Investigation of yellow luminescence intensity of N-polar unintentionally doped GaN

This paper reports that the yellow luminescence intensity of N-polar GaN Epi-layers is much lower than that of Ga-polar ones due to the inverse polarity, and reduces drastically in the N-polar unintentionally-doped GaN after etching in KOH solution. The ratio of yellow luminescence intensity to band-edge emission intensity decreases sharply with the etching time. The full width at half maximum of x-ray diffraction of （10-12） plane falls sharply after etching, and the surface morphology characterized by scanning electron microscope shows a rough surface that changes with the etching time. The mechanism for the generation of the yellow luminescence are explained in details.

Yellow luminescence of co-doped gadolinium oxyhydroxide

Crystals of co-doped gadolinium oxyhydroxide （GdOOH）, Gd0.98Eu0.02-xTbxOOH and Gd1-y-zDyyBizOOH, were synthesized by a flux method. The color coordinates in the Commission Internationale de I＇Eelairage （CIE） chromaticity diagram of Gd0.98Eu0.02-xTbxOOH, obtained under 254 nm irradiation, shifted along a straight line with the changing values ofx to include the yellow region. The CIE coordinates of Dy^3＋ doped in GdOOH were located in the yellow region, while the emission intensity of Dy^3＋ under 286 nm irradiation increased by more than 40 times when co-doped with Bi^3＋.

Effect of High-Temperature Annealing on Yellow and Blue Luminescence of Undoped GaN

The effect of high-temperature annealing on the yellow and blue luminescence of the undoped GaN is investi- gated by photoluminescenee （PL） and x-ray photoelectron spectroscopy （XPS）. It is found that the band-edge emission in the GaN apparently increases, and the yellow luminescence （YL） and blue luminescence （BL） bands dramatically decrease after annealing at 700℃. At the annealing temperature higher than 900℃, the YL and BL intensities show an enhancement for the nitrogen annealed GaN. This fact should be attributed to the increment of the Ga and N vacancies in the GaN decomposition. However, the integrated PL intensity of the oxygen an- nealed GaN decreases at the temperature ranging from 900℃ to 1000℃. This results from the capture of many photo-generated holes by high-density surface states. XPS characterization confirms that the high-density surface states mainly originate from the incorporation of oxygen atoms into GaN at the high annealing temperature, and even induces the 0.34eV increment of the upward band bending for the oxygen annealed GaN at 1000℃.

Luminescence of a GaN grain with a nonpolar and semipolar plane in relation to microstructural characterization

We report on the growth of the high-quality GaN grain on a r-plane sapphire substrate by using a self-organized SiN interlayer as a selective growth mask. Transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy are used to reveal the effect of SiN on the overgrown a-plane GaN growth. The SiN layer effectively terminates the propagation of the threading dislocation and basal plane stacking faults during a-plane GaN regrowth through the interlayer, resulting in the window region shrinking from a rectangle to a ＂black hole＂. Furthermore, strong yellow luminescence （YL） in the nonpolar plane and very weak YL in the semipolar plane on the GaN grain is revealed by cathodoluminescence, suggesting that C-involved defects are responsible for the YL.

Influence of growth rate on the carbon contamination and luminescence of GaN grown on silicon

The unintentional carbon doping concentration of GaN films grown by low pressure metal organic chemical vapor deposition （LP-MOCVD） depends strongly on the growth rate. The concentration of carbon is varied from 2.9 × 1017 to 5.7 × 10^18 cm-3 when the growth rate increases from 2.0 to 7.2 μm/h, as detected by secondary ion mass spectroscopy. It is shown that the presence of N vacancies give rises to high carbon concentration. We show that a reduction of the carbon concentration by one order of magnitude compared to the regular sample with nearly same growth rate can be achieved by operating at an extremely high NH3 partial pressure during growth. The intensity ratios of yellow and blue luminescence to band edge luminescence in the samples are found to depend significantly on carbon concentration. The present results demonstrate direct and quantitative evidence that the carbon related defects are the origin of yellow and blue luminescence.

Highly regular rosette-shaped cathodoluminescence in GaN selfassembled nanodisks and nanorods

Self-assembled GaN nanorods were grown by metal-organic chemical vapor deposition.A highly regular rosette-shaped cathodoluminescence pattern in the GaN nanorods is observed,where its origin is helpful to deepen the understanding of GaN nanorod growth.The pattern forms at the very early stages of nanorod growth,which consists of yellow luminescence at the edges and the non-luminous region at six vertices of the hexagon.To clarify its origin,we carried out detailed cathodoluminescence studies,electron microscopy studies and nanoscale secondary ion mass spectrometry at both the nanorod surface and cross-section.We found the pattern is not related to optical resonance modes or polarity inversion,which are commonly reported in GaN nanostructures.After chemical composition and strain analysis,we found higher carbon and nitrogen cluster concentration and large compressive strain at the pattern area.The pattern formation may relate to facet preferential distribution of non-radiative recombination centers related to excess carbon/nitrogen.This work provides an insight into strain distribution and defect-related emission in GaN nanorod,which is critical for future optoelectronic applications.

Nanocrystalline rare earth fluorides doped with Pr^(3+) ions

Praseodymium（III） doped CeF3, CeF3：Gd, LaF3, GdF3 and YF3 inorganic fluorides were precipitated in an aqueous, sur- factant-free solution, using NH4F as a source of fluoride ions. The as-prepared products were subjected to a hydrothermal treatment, which led to the formation of crystalline nanoluminophores, composed of spherical （30 nm） and elongated （40-200 nm） nanos- tructures. Due to the presence of Pr3＋ ions, the synthesized nanomaterials showed yellow luminescence under a blue light irradiation. The nanoluminophore based on the YF3 host revealed the most promising spectroscopic properties, i.e., bright and intensive emission, hence it was investigated in detail. The photophysical properties of the nanomaterials obtained were studied by powder X-ray diffrac- tion （XRD）, transmission electron microscopy （TEM） and spectrofluorometry, i.e., measurements of excitation/emission spectra and luminescence decay curves.