www.mrs.org/publications/bulletin nologies. The MOCVD method can be used for the relatively cheap fabrication of so- phisticated layer structures with desirable bandgaps and optimized composition and Light-Emitting Diodes: doping profiles. These layered structures are the key element of electroluminescent chips. High rates of light-generation are achieved by confining the injected electrons Progress in Solid- and holes in double heterostructures and single or multiple quantum wells. Other layers are employed for contacts and cur- State Lighting rent spreading in the light-emitting layers. Another crucial issue for high-brightness LEDs is improving their light-extraction Artu¯ras Zˇukauskas, Michael S. Shur, efficiency. Providing for the efficient escape of photons from high-refractive-index and Remis Gaska materials used for light-generation is an important goal of LED chip design. The photons may escape only at incident angles smaller than the critical angle of the total Introduction internal reflection. This critical angle is ␪ ෇ Ϫ1 Until the beginning of the 19th century, problems of low light output and limited given by Snell’s law, c sin (ne/ns), flame produced by combustion was the color range that previously precluded where ne and ns are the refractive indices only source of artificial light. Since then, LED applications in lighting. The bright- of the encapsulating epoxy resin and the physical phenomena other than pyrolumi- ness, efficiency, and color choices of LEDs semiconductor, respectively. The solution nescence have been used to produce light.1 have achieved a level that is leading to widely used at present is to clad the light- Limelight (incandescence of calcium oxide dramatic changes in lighting technology. emitting layer with thick, transparent heated by the flame from an oxyhydrogen In this article, we review the present window layers.5,6 blowpipe), gas mantles (candoluminescence status of solid-state lighting, including dis- Figure 1 demonstrates the difference of gas-flame-heated rare-earth oxides), and cussions of the concept of high-brightness between a conventional LED and a high- the electrical Jablochkoff candle (an early LEDs, materials systems and chip design brightness LED. In a conventional LED type of carbon-arc lamp) were among the for monochrome LEDs, white LED lamps, (Figure 1a), light generated at a certain important milestones that led to modern and, finally, the emerging applications of point in the active layer may only escape lighting technology. In the 21st century, solid-state lighting. upward through a cone with an apex of ␪ most of the residential lighting worldwide A more detailed discussion of many is- 2 c. Almost all of the light emitted in other is provided by tungsten incandescent sues related to solid-state lighting may be directions is totally reflected and absorbed lamps. Compact fluorescent lamps are also found in our upcoming book.4 in the substrate and/or in the active layer. actively promoted because of their higher Ideally, the best performance would be performance—a broader spectrum for High-Brightness LEDs achieved in a spherical LED. In practical higher-quality white light and elimination The development of high-brightness planar high-brightness LEDs, thick window of 100–120-Hz flickering, for example. LEDs relied on the introduction of new layers allow the light to escape through six Most work environments employ fluores- semiconductors with efficiencies of visible cones (see Figure 1b). The thick window cent tubes for general lighting, and street emission much higher than those of early layers allow the light generated at the cen- lighting is dominated by sodium lamps.2 LED materials, such as GaAsP (red), GaP ter of the chip to escape through the lateral Lighting consumes ϳ2000 TWh of energy (yellow-green), and SiC (blue). Semicon- conical paths. Most commercial high- annually, about 21% of the global con- ductors used for high-brightness LEDs brightness LEDs exhibit light-extraction sumption of electricity.3 However, during must exhibit direct transitions with high efficiencies somewhat below 30%. In order the past 20 years, none of the conventional rates of radiative recombination, have to improve the light-extraction efficiency lighting technologies has exhibited a sig- wide bandgaps to emit at visible (or, in further, the LED design can employ non- nificant improvement in efficiency. The certain cases, UV) wavelengths, and pos- rectangular geometries,7 textured surfaces,8 drive to save lighting energy and reduce sess a low density of nonradiative re- and encapsulants with a higher refractive Ϸ its negative environmental impact (i.e., combination centers and high durability. index (at present, epoxy resins with ne 1.6 carbon emissions and the disposal of mer- Novel Group III–V direct-gap ternary and are used). cury contained in discharge lamps) stimu- quaternary compounds and alloys have Advanced solutions for the light- lates the search for new, efficient sources met these requirements. Practical high- extraction problem rely on photon-mode of light. brightness LEDs rely on three semiconduc- engineering. In order to inhibit the gen- This search focused attention on light- tor materials systems: AlGaAs, AlGaInP, eration of light in unfavorable directions, emitting diodes (LEDs), which, prior to and AlInGaN. microcavities,9 photonic crystals (i.e., struc- the last decade of the 20th century, were High-quality compound semiconductors tures with a periodic pattern of the refractive used only as indicator lamps and numerical became available as a result of advances index),10 and emitters with surface-plasmon displays in electronic devices. Today, ma- in epitaxy and especially heteroepitaxy enhancement11 have been introduced. ture methods for fabricating compound- technology. Vapor-phase epitaxy (VPE), semiconductor materials, progress in LED liquid-phase epitaxy (LPE), and metal- AlGaAs Red LEDs design, and the emergence of blue organic chemical vapor deposition The first high-brightness LEDs were de- AlInGaN-based LEDs have resolved the (MOCVD) have all become mature tech- signed for the red spectral region using 764 MRS BULLETIN/OCTOBER 2001 Downloaded from https://www.cambridge.org/core. University of Athens, on 01 Oct 2021 at 17:29:37, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs2001.203 Light-Emitting Diodes: Progress in Solid-State Lighting grown by hydride VPE (HVPE) following MOCVD growth of an AlGaInP double heterostructure on the absorbing GaAs substrate. Then the absorbing GaAs sub- strate is removed from the grown hybrid AlGaInP/GaP structure using conven- tional selective chemical etching, and, fi- nally, a GaP wafer is fused to the revealed AlGaInP layer.16 The highest performances of AlGaInP/ GaP LEDs are achieved using chips that deviate from a conventional rectangular Figure 2. Typical chip structure for shape. A truncated inverted pyramid (TIP) a high-brightness AlGaAs double- shape, which is achieved by dicing a chip heterostructure LED chip using a with a beveled blade to yield side-wall transparent substrate (after Њ 7 Reference 12). angles of 35 with respect to the vertical, greatly improves light-extraction (Figure 3). Such a shape totally redirects internally the reflected photons at small incidence 10 lm/W, which is still three times higher angles that fit the escape cones. As of this than that of a red-filtered incandescent writing, the AlGaInP TIP LED holds the lamp. performance record for an electrolumines- Shifting the emission spectra toward cent visible-light source. In the orange shorter red wavelengths requires an active region (610 nm), it exhibits the highest layer with a wider bandgap and, hence, reported luminous efficiency, exceeding with a higher Al molar fraction. However, 100 lm/W (close to that of sodium lamps) increasing the Al content makes the direct- with a peak luminous flux of 60 lm. In gap and indirect-gap transitions closer, the red region (650 nm), external quantum which results in reduced performance. efficiencies of as high as 55% have been This makes it difficult to match the red achieved. Figure 1. (a) Schematic illustration color with the spectral sensitivity of the of the design of a conventional human eye. Another disadvantage of Blue, Green, and Amber light-emitting diode (LED) chip grown AlGaAs is its low corrosion resistance, on an absorbing substrate. Light InGaN LEDs escapes upward through a single cone which limits LED lifetime, especially under The InxGa1ϪxN alloy exhibits a direct conditions of increased temperature and with an apex of 2␪c. (b) High-brightness bandgap that varies from 1.89 eV to 3.4 eV, LED chip design with thick, transparent humidity. depending on the In molar fraction. This window layers. Light escapes through covers the spectral range from red to near- six cones. Red, Orange, and Yellow UV. At present, the AlInGaN system offers AlGaInP LEDs the most efficient LEDs in the blue to 17 The (AlxGa1Ϫx)0.5In0.5P alloy, which is green region. AlGaN/GaN/AlInGaN/ lattice-matched to GaAs and exhibits a InGaN-based blue LEDs are indispensable AlGaAs/GaAs materials.12 The main ad- direct bandgap in the range of 1.9–2.26 eV for the fabrication of white LEDs (see the vantage of the AlGaAs/GaAs system is (depending on the Al molar fraction), is next section). its very small lattice mismatch (GaAs and the most favorable material for red to yel- For many years, the development of AlAs differ in lattice constant by Ͻ0.2% at low high-brightness LEDs.13 The MOCVD Group III-nitride materials was hindered 25ЊC). This ensures the growth of high- growth of AlGaInP is a mature epitaxial by the lack of a suitable substrate. How- quality AlGaAs films on GaAs substrates. technique. Unfortunately, LPE and VPE ever, the pioneering work of Pankove, LPE can produce the thick, transparent methods, which are suitable for growing Akasaki, Nakamura, and many others led layers (having a sufficiently high Al con- thick window layers, are incompatible the way to the development of a mature tent) required for light-extraction through with the growth of AlGaInP alloys.