A microwave treatment method different from thermal annealing and low-energy electron beam irradiation was proposed to activate Mg dopants in p-type GaN epitaxial layer. From photoluminescence spectra and Hall effect measurements, it was shown that microwave treatment is a very effective way to activate the acceptors in Mg-doped p-type GaN layer. The activation of Mg dopant in p-type GaN layer may be explained as the breaking of magnesium-hydrogen bonding due to the microwave energy absorption.
Nitride-based light-emitting diodes (LEDs) with a reflector at the backside of the sapphire substrates have been demonstrated. It was found that an SiO 2 /TiO 2 distributed-Bragg reflector (DBR) structure could reflect more downward-emitting photons than an Al-mirror layer. It was also found that the 20-mA output power was 2.76 mW, 2.65 mW, and 2.45 mW for the DBR LED, Al-reflector LED, and conventional LED, respectively. With the same 50-mA current injection, the integrated-electroluminescence (EL) intensity of a DBR LED and an Al-reflector LED was 19% and 15% larger than that observed from a conventional LED. INTRODUCTIONThe III-V nitrides have some unique properties, such as wide direct bandgap, high-thermal conductivity, and chemical stability. These properties have made III-V nitrides attractive in recent years. At room temperature, the bandgap energy of AlInGaN varies from 1.95 eV to 6.2 eV, depending on its composition. Therefore, III-V nitride semiconductors are particularly useful for light-emitting devices in the short wavelength region. 1-4 In fact, III-V nitridebased blue and green light-emitting diodes (LEDs) with an InGaN/GaN multiquantum-well (MQW) active region are now commercially available. These nitride-based blue and green LEDs could be used in versatile applications, such as full-color displays, full-color indicators, and traffic lights. The other potential application for nitride-based LEDs is lighting. Currently, light bulbs and fluorescent tubes are the most commonly used light sources in our daily life. However, these conventional light sources consume large power. The lifetime of these conventional light sources is also short. In contrast, nitride-based LEDs are much less power consuming and much more reliable. Thus, much attention has been focused on generating white light with nitride-based LEDs. Although it has been shown that we could combine nitride-based, blue-LED chips with yellow phosphors to generate white light, the output power of the nitride-based, white-LED lamps is still low. In other words, we need to further improve the output intensity of nitride-based, blue-LED chips before we can realize feasible nitride-based, white-LED lamps.Unlike laser diodes, photons generated from LED chips could be emitted in any direction. As a result, a large portion of photons emitted from a LED chip could be lost, particularly for those photons being emitted downward to the substrate. Thus, if we could effectively reflect those photons emitted downward, we should be able to enhance the LED-output intensity significantly. Because nitride-based LED structures are normally grown on transparent-sapphire substrates, we should be able to reflect downwardemitting photons by depositing an Al-mirror layer or a distributed-Bragg reflector (DBR) structure at the backside of the sapphire substrates. A highly reflective DBR mirror is formed from a repeated periodic stack of alternating high and low index quarterwavelength layers. To increase the reflectivity of a DBR structure, we need to precisely contr...
In this study, an AlGaInP/GaP-based heterostructure featuring a silicon substrate and a SiO 2 /indium tin oxide/Ag omnidirectional reflector, using a metal-to-metal bonding technique, serves as a dual-function device operating in light emitting and photovoltaic modes. To enhance the light extraction efficiency and conversion efficiency, AlGaInP/Si heterojunction devices with a periodic texture applied to the n-͑Al 0.5 Ga 0.5 ͒ 0.5 In 0.5 P surface layer using photolithography and a wet etching process are also presented. Using the light emitting mode and a 350 mA current injection, the external quantum efficiencies of AlGaInP/Si light emitting diodes ͑LEDs͒ with ͑LED-I͒ and without ͑LED-II͒ a textured surface are measured at approximately 17.3 and 11.8%, respectively. The enhancement of the output power in LED-I can be attributed to a multitude of bowl-shaped notches on the surface, resulting in a reduction in the reabsorption probability of photons inside the device because the photon path length of LED-I is shorter than LED-II before photons escape into free space. When devices are operated in a photovoltaic mode, measured under an air mass 1.5 condition, the typical efficiency and fill factor are around 4.67 and 83%, respectively, for devices with a textured surface.Surface texturing techniques are available and can be customized to improve light extraction and incident efficiency of semiconductor optoelectronic devices, including light emitting diodes ͑LEDs͒ and solar cells. 1,2 In conventional ͑Al x Ga 1−x ͒ 0.5 In 0.5 P LED emitting wavelengths of 560-670 nm, the light extraction efficiency ͑LEE͒ of AlGaInP/GaAs LEDs is low because the downward light is absorbed by the thick GaAs substrate. 1 Although the problem of substrate absorption can be partially resolved by using distributed Bragg reflectors, these only exhibit a narrow band of high reflectivity. 1 AlGaInP LEDs with high levels of brightness, featuring a metal or a Si substrate and a metal reflector layer ͓an omnidirectional reflector ͑ODR͔͒, are thus developed to further enhance the LEE of AlGaInP LEDs. 3 The transparent substrate ͑TS͒-type AlGaInP LEDs using wafer-direct bonding technology have been applied for more than a decade. However, such TS-LEDs require a critical and costly GaP wafer-bonding process. 4 For the so-called metal bonding ͑MB͒ LEDs, 3 the technical requirements are less strict during the device process vs the wafer-direct bonding process. 5 In addition to the absorption of the GaAs substrate, critical angle loss, due to large differences between the high refractive-index semiconductor material and the lower refractive-index surrounding material, i.e., air or resin, is also a crucial issue. To enhance the escape probability of photons generated in the active layer of the LED, a large critical angle or a rough surface is required. 1,6 Although the refractive index of a semiconductor cannot be changed, one can enhance the LEE by roughening the semiconductor surface. For an LED, the angular randomization of photons can be achieved ...
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