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1 Light-Emitting Diodes and Lighting j1 1 Light-Emitting Diodes and Lighting Introduction Owing to nitride semiconductors primarily, which made possible emission in the green and blue wavelengths of the visible spectrum, light-emitting diodes (LEDs) transmogrified from simple indicators to high-tech marvels with applications far and wide in every aspect of modern life. LEDs are simply p–n-junction devices constructed in direct-bandgap semiconductors and convert electrical power to generally visible optical power when biased in the forward direction. They produce light through spontaneous emission of radiation whose wavelength is determined by the bandgap of the semiconductor across which the carrier recombination takes place. Unlike semiconductor lasers, generally, the junction is not biased to and beyond transparency, although in superluminescent varieties transparency is reached. In the absence of transparency, self-absorption occurs in the medium, which is why the thickness of this region where the photons are generated is kept to a minimum, and the photons are emitted in random directions. A modern LED is generally of a double-heterojunction type with the active layer being the only absorbing layer in the entire structure inclusive of the substrate. Such LEDs have undergone a breathtaking revolution that is still continuing, since the advent of nitride-based white-light generation for solid-state lighting (SSL) applications. Essentially, LEDs have metamorphosed from being simply indicator lamps replacing nixie signs to highly efficient light sources featuring modern technology for getting as many photons as possible out of the package. In the process, packaging has changed radically in an effort to collect every photon generated within the structure. Instead of just employing what used to be the standard 5 mm plastic dome to focus the light, the device package is now a high-tech marvel with even holographically generated (employing laser lithography, which is maskless and convenient for periodic patterns) polymeric photonic crystals placed on top or flip-chip mounts (after peeling the GaN structure from the sapphire substrate) with the blackened N-polarity surface for maximum light collection. Furthermore, the area of the device as well as the shape of the chip is designed for maximum etendue, a measure of the optical size of the device. Furthermore, device packaging also had to adopt strategies not only to remove the heat generated by Handbook of Nitride Semiconductors and Devices. Vol. 3. Hadis Morkoç Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-40839-9 2j 1 Light-Emitting Diodes and Lighting the process but also to deal with the thermal mismatch between the chip and the heat sink owing to the Joule heating effect resulting from the current levels in the vicinity of 350 mA. It should be pointed out that nitride LEDs are fabricated on the polar Ga-face of GaN. Therefore, the quantum wells (QWs) used are subjected to quantum-confined Stark shift (red) due the electric field induced by spontaneous and piezoelectric polarization. The latter is severe for increased InN mole fraction in the lattice, in particular, for green LEDs. This results in reduced emission efficiency because of reduced matrix element (lowered overlap integral between the electron and hole wave functions that are pushed to the opposing sides of the quantum well). In fact, the carrier lifetime increases from some 10 ns in bulk InGaN to as high as about 85 ns in a quantum well corresponding to green wavelength. While the same situation is present in lasers, much lower InN compositions and much higher injection levels mitigate the situation to some extent. A quick fix that helps to some extent is to use vicinal substrates even with tilt angles as small as 1 to reduce the polarization- induced field. To really combat this issue, nonpolar surfaces such as the a-plane GaN is explored. However, the quality of the films is much inferior to those on the c-plane GaN, owing in part to the severe structural mismatch between the r-plane sapphire and a-plane GaN and small formation energy of stacking faults. An additional, aggravating issue is that not much In can be incorporated on this plane, preventing the achievement of blue and green wavelength emission. Research on other orientations such as growth of m-plane GaN has begun. For further information, growth of a- and m-plane GaN is discussed in Volume 1, Chapter 3, and the issue of polarization is discussed in Volume 1, Chapter 2. Elaborating further, as LEDs became brighter and white light generating varieties became available, the role of LEDs shifted from being simply indicator lights to illuminators. The advent of nitride LEDs made white light possible with perfect timing, just when handheld electronic devices such as cell phones and digital cameras became popular, and energy cost increased. In these gadgets, LEDs are used not only for background illumination but also as flashlights, particularly in cell phones. Additionally, LEDs penetrated the automotive industry (aircraft industry is going to follow) in a major way with every indicator and/or background light source, with the exception of headlights, being of LEDs. In the year 2002, with nearly $2 billion in sales worldwide, about 40% accounted for mobile electronics, 23% for signs, and 18% for automotive. The mobile electronics market is mainly of the white- LED type, which is made possible solely by nitride LEDs. The market continues to experience rapid growth. Retail lighting, shelf lighting, flashlights, night lighting, traffic signaling, highway moving signs, outdoor displays, landscape lighting, and mood lighting have all gone the way of LEDs. The power savings made possible by À LEDs in the year 2002 amounted to nearly 10 TW year 1 with potential savings À approaching 35 TW year 1, which will ease the tax on the environment by reducing the greenhouse gas emission. The next frontier for LEDs is to conquer the general illumination, which is underdeveloped, with fierce competition that will bring the best out of those who are going to make this possible. Nitride-based LEDs with InGaN Introduction j3 Figure 1.1 InGaN LEDs spanning the spectral range from violet to orange. Courtesy of S. Nakamura, then with Nichia Chemical Co. Ltd. (Please find a color version of this figure on the color tables.) active regions span the visible spectrum from yellow to violet, as illustrated in Figure 1.1. The three types of LEDs are surface emitters, which are divided into those with plastic domes and those with varieties of flat surface-mount, lacking the dome, edge emitters, generally intended for fiber-optic communications, and superradiant or superluminescent devices, which are biased not quite to the point of lasing but biased enough to provide some gain and narrowing of the spectrum. Antireflec- tion coatings or some other measures are taken to ensure that the device does not lase. Among the applications of LEDs are displays, indicator lights, signs, traffic lights, printers, telecommunications, and (potentially) lighting, which requires emission in the visible part of the spectrum. While saturated-color red LEDs can be produced using semiconductors such as GaP, AlGaAs, and AlGaInP, the green and blue commercial LEDs having brightness sufficient for outdoor applications have so far been manufactured with nitride semiconductors. Figure 1.2 exhibits the various ternary and quaternary materials used for LEDs with the wavelength ranges indi- cated. The color bar corresponds to the visible portion of the spectrum. We should also mention that another wide-gap semiconductor, ZnO, with its related alloys is being pursued for light emission, as it is a very efficient light emitter. However, lack of convincingly high p-type doping in high concentration has kept this approach from reaching its potential so far [1]. Even though there is still some discussion of the fundamentals of radiative recombination in InGaN LEDs, the basics of LEDs will be treated first, assuming that the semiconductors of interest are well behaved. This will be followed by the performance of available nitride LEDs and their characteristics. The discussion is completed with succinct treatments of the reliability of nitride-based LEDs, and of organic LEDs (OLEDs), which have progressed to the point that indoor applications are being considered. 4j 1 Light-Emitting Diodes and Lighting Figure 1.2 The LED materials and range of wavelength of the emission associated with them. The color band indicates the visible region of the spectrum. (Please find a color version of this figure on the color tables.) 1.1 Current-Conduction Mechanism in LED-Like Structures Consider an AlGaN(p)/GaN(p)/AlGaN(n) double-heterojunction device that is for- ward biased. The carrier and light distribution in the active layer are depicted schematically in Figure 1.3. For simplicity, let us assume that a double-heterojunction device is one in which all the carriers recombine in the smaller bandgap active region. In reality, recombination takes place in the active layer, some fraction of the recombination is nonradiative, and at the two heterointerfaces on both sides of the active layer that is nonradiative. Here, the larger bandgap AlGaN n- and p-layers are doped rather heavily so that no field exists in these regions. The treatment here will be developed in a manner similar to that of Lee et al. [2] and Wang [3]. Because the active layer is p-type, we will be dealing with minority electron carriers. The continuity equation for electrons can be written as q2n nÀn qn D À 0 þ g ¼ ; ð1:1Þ qx2 t qt where n and n0 represent the minority-carrier concentration and the equilibrium minority-carrier concentration, respectively. The terms D, g, and t represent the electron diffusion length, the generation rate, and the carrier lifetime, respectively, and x and t have their usual meaning.
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