles. The obtained core-shell phosphors consist of well dispersed submicron spherical particles with narrow size distribution. The thickness of Y2O3:Eu3+ shell could be easily controlled by changing the number of deposition cycles (60nm for three deposition cycles). The SiO2@Y2O3:Eu3+ core-shell particles show a strong red emission corresponding to D05-F27 (611nm) of Eu3+ under the excitation of ultraviolet (250nm) and low-voltage electron beams (2–6kV), which have potential application for field emission displays.

Sol–gel synthesis and photoluminescence properties of spherical SiO2@LaPO4:Ce3+/Tb3+ particles with a core–shell structure

LaPO4:Ce3+ and LaPO4:Ce3+, Tb3+ phosphor layers have been deposited successfully on monodispersed and spherical SiO2 particles of different sizes (300, 500, 900 and 1200 nm) through a sol–gel process, resulting in the formation of core–shell structured SiO2@LaPO4:Ce3+/Tb3+ particles. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microcopy (SEM), transmission electron microscopy (TEM), and general and time-resolved photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting SiO2@LaPO4:Ce3+/Tb3+ samples. The XRD results demonstrate that the LaPO4:Ce3+, Tb3+ layers begin to crystallize on the SiO2 templates after annealing at 700 °C, and the crystallinity increases on raising the annealing temperature. The obtained core–shell phosphors have perfectly spherical shape with a narrow size distribution, non-agglomeration, and a smooth surface. The doped rare-earth ions show their characteristic emission in the core–shell phosphors, i.e. Ce3+ 5d–4f and Tb3+ 5D4–7FJ (J = 6–3) transitions, respectively. The PL intensity of the Tb3+ increased on increasing the annealing temperature and the SiO2 core particle size. The energy transfer process from Ce3+ to Tb3+ in SiO2@LaPO4:Ce3+, Tb3+ core–shell particles was studied using the time-resolved emission spectra.

Sol-gel Synthesis and Characterization of Zn2SiO4 : Mn@SiO2 Spherical Core-Shell Particles

The synthesis and characterization of Zn 2 SiO 4 :Mn phosphor layers on spherical silica spheres, i.e., core-shell particles of Zn 2 SiO 4 :Mn@SiO 2 are described in this paper. First, monodisperse silica spheres with an average size around 750 nm have been obtained via the Stober method by the hydrolysis and condensation of telraethoxysilane Si(OC 2 H 5 )4 under base condition (using NH 4 OH as the catalyst). Second, the silica spheres are coated with Zn 2 SiO 4 :Mn phosphor layers by a sol-gel process. The resulting core-shell particles are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscope, energy-dispersive X-ray spectroscopy, transmission electron microscopy, photoluminescence, low-voltage cathodoluminescence, as well as kinetic decay. The results confirm that the 1000°C-annealed sample consists of crystalline Zn 2 SiO 4 :Mn shells and amorphous SiO 2 cores with spherical morphology and narrow size distribution. The Zn 2 SiO 4 :Mn@SiO 2 particles show the green emission at 521 nm corresponding to 4 T 1 ( 4 G)- 6 A 1 ( 6 S) transition of Mn 2 + under the excitation of UV (250 nm), vacuum UV (172 nm), and electron-beams (1-6 kV). The luminescence intensity has been studied as a function of coating number, accelerating voltage, and filament current, respectively.

Improved GaN film overgrown with a molybdenum nanoisland mask

We report the improved crystalline and optical quality of GaN film overgrown by hydride vapor phase epitaxy adopting a molybdenum (Mo) nanoisland mask (MNM). The MNM is fabricated following thermal annealing of the nanometer-thick Mo film deposited by electron-beam evaporation. The full width at half maximum values of high-resolution x-ray diffraction (HRXRD) rocking curves for the GaN film with MNM are 188arcsec (002 reflection) and 219arcsec (102 reflection), while those for the GaN film without MNM are 256 and 364arcsec, respectively. This result indicates a significant reduction of dislocation density in the overgrown GaN film with MNM. Photoluminescence spectra measurements reveal the compressive strain relaxation and improvement in the quality of the overgrown GaN film with MNM as compared to the regrown GaN film without MNM, which is consistent with the trend observed by HRXRD.

Thick GaN grown on a nanoporous GaN template by hydride vapor phase epitaxy

High-quality thick gallium nitride (GaN) films were overgrown by hydride vapor phase epitaxy (HVPE) on a nanoporous GaN template which was prepared by inductively coupled plasma etching employing an anodized aluminum oxide mask. An obvious reduction of the dislocation density in the thick GaN layer was demonstrated by high-resolution X-ray diffraction, which exhibited the improved crystalline quality in GaN films overgrown on a nanopattemed surface. Moreover, the peak redshift in photoluminescence and micro-Raman spectroscopy indicates a significant strain relaxation in the HVPE-GaN layer. Compared with conventional overgrowth, such a deposition pathway is a more promising technique for the growth of thick GaN layers.

Research on fabrication and properties of nanoporous GaN epilayers

Gallium nitride (GaN) epilayers with nanopore arrays were fabricated by inductive coupled plasma (ICP) etching using anodic aluminum oxide (AAO) as mask. Nanoporous AAO templates were formed by anodizing the Al films deposited on GaN epilayers. The diameter of the perforations in the AAO masks could be easily controlled by tuning the technique parameters of AAO fabrication process. Cl2/Ar and Cl2/He were employed as etching gas. Scanning electron microscopy (SEM) analysis shows that vertical nanoporous arrays with uniform distribution can directly be transferred from AAO masks to GaN films in some proper conditions. Photoluminescence (PL) spectra, X-ray diffraction (XRD) and Raman spectroscopy were applied to assess properties of the nanoporous GaN films with different average pore diameters and interpore distances.

Effect of AlN intermediate layer on growing GaN film by hydride vapor phase epitaxy

Abstract Thick GaN layer deposited by hydride vapor phase epitaxy (HVPE) on a metalorganic chemical vapor deposition (MOCVD) GaN template with a thin low temperature (LT) AIN intermediate layer was investigated. High resolution X-ray resolution diffraction (HRXRD) shows that the crystalline quality of thick GaN layer was improved compared with the template. As confirmed by atomic force microscopy (AFM) observations, the surface morphology of AIN intermediate layer helps to improve the nucleation of GaN epilayer. Photoluminescence (PL) spectra measurement shows its high optical quality and low compressive stress, and micro Raman measurement confirms the latter result. Thus, the deposition of the LT-AIN interlayer has promoted the growth of an HVPE-GaN layer with an excellent crystalline quality.

Growth of high-quality thick GaN using a tungsten interlayer

A high-quality GaN film was (W-GaN) grown by hydride vapor phase epitaxy (HVPE) on metalorganic chemical vapor deposition (MOCVD) GaN templates with a tungsten (W) interlayer. A sample without interlayer was also grown at the same time for comparison. Significant reductions of dislocation density in W-GaN film is confirmed by the result of high-resolution X-ray diffraction and transmission electron microscope (TEM) observation. The improvement of optical properties of the W-GaN is confirmed by photoluminescence (PL) result. A shift of PL peak suggests that the strain is lower in the W-GaN than the film without W interlayer. This technique offers a potential path to obtain high-quality GaN film as free-standing substrate.

High-Quality Thick GaN Overgrown on an Array of SiO2 Nanomasks by HVPE

We report nanoepitaxy of thick GaN films by hydride vapor-phase epitaxy (HVPE) using a uniform array of SiO 2 nanomasks that were prepared by electron-beam evaporation and anodic aluminum oxide membranes. The controllable size and density of the SiO 2 nanoisland mask were about 65 nm and 10 10 cm -2 , respectively. Subsequent overgrowth takes place selectively on the exposed GaN surface without the SiO 2 masks to the formation of the continuous layer. High-resolution X-ray diffraction shows reduction of threading dislocations in the overgrown layers. Improved optical property and significant stress relaxation in the epilayer are demonstrated by photoluminescence and micro-Raman spectra. The deposition method is a more promising technique for obtaining freestanding GaN substrates with high-quality and uniform features.