1 Inorganic Semiconductors for Light-emitting Diodes

During the past 40 years, light-emitting diodes (LEDs) have undergone a significant development. The first LEDs emitting in the visible wavelength region were based on GaAsP compound semiconductors with external efficiencies of only 0.2%. Today, the external efficiencies of red LEDs based on AlGaInP exceed 50%. AlGaInP semiconductors are also capable of emitting at orange, amber, and yellow wavelengths, albeit with lower efficiency. Semiconductors based on AlGaInN compounds can emit efficiently in the UV, violet, blue, cyan, and green wavelength range. Thus, all colors of the visible spectrum are now covered by materials with reasonably high efficiencies. This opens the possibility to use LEDs in areas beyond conventional signage and indicator applications. In particular, LEDs can now be used in high-power applications thereby enabling the replacement of incandescent and fluorescent sources. LED lifetimes exceeding i 10 h compare favorably with incandescent sources (Z 500 h) and fluorescent sources (Z 5000 h), thereby contributing to the attractiveness of LEDs. Inorganic LEDs are generally based on p-n junctions. However, in order to achieve high internal quantum efficiencies, free carriers need to be spatially confined. This requirement has led to the development of heterojunction LEDs consisting of different semiconductor alloys and multiple quantum wells embedded in the light-emitting active region. The light-extraction efficiency, which measures the fraction of photons leaving the semiconductor chip, is strongly affected by the device shape and surface structure. For high internal-efficiency active regions, the maximization of the light-extraction efficiency has proven to be the key to highpower LEDs. This chapter reviews important aspects of inorganic LED structures. Section 1.2 introduces the basic concepts of optical emission. Band diagrams of direct and indirect semiconductors and the spectral shape of spontaneous emission will be discussed along with radiative and nonradiative recombination processes. Spontaneous emission can be controlled by placing the active region in an optical 1

[1]  M. Ibanescu,et al.  An All-Dielectric Coaxial Waveguide. , 2000, Science.

[2]  Zhifang Lin,et al.  Expanding high reflection range in a dielectric multilayer reflector by disorder and inhomogeneity , 2001 .

[3]  Leonard C. Feldman,et al.  Giant enhancement of luminescence intensity in Er‐doped Si/SiO2 resonant cavities , 1992 .

[4]  Roger King,et al.  45% quantum-efficiency light-emitting diodes with radial outcoupling taper , 2000, Photonics West - Optoelectronic Materials and Devices.

[5]  R. A. Logan,et al.  Enhanced spectral power density and reduced linewidth at 1.3 μm in an InGaAsP quantum well resonant‐cavity light‐emitting diode , 1992 .

[6]  E. F. Schubert,et al.  Current crowding and optical saturation effects in GaInN/GaN light-emitting diodes grown on insulating substrates , 2001 .

[7]  R. Fletcher,et al.  High performance AlGaInP visible light‐emitting diodes , 1990 .

[8]  Kazuhiko Itaya,et al.  High-brightness InGaAlP green light-emitting diodes , 1992 .

[9]  Y. Itoh,et al.  Organic photo- and electroluminescent devices with double mirrors , 1993 .

[10]  E. Purcell.  Spontaneous Emission Probabilities at Radio Frequencies , 1995 .

[11]  E. Schubert,et al.  Junction–temperature measurement in GaN ultraviolet light-emitting diodes using diode forward voltage method , 2004 .

[12]  J A Dobrowolski,et al.  Broadband high-reflection multilayer coatings at oblique angles of incidence. , 1997, Applied optics.

[13]  Gustaaf Borghs,et al.  Light propagation through textured surfaces , 1999 .

[14]  Wolfgang Schmid,et al.  Efficient light-emitting diodes with radial outcoupling taper at 980- and 630-nm emission wavelength , 2001, SPIE OPTO.

[15]  E. Fred Schubert,et al.  GaInN light-emitting diodes with RuO2/SiO2/Ag omni-directional reflector , 2004 .

[16]  C. A. Stover,et al.  Giant birefringent optics in multilayer polymer mirrors , 2000, Science.

[17]  A. Scherer,et al.  30% external quantum efficiency from surface textured, thin‐film light‐emitting diodes , 1993 .

[18]  Hiromoto Susawa,et al.  GaAs/GaAlAs surface emitting IR LED with Bragg reflector grown by MOCVD , 1991 .

[19]  Hiroyuki Yokoyama,et al.  Observation of spontaneous emission lifetime change of dye‐containing Langmuir–Blodgett films in optical microcavities , 1991 .

[20]  C. P. Lee,et al.  Wide angle distributed Bragg reflectors for 590 nm amber AlGaInP light-emitting diodes , 2000 .

[21]  E. F. Schubert,et al.  GaInN light-emitting diodes with omnidirectional reflectors , 2003, SPIE OPTO.

[22]  Colin J. Humphreys,et al.  Electron-beam-induced strain within InGaN quantum wells: False indium “cluster” detection in the transmission electron microscope , 2003 .

[23]  J. Gannon,et al.  Optimization of Electroluminescent Efficiencies for Vapor‐Grown GaAs1 − x P x Diodes , 1969 .

[24]  W. B. Joyce,et al.  Steady‐State Junction‐Current Distributions in Thin Resistive Films on Semiconductor Junctions (Solutions of ▿2v = ±ev) , 1970 .

[25]  T. Gmitter,et al.  Inhibited and enhanced spontaneous emission from optically thin AlGaAs/GaAs double heterostructures. , 1988, Physical review letters.

[26]  Lewis J. Rothberg,et al.  Color variation with electroluminescent organic semiconductors in multimode resonant cavities , 1994 .

[27]  G. Borghs,et al.  Light-extraction mechanisms in high-efficiency surface-textured light-emitting diodes , 2002 .

[28]  R. A. Logan,et al.  High Efficiency, Narrow Spectrum Resonant Cavity Light Emitting Diodes , 1995 .

[29]  Mehmet Arik,et al.  Thermal challenges in the future generation solid state lighting applications: light emitting diodes , 2002, ITherm 2002. Eighth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (Cat. No.02CH37258).

[30]  Henri Benisty,et al.  Toward ultrahigh-efficiency aluminum oxide microcavity light-emitting diodes: guided mode extraction by photonic crystals , 2002 .

[31]  M. Osinski,et al.  Life tests and failure mechanisms of GaN-AlGaN-InGaN light emitting diodes , 1998, Conference Proceedings. LEOS'98. 11th Annual Meeting. IEEE Lasers and Electro-Optics Society 1998 Annual Meeting (Cat. No.98CH36243).

[32]  Burak Temelkuran,et al.  External Reflection from Omnidirectional Dielectric Mirror Fibers , 2002, Science.

[33]  K. Streubel,et al.  High brightness visible (660 nm) resonant-cavity light-emitting diode , 1998, IEEE Photonics Technology Letters.

[34]  Gustaaf Borghs,et al.  40% efficient thin-film surface-textured light-emitting diodes by optimization of natural lithography , 2000 .

[35]  Winn,et al.  A dielectric omnidirectional reflector , 1998, Science.

[36]  W. Schmid,et al.  High-efficiency red and infrared light-emitting diodes using radial outcoupling taper , 2002 .

[37]  E. Palik.  Handbook of Optical Constants of Solids , 1997 .

[38]  Chih-Chung Yang,et al.  Dependence of composition fluctuation on indium content in InGaN/GaN multiple quantum wells , 2000 .

[39]  E. F. Schubert,et al.  Efficiency of GaN/InGaN light-emitting diodes with interdigitated mesa geometry , 2001 .

[40]  Hiroyuki Yokoyama,et al.  Enhanced spontaneous emission from GaAs quantum wells in monolithic microcavities , 1990 .

[41]  P. Mataloni,et al.  Anomalous spontaneous emission time in a microscopic optical cavity. , 1987, Physical review letters.

[42]  Roel Baets,et al.  Resonant-cavity light-emitting diodes: a review , 2003, SPIE OPTO.

[43]  C. P. Kuo,et al.  Twofold efficiency improvement in high performance AlGaInP light‐emitting diodes in the 555–620 nm spectral region using a thick GaP window layer , 1992 .

[44]  Gustaaf Borghs,et al.  Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes , 2001 .

[45]  A. A. Allerman,et al.  Junction and carrier temperature measurements in deep-ultraviolet light-emitting diodes using three different methods , 2005 .

[46]  M. Ishikawa,et al.  High-efficiency InGaAlP/GaAs visible light-emitting diodes , 1991 .