Light extraction efficiency is important to the brightness of LEDs. In this study, various texturing and roughing
schemes were formed on the surface or interface of InGaN-based LED structure grown on sapphire substrate to
investigate their effects. Throughout the research, temperature-dependent PL measurement was used to calculate the
internal quantum efficiency so as to derive the light extraction efficiency. The light extraction efficiency is around 60
to 65% while the epitaxy and substrate are flat. On the other hand, the light extraction efficiency reaches an optimal
value of around 85% while the p-GaN surface is textured and the substrate is patterned. However, for LED having
only one-side surface texturing structure optimized on either p- or n-side, the light extraction efficiency can be
already as high as 75 to 80%. Methods for further enhancement, such as use of ZnO nanorod on chip surface, were
also discussed.
The recent breakthrough in high power GaN LED's efficiency makes the adoption of these tiny solid state light
emitting devices into general lighting application earlier than expected before. However, heat management is still an
important issue for these white high power GaN LEDs. So far, the most popular driving current for 1mm square die is
about 350mA but there is a trend to increase the driving current up to 1A or even higher. In order not to degrade the LED
performance at such a high current operation, it is very important to reduce the thermal resistance and keep the junction
temperature below 60 degree centigrade.
In the past, GaN flip chip, thin GaN LED, or GaN on SiC or GaN substrate are some typical structures used to
make high power LEDs with low thermal resistance. However, all of these methods need very complicated chip process
or using very expensive substrates and are difficult to meet general lighting dollar per lumen target. In this study, we
proposed a cheaper way to make a high power LED die with lower thermal resistance. We will report how we can
achieve the thermal resistance of high power GaN LED die less than 1°C/W.
The internal quantum efficiency (IQE) of commercial ultra high brightness AlGaInP red LED has already
reached 90% or higher but the light extraction efficiency was only about 30% to 50%. Through the improvement of
surface texturing structure by nano-imprint technology and current spreading by using narrow width ohmic contact
metal line, the light extraction efficiency of AlGaInP red LED was significantly improved up to 60%. In AlGaInN LED,
the thermal resistance of the LED chip can be reduced by thinning down or totally removing the sapphire substrate and
then replacing it by high thermal conductivity materials. Therefore, the performances of high power AlGaInN LED
chips were improved in high current density operation condition.
We have remarkably improved the Metal Bonding (MB) AlGaInP LED luminous efficiency in the dominant wavelength range form 570 nm to 630 nm. Micro Shaping technology is fabricated on the Surface of LED Chip in order to enhance the extraction efficiency. As a result, the luminous efficiency of the new micro shaping Structure AlGaInP LED can achieve 80 lm/w, at 615 nm dominant wavelength under 20mA injection current. The luminous efficiency increased up to 50% than report valve before.
To investigate the high performance light source for high-speed plastic optical fiber (POF) communication application is important as high-speed short distance communication for the home networks becomes popular. It is straightforward to reduce the size of RCLEDs to increase the small-signal modulation bandwidth (f-3dB). But reduce the size of RCLEDs not only reduce the output power but also decrease lifetime because higher current density flowed through active region. In this paper, we improve. f-3dB of RCLEDs with the aperture of 84μm by reducing the number of quantum wells (QWs) in active region. We found the speed of RCLED inverse proportional to the number of QWs. By reducing the number of QWs to one, the device with standard aperture size exhibits high f-3dB as 235MHz at bias current of 20mA without sacrificing the other performance like maximum output power, high temperature performance, etc. These devices can transmit data rate as high as 500Mb/sec through graded-index POF over 50 meters. Beyond 1Gbits/sec, we have investigated red VCSELs as suitable high-speed light sources. The structure of red VCSELs is similar to RCLEDs except more pairs of DBR yield high reflectivity. Our red VCSEL can have output power as high as 1.5mW at 5mA and transmission data rate up to 2.5Gbits/sec.
A high brightness AlGaInP LED with a high reflectivity metal reflector structure was proposed. The AlGaInP LED layers with metal reflector is bonded to the high thermal conductivity silicon substrate by using indium as a solder. Because the light that would otherwise be absorbed by the opaque GaAs substrate is reflected by the high reflectivity metal reflector, the brightness is significantly improved. The high current operating characteristics are also improved by replacing the GaAs substrate with silicon substrate. The luminous efficiency of the new structure AlGaInP LED can achieve more than 40 lm/W in the dominant wavelength range from 585nm to 625nm.
Future in-house Multimedia networks, based on the IEEE 1394b standards, require low cost and robust optical
transmission system in the range of 100 meter. In this paper, we presented the state of the art 650 nm micro-cavity light
emitting diodes (RCLEDs) for such application. We had made RCLEDs with diameters of the emission window of 84, 60, 40μm for different requirements. Because of excellent epitaxy quality and structure design. Our RCLEDs perform record high power and efficiency. With expoxy encapsulated, the 84μm devices give an efficiency of 12% and yield more than 3.5mW at operation current 20mA. Our 40μm devices exhibit high small-signal modulation-bandwidths (f-3D) as 310MHz at bias current of 20mA. The output power of 40μm devices is still as high as 1.5mW, which is suitable for IEEE 1394b s400 standard. On the other hand, we had developed metal bonding RCLEDs (MBRCLEDs) to improve the high temperature performance of RCLEDs. By proper design the structure and process, the MBRCLEDs can have very low power decay as 0.6dB from 20°C to 100°C.
Semiconductor light emitting diode (LED) has become a promising device for general-purpose illumination applications. LED has the features of excellent durability, long operation life, low power consumption, no mercury containing and potentially high efficiency. Several white LED technologies appear capable of meeting the technical requirements of illumination. In this paper we present a new multi-color white (MCW) LED as a high luminous efficacy, high color rendering index and low cost white illuminator. The device consists of two LED chips, one is AlInGaN LED for emitting shorter visible spectra, another is AlInGaP LED for emitting longer visible spectra. At least one chip in the MCW-LED has two or more transition energy levels used for emitting two or more colored lights. The multiple colored lights generated from the MCW-LED can be mixed into a full-spectral white light. Besides, there is no phosphors conversion layer used in the MCW-LED structure. Therefore, its color rendering property and illumination efficiency are excellent. The Correlated Color Temperature (CCT) of the MCW-LED may range from 2,500 K to over 10,000 K. The theoretical General Color Rendering Index (Ra) could be as high as 94, which is close to the incandescent and halogen sources, while the Ra of binary complementary white (BCW) LED is about 30 ~ 45. Moreover, compared to the expensive ternary RGB (Red AlInGaP + Green AlInGaN + Blue AlInGaN) white LED sources, the MCW-LED uses only one AlInGaN chip in combination with one cheap AlInGaP chip, to form a low cost, high luminous performance white light source. The MCW-LED is an ideal light source for general-purpose illumination applications.
The operating characteristics of ridge-waveguide AlGaAs/GaAs lasers with conventional double heterostructure (DH), separate-confinement heterostructures (SCH), and graded-index separate-confinement heterostructure single quantum well (GRIN-SCH-SQW) active layer structures grown by metalorganic vapor phase epitaxy (MOVPE) technique are reported. For conventional DH and SCH structures, an undoped Al0.15Ga0.85As active layer was used for an emission wavelength of 780 nm. An undoped 30 angstroms GaAs quantum well was centered in the 3000 angstroms thick graded AlxGa1-xAs (x equals 0.2 - 0.55) GRIN-SCH region with linear Al profile. The lasers with GRIN-SCH-SQE active layer exhibited lowest threshold current density among these three structures. The conventional DH laser has been shown to have better beam qualities and lower astigmatism than the other two types of lasers. Reliable operation for more than 1500 hrs was achieved at 50 degree(s)C for conventional DH lasers. However, GRIN-SCH-SQW lasers with very thin active layer (<EQ 30 angstroms) showed much higher degradation rate.
The structures and performances of surface emitting AlGaInP green light emitting diodes (LEDs) with emission wavelength around 565 nm were studied. The AlGaInP green LEDs with epitaxial structure grown on P-type GaAs substrate showed the best performance. This is the first paper ever reported for the fabrication of AlGaInP LEDs using P-type GaAs as a substrate. In AlGaInP LED structure using P-type GaAs substrate, the highly conductive n- type AlInP was consequently used as a top confining layer and the current crowding problem was significantly improved. Therefore, the green electroluminescence (EL) was uniformly emitted from the surface of LED dice.
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