Prospects for LED lighting
Prospects for LED lighting
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DOI：
10.1038/nphoton.2009.32
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发表时间：
2009-04-01
期刊：
影响因子：
35
通讯作者：
Nakamura, Shuji
中科院分区：
文献类型：
--
作者：
Pimputkar, Siddha;Speck, James S.;Nakamura, Shuji
自从19世纪后期白炽灯泡的发展以来，已经研究了更有效地产生白色光的各种方法。其中，基于发光二极管（LED）的白光源似乎将对能源消耗、环境甚至个人健康等问题产生相当大的影响。在美国，大约22%的电力用于照明应用1。如果世界上所有传统的白光源都转换为节能的LED光源，能源消耗可以减少约1，000 TW h yr-1，相当于约230个典型的500 MW燃煤电厂，减少约2亿吨温室气体排放（参考文献2）。基于可靠和节能LED的白光源最近才通过半导体的发展成为可能（图1显示了白光LED的作用）。在半导体材料（例如GaN或AlInGaP）中产生光是可能的，通过将电子注入材料的导带中并在价带中提供它们可以落入其中的较低能量位置（“空穴”），从而产生对应于导带和价带之间的能隙（也称为带隙）的颜色的光。发光二极管是一种电子器件，其集成了对带隙结构的电接入并允许有效的光生成。LED基本上由三种不同类型的材料组成，这些材料彼此层叠。底层具有高浓度的自由电子（例如掺杂有Si的n型GaN），随后是具有较小带隙的材料（InGaN/GaN）的多个交替薄层（1-30 nm），也称为量子威尔斯。在较大带隙材料（GaN）层之间嵌入较小带隙材料（InGaN）会产生一个阱，在空间上捕获电子和空穴，使它们能够有效地重新结合，产生具有较小带隙材料波长的光。在这个“有源层”之上，有一层具有高浓度空穴的材料（掺杂Mg的p型GaN）。直到最近，唯一可用的高亮度LED发出红光。然而，对于白色光，需要两个或更多个波长来产生宽光谱的光，该宽光谱是黑体辐射曲线（诸如太阳的黑体辐射曲线）的更好近似。产生额外颜色的一种方法是使用吸收一种波长的光并发射更长波长的光的材料。磷光体通常用于此任务，并且选择的少数已经受到相当大的关注，例如稀土掺杂的钇铝石榴石（YAG：RE）。例如，铈掺杂的YAG可以吸收蓝光和紫外光，并相对有效地发射黄光3。这一过程的关键是高能量的光（例如紫外线或蓝光）被转换为低能量的光（例如黄光或红光）。因此，发射红光的LED不能用于使用磷光体的白光产生;相反，需要短波长的紫外、紫色或蓝色LED。早期生产蓝光半导体的尝试集中在SiC上，但由于材料的间接带隙，这些器件被证明效率低下（0.03%效率4）。此后，GaN革命提供了高效的紫外线、紫光和蓝光发射器。GaN是一种直接带隙半导体材料，其带隙为3.45 eV，相当于近紫外光（364 nm）。GaN在20世纪60年代末首次被美国无线电公司（RCA）的Paul Maruska和Jacques Pankove作为LED的潜在材料进行研究，并在随后的几年中被...
Since the development of incandescent light bulbs in the late 1800s, various methods of producing white light more efficiently have been investigated. Of these, white-light sources based on light-emitting diodes (LEDs) look set to have a considerable impact on issues such as energy consumption, environment and even the health of individuals. Roughly 22% of the electricity generated in the United States is dedicated to lighting applications1. If all conventional whitelight sources in the world were converted to the energy-efficient LED light sources, energy consumption could be reduced by around 1,000 TW h yr–1, the equivalent of about 230 typical 500-MW coal plants, reducing greenhouse gas emission by about 200 million tonnes (ref. 2). White-light sources based on reliable and energy-efficient LEDs have only recently been made possible through developments in semiconductors (Fig. 1 shows a whitelight LED in action). It is possible to generate light in semiconductor materials (such as GaN or AlInGaP) by injecting electrons into the conduction band of the material and providing lower-energy sites (‘holes’) in the valence band into which they can fall, thereby creating light of a colour corresponding to the energy gap between the conduction band and valence band, also called the bandgap. A light-emitting diode is an electronic device integrating electrical access to the bandgap structure and allowing for efficient light generation. LEDs essentially consist of three different types of materials layered on top of each other. The bottom layer has a high concentration of free electrons (for example n-type GaN doped with Si) followed by multiple alternating thin layers (1–30 nm) of material with a smaller bandgap (InGaN/GaN), also called quantum wells. The sandwiching of a smaller-bandgap material (InGaN) between layers of larger-bandgap material (GaN) creates a well that spatially traps electrons and holes, allowing them to recombine efficiently, generating light with the wavelength of the smaller-bandgap material. Above this ‘active layer’, there is a layer of material with a high concentration of holes (p-type GaN doped with Mg). Until recently, the only high-luminosity LEDs available emitted red light. For white light, however, two or more wavelengths are required to generate a broad spectrum of light that is a better approximation of a blackbody radiation curve, such as that of the Sun. One way to produce additional colours is to use a material that absorbs light of one wavelength and emits at longer wavelength.Phosphors are commonly used for this task and a select few have received considerable attention, such as rare-earth-doped yttrium aluminium garnets (YAG: RE). For example, cerium-doped YAG can absorb blue and ultraviolet light and emit yellow light relatively efficiently3. Crucial to this process is the fact that higher-energy light (for example ultraviolet or blue) is converted to lower energy (for example yellow or red). Therefore, LEDs emitting red light cannot be used for white-light generation using phosphors; instead a short-wavelength ultraviolet, violet or blue LED is required. Early attempts to produce blue-emitting semiconductors focused on SiC, but these devices proved inefficient (0.03% efficiency4) owing to the material’s indirect bandgap. The GaN revolution has since provided efficient ultraviolet, violet and blue light emitters. GaN is a direct-bandgap semiconductor material with a 3.45-eV bandgap, which corresponds to near-ultraviolet light (364 nm). GaN was first investigated as a potential material for LEDs in the late 1960s by Paul Maruska and Jacques Pankove at the Radio Corporation of America (RCA) and in later years additionally by …