The most common method of making white light emitting diode (LED) is to mix the blue light from the LED die and the
wavelength converted yellow light from the phosphor layer. The color conversion efficiency depends on the geometry
and concentration of the phosphor layer including phosphor material. Thus the optimization of the phosphor geometry
and concentration make increase the luminous efficiency of the white LED. In this paper, the remote phosphor scheme is
optimized focusing on increasing the luminous efficiency in high power. The phosphor layer is separated by the silicone
resin from the LED die. The silicone resin covers the LED die with dome shape to increase the extraction efficiency. The
phosphor layer has very large volume with dilute concentration. The separation of phosphor layer from LED die and
very large volumetric dilute phosphor layer were great important role in increasing the luminous flux. The improved
luminous flux was 15% for 1mm2 LED die at 700mA.
We report on the development of GaN-based violet laser diodes (LDs) for the high-capacity optical storage application and blue LDs for the laser projection display application. InGaN LDs with emission wavelength of ~405 nm are already being adopted for next-generation optical-storage systems. We present results on >400 mW single-mode output power under pulsed operation which can be employed in 100 Gbyte multi-layer BD systems. We designed LD layer structures to exhibit high level of catastrophic optical damage (COD) and small beam divergence. In addition, GaN-based blue LDs with emission wavelength of ~450 nm have also been developed for the application to the blue light sources of laser display systems. We demonstrate single-mode blue InGaN LDs with >100 mW CW output power. Interestingly, we observed anomalous temperature characteristics from the blue InGaN LDs, which has shown highly-stable temperature dependence of output power or even negative characteristic temperature (T0) in a certain operation temperature range. This unusual temperature characteristic is attributed to originate from unique carrier transport properties of InGaN QWs with high In composition, which is deduced from the simulation of carrier density and optical gain.
The enhanced output power with improved lifetime is required for the GaN-based blue-violet laser diode (LD) as a light source for Blu-ray Disc or HD-DVD. In this paper, the output power levels and aging behaviors in GaN-based LDs grown on sapphire substrates were compared in epi-up and epi-down bonding. At low current level, the two bondings
show little differences in L-I characteristics. At high current level, however, the epi-up bonding shows a rapidly decreased slope efficiency in L-I characteristics with increasing current injection. On the contrary, the slope efficiency in epi-down bonding is not so much deteriorating as that in epi-up bonding. The differences in junction temperature between epi-up and epi-down bonding are large at higher current levels. The junction temperature of epi-up bonding is
about two times higher than that of epi-down bonding, implying efficient heat dissipation in epi-down bonding. At aging test, the epi-down bonding LD shows lower degradation rate at the aging slope than that of epi-up bonding LD. The degradation rate is accelerated by poor heat dissipation in epi-up bonding. Thus, for the higher power and longer lifetime, it is necessary to employ efficient heat dissipation structures such as epi-down bonding for the GaN-based LD
on sapphire substrate.
Experimental results on a new type of light-emitting device, the light-emitting triode (LET), are presented. The LET is a three-terminal p-n junction device that accelerates carriers in the lateral direction, i.e. parallel to the p-n junction plane, by means of an electric field between two anodes. The lateral field provides additional energy to carriers thereby allowing them to overcome barriers and increasing the carrier injection efficiency into the active region. LETs were fabricated using a ultraviolet LED structure that has an AlGaN/GaN superlattice in the p-type confinement region for high-conductivity 2 dimensional hole gas. LET mesa structures were obtained by standard photolithographic patterning followed by chemically-assisted ion-beam etching using Cl2 and Ar to expose the n-type cladding layer. The n-type contact was fabricated by electron-beam evaporation of Ti/Al/Ni/Au. Ni/Au (50/50 Å) metallization was deposited for both anodes, Anode 1 and Anode 2, and subsequently annealed at 500 oC in an O2 ambient. It is shown that both the current between Anode 1 and the cathode, and the light-output power increase with increasing negative bias to the Anode 2. This is consistent with the expectation that a negative bias to the second anode allows carriers to acquire a high kinetic energy thereby enabling them to overcome the barrier for holes, resulting in high injection efficiency into the active region that lies beyond the barrier.
High power and high efficiency AlInGaN-based laser diodes with 405 nm were fabricated for the post-DVD applications. Magnesium doped AlGaN/GaN multiple quantum barrier (MQB) layers were introduced into the laser diode structure, which resulted in considerable improvement in lasing performances such as threshold current and slope efficiency. Asymmetric waveguide structure was used in order to improve the characteristics of laser diodes. Aluminum content in the n-cladding layer was varied in connection with the vertical beam divergence angle and COD level. By decreasing Al content in the n-cladding layer, the vertical divergence angle was reduced to 17 degree and the COD level was enhanced to over 300mW. The maximum output power reached as high as 470 mW, the highest value ever reported for the narrow-stripe GaN LDs. In addition, the fundamental transverse-mode operation was clearly demonstrated up to 500 mW-pulsed output power.
We report on a 1060 nm single transverse mode operation of an end-pumped vertical external cavity surface emitting laser (VECSEL). End-pumping scheme is enabled by capillary bonding of a VECSEL chip with a diamond heat spreader followed by a GaAs substrate removal by selective wet etching. The VECSEL structure is consisted of 10 periods of resonant periodic gain with an 8 nm InGaAs single quantum well at the antinodes of the standing wave optical field and a 35 pair AlAs/AlGaAs bottom distributed Brag reflector (DBR). Optical pump efficiency through the bottom mirror is enhanced by a modified DBR structure with a reduced reflectance in 808 nm pump wavelength region. A low threshold pump density of 433 W/cm2 and over 45 W/W optical to optical conversion efficiency are achieved with reflectivity of 94 % output coupler at the heat spreader temperature of 20°C. The laser operates in a circular TEM00 mode (M2<1.5) up to 7 W, and maximum power of 9.1 W is limited by our pump laser power.
We have optimized a resonant gain structure of a 920 nm vertical external cavity surface emitting laser. We found that a long saturated carrier lifetime in shallow quantum well (QW) under a high injection level restricts the laser performance. An insertion of non-absorbing laser in the middle of barrier layers with multi QWs is effective to reduce the saturated carrier lifetime and, therefore, to enhance the laser performance. With the optimized laser structure, which has 10 periods of triple In0.09Ga0.91 As QWs located at the anti-standing wave optical field with Al0.3Ga0.7As non-absorbing layers in the middle of GaAs barrier, we achieved 4.9 W operation at 920nm. Subsequently blue laser was achieved by employing an intra-cavity frequency doubling crystal LBO. As a result, we demonstrated 2 W single transverse mode operation in blue (460 nm) with a 20 W pump laser power. The conversion efficiency from 808 nm pump laser to the blue laser is measured to be 10 %.
Enhancement of light extraction in GaN light-emitting diodes (LEDs) employing omnidirectional reflectors (ODRs) is presented. The ODR consists of GaN, ITO nanorod low-refractive-index layer, and an Ag layer. An array of ITO nanorods is deposited by oblique-angle deposition using e-beam evaporation. The refractive index of the ITO nanorods is 1.34 at 461 nm, significantly lower that that of dense ITO, which is n = 2.06 at 461 nm. It is experimentally shown that the GaN LED with GaN/ITO nanorods/Ag ODR show much better electrical properties and higher light-extraction efficiency than LEDs with Ag contact. This is attributed to enhanced reflectivity of the ODR by using an ITO low-refractive-index layer with high transparency, high conductivity, and low refractive index.
The efficiency of a conventional light emitting diode (LED) is limited by coupling of light into guided modes in the structure. Several methods to increase the extraction efficiency of nitride based LEDs are studied from the perspective of the patterned structures in LEDs. The patterned structures are made in the interface between a semiconductor and a sapphire substrate and on the surface of a semiconductor or an indium tin oxide electrode. All of these approaches show an increased light output compared to that of reference samples, which means these kinds of scattering sources are inevitable to make a highly efficient light emitter in nitride-based semiconductor system.
We observed a significant enhancement in light output from GaN-based light-emitting diodes (LEDs) in which two-dimensional photonic crystal (PC) patterns were integrated. We approached two types PC LEDs. One is top loaded PC LEDs. The PC patterns were generated on the top p-GaN layer. The other is bottom loaded PC LEDs. In this LEDs, PC patterns were integrated on the sapphire substrate. Two dimensional square-lattice air-hole array patterns, whose period was varied between 300 and 700nm, were generated by laser holography. Unlike the commonly utilized electron-beam lithographic technique, the holographic method can make patterns over a large area with high throughput. The resultant PC-LED devices with a pattern period of ~500nm had more than double the output power. The experimental observations are qualitatively consistent with three-dimensional finite-difference-time-domain simulation results.
With increasing demands for the development of high power GaN-based blue-violet laser diodes (LDs), thermal management has become an important issue. We present a new method to determine junction temperature of GaN-based LDs for simple, fast, and reliable characterization of thermal performances. The large change of forward operation voltage with temperature is advantageously used to measure junction temperature. Using this method, we compare junction temperature of LD structures with different substrates and chip mounting methods. It is found that the junction temperature can be reduced considerably by employing GaN substrates or epi-down bonding. For epi-down bonded LDs, as much as two-fold reduction in junction temperature is achieved compared to epi-up bonded ones and temperature increase in this case is only about 13 degrees for more than 100 mW-output power.