SOLID-STATE lighting (SSL) based on LEDs is an emerging

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1 1028 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009 LEDs for Solid-State Lighting: Performance Challenges and Recent Advances Mary H. Crawford, Member, IEEE (Invited Paper) Abstract Over the past decade, advances in LEDs have enabled the potential for wide-scale replacement of traditional lighting with solid-state light sources. If LED performance targets are realized, solid-state lighting will provide significant energy savings, important environmental benefits, and dramatically new ways to utilize and control light. In this paper, we review LED performance targets that are needed to achieve these benefits and highlight some of the remaining technical challenges. We describe recent advances in LED materials and novel device concepts that show promise for realizing the full potential of LED-based white lighting. Index Terms Energy conservation, LEDs, lighting, semiconductor devices. I. INTRODUCTION SOLID-STATE lighting (SSL) based on LEDs is an emerging technology with potential to greatly exceed the efficiency of traditional lamp-based lighting systems. Whereas energy efficiency is the primary motivation behind SSL, LEDs are also anticipated to bring entirely new functionalities to lighting systems, greatly enhancing the ways in which we use light. LEDs have already replaced traditional lamps in a number of lighting systems, including traffic lights, signs, and displays. Many of these applications require monochrome light and the narrowband emission properties of LEDs present a clear advantage over filtered-lamp approaches. However, the greatest impact of SSL will likely be in general illumination applications that demand a high-quality white-light source. Coupled with the need for high efficiency, the stringent color requirements of white-light illumination place new challenges on LED technology, many of which have yet to be overcome. One of the obvious challenges specific to white lighting is the need to have highly efficient light emission spanning the visible spectrum. Whether the light is generated from inorganic LEDs, organic LEDs, or phosphors, this requirement demands development of a range of materials and/or material alloys with exceptional optical performance. In addition, many illumination Manuscript received December 19, First published May 27, 2009; current version published August 5, This work was supported in part by the Department of Energy Office of Basic Energy Sciences and in part by Sandia s Laboratory Directed Research and Development Program. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy s National Nuclear Security Administration, under Contract DE-AC04-94AL The author is with Sandia National Laboratories, Department of Semiconductor Material and Device Sciences, Albuquerque, NM USA ( mhcrawf@sandia.gov). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JSTQE applications require significantly higher brightness levels than indicator applications, which has motivated the development of large-area, high-power LEDs [1]. A major challenge for LEDs is delivering highest efficiency performance at the current densities and temperatures relevant to high-power operation. LED-based illumination systems must also provide efficient and flexible approaches to extract and distribute light. As LEDs have entirely different form factors and dielectric properties than traditional lamps, new strategies for efficiently extracting light from the LED chip as well as controlling the light emission pattern are being explored. While there are certainly other challenges to LED-based white lighting, these requirements present some of the most significant challenges on the LED chip level. In this paper, we examine these roadblocks to realizing the full potential of LED-based white lighting, and highlight recent research advances toward overcoming them. In keeping with the multidisciplinary and global nature of SSL research, our review encompasses insights and advances in materials development, device physics, and novel device concepts, as accomplished by many groups around the world. The remainder of this paper is organized into three sections. Section II provides a brief background on solid-state white lighting, presenting LED performance targets and describing approaches to generating white light from LED-based systems. A review of LED performance challenges and recent research advances is given in Section III. To provide sufficient technical detail, we have limited the scope of that section to inorganic semiconductor LEDs, omitting organic LEDs, which have recently been reviewed in [2]. Finally, Section IV provides a summary and outlook for LED-based white lighting. II. SOLID-STATE WHITE LIGHTING The potential for SSL, and indeed the optimism that it will succeed, is based on dramatic LED performance advances over the past two decades. AlGaInP semiconductor alloys, developed and matured throughout the 1990s [3], have emerged as the most efficient LED materials in the yellow, orange, and red regions, with peak external quantum efficiencies (EQEs) as high as 55% at 650 nm [4]. The commercialization of blue LEDs based on InGaN semiconductor alloys was a major breakthrough in the early 1990s, providing, for the first time, the potential for efficient and reliable blue and green LEDs [5]. Extensive development of InGaN LEDs over the past 15 years has yielded commercial high-power blue LEDs with EQEs as high as 56% [6]. And, as described in the following section, high-power white X/$ IEEE

2 CRAWFORD: LEDs FOR SOLID-STATE LIGHTING: PERFORMANCE CHALLENGES AND RECENT ADVANCES 1029 Fig. 1. Approaches for generating white light from LEDs and representative power spectra. (a) R B G B B system that employs a blue LED to pump red and green phosphors. (b) Blue LED pumping a green phosphor combined with a red LED (RG B B). (c) Four-color RYGB all-led system. LEDs based on InGaN blue LEDs with phosphor coatings have already surpassed the luminous efficacy 1 of incandescent lamps and are approaching that of fluorescents [6]. For SSL to realize decisive energy conservation benefits, however, even greater performance must be achieved. The U.S. Department of Energy s Multi-Year Program Plan for SSL has targeted a white LED luminous efficacy of 163 lm/w by 2015, which would surpass those of incandescent and fluorescent lamps by 10 and 2, respectively [8]. Realizing this LED performance would have a tremendous impact, as widescale adoption of such LEDs would have the potential to reduce electricity consumption for lighting by 2 or more, with projected year 2025 savings in the U.S. alone of 620 TWh/year (or roughly $42 billion/year at current prices). Translated into environmental benefits, these energy savings would serve to reduce carbon-equivalent emissions by about 100 Megatons per year [9]. These benefits have provided substantial motivation for research toward advancing LED performance. A major challenge of reaching these goals is that LED-based white lighting must achieve such high efficiencies while simultaneously delivering exceptional color quality at low cost. Fig. 1 illustrates several approaches that are being explored to achieve these qualities in LED-based white lighting. Presently, the most common white LED design employs an InGaN LED to provide blue emission and to pump phosphors that emit at longer wavelengths where LED performance is deficient. These phosphor-converted or pc-leds are illustrated in Fig. 1(a). The simplest pc-leds combine a blue LED (λ nm) with a YAG:Ce 3+ phosphor (λ 560 nm) to create a 1 Luminous efficacy, measured in lumens per Watt of electrical power, is a metric defining the power conversion efficiency of the LED weighted by the human eye response. Incandescent and fluorescent lamps have values of 14 and 75 lm/w, respectively [7]. two-color white LED with a color rendering index (CRI) 2 of and a correlated color temperature (CCT) 3 of K. While suitable for less demanding applications, including outdoor lighting, these LEDs are not adequate for indoor illumination applications that generally require CRI > 80 [6]. The color characteristics may be substantially improved by adding a second, red-emitting phosphor, creating a three-color system that we designate as R B G B B in Fig. 1(a) (the subscript B indicating that the red and green light is produced via optical pumping by the primary blue LED). Recently developed commercial white LEDs employing nitrodosilicate red-emitting phosphors have demonstrated CRI of 90, with luminous efficacies of 55 lm/w and CCT of K (350 ma, 1 mm 1 mm chip [6]). As mentioned before, the luminous efficacy of these warm white LEDs have already surpassed that of incandescents ( 14 lm/w [7]) and are approaching that of fluorescents ( 75 lm/w [7]). Moving beyond this performance will require further optimization of phosphors, particularly narrowband phosphors in the red region, as well as improvements in blue LED efficiencies. In contrast to phosphor approaches, the multichip approach utilizes only LEDs to generate white light [Fig. 1(c)]. Threecolor systems with components at red, green, and blue (RGB) wavelengths are limited to CRI values of 85 or less [12], whereas a four-color (RYGB) system can achieve CRI > 95 [13]. As described in the following section, highly efficient LED emission has not yet been demonstrated across the entire visible spectrum, and significant semiconductor material advances are needed to realize efficient multichip white LEDs. As present-day LED performance is most limited in the green yellow region, hybrid approaches, employing a green phosphor in combination with red and blue LEDs [Fig. 1(b)], are also being explored. In assessing the merits of these LED architectures, we note that pc-leds may provide a simpler and lower cost solution. Requiring only a blue LED, the pc-led avoids some of the complexities of multichip systems, including aspects of assembly, color mixing, and feedback circuitry to maintain color quality, given different thermal and degradation properties of the individual LEDs. However, in terms of energy efficiency, the multichip approach offers a clear advantage. By providing direct emission at the necessary visible wavelengths, multichip LEDs avoid the absorption and emission losses of the phosphor as well as down-conversion losses associated with generating lower energy phosphor emission from a higher energy blue source. The multichip approach also has greater potential for actively controlling the light s spectral distribution, providing smart lighting capabilities far beyond traditional lamp systems. Thus, although success of the multichip approach requires overcoming significant LED materials challenges, the compelling benefits have inspired concerted pursuit of that design. 2 Color rendering index (CRI) describes the ability of a light source to accurately render the colors of objects in the environment as seen by the human eye [10]. The maximum CRI of 100 is achieved by incandescent lamps; a CRI of 90 is considered excellent for illumination applications. 3 Correlated color temperature (CCT) is defined by the chromaticity position along the Planckian locus of the CIE chromaticity diagram [11]. Incandescent and fluorescent lamps are typically in the K range.

3 1030 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009 Fig. 2. State-of-the-art EQEs for LEDs (T j = 25 C) emitting at various wavelengths and made from two material systems. The first material system is InGaN: (1) InGaN TFFC LEDs, 350 ma; (2) InGaN vertical thin-film (VTF) LED, 1000 ma; and (3) InGaN conventional chip (CC) LEDs employing patterned substrates. The second material system is AlGaInP: (4) Production performance AlGaInP truncated-inverted-pyramid (TIP) LEDs, Philips Lumileds Lighting Company, 350 ma. V (λ) is the luminous eye response curve from CIE. Dashed lines are guides to the eye. (reprinted from [6], IEEE). III. LED PERFORMANCE CHALLENGES AND RECENT ADVANCES All of the approaches described in the previous section rely on LED emission for a major part of the white-light production. This section discusses LED performance limitations that must be overcome to realize the full potential of SSL. Our first topic examines the LED material challenges in developing multichip white LED platforms. We next examine the present limitations of achieving high-efficiency LED operation at high operating currents and output powers, relevant to all of the white LED platforms under consideration. Finally, we review approaches to achieve high light extraction efficiencies from LEDs and the related topic of advanced photon management. Each of these areas brings unique challenges, but notable advances are being made, as described next. A. The Green Yellow Gap A challenge for SSL is the fact that high optical efficiencies must be achieved across the visible spectrum to match the color quality of traditional white-light sources. While much progress has been made, semiconductors and down-conversion materials still show major performance limitations in particular spectral regions. The most notable challenge for semiconductor materials is the efficiency gap in the green yellow region of the spectrum. As shown in Fig. 2, InGaN LEDs exhibit high EQEs in the violet and blue regions, but longer wavelength devices, achieved by increasing indium concentration in the InGaN active region, show a notable drop in efficiency. This efficiency loss could be attributed to a number of material issues, but the dominant cause is still unclear. AlGaInP-based LEDs, on the other hand, have shown impressive performance in the red region, but lose efficiency at shorter wavelengths. An indirect-bandgap crossover in the green yellow region provides an explanation for efficiency loss in these AlGaInP alloys, and their improved performance in the green yellow region is therefore far less likely. Improving LED performance within the green yellow gap is a major focus of SSL research efforts, and a number of approaches are being explored. To attain the long-term SSL performance targets [8], these efforts will need to achieve high internal quantum efficiencies (IQEs) of 90% at particular green and yellow wavelengths. The ultimate solution may include innovative approaches to improved InGaN materials, the exploration of alternative III V or II VI semiconductors [14], [15], or the development of suitable broadband green phosphors to replace green semiconductor LEDs [13]. In this paper, we narrow our scope to InGaN materials, given the tremendous potential that they have demonstrated to date. InGaN alloys are largely believed to be the most promising semiconductors for bridging the green yellow gap. Unlike AlGaInP alloys, they possess a direct energy bandgap across the visible spectrum and present no intrinsic roadblock to highefficiency optical emission. While efficiency clearly drops off in the green region, state-of-the-art commercial InGaN LEDs near 530 nm have now demonstrated 100 lm output and 80 lm/w performance (at 35 A/cm 2 [16]), and IQE values have been estimated to be greater than 30% (at 50 A/cm 2 [6]). These achievements have provided strong motivation for pushing the limits of high-performance InGaN to longer visible wavelengths. The efficiency loss in the green yellow region is inextricably linked with the evolution of InGaN material properties with increasing indium content of the alloy. Substantial research over the past decade has revealed InGaN alloys to have a complex array of properties that evolve with indium composition and negatively impact material quality and optical efficiency. Here, we focus on two of the most significant factors that influence high-in-content InGaN alloys in LED heterostructures: polarization effects and strain effects. 1) Polarization Effects: One factor that has long been thought to limit the efficiency of green InGaN materials is the strong polarization effects that arise from their wurtzite crystal structure. In particular, piezoelectric and spontaneous polarization along the polar[ ](c-axis) crystal direction leads to large (>1 MV/cm) electrostatic fields in the quantum well (QW) active regions of InGaN LEDs. These fields cause a spatial separation of electron and hole wavefunctions in the QWs, thereby reducing radiative recombination rates and related radiative efficiency. Because the piezoelectric polarization depends on strain, the reduction in radiative recombination rate is larger for longer emission wavelengths (moving toward the green and yellow) where higher In-concentration InGaN is grown coherently strained on GaN. Polarization effects also impact LED emission wavelength as well as the stability of the emission wavelength with injection current. Interestingly, polarization-induced electrostatic fields in InGaN QWs produce a red shift of the emission wavelength, which is beneficial as we seek to achieve longer wavelengths with a given alloy composition. However, this red shift is primarily realized at lower currents and is counteracted at increasing currents by carrier-induced screening of the internal fields. The

4 CRAWFORD: LEDs FOR SOLID-STATE LIGHTING: PERFORMANCE CHALLENGES AND RECENT ADVANCES 1031 Fig. 3. Piezoelectric polarization of an InGaN QW grown pseudomorphically on GaN as a function of crystal orientation. Zero degree represents c-plane (0 0 01) growth, (11 2 2) is a semipolar crystal plane, and m-plane ( ) is a nonpolar crystal plane. P z is the polarization perpendicular to the growth plane (reprinted from [17]). Fig. 4. Electroluminescence output powers at 20 ma dc drive current as a function of emission wavelength of m-plane InGaN LEDs. Circle and triangle symbols represent InGaN LEDs with 2.5 and 8.0 nm QW thickness, respectively (reprinted from [26]). overall effect is a blue shift of emission wavelength with increasing current, which is problematic for achieving a color-stable white-light source. Given these undesirable effects, significant effort has been expended in developing InGaN LEDs on GaN templates with nonpolar [e.g., m-plane ( ) or a-plane ( )] or semipolar [e.g., ( )] crystal orientations. As shown in Fig. 3, moving to these nontraditional crystal orientations affords the opportunity to greatly reduce, and even eliminate, polarization effects in wurtzite nitride materials [17], [18]. However, early attempts to achieve nonpolar GaN templates by heteroepitaxial growth on foreign substrates, such as a-plane GaN on r-plane sapphire, resulted in unacceptably high densities of extended defects, including threading dislocations (>10 10 cm 2 ) and stacking faults (>10 5 cm 1 ) [19]. More recently, the commercial availability of high-quality, free-standing, nonpolar GaN substrates grown by hydride vapor-phase epitaxy has enabled major advances. These substrates are formed by slicing small-area wafers with nonpolar orientation from thick, high-quality, c-plane-oriented substrates [20] [22]. By employing such m-plane GaN substrates with threading dislocation densities < cm 2,the University of California at Santa Barbara group reported remarkable progress in LED performance: an EQE of 45% at 402 nm, comparable to the best results from polar (c-plane) InGaN LEDs [23]. With these advances, nonpolar InGaN LEDs have reached the material quality where their advantages and limitations can be more clearly examined. Many reports have confirmed that nonpolar InGaN LEDs show improved wavelength stability with increasing current compared to c-plane LEDs [23] [25]. And, as mentioned before, efficiencies of the best m-plane InGaN LEDs at near-uv wavelengths have met or exceeded that of c-plane LEDs [23]. However, indium incorporation efficiency in nonpolar orientations is reported to be 2 3 less than for c-plane, making it more difficult to reach blue and longer wave- lengths [26]. Increased indium incorporation could be achieved by using lower growth temperatures than those used for c-plane growth, but this approach is expected to reduce material quality. Therefore, various active region designs, including wide QWs and thick barrier layers, have been employed to achieve longer wavelengths. These efforts have enabled blue (468 nm) LEDs on m-plane GaN substrates with EQEs of 16.8% at 20 ma [27]. The development of nonpolar InGaN LEDs with reasonable efficiencies into the blue region provides important insights into the green yellow gap problem. In Fig. 4, we show wavelengthdependent output powers reported for a series of m-plane InGaN LEDs with different indium compositions and with two different QW widths [26]. Similar to the c-plane LED EQE performance in Fig. 2, the nonpolar LED output power is seen to peak in the near-uv/violet region and is reduced significantly with increasing indium composition and emission wavelength. Notably, this strong drop in LED output power at longer wavelengths is present even in the absence of polarization effects, indicating that polarization effects alone do not account for the green yellow efficiency gap. Yamada et al. hypothesize that nonradiative centers caused by strain may be a remaining factor [26]. It should be noted that semipolar LEDs provide an interesting compromise for achieving emission in the green yellow region. As shown in Fig. 3, the semipolar ( ) plane has substantially reduced piezoelectric polarization compared to the c-plane, and indium incorporation efficiency on the ( ) plane is reported to be comparable to that of c-plane [28]. These factors have enabled the demonstration of ( ) LEDs at blue, green, and amber wavelengths [29]. Recent performance advances include EQEs of 10.5% at 20 ma in the green region (516 nm) [28] and EQEs as high as 9.5% at 20 ma in the green yellow region (562 nm) [30]. While the results in the green are not yet competitive with c-plane LEDs, the green yellow LED performance reportedly surpasses that of any LEDs in this wavelength region, representing an important advance.

5 1032 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009 Ultimately, nonpolar and semipolar nitrides may play a role in enabling both LEDs and laser diodes in the green yellow region. Continued improvements in longer wavelength device efficiencies as well as sustained development of larger-area, high-quality, nonpolar substrates [23], will be critical for their commercial success. 2) Strain Effects: As discussed before, other factors, beyond polarization effects, likely contribute to the low efficiencies of InGaN LEDs at longer wavelengths. One candidate is the lattice mismatch strain between InGaN QWs and commonly employed GaN templates that increases with increasing indium composition of the QWs. Strain is, of course, directly responsible for the piezoelectric polarization properties described before. But a number of additional phenomena are related to material strain. First, it is well known that the growth of higher indium composition InGaN alloys requires lower growth temperatures due to reduced thermal stability [31]. At a given growth temperature, the strain in an InGaN/GaN heterolayer limits the further incorporation of indium by so-called composition pulling effects [32], [33]. In this way, strain inhibits the higher indium concentrations needed for longer wavelength devices and contributes to employing even lower growth temperatures for adequate indium incorporation. The combination of lower growth temperatures and increasing strain energy enhances the formation or incorporation of a number of structural defects (e.g., point defects [34], impurities [35], V-defects [36], [37]) with accompanying degradation of optical efficiency. Strain may also play a role in the increased compositional instabilities that have been observed in higher indium composition alloys [38]. Thus, beyond contributing piezoelectric effects, strain is a major factor that limits the emission wavelength and optical performance of InGaN LEDs. An effective solution to mitigating these strain-related factors is to replace GaN templates with InGaN templates that are more closely lattice-matched to overlying InGaN QW structures. While this approach sounds simple, the need to realize thick, strain-relaxed InGaN templates with both low defect densities and smooth surface morphologies presents a tremendous challenge. Given that bulk InGaN substrates are not yet available, alternative substrates or growth strategies must be employed. To date, several alternative substrates have been explored to find a suitably lattice-matched condition for InGaN growth. ZnO is an interesting candidate for green materials, having a similar wurtzite crystal structure to InGaN and providing a lattice-matched condition for In 0.18 Ga 0.82 N alloys grown in the [ ] crystal orientation [39]. However, a major roadblock to achieving high-quality InGaN on ZnO is the thermal incompatibility of the two materials, as metal organic vapor-phase epitaxy (MOVPE) of InGaN is ideally performed at temperatures >700 C for higher material quality, whereas ZnO decomposition and formation of interfacial alloys is seen at temperatures <600 C [39]. Although this incompatibility seems formidable, two recent areas of exploration have been fruitful. First, nonpolar ZnO substrates have been shown to resist decomposition up to higher growth temperatures than c-plane ZnO. Accordingly, MOVPEgrown In 0.25 Ga 0.75 N films on both a-plane and m-plane ZnO have shown no degradation in structural quality up to 650 C [40]. Second, the application of pulsed-layer deposition (PLD) has demonstrated distinct advantages for InGaN/ZnO growth. Here, laser ablation of an In x Ga 1 x eutectic target in the presence of atomic nitrogen provides the source materials for InGaN growth. As one benefit, the ablation process provides enhanced kinetic energy of group III atoms on the growing surface, enabling improved material quality at lower growth temperatures. Notably, atomically smooth InGaN films on ZnO have been realized by PLD at room temperature [39]. PLD growth of GaN on m-plane ZnO further suggests that GaN or InGaN epilayers deposited at room temperature are viable buffer layers, suppressing ZnO decomposition for subsequent growth temperatures up to 700 C [41]. Overall, these reports present clear progress toward higher temperature growth conditions, but structural data suggest considerable advances in InGaN material quality are still needed. Given continuing challenges of high-quality InGaN growth on lattice-matched substrates, a greater emphasis has been placed on studies to examine the limits of InGaN growth on lattice-mismatched GaN templates. The goal of these efforts is to gain insight into strain relaxation mechanisms and to devise strategies to relax strain while mitigating the defect formation that is typically present. One of the recent advances in this area is the application of epitaxial lateral overgrowth (ELO) techniques to InGaN alloys [42]. ELO approaches involve epitaxial growth on templates (e.g., GaN) that are patterned, typically with dielectric stripes or by etching a stripe pattern directly into the template. Vertical growth combined with lateral growth between the stripes results in a coalesced epitaxial film and provides a mechanism for turning or redirecting threading dislocations away from the overlying active region. When applied to GaN or AlGaN materials, such strategies have enabled thick, coalesced films with dislocation densities < cm 2 [43], [44]. The aforementioned challenges of strained InGaN epilayers have made it more difficult to effectively apply ELO strategies to InGaN; however, InGaN ELO on periodically grooved GaN templates has recently shown promise [45]. The ELO process employs GaN templates with low dislocation density regions (<10 7 cm 2 ) achieved through sidewall lateral epitaxial overgrowth methods [46]. Subsequent reactive ion etching yields periodically grooved structures on the GaN surface for InGaN nucleation, growth, and coalescence into a planar film. The selection of m-plane GaN templates has proven to be a key factor in enabling a flat and smooth coalesced InGaN film, as c-plane and a-plane GaN orientations led to faceted InGaN surfaces [42]. By combining these components of low-dislocation m-plane GaN templates with InGaN lateral overgrowth, Iwaya et al. have realized a 7-µm-thick coalesced In 0.07 Ga 0.93 N film with almost complete strain relaxation and threading dislocation densities <10 8 cm 2 in selected regions (Fig. 5) [45]. While ELO adds complexity and cost, this effort is an important advance toward realizing high-quality strain-relaxed InGaN templates. Further efforts to extend this approach to higher indium compositions and/or growth of InGaN LED structures should yield valuable

6 CRAWFORD: LEDs FOR SOLID-STATE LIGHTING: PERFORMANCE CHALLENGES AND RECENT ADVANCES 1033 Fig. 5. Cross-sectional transmission electron micrograph of a strain-relaxed In 0.07 Ga 0.93 N film on a periodically grooved m-plane GaN template. The GaN template itself was grown by sidewall epitaxial lateral overgrowth to provide regions of low threading dislocation density The crystal orientation of the sample is shown in the inset (reprinted from [45]). insights into the strain limitations of present-day green yellow InGaN LEDs. Finally, we note that the growth of InGaN alloys as lower dimensional structures is another promising strategy for providing strain relaxation and defect reduction. 1-D nanowires (or nanorods) with small radial dimensions ( nm) and typical axial dimensions >1 µm, are one such candidate that is currently under investigation (Fig. 6). Unlike 2-D planar heterostructures that are laterally constrained during growth, 1-D nanowires can relieve strain by lateral relaxation, and therefore accommodate higher mismatch strains before misfit dislocations are formed [48]. Single-crystal nanowires can be grown entirely free of threading dislocations on a wide range of substrates, including silicon [49]. These benefits, coupled with potential light extraction benefits of the 1-D geometry, may ultimately be applied to enhance both IQE and EQE of InGaN LEDs over a wide compositional range. Recent research efforts on InGaN nanowires have already confirmed some of these impressive properties. Through the application of halide vapor-phase epitaxy, Kuykendall et al. have demonstrated single-crystal In x Ga 1 x N nanorods over the entire compositional range [50]. Notably, the nanowire photoluminescence intensity did not show a dramatic drop at longer wavelengths, as only a 1.7 drop in photoluminescence intensity for x = (spanning near-uv to red wavelengths) was reported. Other growth techniques, including RF-plasmaassisted molecular beam epitaxy [49] and MOVPE [51], have yielded InGaN nanowires at green and longer wavelengths. Transitioning these nanostructured materials into a viable LED device presents obvious challenges. Nevertheless, several proof-of-concept device geometries and fabrication strategies have been demonstrated. One such strategy involves growth of 3-nm-thick InGaN quantum disks along the growth direction of a GaN nanowire followed by lateral growth and coalescence of the overlying p-gan layers to present a planar top layer for Fig. 6. Cross-sectional scanning electron micrograph showing a high-density and highly aligned GaN nanowire array catalyzed by a 0.8 Å layer of Ni on an r-plane sapphire substrate (reprinted from [47], 2008, American Institute of Physics). electrical contacting [49]. Similar axially grown LEDs use a spin-on-glass layer to encase the nanowire array, thereby providing a support structure for top metal contacting [52]. Finally, LEDs with a radial geometry, employing a GaN core and an InGaN-QW/p-AlGaN/p-GaN shell structure, have been demonstrated at wavelengths into the yellow region (577 nm) [51]. Electroluminescence characterization of individual core/shell LEDs employing a single InGaN QW has yielded 3.9% EQE in the 540-nm green region, and improved performance is expected with multi-qw (MQW) structures and optimized light extraction. Despite still being largely in the research realm, these demonstrations show the potential for nanostructured InGaN LED solutions in the green yellow region. In the near term, lower dimensional structures present a unique opportunity for studying the properties of InGaN heterostructures with light emission across the entire visible region, and with distinctly different defect, strain, and polarization properties compared to 2-D planar structures. B. Efficiency Droop of InGaN LEDs As described in Section II, InGaN-based LEDs promise to be key elements in any white-led system, whether based on the multichip or phosphor conversion approach. One of the most critically important challenges to achieving ultraefficient SSL relates to the loss of radiative efficiency of InGaN LEDs at high injected carrier densities. As shown in Fig. 7, although nitride LEDs have demonstrated high efficiencies at low currents, highpower LEDs can suffer more than 70% loss of optical efficiency at desired operating currents [53]. This phenomenon, called efficiency droop, is not due to simple heating, but occurs under pulsed as well as continuous wave (CW) conditions. While seen in virtually all InGaN-based LEDs, the effect is found to be less severe in shorter wavelength (near-uv) InGaN LEDs and more pronounced at longer wavelengths. Nonradiative mechanisms proposed to explain

7 1034 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009 Fig. 7. External quantum efficiency of commercial green LEDs (Philips Lumileds Luxeon line) as a function of operating current. Maximum rated operating currents of three different Luxeon models are indicated [53]. Note that more recent device designs have yielded improved efficiency droop, as described in the text. efficiency droop are varied, and include carrier delocalization [54], screening of excitons [55], defect-assisted tunneling of carriers out of the active region [56], Auger recombination [57], [58], and inefficient carrier injection, particularly due to internal polarization fields [59], [60]. Despite some improvements in efficiency droop, a consensus on the mechanism behind it remains elusive. Of the many possible theories that have been proposed, two hypotheses have gained considerable momentum over the past year and are now supported by multiple research groups. The first hypothesis, proposed by Philips Lumileds, identifies Auger recombination as the dominant nonradiative mechanism leading to efficiency loss at high carrier densities [57]. This proposal was not without controversy, as wide-bandgap semiconductors are thought to have small Auger coefficients, indeed some four orders of magnitude lower than for near-ir materials such as InGaAsP. Further, the Auger process, with its higher order ( n 3 ) dependence on carrier density, was not expected to be dominant at the relatively low carrier densities of typical LED operation. Several factors, however, may support the hypothesis of an Auger-related nonradiative mechanism in InGaN heterostructures. As pointed out by Hader et al. [61], while the coupling between electron and hole bands is strongly reduced with increasing bandgap, the low dielectric constants of wide-bandgap materials contribute to enhanced Coulomb interaction, as seen in exciton binding energies that are 10 stronger than for near-ir materials. Given these competing effects, the probabilities of electron hole scattering processes underlying Auger recombination are not intuitively obvious, leaving open the possibility for reasonably large Auger losses in InGaN materials. Furthermore, it has been shown that low hole mobility and large hole confinement energies lead to nonuniform hole distribution across the QWs in InGaN LEDs, and primarily population of the QW closest to the p-side of the junction [62]. Such a build-up of carriers in an ultrathin ( 2.5 nm) QW translates to fairly high carrier densities of >10 18 cm 3 at currents as low as 50 ma in standard high-power LEDs. Efforts to test the Auger recombination hypothesis have been carried out on both theoretical and experimental fronts. The application of microscopic many-body models by the University of Arizona and Philipps University Marburg groups has provided theoretical predictions of the impact of Auger recombination, and initial reports predict that direct Auger recombination is negligible in InGaN LEDs under relevant operating conditions [61]. However, indirect processes, including phonon-assisted or defect-assisted Auger recombination, were proposed as potentially yielding higher losses and are now being evaluated as possible mechanisms behind efficiency droop [61]. Experimental investigations of Auger processes have involved both photoluminescence and electroluminescence studies of InGaN heterostructures. Notably, photoluminescence studies employing direct excitation of the InGaN epilayers through selective optical pumping was reported to show efficiency droop with increasing pump powers [57]. This observation is consistent with an intrinsic recombination mechanism, such as Auger recombination, while being contrary to proposed carrier transport and injection mechanisms. We note, however, that other groups have reported no efficiency droop in such measurements [59], [63], with opposing conclusions, and consensus on the carrier densities achieved in these measurements is needed. LED development to mitigate the effects of Auger recombination has focused on new heterostructure designs to better distribute hole populations, including double heterostructures designs with thick ( 10 nm) InGaN active layers [58] as well as MQW designs that enhance hole injection into multiple wells in the QW stack [16]. These strategies have succeeded in reducing efficiency droop and further advances are anticipated with optimized designs. Coexisting with the Auger recombination theory is a second hypothesis that more directly attributes droop to carrier transport and injection limitations, without invoking Auger processes [60], [64]. At the core of this hypothesis are the related phenomena of ineffective hole injection into the active region and electron leakage out of the active region; both of which lead to parasitic carrier recombination on the p-side of the device. The cause and effect relationship of these phenomena at high injection currents whether poor hole injection drives increased electron leakage or whether electron leakage further limits hole injection is not entirely clear, but both effects may contribute to reduced radiative recombination in the active region at high current densities [65]. As a major proponent of this hypothesis, a group from Rensselaer Polytechnic Institute (RPI) has identified polarization-induced electron leakage out of the active region as a major contributor to efficiency loss at high carrier densities [59], [64], [65]. The phenomenon is depicted in Fig. 8, which shows the substantial modification of InGaN LED bandstructure due to interfacial polarization charges and related internal fields (occurring for polar crystal orientations as discussed in the previous section). Triangular band profiles of QWs and barriers as well as a reduced effective bandgap of electron block layers on the p-side of the device are all manifestations of these strong polarization effects.

8 CRAWFORD: LEDs FOR SOLID-STATE LIGHTING: PERFORMANCE CHALLENGES AND RECENT ADVANCES 1035 Fig. 8. Schematic of the conduction band (E c ) and valence band (E v ) energies of an InGaN-based LED heterostructure, including n-type GaN, InGaN MQWs, AlGaN EBL, and p-type GaN regions. Arrows indicate regions of electron and hole injection and recombination (figure courtesy of E. F. Schubert). > This carrier leakage hypothesis predicts that the excess bias voltage needed to overcome the triangular barrier profiles as well as the reduced effectiveness of the electron block layer (EBL) combine to yield increased carrier leakage and reduced LED efficiency at high injection currents. The RPI team has proposed that mitigation of these polarization effects through polarizationmatched barriers and EBLs is a method to eliminate efficiency droop. Recently, they have succeeded in reducing efficiency droop by 18% in blue LEDs that employ AlInGaN quaternary barriers to independently engineer bandgap and polarization [65]. Further benefits, including reduced operating voltages and related 25% improvement in wall plug efficiency, were attained with these polarization-engineered designs. Other groups have emphasized a more direct improvement in hole injection as an approach to eliminating efficiency droop [60], [63], [66]. In particular, improved performance is predicted from a p-type MQW structure, or a single p-type InGaN active layer, which would ideally mitigate both hole injection and hole transport limitations [63]. The practical challenges of realizing such a structure include the fact that Mg dopants act as nonradiative centers and reduce the radiative efficiency of the InGaN active layer. As a compromise, p-type doping of only the barrier layers has been explored [63], which yielded peak efficiencies at much higher current densities ( 900 A/cm 2 ) than standard LEDs ( 35 A/cm 2 ). However, the proximity of Mg dopants to the QWs still leads to some reduction of radiative efficiency, and an optimal solution will have to mitigate those nonradiative effects. Various new device designs, including p-type MQW devices with thin barriers or lower bandgap InGaN barriers to promote hole transport [67], as well as n-p-n device structures [60], are examples of related solutions being explored. Interestingly, the study of efficiency droop in LEDs grown in nonpolar crystal orientations, where deleterious effects of polarization are avoided, has not sufficiently clarified the efficiency droop mechanism. Early nonpolar LED studies demonstrated minimal droop, potentially confirming polarization effects as the cause of droop in standard c-plane LEDs [68]. However, these studies were carried out on LEDs at shorter (407 nm) wavelengths where droop is less pronounced, and further employed relatively thick (8 nm) QWs, which would serve to reduce carrier density and thus Auger recombination. More recent studies on nonpolar near-uv LEDs have verified increased efficiency droop for narrower ( 2.2 nm) near-uv QWs [69], consistent with Auger processes. However, electroluminescence studies of LEDs over a range of temperatures suggest hole transport limitations may be the cause of efficiency loss at high currents in both c-plane and nonpolar LEDs [70]. Overall, the origin of efficiency droop is still under active investigation, but improved hole distribution in the LED QW active region is a common strategy to avoid both higher order nonradiative processes and carrier transport limitations, and will likely have continued emphasis in future LED designs. Resolution of the cause of efficiency droop will require further application of advanced modeling to quantify Auger recombination losses, including treatment of indirect (e.g., phonon-assisted) Auger processes. In addition, reevaluation of selective optical pumping studies of InGaN MQW structures at high carrier densities is in order, as contradictory results are undoubtedly contributing to the sustained efficiency-droop controversy. C. Advanced Photon Management Beyond requiring highly efficient generation of light within the semiconductor material, advanced LED devices must employ approaches to efficiently extract photons and control light distribution from the chip. Light extraction solutions must surmount the significant trapping of light that occurs within the chip due to the relatively high (n ) refractive indexes of semiconductors compared to surrounding encapsulants (n ) or air. Requirements for controlling light properties are highly dependent on the particular application and may include achieving highly directional (and high radiance) sources for projection systems as well as polarization-controlled sources for display applications. In some cases, the compatibility of LED materials with nanoscale patterning affords unique opportunities for advanced photon management, which may simultaneously enhance photon generation (and IQE) and extraction, and further allow for tailored light distributions. In this section, we review recent progress in LED light extraction and highlight novel strategies for reaping the benefits of advanced photon management. 1) Thin-Film Flip-Chip (TFFC) LEDs: The latest advanced chip design adopted by the SSL industry is the TFFC LED [71]. This design has been applied to both InGaN [72], [73] and AlInGaP LEDs [74], and employs several features to enhance light extraction. InGaN-based thin-film LEDs include Philips Lumileds TFFC [72] and Osram s Thin-GaN structures [73], and in Fig. 9, we illustrate the general features of these designs. Device fabrication includes flip-chip mounting of the InGaN-based LED followed by removal of the sapphire or SiC substrate. Texturing of the n-gan surface improves outcoupling of light while the thin epilayer design restricts the number of modes that are trapped within the LED by total internal reflection. The design also employs a low-loss reflective

9 1036 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009 Fig. 9. General design of TFFC InGaN LEDs. Enhanced light extraction is achieved through substrate removal, n-gan surface texturing, and use of a reflective p-contact. p-contact underneath the InGaN epilayers to further enhance light extraction. Notably, these designs have achieved LED light extraction efficiencies as high as 80% [6]. Given the success of these approaches, relatively little improvement is now needed to reach the 90% extraction efficiencies targeted in the U.S. Department of Energy Multi-Year Program Plan for SSL [8]. However, surface texturing approaches may not bridge this final gap and do not provide highly tailored light distributions or the potential for enhanced IQE. In addition, the 80% extraction efficiency has been achieved with the use of encapsulants that degrade at higher currents (temperatures), and improved extraction designs that avoid encapsulants are of interest. A number of alternative designs are being explored to meet these needs, and next, we specifically highlight progress in photonic crystal LEDs and surface-plasmon-enhanced LEDs. 2) Photonic Crystal LEDs (PXLEDs): PXLEDs are fabricated by periodic patterning of the semiconductor material, or an overlying dielectric layer, with feature sizes on the order of the wavelength of emitted light [75]. Consistent with the concept of advanced photon management, the photonic crystal (PX) structure has the potential to improve LED performance in several ways. First, it enables enhanced light extraction through scattering of waveguided (trapped) light into radiation modes, and inhibits the emission of particular waveguided modes through the creation of a photonic bandgap [75], [76]. Modification of the photonic density of states by the PX structure can also enhance spontaneous recombination rates (and related IQE) by the so-called Purcell effect [77]. Lastly, PXs can enable control of the light emission pattern, and in particular, can be employed to enhance forward-directed light for LED applications that require high radiance [78]. Each of these enhancements has been demonstrated in LEDs, but achieving all of them in the same design has proven difficult. The most widely used PX design for LEDs is a 2-D structure employing periodic patterns of holes ( nm diameter) created by nanolithography and plasma etching. Early theoretical modeling predicted that 2-D PXs could enable extraction efficiencies as high as 90% [76]. These model structures consist of a semiconductor slab with a 2-D PX penetrating the entire slab, providing substantial control of light emission and extraction. High extraction efficiencies and Purcell enhancement have also been demonstrated experimentally, through photoluminescence of a similarly designed PX [69]. SSL applications require electrically injected devices and this adds challenges to employing PX structures. Since PXs are typically etched into the semiconductor, there is potential for degradation of LED performance through current leakage, surface recombination, or damage to the QW layers. To avoid these effects, PX patterning in InGaN LEDs is often limited to the topmost (typically p-type) epilayers of the LED structure, leaving the QWs intact. To date, there have been a number of reports of InGaN-based PXLEDs using this approach [78], [80]. The surface patterning leads to fairly poor interaction between confined optical modes and the PX, but has enabled the demonstration of 1.5 increased light extraction efficiency compared to nonpatterned planar structures [78]. More recently, it has been shown that a reduction in the number of optical modes and increased optical mode PX interaction can be achieved by incorporating a lower index AlGaN confining layer beneath the InGaN MQW region [81], [82] or using thin-film structures [83]. The realization of PX structures that are embedded in the LED heterostructure, rather than limited to surface layers, is anticipated to have several advantages, including stronger mode interaction with the PX structure, avoidance of QW etch damage, and simplified electrical injection by maintaining a planar top layer. Recently, InGaN LEDs with embedded PXs have been explored through lateral overgrowth of GaN on periodically patterned SiO 2 layers [84], [85]. These strategies achieve full planarization of the periodic structure, allowing subsequent planar growth of high-quality QW and p-gan layers. Here, the embedded SiO 2 acts as the PX structure as well as a lower index confining layer. Although further optimization is needed, preliminary LED results include up to 33% increased output power at 480 nm with the implementation of embedded 2-D SiO 2 pillar patterns [85]. Importantly, the enhanced light extraction and radiance achieved by PXLEDs can eliminate the need for low-refractiveindex encapsulants, resulting in longer LED operational lifetimes at high currents. To date, commercialization of PXLEDs is not yet widespread, and little data on photonic-crystal-enabled performance enhancements compared to conventional designs are available. However, existing PXLED products highlight the advantages of long life and high brightness [86]. To enable 90% extraction efficiencies from LEDs, future PXLED designs may control light propagation in all directions through the growth or assembly of 3-D PX structures [87] with embedded light emitting layers. Strategies to etch 2-D PX structures through the entire LED heterostructure, without degradation of QW efficiency, should also enable further advances over existing PXLED designs. 3) Surface plasmon (SP) enhanced LEDs: SP-enhanced LEDs are also being explored for achieving enhanced light extraction and improved IQE at visible wavelengths. As shown schematically in Fig. 10(a), these devices circumvent radiative recombination in the QW region, and instead, invoke ultrafast energy transfer from the QW to SP modes in a neighboring metal surface coating [88]. With appropriate nanoscale structure in the metal, the SP excitations can then scatter, lose momentum, and couple to radiation modes [Fig. 10(b)].

10 CRAWFORD: LEDs FOR SOLID-STATE LIGHTING: PERFORMANCE CHALLENGES AND RECENT ADVANCES 1037 Fig. 10. (a) Schematic of an SP-enhanced InGaN LED structure employing a thin p-gan contact layer for effective QW SP coupling and metal patterning for surface plasmon scattering. (b) Surface plasmon energy dispersion curve for a metal interface with air and a dielectric, yielding surface plasmon frequencies ω air and ω n, respectively. Scattering of the surface plasmon mode enables momentum loss (g) and photon emission (figure courtesy of A. J. Fischer). have been made [96], [97]. In particular, nanostructured Ag films were applied to green (550 nm) InGaN LEDs with an originally low IQE of 7%. Electroluminescence intensity enhancements of 2.2 were observed and attributed to SP-mediated effects [97]. At this stage, dramatic efficiency improvements have not yet been realized for InGaN LEDs employing SP-mediated emission. Significant opportunity exists for demonstrating greater efficiency enhancements through improved QW SP coupling, reduced SP losses, and more effective extraction of light from the SP mode. However, it remains to be seen whether SP-enhanced approaches can yield the ultrahigh IQE and EQE necessary to reach SSL performance targets. Through this SP-mediated process, LED efficiency can potentially be enhanced in multiple ways. Coupling to SP modes aids in extracting photons that would have otherwise suffered total internal reflection [89]. In addition, the fast energy transfer to the SP, promoted by the high SP density of states at resonance, may increase IQE by allowing more effective competition with nonradiative processes in the QW. Whether SPs ultimately enhance or reduce the overall emission efficiency is critically dependent upon the losses of the SP mode and the efficiency with which radiation can be extracted from SP modes compared to the intrinsic radiative efficiency of the semiconductor material. The greatest enhancements are therefore expected for QW structures with low radiative efficiency, where SP-mediated emission can enable a clear advantage [90]. Conversely, the advantages of SP-mediated emission are reduced for structures with higher IQE, and are likely to be minimal for state-of-the-art blue LEDs (IQE 70%). Given the landscape of LED efficiencies depicted in Fig. 2, we see that SSL materials near the green yellow-gap region have the most to gain from SP-mediated emission strategies. As a first step to achieving SP-enhanced InGaN LEDs, several groups have explored optical pumping of InGaN QWs with various metal coatings [91] [93]. Ag and Au metals on GaN surfaces are expected to have SP resonances at 440 and 560 nm, respectively [94], and are therefore particularly well-matched to InGaN visible emitters. To date, application of Ag coatings to InGaN QWs has shown evidence of SP-enhanced emission, including a 6.8 enhancement of IQE, from 6% to 41%, at a wavelength of 470 nm [93]. Time-resolved photoluminescence studies of similar structures have further verified spontaneous emission rate enhancement up to 32 compared to emission directly in air [95], establishing effective QW SP coupling. Transitioning these optical pumping results to electrically injected structures adds formidable challenges. Unlike InGaN QW structures with thin ( 10 nm) GaN cap layers, InGaN LEDs typically employ thick (>100 nm) p-type layers between the QWs and the surface metal. Substantially thinner p-type layers are needed to place the QW within the fringing field of the SP mode ( 40 nm for Ag on GaN and λ = 425 nm [91]); however, thin p-type layers can lead to device shorting and electrical performance degradation. In spite of these challenges, a few reports of SP emission in InGaN-based LEDs employing thin p-type layers IV. SUMMARY AND OUTLOOK In summary, the recent efforts of many groups worldwide have continued the impressive advancement of LED performance. A major emphasis has been on the development of InGaN-based materials and heterostructures to overcome LED efficiency limitations. At this stage, there is still much work to be done to achieve ultrahigh ( 90%) IQEs from LED materials at all desired wavelengths. Blue LEDs have particular importance, as they are a likely component of either multichip or phosphor-converted white LED approaches. Given their high IQEs to date ( 70%), and expected advances in nitride materials, that goal appears to be attainable. The greater challenge, necessary for realizing the benefits of multichip approaches, is to fill the green yellow gap where present IQEs are 30% and lower. In this paper, we have highlighted a number of innovative materials strategies, including nonpolar crystal orientations, strain-relaxed InGaN templates, and nanowire-based emitters, that show particular promise for overcoming the challenges of longer wavelength InGaN LEDs. Most compelling is the possibility that these and other approaches may ultimately yield high-performance InGaN emitters all the way to the red region, providing performance advantages over AlInGaP LEDs (e.g., efficiency at elevated temperatures) and enabling a multichip platform based on a single-alloy system. The loss of InGaN LED efficiency at high operating currents is relevant to both multichip and phosphor-converted white LEDs, and has been a topic of active investigation. Many mechanisms for efficiency droop have been proposed and both Auger recombination and carrier injection/leakage mechanisms have emerged as strong candidates. LED heterostructure designs inspired by these insights have already reduced efficiency droop. It is now very likely that focused theoretical and experimental efforts will enable final consensus on the underlying mechanism in the near future, clarifying the path to even greater performance advances. LED light extraction efficiencies have seen notable improvement through the application of TFFC LED designs, coming within reach of the 90% extraction efficiency targets. Although not yet fully optimized for high efficiency, advanced photonic structures, including PXLEDs and SP-enhanced LEDs, are being explored to provide combined IQE and EQE benefits and new strategies for controlling light emission.

11 1038 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009 In this paper, we have narrowed our scope to primarily InGaNbased materials and LEDs, and the significant advances achieved by many groups throughout the past few years. SSL efforts are of course widespread and encompass many technical areas. Important light-emitting materials research and development topics that we have not emphasized include alternative greento-red-emitting semiconductors, high-performance phosphors, quantum dots and other down-conversion materials, as well as organic LED materials. Similarly, research on next-generation LED device concepts for SSL covers many additional topics, including systems employing strong light matter interactions for tailoring the light emission process and novel nanoscience approaches to white-light generation. Details on these and other key SSL research topics may be found in this journal edition and also in [9]. 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Kawakami, Surface plasmon enhanced spontaneous emission rate of InGaN/GaN quantum wells probed by time-resolved photoluminescence spectroscopy, Appl. Phys. Lett., vol. 87, pp , [96] D.-M. Yeh, C.-F. Huang, C.-Y. Chen, Y.-C. Lu, and C. C. Yang, Surface plasmon coupling effect in an InGaN/GN single-quantum-well lightemitting diode, Appl. Phys. Lett., vol.91,pp ,2007. [97] D.-M. Yeh, C.-F. Huang, C.-Y. Chen, Y.-C. Lu, and C. C. Yang, Localized surface plasmon-induced emission enhancement of a green light-emitting diode, Nanotechnology, vol. 19, pp , Mary H. Crawford (M 05) received the B.A. degree from Holy Cross College, Worcester, MA, in 1985, and the M.S. and Ph.D. degrees from Brown University, Providence, RI, in 1987 and 1993, respectively, all in physics. Her Ph.D. research involved gain spectroscopy and device physics of II VI semiconductor laser diodes and LEDs. In 1993, she joined Sandia National Laboratories, Albuquerque, NM, where she is currently a Distinguished Member of Technical Staff in the Semiconductor Material and Device Sciences Department. Her work at Sandia has involved the development of vertical-cavity surfaceemitting lasers (VCSELs), including AlInGaP-based red VCSELs, and the study of wide-bandgap nitride materials for deep UV and visible LED applications. From , she was a Senior Scientist and the Director of Research and Development at Uniroyal Optoelectronics, a start-up company developing InGaN and AlInGaP LEDs for a range of applications. Her current research interests include optical spectroscopy of wide-bandgap semiconductor materials and the development of novel semiconductor light-emitting devices.