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Effects of Patterned Sapphire Substrates on Piezoelectric Field in Blue-Emitting InGaN Multiple Quantum Wells

The strain and piezoelectric fields in InGaN blue light-emitting diodes on a GaN layer, which is grown on a planar sapphire substrate or patterned sapphire substrates (PSSs), such as a microsized PSS and a nanosized PSS (NPSS), are investigated by micro-Raman spectroscopy and electroreflectance (ER) spectroscopy. The obtained piezoelectric field in InGaN multiple quantum wells (QWs) grown on the planar substrate is 0.83 MV/cm, and it is 0.70 MV/cm for the case of the NPSS. These results are attributed to the fact that the GaN layers on the PSSs have a smaller residual strain compared to that on the planar sapphire, and thus, strain reduction in the GaN layer can reduce the piezoelectric field in the InGaN QWs grown on top of it.

High Efficiency InGaN Blue Light-Emitting Diode With

This letter reports high-power and high-efficiency characteristics of the InGaN-based blue light-emitting diode (LED) operating at > 10-W electrical input power in a single-chip package. The LED chip is fabricated as a vertical-injection structure with chip dimensions of 1.8 mm × 1.8 mm. InGaN/GaN short-period superlattice (SL) structures are employed below multiple-quantum-well active region as current spreading layers. It is found, by simulation, that SL layers are quite effective in improving current spreading and uniformity in carrier distribution. When the characteristics of the fabricated LED package are measured under pulsed operation conditions, efficiency droop is found to be greatly reduced in the LED structure with SL layers. A record high light output power of 4.18 W and external quantum efficiency of 51% are demonstrated at 3-A injection current.

{>}{\rm 4}\hbox{-}{\rm W}

For developing high-power resonant cavity light-emitting diodes (RCLEDs) appropriate for visible light communications, we have investigated the effect of reflectivity of a p-side distributed Bragg reflector (p-DBR) and the number of quantum wells (QWs) in active layers on the spectral characteristics and optical power of RCLEDs. As the reflectivity of p-DBR increased, the full width at half maximum (FWHM) of the electroluminescence (EL) spectrum was reduced from 12.3 to 3.6 nm, whereas the relative integrated intensity decreased from 1.0 to 0.37, which can be attributed to the improvement of spectral purity of peaks with in-phase condition. As the number of QWs decreased, optical power increased owing to the reduction of the optical loss of recycling light in the active region. Using the optimized structural conditions, we demonstrated RCLEDs having a modulation speed up to 130 MHz in free space, which shows that the optimized RCLED structure is a promising candidate for visible light communications (VLCs).

Output Power at 3 A

We have investigated the effects of well strain on optical and device characteristics of AlGaInP-based multi-quantum well (MQW) light-emitting diodes (LEDs) according to the variation of well strain and the corresponding change of well thickness in order to maintain the operating wavelengths. As the compressive well strain increased from +0.04 to +0.41% and the corresponding well thickness decreased from 7.2 to 4.5 nm, the relative photoluminescence (PL) intensity increased from 1.0 to 1.21, and the radiative carrier lifetime measured from the time-resolved PL (TRPL) was shortened from 4.43 to 1.77 ns. The compressive well strain may cause the strain-induced heavy-hole-assisted recombination process, followed by reduction of the radiative decay time due to a reduced effective-heavy-hole mass. In addition, a thinner well thickness may enhance the spatial carrier confinement in the well regions, which greatly improved light output power of the LEDs.

Structural Optimization of High-Power AlGaInP Resonant Cavity Light-Emitting Diodes for Visible Light Communications

We developed light-emitting diodes (LEDs) having a shunt diode in order to improve their negative-voltage electrostatic discharge (ESD) characteristics. To make the discharge path, the n-electrode of the LED was extended over a p-GaN surface. The leakage current at reverse bias owing to the shunt diode can be significantly reduced via plasma treatment on the p-GaN surface prior to the extended n-electrode. In this design, the finger type extended n-electrode was implemented to minimize the light absorption by the n-electrode. The negative-voltage ESD threshold of the LED with the shunt diode significantly increased from 300–500 to 3000 V.

Investigation of Strained Multi-Quantum Well Structures for High Bright AlGaInP-Based Light Emitting Diodes

This chapter deals with methods for fabricating high-efficient light-emitting diodes (LEDs) with higher light extraction efficiency (LEE). Some LED prototypes are then reviewed to investigate how their performance was enhanced by utilizing a variety of chip processes. The most efficient devices were found to be produced with unique fabrication processes, having at least one patterned sapphire substrate, chip shaping, vertical configuration, and/or flip-chip packaging, among others.

Negative-Voltage Electrostatic Discharge Characteristics of Blue Light-Emitting Diodes Using an Extended n-Electrode onto Plasma Treated p-GaN

This chapter deals with methods for fabricating high efficient light-emitting diodes (LEDs) with higher light extraction efficiency (LEE). Some LED prototypes are then reviewed to investigate how their performance was enhanced by utilizing a variety of chip processes. The most efficient devices were found to be produced with unique fabrication processes, having at least one patterned sapphire substrate, chip shaping, vertical configuration, and/or flip-chip packaging, among others.

Light Extraction of High-Efficient Light-Emitting Diodes

We investigated high-brightness light emitting diodes appropriate for general lighting applications in terms of their optical behaviors and device performances according to the insertion of the sloped barrier between the well and the barrier and changing the sloped barrier thickness. As the sloped barrier thickness was increased from 0 to 5 nm, radiative recombination efficiency and device performances significantly improved, due to the suppression of carrier overflow by the stronger capture of carriers and the shortening of the carrier lifetime in the active region owing to the built-in quasi-electric field. At a further increase in the sloped barrier thickness to 10 nm, however, the optical and device performances started to degrade because of the loosening of the quantum confinement effect in the active region and due to the saturation of the improvement of the carrier capture by the sloped barrier region.

Light Extraction Efficiency Part B. Light Extraction of High Efficient LEDs

We fabricated InxGa1−xN multiple quantum well (MQW) light-emitting diodes (LEDs) on patterned sapphire substrates (PSSs); the nano-sized PSS (NPSS) and the micro-sized PSS (MPSS), and investigated to enhance the light extraction efficiency (LEE) of LEDs. Furthermore, the micro-Raman and ER spectroscopy were used to compare the strain and piezoelectric fields in the InxGa1−xN MQWs grown on the different substrate conditions. The light output power (at 20mA) of the MPSS and NPSS are 11.4 mW and 11.6 mW, which are enhanced by 39% and 41% compared with that of the conventional LED, respectively.

Investigation of Trapezoidal Well for Improving the Light Efficiency in AlGaInP-Based Light-Emitting Diodes.

Recently, AlGaInP-based light emitting diodes (LEDs) have experienced an impressive evolution in both device performance and market volume. However, development of new applications is required in order to realize their full potential in areas such as use as a light source for auto focusing in digital cameras, special illumination for particular functions in agriculture, and in full color displays. To enlarge their utility in these applications, it is necessary to fabricate and understand a new structure capable of emitting longer wavelengths of around 700 nm. In particular, AlGaInP heterostructure LEDs are lattice-matched with respect to the GaAs substrate, which limits the emitting spectrum to around 650 nm at the longer peak wavelength side. To fabricate an LED structure capable of emitting at a 700 nm peak wavelength, the composition (x) of GaxIn1−xP material in the active layer requires a compressive strain of larger than 1 %. This large lattice mismatch, however, causes significant problems in terms of both growth and device properties due to the formation of defects. To overcome these problems, it is necessary to relieve the well strain via the formation of islands, referred to as a Stranski-Krastanow (S-K) growth mode, in order to prevent the generation of dislocations [1,2]. However, in AlGaInP-based LEDs emitting at a 700 nm peak wavelength, the effects of well strain on the epitaxial growth and the realization of device performance has yet to be extensively studied. In this study, we investigate the behaviors of morphological and optical characteristics on the composition of Ga0.33In0.67P material and demonstrate the performance of a device emitting at around 700 nm using quantum dot (QD)-based LEDs.