Development of Nonpolar and Semipolar InGaN/GaN Visible Light-Emitting Diodes
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Nonpolar and Semipolar InGaN/GaN Visible Light-Emitting Diodes
Daniel F. Feezell, Mathew C. Schmidt, Steven P. DenBaars, and Shuji Nakamura Abstract This article reviews the development of nonpolar and semipolar InGaN/GaN lightemitting diodes (LEDs), emphasizing structures on freestanding bulk GaN. A brief history of LED development on each orientation is provided, followed by a discussion of the most relevant and recent results. The context is related to several current LED issues, such as the realization of high-efficiency white solid-state lighting, potential solutions to the “green gap,” and applications for polarized emitters. The section on nonpolar LEDs highlights high-power violet and blue emitters and considers the effects of indium incorporation and substrate miscut. The section on semipolar GaN reviews the development of LEDs in the violet, blue, green, and yellow regions and highlights the potential of InGaN/GaN LEDs as an alternative technology to AlInGaP for yellow emitters. A brief review of polarization anisotropy also is included for each orientation. Finally, a two source white light system utilizing a nonpolar blue LED and a semipolar yellow LED is presented.
Introduction and Background Based on semiconductor light-emitting diodes (LEDs), solid-state lighting (SSL) is a promising approach for the realization of highly efficient white light sources. Recent progress in SSL has led to white lighting approaches with luminous efficiencies beyond 100 lumens per watt (lm/W), and high-performance commercial products are now available.1 While the luminous efficiencies of conventional incandescent bulbs, halogen lamps, compact fluorescents, and large-area fluorescent tubes hover around 15 lm/W, 25 lm/W, 60 lm/W, and 80–100 lm/W respectively,1,2 the proposed targets for SSL exceed 200 lm/W.2,3 An efficiency increase of this magnitude has the potential to significantly reduce world energy consumption and enable huge costs savings. Consequently, considerable research 318
has been invested into various approaches for generating white SSL. One commonly employed approach involves using LEDs emitting in the blue range (440–460 nm) to pump down-converting phosphors, such as Y3Al5O12:Ce3+ (YAG). These phosphors consist of an inorganic host material (e.g., Y3Al5O12) and an optically active element (e.g., Ce3+) that absorb light in the higherfrequency blue region of the spectrum and emits light in the lower-frequency yellow region of the spectrum. If the resulting yellow phosphor-emission is mixed with a portion of the blue “pump” source, white light is generated. Although this approach is simple and efficient, it typically produces white light with a color rendering index (CRI) below 80 and a correlated color temperature (CCT) above 4000 K. CRI refers to a light source’s ability to
properly render the colors of an illuminated object and ranges from 0 to 100, with 100 being perfect color rendering, while CCT is the temperature of a Planckian black-body radiator whose color is closest to the color of the white lig
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