Ian T. Ferguson

Optical and Structural Investigations on Mn-Ion States in MOCVD-grown Ga1-xMnxN

The incorporation of Mn into GaMnN epilayers by MOCVD growth was investigated. Samples with high Mn concentrations lead to room temperature ferromagnetism. In addition an absorption band around 1.5 eV was observed. Intensity and linewidth of this band scaled with the Mn concentration and with the room temperature (RT) saturation magnetization. This band is assigned to the internal Mn 3+ transition between the 5 E and the partially filled 5 T2 levels of the 5 D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn 3+ to Mn 2+ was found to effectively suppress these transitions resulting also in a significant reduction of the RT magnetization. The pronounced sensitivity of the relative position of the Fermi level and 1.5 eV absorption band can be used to predict the magnetization behavior of the Ga1-xMnxN epilayers. The absence of doping-induced strain was observed by Raman spectroscopy. The structural quality, the presence of Mn 2+ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions were observed.

The optical properties of phosphor converted white LED with adding Zirconium dioxide particles (Conference Presentation)

We performed the simulation and experiment to investigate the influence of Zirconium dioxide (Zirconia, ZrO2) particles on the optical properties of phosphor converted white LED (pcW-LED). An efficient optical model was developed and applied to the incorporation diffuse particle of ZrO2 into a hemisphere package containing YAG phosphors. The optical properties (chromaticity, packaging efficiency) were estimated as a function of phosphor and ZrO2 particles, through the calculation of effective radius, refractive index, and absorption and conversion efficiency, in a range of correlated color temperature 4500 K to 6500 K. In the same way, the amount of phosphor and ZrO2 can be calculated accurately to obtain a targeted optical property in a hemisphere LED design. Especially, the angular distribution of CCT was also diminished, and even almost inexistent for low CCT design. In addition, the adding of ZrO2 particles allows clearly decreasing the amount of phosphor for an identical target CCT. It is really suitable in the context of decreasing the amount of phosphor or in some applications where the color uniformity is an important parameter, like indoor down-lighting.

New phosphors Eu2+ and Ce3+ doped Sr4-x(Si,Al)19+x(N,O)29+x for white LED applications (Conference Presentation)

Light-emitting diodes (LEDs) have been steadily consolidating their share in the lighting and display market. Phosphor-converted (pc) white LED becomes the preferred way to generate white light especially for general lighting, as it is much cheaper and simpler than RGB system. Phosphors are essential to high color quality and luminous efficacy. However, the number of commercially available phosphors is very limited. Therefore, developing new phosphors suitable for various white LED applications is very important. Recently, our group developed the single-particle-diagnosis approach [1-2] to discover new phosphors, with which a tiny luminescent microcrystalline particle down to 5-10 μm can be selected from powder mixtures. In this work, we report a new green emitting Sr-sialon:Eu phosphor discovered by this approach. The crystal structure was solved and refined from single crystal X-ray diffraction data. Sr-sialon:Eu crystallizes in the trigonal space group P3m1 (no. 156) with a = b = 12.1054 A, c = 4.8805 A and Z = 1, and consists of a network of corner sharing (Si,Al)(N,O)4 tetrahedra. Upon doping with Eu2+, the emission band can be tuned from 487 nm to 541 nm with fwhm = 96-124 nm. Ce3+ doped Sr-sialon phosphor shows strong blue emission around 435 nm with a fwhm ≈ 90 nm after 355 nm light excitation. The blue luminescence exhibits a small thermal quenching behavior at high temperature. These performances show that the new Eu2+ and Ce3+ doped Sr-sialon phosphors are promising for white LED applications. [1] N. Hirosaki, T. Takeda, S. Funahashi and R.-J. Xie, Chemistry of Materials, 2014, 26, 4280-4288. [2] T. Takeda, N. Hirosaki, S. Funahashi and R.-J. Xie , Materials Discovery, 2015, 1, 29-37.

Comparison of methods for measurement of HP-LEDs based on the junction temperature (Conference Presentation)

High power LEDs (HP-LEDs) are key building blocks of solid-state lighting products, therefore, it is important for LED manufacturers, lamp/luminaire manufactures, and testing/calibration laboratories to measure their optical and electrical properties with high accuracy. Measuring HP-LEDs has been difficult because they are highly sensitive to their junction temperatures, which rise rapidly when they are turned on. Various methods have been proposed and used to measure HP-LEDs, but most of them are only useful for particular applications and unable to produce accurate and reproducible measurement results. To address the measurement need, the Illuminating Engineering Society (IES) recently approved three methods that can be used for the measurement of HP-LEDs, which are the DC method, single-pulse method, and continuous-pulse method [1]. All three measurement methods refer to the junction temperature of an HP-LED as the thermal condition and thus, the measured results are considered to be equivalent as long as the junction temperature is set to be the same. However, our recent study shows that the difference in the measurement results of the three different methods can be significant (e.g., 5 % in total luminous flux) due to significant heating of the junction and/or phosphor material of the HP-LED during the period of a measurement. In this paper, we will describe the measurement of HP-LEDs using the three different methods, compare the measurement results, and discuss the cause that results in the significant difference. [1] Illuminating Engineering Society, “IES LM-85-14: Approved Method: Electrical and Photometric Measurements of High-Power LEDs.” (2014)

Epitaxial Growth of GaN-based LEDs on Simple Sacrificial Substrates

The objective of this project is to produce alternative substrate technologies for GaN-based LEDs by developing an ALD interlayer of Al{sub 2}O{sub 3} on sacrificial substrates such as ZnO and Si. A sacrificial substrate is used for device growth that can easily be removed using a wet chemical etchant leaving only the thin GaN epi-layer. After substrate removal, the GaN LED chip can then be mounted in several different ways to a metal heat sink/reflector and light extraction techniques can then be applied to the chip and compared for performance. Success in this work will lead to high efficiency LED devices with a simple low cost fabrication method and high product yield as stated by DOE goals for its solid state lighting portfolio.