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- Monemar, Bo, et al.
(författare)
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Nanowire-Based Visible Light Emitters, Present Status and Outlook
- 2016
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Ingår i: Semiconductor Nanowires : Properties and Applications - Properties and Applications. - : Elsevier. - 0080-8784. - 9780128040164 ; 94, s. 227-271
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Bokkapitel (refereegranskat)abstract
- So far, the semiconductor nanowire research area has mainly delivered results on growth procedures and related material properties. As the development lately has been successful in producing novel nanowire-based structures for optical or electronic applications, the time is ripe to review the device work that has been done and in some cases has produced devices ready for the market. In this chapter, we shall review the specific area of nanowire-based LEDs (NW-LEDs) for visible light, including the application area of “solid state lighting” (SSL). A brief review of the progress in the area of visible light LEDs over the last half century is presented, this also mentions some of the progress made in the planar technology so far. The most successful way of producing white light is still based on the use of phosphors, just like in the present compact fluorescence lamps (CFLs). The reason for this is the high efficiency (external quantum efficiency > 80%) possible at low currents in the violet planar InGaN-based LEDs used to excite the phosphors. These LEDs are presently mainly produced on foreign substrates, leading to a high dislocation density (DD) and a sizeable droop at high injection currents (25–40%). This droop and the down conversion energy loss in the phosphors (20–25%) has motivated the interest for a phosphor-less white light source based on direct mixing of light of different wavelength (such as red, green, and blue; RGB). To be competitive, this solution must be based on highly efficient LEDs for all RGB (red, green, and blue) colors. Since NW-LED structures can be produced basically free of structural defects (even if grown on a foreign substrate), the idea of using the RGB mixing concept for the production of white light sources with an ultimately higher efficiency than for the phosphor-based lamps is a major technical target for a second generation of light sources in the SSL field. Basic concepts behind the design and optical properties of NW-LED structures are discussed in this chapter, with emphasis on the present developments of III-nitride-based structures. The growth procedure relevant for such NW-LED structures is reported in some detail, specifically the core–shell configuration readily produced with metalorganic vapor phase epitaxy (MOVPE). The first generation processing technology for NW-LED structures is briefly described; this is naturally quite different from the established routines for planar LED chips. Experimental data for nitride-based NW-LEDs for blue, green, and even longer wavelengths are given in terms of radiative efficiencies, light outcoupling, droop, and long-term reliability. The experience so far is that for these NW-based emitters, efficiencies can be obtained that are close to those for the corresponding planar LEDs. There are still problems with the reproducibility of the radiative output, as well as a significant droop that would not be expected for m-plane emitters. More work is needed to pinpoint the cause of these problems. Finally, we briefly discuss various applications (also other than white lamps) where the NW-LEDs may have a specific advantage.
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