Natalie Fellows

High Power and High External Efficiency m-Plane InGaN Light Emitting Diodes

High power and high efficiency nonpolar m-plane (1100) nitride light emitting diodes (LEDs) have been fabricated on low extended defect bulk m-plane GaN substrates. The LEDs were grown by metal organic chemical vapor deposition (MOCVD) using conditions similar to that of c-plane device growth. The output power and external quantum efficiency (EQE) of the packaged 300 ×300 µm2 was 23.7 mW and 38.9%, respectively, at 20 mA. The peak wavelength was 407 nm and <1 nm redshift was observed with change in drive current from 1–20 mA. The EQE shows a minimal drop off at higher currents.

Optical properties of yellow light-emitting diodes grown on semipolar (112¯2) bulk GaN substrates

We demonstrate high power yellow InGaN single-quantum-well light-emitting diodes (LEDs) with a peak emission wavelength of 562.7nm grown on low extended defect density semipolar (112¯2) bulk GaN substrates by metal organic chemical vapor deposition. The output power and external quantum efficiency at drive currents of 20 and 200mA under pulsed operation (10% duty cycle) were 5.9mW, 13.4% and 29.2mW, 6.4%, respectively. It was observed that the temperature dependence of the output power of InGaN LEDs was significantly smaller than that of AlInGaP LEDs.

High Brightness Blue InGaN/GaN Light Emitting Diode on Nonpolar m-plane Bulk GaN Substrate

Improved nonpolar m-plane (1100) light emitting diode (LED) with a thick InGaN active layer of 8 nm and a thick GaN barrier layer of 37.5 nm for multi-quantum-well (MQW) structure have been fabricated on low extended defect bulk m-plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence (EL) emission from the packaged LED was 468 nm. The output power and external quantum efficiency (EQE) were 8.9 mW and 16.8%, respectively, at a DC driving current of 20 mA.

A yellow-emitting Ce3+ phosphor, La1−xCexSr2AlO5, for white light-emitting diodes

A yellow-emitting phosphor, La1−xCex3+Sr2AlO5, is reported that displays a peak in the excitation at 450nm and a peak in the emission at 556nm. When this phosphor is pumped by a blue InGaN light-emitting diode (λmax=450nm) we obtain white light with color rendering index (Ra) between 81 and 85 and color temperatures between 4200 and 5500K, suggesting that this material is competitive as a blue-pumped yellow phosphors.

High brightness violet InGaN/GaN light emitting diodes on semipolar (1011) bulk GaN substrates

We report the fabrication of violet InGaN/GaN light-emitting diodes (LEDs) on semipolar (1011) GaN bulk substrates. The LEDs have a dimension of 300 ?300 ?m2 and are packaged in an epoxy resin. The output power and external quantum efficiency (EQE) at a driving current of 20 mA were 20.58 mW and 33.91% respectively, with peak electroluminescence (EL) emission wavelength at 411 nm. The LEDs show minimal shift in peak EL wavelength with increasing drive current along with a high EQE.

High power and high efficiency blue light emitting diode on freestanding semipolar (101¯1¯) bulk GaN substrate

Blue InGaN∕GaN multiple-quantum-well light emitting diodes with a peak emission wavelength of 444nm were grown on low extended defect density semipolar (101¯1¯) bulk GaN substrates by conventional metal-organic chemical vapor deposition. The calculated external quantum efficiency and output power at a drive current of 20mA under pulsed operations (10% duty cycle) were 29% and 16.21mW, respectively. The device exhibited virtually no peak electroluminescence wavelength shift with increasing drive currents, indicating a significant reduction of polarization-related internal electric fields.

La1−x−0.025Ce0.025Sr2+xAl1−xSixO5 solid solutions as tunable yellow phosphors for solid state white lighting

Solid solutions of two isotypic compounds from the end members LaSr2AlO5 and Sr3SiO5 are hosts for Ce3+ activator ions, allowing for careful tuning of the various parameters associated with efficient solid state (blue + yellow =) white lighting. By incorporating this phosphor with an encapsulant on InGaN light-emitting diodes (λmax = 430 nm), we obtain white light with a color rendering index (Ra) between 67 and 70 and color temperatures between 6550 and 7345 K, with relatively high efficacies, suggesting these solid solutions are promising for applications in solid state white lighting.

Study of nonpolar m-plane InGaN∕GaN multiquantum well light emitting diodes grown by homoepitaxial metal-organic chemical vapor deposition

Nonpolar m-plane (11¯00) InGaN-based light emitting diodes (LEDs) grown on low-extended defect density bulk m-plane GaN substrates offer great potential for high performance devices due to the absence of polarization-related internal electric fields. To optimize the quantum well (QW) structure, systematic sets of near blue-ultraviolet LEDs using different well widths, barrier widths, and QW periods were packaged and tested. With increasing current, high power LEDs were realized with fairly flat external quantum efficiency and blueshift-free peak wavelength for QWs with thicknesses from 8to20nm, barrier widths from 10to22nm, and QW numbers from 4 to 10.

Hexagonal pyramid shaped light-emitting diodes based on ZnO and GaN direct wafer bonding

The authors report on hexagonal pyramid shaped light-emitting diode (LED) based on ZnO and GaN wafer bonding. After direct wafer bonding of an n-type ZnO substrate to a III-nitride LED wafer, O-plane ZnO was selectively etched to form an electrode having a truncated hexagonal pyramid shape. This wafer bonded LED chip was evaluated with optical output power as a function of forward current and was 2.2 times higher than a conventional-type LED chip having thin Ni (5nm)∕Au (10nm) p-type electrode at forward current condition of 20mA.

Luminescence Characteristics of N-Polar GaN and InGaN Films Grown by Metal Organic Chemical Vapor Deposition

Nitrogen- and Ga-polar GaN and InGaN/GaN multiple quantum-well (MQW) films were prepared via metal organic chemical vapor deposition and evaluated via photoluminescence at reduced temperatures in order to compare their optical characteristics. While N- and Ga-polar GaN films grown at standard high temperatures were comparable in terms of photoluminescence at tested temperatures, the N-polar InGaN MQW quality was inferior to their Ga-polar counterparts, confirmed with greater enhancement in luminescence intensity from 300 to 10 K and unobservable phonon replicas at 10 K for the N-polar InGaN MQW due to the broad emission peak. Additionally performed electroluminescence studies on N-polar light-emitting diode samples indicated that the poor luminescence of the N-polar samples was not related to electric field effects. Influence of residual impurities (C and O) was strongly suggested via secondary ion mass spectroscopy, leading us to conclude that the poor luminescence properties of the N-polar InGaN MQWs were predominantly caused by the elevated residual impurity concentrations in the N-polar (In,Ga)N layers grown at low temperatures.