ARTICLE IN PRESS 1.Introduction phosphors for LEDs,it is essential to modify existing on phosphors or to explore new host crystals for phosphors nitrid earth-doped III-V g nitride ena are associated with large energy losses that occu such as AIN,GaN, large Stokes shifts ntensively i operation of LEDs is based on spontaneous light emissior the luminescence of silicon-based oxynitride and nitride in semiconductors,which is due to the radiative recombi nation of excess el tron and [that are produce con N2 pres sure,an Sub the ly,the radi that they used as As a high-temperature structural materials;and (iv)the limited with conventional lamp dersta struct ures as a resul of th hav nsumption and pollution from fossil fuel power plants recent vears becar of their enc raging luminescent ]Currently,LEDs are widely used ndicators. rea properties (excitability blue light,high conversion quid de hts Io the of ful in the brighu and high otential for use i yhite I ED it is generally accepted that they [11-14.In this review,we discuss recent developments in replace conventional lamps for general lighting in the rare-earth-activated oxynitrid and nitride ding th phosphor ral ther are three methods of creating light in LEDs:(i)using three individual monochromatic green and red colors;(ii)combining an olet (UV) and red pho 2.Cla sification and crystal chemistry of nitride compound nhosnhors 2 In the latter two case Nitride compounds are a large family of nitrogen- phosphorsre used as downconversion luminescent mat containing that are formed by combining sourc hors in ED electro J:0 for pho for p e in I EDs ar (in chemical characteristics of the bonds between nitr en and c phosphors in cathode-ray Meta llic nitrides. such as TiN 254 aps are are u by the form of M-N.with M being an alkali-.alkaline- addition,they should also have the following ch cte metal,and/or rare arth metal;examples include Li; stabilit n as B. AIN.GaN y thermal a ith IB-VB om the size(5-20um片and(w）appr phor ut white be co ed as host lattices for phosphors because the (Y1 ol an mn orthosilicates 3.aluminates 5 and sulfides 5.6]have either electrical or ionic conductors and both have narrow d in white LEDs.How band the gaps.Furthermore,the covalent chemical bonding low a nit to a ene hlue i EDs On the other hand sulfide-hased phosphors e cited of the sd electrons of the activato thermally unstable and very sensitive to moisture,and thei (e.g..Eu Ce )[16-20].This results in long excitation/ nce degrades significantly unde ambient emi wave engths and low therma in conventiona ors usec nd CRTs Please cite this article as:R.Xie,N.Hirosaki Sci.Technol Adv.Mater.(2)doi:.1016/.00 1. Introduction Conventional incandescent or fluorescent lamps rely on either incandescence or discharge in gases. Both phenom￾ena are associated with large energy losses that occur because of the high temperatures and large Stokes shifts involved. Light-emitting diodes (LEDs) using semiconduc￾tors offer an alternative method of illumination. The operation of LEDs is based on spontaneous light emission in semiconductors, which is due to the radiative recombi￾nation of excess electrons and holes [1] that are produced by the injection of current with small energy losses. Subsequently, the radiative recombination of the injected carriers may attain quantum yields close to unity. As a result, compared with conventional lamps, LED-based light sources have superior lifetime, efficiency, and reliability, which promise significant reductions in power consumption and pollution from fossil fuel power plants [1]. Currently, LEDs are widely used as indicators, rear lamps for vehicles, decorated lamps, backlights for cellular phones and liquid crystal displays, and small-area lighting. With advances in the brightness and color-rendering properties of LEDs, it is generally accepted that they will replace conventional lamps for general lighting in the near future. In general, there are three methods of creating white light in LEDs: (i) using three individual monochromatic LEDs with blue, green, and red colors; (ii) combining an ultraviolet (UV) LED with blue, green, and red phosphors; and (iii) using a blue LED to pump yellow or green and red phosphors [2]. In the latter two cases, appropriate phosphors are used as downconversion luminescent mate￾rials. The excitation sources used for phosphors in LEDs differ greatly from those of phosphors in conventional lighting. The excitation sources for phosphors in LEDs are UV (360–410 nm) or blue light (420–480 nm), whereas those for conventional inorganic phosphors in cathode-ray tubes (CRTs) or fluorescent lamps are electron beams or mercury gas (lem ¼ 254 nm). Therefore, the phosphors in LEDs should have high absorption of UV or blue light. In addition, they should also have the following character￾istics: (i) high conversion efficiency; (ii) high stability against chemical, oxygen, carbon dioxide, and moisture; (iii) low thermal quenching; (iv) small and uniform particle size (5–20 mm); and (v) appropriate emission colors. The phosphor most commonly utilized in bichromatic white LEDs is the yellow-emitting (Y1aGda)3(Al1bGab) O12:Ce3+ (YAG:Ce)[1]. Other types of phosphor such as orthosilicates [3,4], aluminates [5], and sulfides [5,6] have also been used in white LEDs. However, most oxide-based phosphors have low absorption in the visible-light spec￾trum, making it impossible for them to be coupled with blue LEDs. On the other hand, sulfide-based phosphors are thermally unstable and very sensitive to moisture, and their luminescence degrades significantly under ambient atmo￾sphere without a protective coating layer. Consequently, to solve these problems and develop high-performance phosphors for LEDs, it is essential to modify existing phosphors or to explore new host crystals for phosphors such as nitrides. Luminescence in rare-earth-doped III–V group nitrides such as AlN, GaN, InGaN, and AlInGaN has been intensively investigated because of their potential applica￾tions in blue-UV optoelectronic and microelectronic devices [7–10]. However, less attention has been paid to the luminescence of silicon-based oxynitride and nitride compounds, perhaps due to (i) their critical preparation conditions (high temperature, high N2 pressure, and air￾sensitive starting powders); (ii) the lack of general synthetic routes; (iii) the strong impression that they are used as high-temperature structural materials; and (iv) the limited understanding of their crystal structures as a result of the difficulties in crystal growth. Silicon-based oxynitride and nitride phosphors have received significant attention in recent years because of their encouraging luminescent properties (excitability by blue light, high conversion efficiency, and the possibility of full color emission), as well as their low thermal quenching, high chemical stability, and high potential for use in white LEDs [11–14]. In this review, we discuss recent developments in rare-earth-activated oxynitride and nitride phosphors, including their crystal structure, preparation, luminescent properties, and applications in white LEDs. 2. Classification and crystal chemistry of nitride compounds Nitride compounds are a large family of nitrogen￾containing compounds that are formed by combining nitrogen with less electronegative elements. Generally, nitrides can be grouped into three types: (i) metallic, (ii) ionic, and (iii) covalent compounds, based on the chemical characteristics of the bonds between nitrogen and other elements [15]. Metallic nitrides, such as TiN, ZrN, VN, CrN, and FeN, are usually produced by combining nitrogen with transition metals. Ionic nitrides are usually of the form of M–N, with M being an alkali-, alkaline-earth metal, and/or rare-earth metal; examples include Li3N, Ca3N2, CeN, and LiMnN2. Covalent nitrides, such as BN, AlN, GaN, silicon nitride (Si3N4), and P3N5, are formed by combining nitrogen with IIIB–VB group metals. From the viewpoint of luminescent materials, covalent nitrides can be considered as host lattices for phosphors because they have the characteristics of an insulator or semiconductor and wide band gaps, whereas metallic and ionic nitrides are either electrical or ionic conductors and both have narrow band gaps. Furthermore, the covalent chemical bonding in nitrides gives rise to a strong nephelauxetic effect (i.e., electron cloud expansion), reducing the energy of the excited state of the 5d electrons of the activators (e.g., Eu2+, Ce3+) [16–20]. This results in long excitation/ emission wavelengths and low thermal quenching, which cannot be achieved in conventional phosphors used in lamps and CRTs. ARTICLE IN PRESS 2 R.-J. Xie, N. Hirosaki / Science and Technology of Advanced Materials ] (]]]]) ]]]–]]] Please cite this article as: R.-J. Xie, N. Hirosaki, Sci. Technol. Adv. Mater. (2007), doi:10.1016/j.stam.2007.08.005