UC San Diego
Synthesis and characterization of visible emission from rare-earth doped aluminum nitride, gallium nitride and gallium aluminum nitride powders and thin films
- Author(s): Tao, Jonathan Huai-Tse
- et al.
A three-step solution-based process had been used synthesize powders of GaN, AlN and their alloys. The complete solid solubility and tunable nature of these nitride band gaps in the visible spectrum were the motivation of these studies due to their application in solid state lighting. Energy dispersive X-ray spectroscopy confirmed the reduction in oxygen content for the GaN powders to as low as 4 atom % with an 8 % oxygen to nitrogen ratio. Relative to commercial GaN powders, the bandedge of the powders synthesized by such approach also shifted to higher energy, which indicated fewer defects, as observed from reflectance measurements. Inspired by the use of rare-earth elements as color emitters in fluorescent lamp phosphors, these elements were also used as activators in our nitride material. Visible emission was demonstrated through photoluminescence measurements in AlN powders activated with rare-earth elements Eu³⁺, Tb³⁺, Tm³⁺. These ions showed emission in the red, green and blue regions of the visible spectrum, respectively. Eu³⁺ and Tb³⁺ co-activation was also observed in an AlN sample that indicated successful energy transfer from the host to sensitizer, and subsequently to another activator. Tb³⁺ emission was observed under cathodoluminescence in GaN powders synthesized by the same method, and a concentration study showed no effect of concentration quenching up to 8 atom %. Using the same source powder, a pulsed-laser deposited thin film was fabricated that showed both band gap emission and activator-related emission, suggesting a reduction of defects when the powders were deposited as thin films. Additionally, GaN: Tb³⁺ films were also fabricated using metallorganic vapor phase epitaxy using precursors with and without oxygen ligands. Tb³⁺ emission was only observed in the sample fabricated from the precursor with oxygen ligand, suggestion that oxygen may be required for effective rare earth luminescence. Finally, Ga₁-xAl/xN alloy powders ( x = 0.5) and Ga₁-x-yAl/xDy/yN (x = 0.10, 0.30, y = 0.01) powders were synthesized using the solution method while incorporating a stainless steel pressure vessel, which increased the synthesis pressure and aided the formation of a single phase hydroxide precursor. This in turn produced a single phase alloy nitride in the final step. Dy³⁺ emission that was not observed in GaN powders was also observed in the Ga₁-x-yAl/xDy/yN powder. This suggested that the incorporation of aluminum enabled rare- earth emission in the nitrides synthesized for these experiments. However, attempts to sputter nitride alloy thin films via radio frequency sputtering were unsuccessful; only very minor peak shifts in the X-ray diffraction patterns were observed. Nevertheless, energy dispersive X-ray spectroscopy indicates the presence of Al in the Ga₀.₅Al₀.₅N film deposited on a Si substrate. This suggested that Al atoms may have segregated from the alloy lattice during the deposition process, with only a small amount of Al atoms incorporated into the GaN lattice