Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.1 Efficiency of LEDs achieved as a function of time since the 1960s. The photometric quantity lmW −1 (lumens per watt) was introduced in Chapter 3

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.2 Forward-biased LED p-n junction. When one electron and one hole recombine near the junction one photon of light may be emitted. The achievement of high efficiency requires that there is a good chance that recombination events are radiative and that the generated photons are not reabsorbed or trapped in the device

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.3 LED packaging includes a transparent lens, which is usually made from an epoxy material, and a reflector cup into which the LED die is mounted. The radiation pattern is determined by the combination of the die emission pattern, the reflector cup design, and the shape and refractive index of the polymer lens. Reproduced by permission of Avago Technologies

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.4 LED die consists of a single crystal substrate on which a series of epitaxial layers is grown forming the active layers. (a) For conductive substrates a current spreading layer and the top and bottom contacts are shown as well as an n-type current blocking layer discussed in section 5.4. The notations DH and DBR refer to Double Heterostructure and Distributed Bragg Reflector, which will be discussed in sections 5.8 and 5.11 respectively. (b) For LEDs grown on insulating substrates such as sapphire (see section 5.10) the second contact is made via a buried current spreading layer as shown

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.5 Emission spectra of AlGaInP LEDs. The linewidth of the amber LED is measured as the full width at half maximum as shown and is 13.5 nm. Reproduced by permission of Avago Technologies

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.6 Observed luminescence at the junction of a GaInAs/GaAs LED near a cleaved surface showing that carrier diffusion lengths are in the range of a few microns in GaInAs. Reprinted from E. Fred Schubert, Light-Emitting Diodes, 2e ISBN Copyright (2006) with permission from Cambridge University Press

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.7 Light generated in the semiconductor will reach the surface and either reflect or be able to exit depending on the angle of incidence. The critical angle is θ c

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.8 Light that escapes will be diffracted from an incident angle θ to an emitted angle Θ

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.9 Typical radiation pattern for a LED. This AlGaInP red LED has a radiation pattern with a 30 ◦ (±15 ◦ ) beam divergence determined at half maximum intensity. Reproduced by permission of Avago Technologies from data sheet file AV EN, P0000.pdf, Copyright (2008) with permission from Avago Technologies, USA

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.10 Photon energy plotted as a function of phosphorus mole fraction x in GaAs 1−x P x and GaAs 1−x P x :N. Reprinted with permission from Craford, M. G., et al, Radiative recombination mechanisms in GaAsP diodes with and without nitrogen doping, J. Appl Phys, 43: 10,4075. Copyright (1972) with permission from American Institute of Physics

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.11 Band gaps of double heterojunction using Al x Ga 1−x As layers grown epitaxially on a GaAs substrate. Carriers recombine in the active layer of width W. The cladding layers are doped such that one layer is n-type and one layer is p-type. Similar structures are used in GaInN LEDs. See Section 5.10

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.12 Electron and hole energy levels within the double heterojunction wells of height  E c in the conduction band and  E v in the valence band

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.13 Double heterojunction showing the exponential decay of excess carriers into the cladding layer. Only one side is shown for simplicity

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.14 (Al x Ga 1−x ) y In 1−y P bandgap versus lattice constant graph showing the composition ranges in this quaternary system. By adjusting the two available parameters, x and y, a field of compositions is possible represented by the shaded areas. A range of energy gaps from 1.89 eV to 2.33 eV is available in the direct bandgap region while matching the GaAs lattice constant. Reproduced with permission from Elsevier from OMVPE Growth of AlGalnP for High-Efficiency Visible Light-Emitting Diodes, Semiconductors and Semimetals, Volume 48, C.H. Chen, S.A. Stockman, M.J. Peanasky, C.P. Kuo Copyright (1997) Elsevier Ltd

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.15 Radiative efficiency as a function of dislocation (etch pit) density for a variety of III-V semiconductors. Dislocation density is determined by etching the crystal surface and then counting the number of resulting etch pits per unit area. Etch pits form at the dislocations. Reprinted from E. Fred Schubert, Light-Emitting Diodes, 2e ISBN Copyright (2006) with permission from E. Fred Schubert

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.16 Dislocations in GaN epitaxial layer grown on sapphire. In addition to these factors, the 12% lattice mismatch of GaN with respect to sapphire is effectively much less apparent since it turns out that a rotation about the c-axis of GaN relative to the sapphire substrate allows a far better lattice match of the GaN system relative to the sapphire in the plane normal to the c-axis. See Problem Both sapphire and SiC are very stable substrate materials that may be heated to over 1000 ◦ C during GaN growth, and both substrates are used in the high-volume production of GaInN LEDs. Reprinted with kind permission from The Blue Laser Diode, S. Nakamura, S. Pearton, G. Fasol, page 14 fig 2.2, Copyright (2000) Springer Science & Business Media

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.17 Emission spectra of blue, green and red LEDs having the highest available efficiencies. Reprinted with permission of Toyoda Gosei

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.18 Output intensity versus ambient temperature for GaInN and AlGaInP LEDs. Note the decreased thermal quenching in GaInN. Reprinted with permission of Toyoda Gosei

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.19 Forward intensity versus current characteristics for GaInN and AlGaInP LEDs. Reprinted with permission of Toyoda Gosei

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.20 The nitride alloy semiconductor systems Al x Ga 1−x N, Ga 1−x In x N and Al x In 1−x N plotted to show energy gap as a function of lattice constant. Reproduced from Schubert EF. Light Emitting Diodes, 2nd edn. Cambridge University Press, 2006, p. 223, with permission

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.21 (a) Band diagram of a double heterostructure using In x Ga 1−x N active layer grown epitaxially. Carriers recombine in the active layer. The cladding layers are doped such that one layer is n-type and one layer is p-type. (b) Band diagram including the effect of polarization

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.22 GaN planes are generally (0001) planes that are formed as a result of c-axis growth; however, alternative growth directions and planes may also be achieved. This reduces or eliminates polarization in the growth directions. M-planes (1100) or A-planes (1120) as shown are non-polar and semi-polar planes are also available. Growth of high quality GaNin directions resulting in non-polar and semi-polar quantumwell structures is an area of current LED development. Reprinted from Speck, J. S., New Faces of GaN: Growth, Doping and Devices from INSIGHTS 2006, Copyright (2006) with permission from James Speck

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.23 Emission spectrum of white-emitting LED. Blue light from the LED die is downconverted using YAG:Ce to produce a broadband yellow emission. When combined with the blue LED emission white light results. Reprinted with permission from Nichia Corp

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.24 Three of the six possible escape cones for light emission from a LED die with vertical sidewalls. Reprinted from E. Fred Schubert, Light-Emitting Diodes, 2e ISBN Copyright (2006) with permission from Cambridge University Press

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.25 Light can outcouple more efficiently if the sidewalls of the die are tilted as shown. The tilted walls can be applied to a variety of substrates. Reproduced from M.R. Krames et al., High- power truncated-inverted-pyramid (AlxGa1-x)0.5In0.5P/GaP light-emitting diodes exhibiting > 50% external quantum efficiency, Applied Physics Letters Vol. 75, No. 16. Copyright (2000) with permission from American Institute of Physics

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.26 Light propagation through a textured surface. The light path shown exceeds the critical angle of the surface but the beam can pass through due to surface texturing. Up to a 50% improvement in outcoupling has been achieved through the use of texturing

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.27 Structure of high-power LED showing the die mounted on a base suitable for mounting on a heatsink. The lens is made of a silicone polymer, which withstands higher optical flux without yellowing compared to epoxy lenses. Reproduced fromwww.luxeon.com. Copyright (2011) LUXEON

Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 5.28 Photograph of high-power LED capable of over 300 lumens output. The rectangular ceramic plate is designed for ease of mounting and heatsinking. Specific applications of these LEDs include street lighting, retail lighting and automobile headlights. Reproduced from Copyright (2011) LUXEON