Madis Raukas

Investigation of Fluorescence Yield of Phosphors in Vacuum Ultraviolet Region

A theoretical model describing the dependence of quantum efficiency of a phosphor on the intensity of exciting radiation is developed. This model takes into account three dominant processes contributing to the loss of radiation when the host is excited by vacuum ultraviolet radiation above its bandgap energy, and fluorescence occurs through transfer of radiation from the host to the activator ions. The three processes are depletion of the ground state, nonradiative energy transfer in the excited state, and surface recombination of electron-hole pairs. It has been shown that the surface recombination effect can be treated adequately by assuming that the excited state lifetime of the host material is modified by a loss factor associated with this effect.

Investigation of Luminescence from Dy3 + in AlN

The preparation of powder samples of aluminum nitride activated by dysprosium, AlN/Dy 3+ , was achieved for the first time using a low-temperature, solution-based approach. Aqueous solutions of aluminum and dysprosium nitrates was first converted to hydroxides and then to ammonium-metal hexafluorides (Al 1-x Dy x (NH 4 ) 3 F 6 . Finally, this complex ammonium salt was converted to finely divided powders of Al 1-x Dy x N via a pyrolysis reaction in a high-purity ammonia flow inside a tubular quartz furnace at 900°C. The phase purity and crystallinity of the material were determined using X-ray diffraction and energy dispersive spectroscopy. The intraconfigurational f-f transitions of Dy 3+ were investigated using photoluminescence (PL) and photoluminescence excitation (PLE) measurements. The excited states of Dy 3+ in this large-gap nitride were identified from a careful analysis of PL and PLE results. It was also observed from the PLE measurements that the Dy 3+ ions in this host are excited through energy transfer from both the excited state of the host lattice and defects. Measurements using site-selective spectroscopy suggest multiple sites for Dy 3+ ions in AlN. A theoretical model describing the excitation process and saturation of excited states of Dy 3+ at higher intensity of the exciting radiation is also proposed.

Study of GaN : Eu3 + Thin Films Deposited by Metallorganic Vapor-Phase Epitaxy

Using metallorganic vapor-phase epitaxy, thin films of gallium nitride activated by Eu 3+ (GaN:Eu 3+ ) have been deposited on sapphire substrates at atmospheric pressure. Luminescence from Eu 3+ ions in GaN has been investigated using photoluminescence (PL) and PL excitation spectroscopy. Experimental results show that Eu 3+ ions are excited via energy transfer from the host. Analyses of the observed emission and excitation spectra indicate occupancy of multiple sites in the nitride lattice. Using a pulsed laser source, variation of emission intensity with increasing excitation intensity has also been examined. The possibility of emission saturation at high excitation intensity is discussed from the perspective of application in light-emitting diode sources.

A Study of Luminescence from Tm3 + , Tb3 + , and Eu3 + in AlN Powder

Nitride alloys of Ga, In, and Al activated by rare-earth ions (Re 3+ ) are being considered for application in nitride-based solid-state light sources. The potential applications involve using such materials as the active layer in a heterostructure design or as a fluorescent material for converting the emission from a light-emitting diode to white light. In this paper, we report luminescence from Tm 3+ , Tb 3+ , Eu 3+ , and Tb 3+ -Eu 3+ couple in A1N powder samples synthesized for this purpose. Using the photoluminescence and photoluminescence excitation responses of Re 3+ -doped A1N samples, the multiplet structures of these Re 3+ ions in A1N with tetrahedral coordination have been determined. The excitation energy transfer processes from the host and defects to Re 3+ and from Tb 3+ to Eu 3+ have been observed. These processes are critical for developing nitride-based luminescent materials for white-light emission.

Enhancement of Yellow Light Extraction Efficiency of Y 3 Al 5 O 12 :Ce 3+ Ceramic Converters Using a 2-D TiO 2 Hexagonal-Lattice Nanocylinder Photonic Crystal Layer

The effects of 2-D hexagonal-lattice TiO₂ photonic crystal layers on the directional light extraction efficiency of low-scattering (Y_1-xCe_x)₃Al₅O₁₂ (YAG:Ce ³⁺) ceramic converters have been investigated. The hexagonal-lattice TiO₂ patterns were fabricated by an electron beam lithography process. The optimized yellow directional light extraction enhancement by a factor of 4.4 is achieved with a 2-D photonic crystal layer having a diameter of 430 nm, a lattice constant of 590 nm, and a height of 350 nm. A 3-D finite-difference time-domain model has also been developed to study the geometric parameters of the photonic crystal layer, yielding results that are consistent with our measured data.

Study of Luminescence from GaN : Tb3 + Powders and Thin Films Deposited by MOVPE and PLD Methods

accompanied by near-band-edge emission and defect emissions from the GaN host was observed for the MOVPE films made using tris2,2,6,6-tetramethyl-3,5-heptanedionatoterbium but not films made with trisisopropylcyclopentadienylterbium. Despite visible luminescence from Tb 3+ in GaN powders and thin films, no energy transfer from the host to activator ions was observed. This suggests that Tb 3+ is unlikely to fluoresce if used in a GaN-based optoelectronic device.

Luminescent ceramics for LED conversion

Many LED-based applications would benefit from more efficient and/or high lumen output devices that enable usage in both white and single color illumination schemes. In the present article we briefly review the materials research history leading to optical ceramic converters and discuss their typical characteristics. Recently demonstrated high performance values in terms of efficacy and external quantum efficiency in orange (amber) spectral region are described.

A Study of Oxygen Content in GaN, AlN, and GaAlN Powders

A three-step solution-based method has been adopted and improved to synthesize AlN, GaAIN, and GaN powders with low oxygen content by sequential conversion of nitrates to hydroxides to fluorides and finally into nitrides. The synthesis parameters for rare-earth-activated AlN powders, high-purity single-phase GaN, and GaAIN were determined. Oxygen content of approximately 4 atom % in GaN as determined by energy-dispersive spectroscopy was achieved by optimizing the experimental process. X-ray diffraction observed single-phase AlN, GaN, and GaAIN powders; reflectance measurements indicated a slightly increased bandgap for the synthesized GaN powder with the highest purity compared to those with higher oxygen content, and the GaAIN powder with more incorporated aluminum.