Aoi Okada

Luminescence Properties of Eu2+-Doped Red-Emitting Sr-Containing Sialon Phosphor

We developed a Eu2+-doped red-emitting Sr-containing sialon phosphor Sr2Si7Al3ON13:Eu2+ that could play a very important role in high color rendering of white light-emitting diodes (LEDs) for solid-state lighting. It realizes both high efficiency and small thermal quenching under excitation by blue light, which are essential for operation in a high-temperature atmosphere. It shows a highly efficient red luminescence whose external quantum efficiency reaches 73% for 450 nm excitation. These features show that this red-emitting phosphor has high potential for application to white LEDs.

White Light-Emitting Diodes for Wide-Color-Gamut Backlight Using Green-Emitting Sr-Sialon Phosphor

We have successfully developed a white light-emitting diode (LED) for a wide-color-gamut backlight composed of a green-emitting phosphor Sr3Si13Al3O2N21:Eu2+ combined with a blue LED and a red-emitting phosphor CaAlSiN3:Eu2+. This white LED showed a discrete spectrum with distinct separation of red, green, and blue primary colors due to a narrow emission band of around 525 nm for the green phosphor. 94.2% of the wide color gamut of the National Television System Committee standard was attained by applying typical color filters of LCDs. The power LED module composed of 16 of these white LEDs revealed their excellent power dependence. The LED is expected to replace cold cathode fluorescent lamp (CCFL), and find a suitable application as a backlight in large-scale LCDs for in-vehicle use or for flat-panel television sets.

Performance enhancement of blue light-emitting diodes with InGaN/GaN multi-quantum wells grown on Si substrates by inserting thin AlGaN interlayers

We have grown blue light-emitting diodes (LEDs) having InGaN/GaN multi-quantum wells (MQWs) with thin AlyGa1−yN (0 < y < 0.3) interlayers on Si(111) substrates. It was found by high-resolution transmission electron microscopy observations and three-dimensional atom probe analysis that 1-nm-thick interlayers with an AlN mole fraction of less than y = 0.3 were continuously formed between GaN barriers and InGaN wells, and that the AlN mole fraction up to y = 0.15 could be consistently controlled. The external quantum efficiency of the blue LED was enhanced in the low-current-density region (≤45 A/cm2) but reduced in the high-current-density region by the insertion of the thin Al0.15Ga0.85N interlayers in the MQWs. We also found that reductions in both forward voltage and wavelength shift with current were achieved by inserting the interlayers even though the inserted AlGaN layers had potential higher than that of the GaN barriers. The obtained peak wall-plug efficiency was 83% at room temperature. We suggest...

High-efficiency blue LEDs with thin AlGaN interlayers in InGaN/GaN MQWs grown on Si (111) substrates

We demonstrate high-efficiency blue light-emitting diodes (LEDs) with thin AlGaN interlayers in InGaN/GaN multiquantum wells (MQWs) grown on Si (111) substrates. The peak external quantum efficiency (EQE) ηEQE of 82% at room temperature and the hot/cold factor (HCF) of 94% have been obtained by using the functional thin AlGaN interlayers in the MQWs in addition to reducing threading dislocation densities (TDDs) in the blue LEDs. An HCF is defined as ηEQE(85°C)/ηEQE(25°C). The blue LED structures were grown by metal-organic chemical vapor deposition on Si (111) substrates. The MQWs applied as an active layer have 8- pairs of InGaN/AlyGa1-yN/GaN (0≤y≤1) heterostructures. Thinfilm LEDs were fabricated by removing the Si (111) substrates from the grown layers. It is observed by high-resolution transmission electron microscopy and three-dimensional atom probe analysis that the 1 nm-thick AlyGa1-yN interlayers, whose Al content is y=0.3 or less, are continuously formed. EQE and the HCFs of the LEDs with thin Al0.15Ga0.85N interlayers are enhanced compared with those of the samples without the interlayers in the low-current-density region. We consider that the enhancement is due to both the reduction of the nonradiative recombination centers and the increase of the radiative recombination rate mediated by the strain-induced hole carriers indicated by the simulation of the energy band diagram.

Cation and anion ordering in Sr2Si7Al3ON13 phosphors

A series of photoluminescent Ce3+ doped samples with compositions close to Sr2Si7Al3ON13:Ce have been studied by neutron powder diffraction to determine the Si4+/Al3+ and N3−/O2− site ordering. Contrary to a commonly held assumption that the edge sharing tetrahedral sites in this structure are occupied exclusively by Al3+, we find a partial occupancy of Al3+ on these site but also an unexpected preference for Al3+ to occupy 2 other tetrahedral sites which are only corner sharing. From the crystal structures and local structures, as determined by pair distribution function (PDF) analysis, we also find evidence for alternating Si–Al site ordering within the edge sharing chains as well as dimerization of the Si4+ and Al3+ cations within these chains. The O2− are found to be partially ordered onto 2 of the anion sites, although small amounts of O2− are found on other sites as well. The cation and anion ordering found by neutron diffraction is supported by theoretical calculations. Understanding cation and anion ordering is essential for optimizing the photoluminescence properties of this promising class of phosphor materials.

Demonstration of novel high-efficiency blue LEDs on silicon substrates

High-efficiency gallium-nitride-based LEDs have become widely popular as these devices have found several applications, e.g., as white LEDs in general lighting.1 Many of these LEDs are grown on aluminum oxide (Al2O3), silicon carbide (SiC), or gallium nitride (GaN) substrates.2 In addition, GaN-based LEDs grown on silicon (Si) substrates have attracted much attention because of their low fabrication cost. These GaN-based LEDs, however, tend to include high densities of dislocations and cracks because of the large mismatch of lattice constants and thermal expansion coefficients between the GaN-based layers and the Si wafers.3 Many groups have attempted to tackle this issue and have reported improvements in the manufacture of GaN-based LEDs.4, 5 For example, gas flow conditions and precise control of temperature in a reactor have been optimized.5 In addition, we have previously developed LEDs with low threading dislocation densities (TDDs), i.e., of less than 2 108cm2 on Si (111 crystallographic plane) substrates.6, 7 Those TDD values were almost the same as those of LEDs grown on Al2O3 (0001) substrates. We achieved this reduction of TDDs by using multiple modulations of dislocations during the formation of GaN islands on a silicon nitride (SiN) interlayer, and from the growth of the SiN cap layers on the GaN island surfaces. In the past, we have also confirmed that the external quantum efficiency (EQE) and the hot/cold factors (HCFs) of LEDs grown on Si (111) substrates increase with decreasing edge-type dislocations in the low-current-density region.8 We also showed that the EQE of a sample with a low TDD is almost as high as that of an LED grown on an Al2O3 (0001) substrate at room temperature.8 Figure 1. Schematic diagrams of the multi-quantum well structures investigated in this study. (a) A reference structure that consists of indium gallium nitride (InGaN) well layers and gallium nitride (GaN) barrier layers. (b) A structure with thin aluminum gallium nitride interlayers that partly replace the GaN barrier layers.