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Innovative advances in LED technology
Microelectronics Journal 36 (2005) 129–137 www.elsevier.com/locate/mejo
Innovative advances in LED technology F.K. Yam*, Z. Hassan School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 27 July 2004; received in revised form 19 October 2004; accepted 9 November 2004
Abstract An overview of the rapid progress in the developments of the inorganic light emitting diode (LED) technology is presented. Innovative structures and designs of the device have led to dramatic improvements of the performance in LED technology, groundbreaking performance records are being reported constantly. This article summaries the recent progress of the high brightness LEDs, and describes the LED structures from the basic pn homojunction, to heterojunction, and eventually the use of nano-scale low-dimensional structures in the device fabrication. Some of the novel structures and designs of the most recent developed high brightness LEDs, as well as the conventional and innovative ways of producing white LEDs are briefly discussed. q 2004 Elsevier Ltd. All rights reserved. Keywords: III–V semiconductors; High brightness LEDs; White LEDs
1. Introduction Light emitting diodes (LEDs) are the ultimate light source in the lighting technology. The LED technology has flourished for the past few decades. High efficiency, reliability, rugged construction, low power consumption, and durability are among the key factors for the rapid development of the solid-state lighting based on highbrightness visible LEDs. Conventional light sources, such as filament light bulbs and fluorescent lamps depend on either incandescence or discharge in gases. These two processes are accompanied by large energy losses, which are attributed to the high temperatures and large Stokes shift characteristics. On the other hand, semiconductors allow an efficient way of light generation. LEDs made of semiconductor materials have the potential of converting electricity to light with near unity efficiency. For decades, semiconductor device researchers have dreamed of obtaining blue LED. The stunning technological breakthroughs by Nakamura in producing GaN-based blue and green LEDs in early 1990s [1,2] have a profound impact on the LED technology. Since the development of the high
* Corresponding author. Tel.: C60 4 6533673; fax: C60 4 6579150. E-mail address: [email protected] (F.K. Yam). 0026-2692/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2004.11.008
brightness blue LEDs, the LED market has grown significantly [3,4]. By combining red, green and blue; the three primary colors, full-color display and white light source could be realized. Currently, LEDs play an important role in many applications including large area displays, automotive and aircraft lighting, traffic signals. LEDs will soon play a much larger role in the future of architectural lighting and general illumination. On the other hand, with the technology advances in the nano-scale dimension fabrication, the applications of the LEDs are no longer limited to the above-mentioned roles. The new generation high brightness planar LEDs, i.e. resonant cavity LEDs, show promise in many advanced applications, such as optical communication, as well as non-communication application, e.g. sensors, printers and scanners [5–7]. The invention of first LED based on GaAsP semiconductor material by Holonyak and Bevacqua in 1962 [8], which emitted red light with low luminous efficient of about 0.1 lm/W. Since then a remarkable progress in the LED performance has been reported. Fig. 1 shows the historical development of the luminous efficiency of visible-spectrum LEDs. It is found that the light output performance of the LEDs has been improving by nearly an order of magnitude for every 10 years [9]. Such a rapid progress in LEDs performance mainly can be attributed to the advancement of the semiconductor growth technique, which leads to higher
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Fig. 1. Historical progress in the development of the luminous efficiency of visible-spectrum LEDs. (After Ref. [9]. Reprinted from Semiconductors and Semimetals, vol. 48, ed. by G.B. Stringfellow and M. G. Craford, series eds. R.K. Willardson and E.R. Weber, ‘Overview of Device Issues in HighBrightness Light-Emitting Diodes,’ High Brightness Light Emitting Diodes, Academic Press, New York, 1997, p. 47, with permission from Elsevier).
purity material and lower defect density, as well as the use of novel LEDs structures resulting in higher quantum efficiencies.
2. Semiconductor material systems for high brightness LEDs Human eyes are sensitive to light with output wavelengths lie in the range 390!l!720 nm which is corresponding to photon energy of 1.7!hn!3.2 eV. Table 1 shows the colors associated with range of the wavelengths and photon energies. From the operational description and spectral sensitivity of human eye, semiconductor materials used to generate visible light should fulfill certain criteria fundamentally. The semiconductors should be direct with a bandgap lie between 1.7 and 3.2 eV, and they can be doped to form a p–n junction. However, from the literature, not many semiconductors are meeting these requirements. Fig. 2 shows the bandgap energies of common elementary and binary semiconductors compared with spectral sensitivity of human eye [10]. Presently, most of the LEDs are made of III–V compound semiconductor materials with a small number fabricated by II–VI and group IV semiconductor materials, Table 1 The colors and the corresponding wavelengths and photon energy Color
Wavelength, l (nm)
Photon energy hn (eV)
Ultraviolet Violet Blue Cyan Green Yellow Orange Red Infrared
!390 390–455 455–490 490–515 515–570 570–600 600–625 625–720 O 720
O3.18 2.72–3.18 2.53–2.72 2.41–2.53 2.18–2.41 2.06–2.18 1.98–2.06 1.72–1.98 !1.72
Fig. 2. Bandgap energies of common elementary and binary semiconductors compared with spectral sensitivity of human eye. Black bars represent indirect bandgap semiconductors. (After Ref. [10]. Reprinted from ‘Introduction to Solid-State Lighting’, A. Zukauskas, M.S. Shur and R. Gaska, Wiley, New York, 2002. This material is used by permission of John Wiley and Sons, Inc.).
i.e. ZnCdSe, ZnTeSe, MgS, MgSe and SiC. LEDs fabricated by these materials normally have low durability or poor efficiency, which prevented them from large-scale production for commercial applications. The new generation high brightness LEDs are mainly produced by three important semiconductor material systems in which, bandgap engineering is being employed to fabricate suitable semiconductor materials with desired emission wavelength. AlGaAs materials system which is able to generate light from red to IR, on the other hand, AlInGaP emits light range from amber to red-orange. AlGaInN materials have a wider bandgap than both of the AlGaAs and AlInGaP material systems, this allows the system to access to the higher energy green, blue and UV regions of the spectrum. With these three material systems, the spectral regions from UV to IR are widely covered. By combining high power LEDs from these material systems, full color displays and white light for lighting and illumination purposes can be realized. Table 2 shows the LEDs fabricated from different material systems and their characteristics.
3. Structures of LEDs 3.1. p–n homojunction The LEDs are fabricated from semiconductor materials. The basic LED consists of a p–n junction. Under forward bias condition, electrons are injected into the p-type region, and holes are injected into the n-type region. Recombination of these minority carriers with the majority carriers at
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Table 2 Characteristics of different types of high-brightness LEDs Color range
Peak emission (nm)
Structure
External quantum efficiency (%) at 20 mA
Luminous efficiency at 20 mA (lm/w)
Infrared Red Redorange Amber Green Green Blue Green
780, 880 650 636
DH DH DH
27 16 24
N.A 8 35
590 570 525 450 517
10 2 6.3 9.1 2.6
40 14 w18 w2 6.5
Blue
450
5.5
5
ZnTeSe
Green
512
DH DH SQW SQW DH: co-doped with Zinc and silicon DH: co-doped with Zinc and silicon SQW
17
ZnCdSe
Blue
489
MQW
5.3 (at 10 mA) 1.3 (at 10 mA)
Material AlGaAs AlGaInP AlGaInN
AlInGaN
1.6
DH, double heterostructure; NA, not applicable; SQW, single quantum well; MQW, multiple quantum well. (After Ref. [9]. Reprinted from Semiconductors and Semimetals, vol. 48, ed. by G.B. Stringfellow and M. G. Craford, series eds. R.K. Willardson and E.R. Weber, ‘Overview of Device Issues in High-Brightness Light-Emitting Diodes,’ High Brightness Light Emitting Diodes, Academic Press, New York, 1997, p. 47, with permission from Elsevier).
the p–n junction leads to light generation. The wavelength and color of the light is determined by the difference in the energy levels of the electrons and holes. This simplest type of LED design is no longer used in current LED technology since there are some drawbacks which could lower the efficiency in the lighting and illumination applications. First, electron injection into the p-type region is desired for achieving high internal quantum efficiency, therefore, a low injection level of holes into the n-region is required. Second, self-absorption of the generated light is high due to the entire structure possesses same composition; this reduces the light extraction efficiency. Third, the radiative recombination in such LEDs is monomolecular, so that only increasing the doping can increase the emission rate [10,11]. 3.2. Heterostructures Maximization of light generation is the primary goal in LED fabrication; this can be done by the application of various epitaxial structures in the LED. Currently, the fabrication of high intensity LEDs are based on heterostructures. Structures consist of dissimilar semiconductor materials which have different bandgaps because of different chemical composition are called heterostructures. The typical energy band profile of single heterostructure
Fig. 3. Potential profile in (a) p–n heterojunction, (b) double-heterojunction LED. (After Ref. [10]. Reprinted from ‘Introduction to Solid-State Lighting’, A. Zukauskas, M.S. Shur and R. Gaska, Wiley, New York, 2002. This material is used by permission of John Wiley and Sons, Inc.).
(SH) is illustrated in Fig. 3(a). The SH is created by the formation of a p–n junction between two different types of semiconductors; the n-type region is composed of semiconductor with bandgap Eg1 which is wider than the p-type region, Eg2. The n-type region with wider bandgap forms a sufficiently large hole potential barrier resulting in singlesided injection. In addition, this wider bandgap also offers transparency to the photons generated in the p-type region (active layer); this minimizes the internal absorption of photon emitting toward the n-type region and improves the light extraction efficiency [10,11]. Double heterostructure (DH) can be employed to further enhance the performance of the LED. Fig. 3(b) shows the typical structure of a double-heterostructure. DH consists of a narrow bandgap of p-type active region sandwiched between wide bandgap of n- and p-types regions. This structure provides effective confinement by means of potential barriers for both directional injections of excess carriers into the active layer, where electrons and holes recombine. This leads to the increase of the excess carrier density, as well as the recombination rate. In addition, the n- and p-epitaxial layers with wider bandgap are transparent to the light emitted by the active region, and therefore internal absorption effect is minimized [10,11]. The efficiency of DH AlGaAs red LEDs was reported to be approximately twice of the SH [11]. 3.3. Low-dimensional structures The radiative recombination in heteroepitaxial layers can be further enhanced by the creation of low-dimensional structures such as quantum wells and quantum dots. A quantum well could be formed as a thin layer of semiconductor with a given energy gap is sandwiched between two slabs of higher bandgap semiconductors. When the thickness of the epitaxial layer is smaller than the de Broglie wavelength of the electrons in the semiconductor, the energy spectrum of the carriers is modified and quantum confinement is formed. This type of heteroepitaxial structure is called quantum well (QW). The QW thickness usually ranges from a fraction of a nanometer to a 10 or 20 nm [12]. The improvement of the efficiency of
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LEDs by the introduction of low-dimensional structures can be attributed to the forced increase in overlap of the wavefunctions for electrons and holes, and the higher electronic density of states near the band gap of the lowdimensional structures as compared to the bulk (3-D) material. This leads to higher recombination rates and a narrowing of the gain spectrum [13]. In addition to improved quantum confinement, these low-dimensional structures are able to reduce the lattice-matching problem because the thickness of the well and barrier are sufficiently thin to accommodate the compressive strain without the formation of misfit dislocations. 3.4. Innovative and novel LED structures 3.4.1. LEDs with tensile and compressive strained With the introduction of low-dimensional structure, the performance of the LEDs with novel designs has been improved significantly. A novel tensile strain barrier cladding (TSBC) structure has been introduced by Chang et al. [14] which can effectively increase the potential barrier and reduce the leakage current for the AlGaInP yellow-green LEDs. The schematic LED structure is shown in Fig. 4, it consists of a GaAs–AlAs distributed Bragg reflector (DBR), an n-Al0.5In0.5P cladding layer, an undoped MQW active layer, a thin K1% tensile strained p-Al0.65In0.35P TSBC layer, a 0.75 mm p-Al0.5In0.5P cladding layer and a 15 mm p-GaP window layer. The adoption of the combined MQW and TSBC structure has led to higher efficient of the device. It is found that the electroluminescence intensity is twice of the conventional MQW AlGaInP LED emitting at the same wavelength. In addition, this novel structure is less temperature sensitive; therefore, it is useful under high temperature operation. AlGaInP/GaInP LEDs with a compressively strained MQW (CS MQW) active layer emitting at 650 nm, was also investigated by Chang et al. [15,16]. By incorporation of
Fig. 4. The schematic cross-sectional view of the AlGaInP MQWCTSBS LED. (After Ref. [14]. Reprinted from ‘AlGaInP Yellow-Green LightEmitting Diodes with a Tensile Strain Barrier Cladding Layer’, S.J. Chang, C.S. Chang, Y.K. Su, P.T. Chang, Y.R. Wu, K.H. Huang, T.P. Chen, IEEE Photonic Tech. Lett, 9, (9) (1997) 1199, q 2004 IEEE.)
a compressive strain into the MQW active layer, it can increase the output power of the LEDs significantly. Moreover, the C0.33% compressive strain can reduce 10–90% rise time and fall time of the device, which is particularly important in the optical communication. 3.4.2. Tunnel junction LED with pC/nC tunnel junction (TJ) structure is formed on top of the p-type Mg–GaN confinement layer. This TJ provides lateral current spreading and eliminates the need for the semitransparent electrode and a highly resistive ptype AlGaN as compared to the conventional LEDs [17]. The adoption of tunnel junction has led to the formation of two n-electrodes in this LED structure. Jeon et al. [18,19] reported that the light output power of TJ LED is two times higher than the conventional LED, however, the forward voltage is higher than the conventional LED, and this is due to the series resistance across the TJ which causes a voltage drop. 3.4.3. Inverted p-down LED Typical commercially available LEDs are designed with the active region grown on top of the n-type cladding layer, and the p-type cladding layer is grown on top of the active region. Such design is called ‘n-down structure’. The reasons of adopting such structure generally can be attributed to a relatively high crystal quality of n-type cladding layer as compared to p-type cladding layer, which leads to better crystal quality of the active region, and also, the conductivity of n-type layer is much larger than p-type layer, therefore, better current spreading can be achieved in the bottom n-layer, resulted a smaller turn on voltage [20]. However, recently a InGaN/GaN MQW LED with p-down structure and a nC/pC tunnel junction for current spreading has been developed [20,21]. A conventional n-down has been fabricated for comparison purpose. It is found that the optical output power of the inverted LED is almost twice of the conventional LED when operated at 20 mA, which corresponds to an external quantum efficiency as high as 17% compared to 9.5% for the conventional LED. However, the operating voltage of the inverted LED is 7.5 V at 20 mA which is much larger than 3.5 V of the conventional LED. This could be due to high tunnel voltage and high series resistance of the inverted LED [21]. 3.4.4. LEDs with electron reservoir layer An enhanced radiative efficiency of MQW LEDs with the introduction of an electron reservoir layer in between the n-GaN cladding layer and the MQW layers has been fabricated by Takahashi [22]. Fig. 5 illustrates the schematic diagram of InGaN/GaN MQW LED with the electron reservoir layer. The electroluminescence spectral and current-voltage characteristics are enhanced substantially with the addition of the n-doped In0.18Ga0.82N electron reservoir layer. The enhancement of the efficiency of this
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brightness InGaN/GaN MQW LEDs have been commercially available, the output intensity could be enhanced by the exploration of QDs instead of MQW, to further confine the carriers. In this device, nanoscale InGaN self-assembled QDs have been formed in the well layers of the active region. The self-assembly strain-induced islands offer the formation of zero-dimensional quantum structures without the use of current limitation of lithography, however, the size fluctuation of the QDs leads to inhomogeneous optical and electrical characteristics.
Fig. 5. Schematic structure of the InGaN/GaN MQW LED with an additional n-type In0.18Ga0.82N electron reservoir layer. (After Ref. [22]. Reprinted from Physica E, vol. 21, Y. Takahashi, A. Satake, K. Fujiwara, J.K. Shue, U. Jahn, H. Kostial, H.T. Grahn, ‘Enhanced radiative efficiency in blue (In,Ga)N multiple-quantum-well light-emitting diodes with an electron reservoir layer’, p. 876, 2004, with permission from Elsevier).
electroluminescent structure is attributed to the improvement of electron capture process by radiative recombination centres in the MQW layers. 3.4.5. Quantum dots LEDs QD demonstrates a number of unique properties as compared to bulk materials and QW structures. The electronic states in a QD are spatially localized and the energy is fully quantized, therefore, this QD structure is more stable against any thermal perturbation. Moreover, the electron localization may dramatically reduce the scattering of electrons by bulk defects and reduce the non-radiative recombination rate [23]. InGaN/GaN blue LEDs and InAs/ GaAs infrared LED with multiple quantum dots (MQD) were successfully fabricated [24,25]. Fig. 6 shows the schematic structure of the MQD blue LED. Although, high
Fig. 6. Schematic structure of the blue LED with InGaN/GaN MQD. (after Ref. [24]. Reprinted from J. Crystal Growth, vol. 263, L.-W. Ji, Y.K. Su, S.J. Chang, C.S. Chang, L.W. Wu, W.C. Lai, X.L. Du, H. Chen, ‘ InGaN/GaN multi-quantum dot light-emitting diodes’, p. 114, 2004, with permission from Elsevier.)
3.4.6. LEDs with flip–chip structure Conventional LED structure possesses several shortcomings which reduce the efficiency of the LEDs. The light extraction efficiency is enhanced substantially for LEDs built in flip–chip geometry. Since flip–chip structure offers light extraction from substrate, therefore, no attenuation of light by the semi-transparent metal electrode. Absorption of wave-guided light is minimized significantly by the adoption of highly reflective metallization. Fig. 7 shows the schematic structure of a commercially available GaN-based flip–chip LED. As compared to thin currentspreading layer in conventional LEDs, the use of thick metallization contacts in flip–chip structure allows LEDs operate with higher current densities, and the obscuration of light is reduced substantially with the removal of the wirebonds on the top of the device [26,27]. The structure of this device also offers the heat generated to be dissipated from the semiconductor through the thick metallization contacts. 3.4.7. Non-resonant cavity LEDs The maximization of light extraction is one of the primary goals in the fabrication of LEDs. The external efficiency of conventional LEDs is restricted by total internal reflection of the light generated which occurs at the interface of the semiconductor and air. This is due to
Fig. 7. Schematic structure of a commercially available GaN-based flip– chip LED. (After Ref. [26]. Reprinted from D.A. Steigerwald, J.C. Bhat, D. Collins, R.M. Fletcher, M.O. Holcomb, M.J. Ludowise, P.S. Martin, S.L. Rudaz, ‘Illumination with Solid State Lighting Technology’, IEEE, Journal on Selected Topics in Quantum Electronics 8 (2) (2002) 310, q 2004 IEEE).
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Fig. 8. Schematic structure of the NRC-LED (After Ref. [33]. Reprinted from R. Windisch, P. Heremans, B. Dutta, M. Kuijk, S. Schoberth, P. Kiesel, G.H. Dohler, G. Borghs, ‘High-efficiency non-resonant cavity lightemitting diodes’, IEEE Electronics Letters 34 (11) (1998) 1153, q 2004 IEEE.)
the large difference in the refractive index between the semiconductor and air [28]. A number of methods have been developed to enhance the external efficiency, one of the approaches is to design chip with a geometry that facilitates the escape of the photons onto the surface of the chip, for example, the introduction of the hemispherical LED [29]. Other methods include the use of transparent substrate to replace the absorbing substrate [30,31], however, transparent substrate could be difficult to implement in some of the material/substrate systems, and therefore, another option is to incorporate the distributed Bragg reflector (DBR) in the device structure [31]. An alternative successful method is the adoption of the surface-texture thin-film LED [28,32, 33], which has a textured top surface and a back mirror. The back mirror offers photons multiple chances to escape from the semiconductor medium. The device consists of GaAs active layer sandwiched by AlGaAs layers. A current aperture made of oxidized AlGaAs is introduced in order to reduce the generation of light below the top contact. Fig. 8 shows schematic structure of the GaAs non-resonant cavity (NRC) LED. High external quantum efficiency (68%) of the NRC-LED measured at 90 K was reported [32].
3.4.8. Resonant cavity and photonic crystal LEDs Spontaneous emission in the LED is a phenomenon where the recombination process occurs spontaneously. Spontaneous emission was thought to be uncontrollable; nevertheless, recent research and development of the nanotechnologies in the semiconductor industry suggests that there are possibilities of controlling the spontaneous emission properties by the adoption of microcavity and photonic crystal structures in the optoelectronic devices. The optical mode density could be modified by the interference due to the optical environment. The main advantages of these structures compared to conventional LEDs are higher emission intensities, better spectral purity and more directional emission patterns [34]. The first GaInAs/GaAs resonant cavity LEDs (RC LEDs) emitting at 930 nm was realized in early 1990s by Schubert et al. [35]. However, the overall external efficiency remained low. The high efficient RC LED was made by De Neve et al. [36] with external quantum efficiency, hextw23%. The idea of resonant cavity is being further studied and eventually extended to two and three dimensions, which lead to the concept of photonic crystal LEDs and photonic bandgap enhanced LEDs [5]. Photonic crystal (PC) is an one-, two, or three-dimensional (1D, 2D, 3D) periodic corrugated medium. Its wavelength-scale period and high refractive index contrast can strongly influence the optical mode density [6]. Baba et al. [37] showed that 2D photonic crystal emitter exhibited a more than 10-fold increase in photoluminescence extraction compared to a planar wafer by creating a honeycomb array of InGaAsP/InP micropillars in the device. Most recently, the demonstration of PC on III-nitride PC with the intention to exploit the green, blue and UV emitter has been reported. Under optical pumping, InGaN/GaN quantum well LEDs with PC produced an unprecedented 20-fold increase in the intensity of the emitted light at 475 nm [38,39]. Fig. 9 shows the SEM image and near-field
Fig. 9. SEM and NSOM intensity images collected above the patterned PC region. 20 times enhancement is observed under optical pumping. (After Ref. [39]. Reprinted from III–Vs Review: The advanced Semiconductor Magazine, vol. 17 (1), Alan Mills ‘First time III-nitride photonic crystals’, p.40 (Feb. 2004), with permission from Elsevier.)
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scanning optical microscopy (NSOM) intensity image collected above the patterned PC region.
4. White LEDs The rapid progress of the LED technology, especially with the development of the high efficient blue and UV LEDs from the AlGaInN material system has great impact on the lighting technology. Since then white light LEDs could be realized by the introduction of these high performance blue and UV LEDs [40]. Basically, there are some approaches for white light generation: (a) Phosphor Conversion (PC) LEDs [1] Dichromatic PC LEDs: blue LED with phosphor converters. [2] Polychromatic PC LEDs: UV LED with phosphor converters. (b) RGB LEDs. The mixture of three primary light colors to form white light. Each of these approaches has potential advantages and shortcomings. 4.1. Phosphor conversion LEDs Currently, most of the commercial available white LEDs are dichromatic PC LEDs which are made from blue GaN-based chip and phosphor wavelength converter. The common phosphor converter is Ce-doped yttrium aluminum garnets (YAG), with chemical formula of (Y1KaGda)3 (Al1KbGab)5O12:Ce [41]. White LED usually consists of an AlInGaN blue LED chip and a phosphor-containing epoxy [41]. When current is applied to the LED, the chip starts emitting blue light; part of light is absorbed by phosphor and reemitted to amber light. The amber colored light is the complimentary color of the blue light emitted by the LED, thereby producing white emission. However, the ‘halo effect’ and low level of absorption of blue light by the phosphor are the two major problems [42]. The halo effect or bleed-through effect occurs because the light from the blue LED is directional while the amber light from the phosphor radiates over a 2p solid angle. Therefore, the light appears in multicolor when an observer looks from the side. Polychromatic PC LEDs employ a GaN-based UV chip to pump a combination of red, green and blue phosphors; this method offers high color rendering indices, similar to fluorescent lamps. However, the use of UV pump source will lead to a reduced radiant efficiency due to the Stokes shift characteristic of the photonic down conversion process of the phosphors [26].
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4.2. RGB LEDs The mix of three single primary color (red, green, blue) LEDs is another technique used to generate white light RGB mixing is the most efficient way to produce white light, this approach offers excellent color rendering of white light. By changing the relative intensity of the different color LEDs it is relatively easy to change the hue of this light source for different applications, and no quantum deficit arising from the Stokes shift loss. However, a relatively complicated external detector and feedback system are required to control the intensity of the light. Since each LED will degrade at different rate over a period of time, therefore, certain light intensity ratio emitted by each of these LED must be maintained. 4.3. Other approaches In addition to the above mentioned methods, there are few novel approaches for producing white light LEDs. The poor reproducibility, complexity of fabrication, and low performance of these LEDs are among the major technical problems, which need to be overcome; therefore, they are still in the research and development stage. A brief overview on these methods is presented here. The first approach explores the nanotechnology. It is well known that the fabrication of different sizes of lowdimensional structures (i.e. QDs or QWs) in the device could lead to the variation of the wavelengths. The shift of energy due to the change of the size or thickness is ascribed to the quantum confined Stark effect which is induced by the presence of large piezoelectric field [43,44]. By combining two or more suitable wavelengths emitted from different sizes or thickness of the low-dimensional structures, white light could be generated. The generation of white light by stacking the GaN/AlGaN QDs with different sizes, as well as the incorporation of InGaN/GaN QWs with different thicknesses in the active region had been intensively investigated by Damilano et al. [44–47].
Fig. 10. Schematic structure of the PR-LED. (After Ref. [48]. Reprinted from X. Guo, J. Graff, E.F. Schubert, ‘Photon Recysling Semiconductor Light Emitting Diode’, IEEE IEDM Technical Digest, IEDM-99, (1999) 600, q 2004 IEEE.)
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Photon recycling semiconductor (PRS) LEDs is another potential approach used to generate white light. The principle of this PRS-LED is similar to the phosphor conversion LEDs. Fig. 10 depicts the schematic structure of the PRS-LED. The device consists of InGaN/GaN LED, the primary light source which emits light in blue spectral region. The secondary light source, PRS is made of AlGaInP, and bonded to the InGaN/GaN LED. In this method, PRS layer absorbs the light produced by the primary light source and re-emits with a complementary light color [48].
[10] [11]
[12]
[13]
5. Conclusion [14]
The performance of the high brightness LEDs is determined by the internal and external efficiency, as well as the operating voltage. All these parameters are fundamentally dependent on the device structure and the chip design. An overview of rapid progress in the development of high brightness LEDs has been presented. The semiconductor material systems for high brightness LEDs; the LED structures from simple pn junction, to advanced nano scale low-dimensional structures as well as the recent development of the innovative and novel structures for new generation LEDs are briefly discussed. In this article, conventional and innovative ways of producing white LEDs are also reported. With the advances and breakthroughs of new technologies, the performance and lifetimes of LEDs could be further improved and enhanced. The potential of LEDs in future looks bright.
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Acknowledgements [24]
The authors would like to thank Universiti Sains Malaysia for the technical support.
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Innovative advances in LED technology F.K. Yam*, Z. Hassan School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 27 July 2004; received in revised form 19 October 2004; accepted 9 November 2004
Abstract An overview of the rapid progress in the developments of the inorganic light emitting diode (LED) technology is presented. Innovative structures and designs of the device have led to dramatic improvements of the performance in LED technology, groundbreaking performance records are being reported constantly. This article summaries the recent progress of the high brightness LEDs, and describes the LED structures from the basic pn homojunction, to heterojunction, and eventually the use of nano-scale low-dimensional structures in the device fabrication. Some of the novel structures and designs of the most recent developed high brightness LEDs, as well as the conventional and innovative ways of producing white LEDs are briefly discussed. q 2004 Elsevier Ltd. All rights reserved. Keywords: III–V semiconductors; High brightness LEDs; White LEDs
1. Introduction Light emitting diodes (LEDs) are the ultimate light source in the lighting technology. The LED technology has flourished for the past few decades. High efficiency, reliability, rugged construction, low power consumption, and durability are among the key factors for the rapid development of the solid-state lighting based on highbrightness visible LEDs. Conventional light sources, such as filament light bulbs and fluorescent lamps depend on either incandescence or discharge in gases. These two processes are accompanied by large energy losses, which are attributed to the high temperatures and large Stokes shift characteristics. On the other hand, semiconductors allow an efficient way of light generation. LEDs made of semiconductor materials have the potential of converting electricity to light with near unity efficiency. For decades, semiconductor device researchers have dreamed of obtaining blue LED. The stunning technological breakthroughs by Nakamura in producing GaN-based blue and green LEDs in early 1990s [1,2] have a profound impact on the LED technology. Since the development of the high
* Corresponding author. Tel.: C60 4 6533673; fax: C60 4 6579150. E-mail address: [email protected] (F.K. Yam). 0026-2692/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2004.11.008
brightness blue LEDs, the LED market has grown significantly [3,4]. By combining red, green and blue; the three primary colors, full-color display and white light source could be realized. Currently, LEDs play an important role in many applications including large area displays, automotive and aircraft lighting, traffic signals. LEDs will soon play a much larger role in the future of architectural lighting and general illumination. On the other hand, with the technology advances in the nano-scale dimension fabrication, the applications of the LEDs are no longer limited to the above-mentioned roles. The new generation high brightness planar LEDs, i.e. resonant cavity LEDs, show promise in many advanced applications, such as optical communication, as well as non-communication application, e.g. sensors, printers and scanners [5–7]. The invention of first LED based on GaAsP semiconductor material by Holonyak and Bevacqua in 1962 [8], which emitted red light with low luminous efficient of about 0.1 lm/W. Since then a remarkable progress in the LED performance has been reported. Fig. 1 shows the historical development of the luminous efficiency of visible-spectrum LEDs. It is found that the light output performance of the LEDs has been improving by nearly an order of magnitude for every 10 years [9]. Such a rapid progress in LEDs performance mainly can be attributed to the advancement of the semiconductor growth technique, which leads to higher
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Fig. 1. Historical progress in the development of the luminous efficiency of visible-spectrum LEDs. (After Ref. [9]. Reprinted from Semiconductors and Semimetals, vol. 48, ed. by G.B. Stringfellow and M. G. Craford, series eds. R.K. Willardson and E.R. Weber, ‘Overview of Device Issues in HighBrightness Light-Emitting Diodes,’ High Brightness Light Emitting Diodes, Academic Press, New York, 1997, p. 47, with permission from Elsevier).
purity material and lower defect density, as well as the use of novel LEDs structures resulting in higher quantum efficiencies.
2. Semiconductor material systems for high brightness LEDs Human eyes are sensitive to light with output wavelengths lie in the range 390!l!720 nm which is corresponding to photon energy of 1.7!hn!3.2 eV. Table 1 shows the colors associated with range of the wavelengths and photon energies. From the operational description and spectral sensitivity of human eye, semiconductor materials used to generate visible light should fulfill certain criteria fundamentally. The semiconductors should be direct with a bandgap lie between 1.7 and 3.2 eV, and they can be doped to form a p–n junction. However, from the literature, not many semiconductors are meeting these requirements. Fig. 2 shows the bandgap energies of common elementary and binary semiconductors compared with spectral sensitivity of human eye [10]. Presently, most of the LEDs are made of III–V compound semiconductor materials with a small number fabricated by II–VI and group IV semiconductor materials, Table 1 The colors and the corresponding wavelengths and photon energy Color
Wavelength, l (nm)
Photon energy hn (eV)
Ultraviolet Violet Blue Cyan Green Yellow Orange Red Infrared
!390 390–455 455–490 490–515 515–570 570–600 600–625 625–720 O 720
O3.18 2.72–3.18 2.53–2.72 2.41–2.53 2.18–2.41 2.06–2.18 1.98–2.06 1.72–1.98 !1.72
Fig. 2. Bandgap energies of common elementary and binary semiconductors compared with spectral sensitivity of human eye. Black bars represent indirect bandgap semiconductors. (After Ref. [10]. Reprinted from ‘Introduction to Solid-State Lighting’, A. Zukauskas, M.S. Shur and R. Gaska, Wiley, New York, 2002. This material is used by permission of John Wiley and Sons, Inc.).
i.e. ZnCdSe, ZnTeSe, MgS, MgSe and SiC. LEDs fabricated by these materials normally have low durability or poor efficiency, which prevented them from large-scale production for commercial applications. The new generation high brightness LEDs are mainly produced by three important semiconductor material systems in which, bandgap engineering is being employed to fabricate suitable semiconductor materials with desired emission wavelength. AlGaAs materials system which is able to generate light from red to IR, on the other hand, AlInGaP emits light range from amber to red-orange. AlGaInN materials have a wider bandgap than both of the AlGaAs and AlInGaP material systems, this allows the system to access to the higher energy green, blue and UV regions of the spectrum. With these three material systems, the spectral regions from UV to IR are widely covered. By combining high power LEDs from these material systems, full color displays and white light for lighting and illumination purposes can be realized. Table 2 shows the LEDs fabricated from different material systems and their characteristics.
3. Structures of LEDs 3.1. p–n homojunction The LEDs are fabricated from semiconductor materials. The basic LED consists of a p–n junction. Under forward bias condition, electrons are injected into the p-type region, and holes are injected into the n-type region. Recombination of these minority carriers with the majority carriers at
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Table 2 Characteristics of different types of high-brightness LEDs Color range
Peak emission (nm)
Structure
External quantum efficiency (%) at 20 mA
Luminous efficiency at 20 mA (lm/w)
Infrared Red Redorange Amber Green Green Blue Green
780, 880 650 636
DH DH DH
27 16 24
N.A 8 35
590 570 525 450 517
10 2 6.3 9.1 2.6
40 14 w18 w2 6.5
Blue
450
5.5
5
ZnTeSe
Green
512
DH DH SQW SQW DH: co-doped with Zinc and silicon DH: co-doped with Zinc and silicon SQW
17
ZnCdSe
Blue
489
MQW
5.3 (at 10 mA) 1.3 (at 10 mA)
Material AlGaAs AlGaInP AlGaInN
AlInGaN
1.6
DH, double heterostructure; NA, not applicable; SQW, single quantum well; MQW, multiple quantum well. (After Ref. [9]. Reprinted from Semiconductors and Semimetals, vol. 48, ed. by G.B. Stringfellow and M. G. Craford, series eds. R.K. Willardson and E.R. Weber, ‘Overview of Device Issues in High-Brightness Light-Emitting Diodes,’ High Brightness Light Emitting Diodes, Academic Press, New York, 1997, p. 47, with permission from Elsevier).
the p–n junction leads to light generation. The wavelength and color of the light is determined by the difference in the energy levels of the electrons and holes. This simplest type of LED design is no longer used in current LED technology since there are some drawbacks which could lower the efficiency in the lighting and illumination applications. First, electron injection into the p-type region is desired for achieving high internal quantum efficiency, therefore, a low injection level of holes into the n-region is required. Second, self-absorption of the generated light is high due to the entire structure possesses same composition; this reduces the light extraction efficiency. Third, the radiative recombination in such LEDs is monomolecular, so that only increasing the doping can increase the emission rate [10,11]. 3.2. Heterostructures Maximization of light generation is the primary goal in LED fabrication; this can be done by the application of various epitaxial structures in the LED. Currently, the fabrication of high intensity LEDs are based on heterostructures. Structures consist of dissimilar semiconductor materials which have different bandgaps because of different chemical composition are called heterostructures. The typical energy band profile of single heterostructure
Fig. 3. Potential profile in (a) p–n heterojunction, (b) double-heterojunction LED. (After Ref. [10]. Reprinted from ‘Introduction to Solid-State Lighting’, A. Zukauskas, M.S. Shur and R. Gaska, Wiley, New York, 2002. This material is used by permission of John Wiley and Sons, Inc.).
(SH) is illustrated in Fig. 3(a). The SH is created by the formation of a p–n junction between two different types of semiconductors; the n-type region is composed of semiconductor with bandgap Eg1 which is wider than the p-type region, Eg2. The n-type region with wider bandgap forms a sufficiently large hole potential barrier resulting in singlesided injection. In addition, this wider bandgap also offers transparency to the photons generated in the p-type region (active layer); this minimizes the internal absorption of photon emitting toward the n-type region and improves the light extraction efficiency [10,11]. Double heterostructure (DH) can be employed to further enhance the performance of the LED. Fig. 3(b) shows the typical structure of a double-heterostructure. DH consists of a narrow bandgap of p-type active region sandwiched between wide bandgap of n- and p-types regions. This structure provides effective confinement by means of potential barriers for both directional injections of excess carriers into the active layer, where electrons and holes recombine. This leads to the increase of the excess carrier density, as well as the recombination rate. In addition, the n- and p-epitaxial layers with wider bandgap are transparent to the light emitted by the active region, and therefore internal absorption effect is minimized [10,11]. The efficiency of DH AlGaAs red LEDs was reported to be approximately twice of the SH [11]. 3.3. Low-dimensional structures The radiative recombination in heteroepitaxial layers can be further enhanced by the creation of low-dimensional structures such as quantum wells and quantum dots. A quantum well could be formed as a thin layer of semiconductor with a given energy gap is sandwiched between two slabs of higher bandgap semiconductors. When the thickness of the epitaxial layer is smaller than the de Broglie wavelength of the electrons in the semiconductor, the energy spectrum of the carriers is modified and quantum confinement is formed. This type of heteroepitaxial structure is called quantum well (QW). The QW thickness usually ranges from a fraction of a nanometer to a 10 or 20 nm [12]. The improvement of the efficiency of
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LEDs by the introduction of low-dimensional structures can be attributed to the forced increase in overlap of the wavefunctions for electrons and holes, and the higher electronic density of states near the band gap of the lowdimensional structures as compared to the bulk (3-D) material. This leads to higher recombination rates and a narrowing of the gain spectrum [13]. In addition to improved quantum confinement, these low-dimensional structures are able to reduce the lattice-matching problem because the thickness of the well and barrier are sufficiently thin to accommodate the compressive strain without the formation of misfit dislocations. 3.4. Innovative and novel LED structures 3.4.1. LEDs with tensile and compressive strained With the introduction of low-dimensional structure, the performance of the LEDs with novel designs has been improved significantly. A novel tensile strain barrier cladding (TSBC) structure has been introduced by Chang et al. [14] which can effectively increase the potential barrier and reduce the leakage current for the AlGaInP yellow-green LEDs. The schematic LED structure is shown in Fig. 4, it consists of a GaAs–AlAs distributed Bragg reflector (DBR), an n-Al0.5In0.5P cladding layer, an undoped MQW active layer, a thin K1% tensile strained p-Al0.65In0.35P TSBC layer, a 0.75 mm p-Al0.5In0.5P cladding layer and a 15 mm p-GaP window layer. The adoption of the combined MQW and TSBC structure has led to higher efficient of the device. It is found that the electroluminescence intensity is twice of the conventional MQW AlGaInP LED emitting at the same wavelength. In addition, this novel structure is less temperature sensitive; therefore, it is useful under high temperature operation. AlGaInP/GaInP LEDs with a compressively strained MQW (CS MQW) active layer emitting at 650 nm, was also investigated by Chang et al. [15,16]. By incorporation of
Fig. 4. The schematic cross-sectional view of the AlGaInP MQWCTSBS LED. (After Ref. [14]. Reprinted from ‘AlGaInP Yellow-Green LightEmitting Diodes with a Tensile Strain Barrier Cladding Layer’, S.J. Chang, C.S. Chang, Y.K. Su, P.T. Chang, Y.R. Wu, K.H. Huang, T.P. Chen, IEEE Photonic Tech. Lett, 9, (9) (1997) 1199, q 2004 IEEE.)
a compressive strain into the MQW active layer, it can increase the output power of the LEDs significantly. Moreover, the C0.33% compressive strain can reduce 10–90% rise time and fall time of the device, which is particularly important in the optical communication. 3.4.2. Tunnel junction LED with pC/nC tunnel junction (TJ) structure is formed on top of the p-type Mg–GaN confinement layer. This TJ provides lateral current spreading and eliminates the need for the semitransparent electrode and a highly resistive ptype AlGaN as compared to the conventional LEDs [17]. The adoption of tunnel junction has led to the formation of two n-electrodes in this LED structure. Jeon et al. [18,19] reported that the light output power of TJ LED is two times higher than the conventional LED, however, the forward voltage is higher than the conventional LED, and this is due to the series resistance across the TJ which causes a voltage drop. 3.4.3. Inverted p-down LED Typical commercially available LEDs are designed with the active region grown on top of the n-type cladding layer, and the p-type cladding layer is grown on top of the active region. Such design is called ‘n-down structure’. The reasons of adopting such structure generally can be attributed to a relatively high crystal quality of n-type cladding layer as compared to p-type cladding layer, which leads to better crystal quality of the active region, and also, the conductivity of n-type layer is much larger than p-type layer, therefore, better current spreading can be achieved in the bottom n-layer, resulted a smaller turn on voltage [20]. However, recently a InGaN/GaN MQW LED with p-down structure and a nC/pC tunnel junction for current spreading has been developed [20,21]. A conventional n-down has been fabricated for comparison purpose. It is found that the optical output power of the inverted LED is almost twice of the conventional LED when operated at 20 mA, which corresponds to an external quantum efficiency as high as 17% compared to 9.5% for the conventional LED. However, the operating voltage of the inverted LED is 7.5 V at 20 mA which is much larger than 3.5 V of the conventional LED. This could be due to high tunnel voltage and high series resistance of the inverted LED [21]. 3.4.4. LEDs with electron reservoir layer An enhanced radiative efficiency of MQW LEDs with the introduction of an electron reservoir layer in between the n-GaN cladding layer and the MQW layers has been fabricated by Takahashi [22]. Fig. 5 illustrates the schematic diagram of InGaN/GaN MQW LED with the electron reservoir layer. The electroluminescence spectral and current-voltage characteristics are enhanced substantially with the addition of the n-doped In0.18Ga0.82N electron reservoir layer. The enhancement of the efficiency of this
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brightness InGaN/GaN MQW LEDs have been commercially available, the output intensity could be enhanced by the exploration of QDs instead of MQW, to further confine the carriers. In this device, nanoscale InGaN self-assembled QDs have been formed in the well layers of the active region. The self-assembly strain-induced islands offer the formation of zero-dimensional quantum structures without the use of current limitation of lithography, however, the size fluctuation of the QDs leads to inhomogeneous optical and electrical characteristics.
Fig. 5. Schematic structure of the InGaN/GaN MQW LED with an additional n-type In0.18Ga0.82N electron reservoir layer. (After Ref. [22]. Reprinted from Physica E, vol. 21, Y. Takahashi, A. Satake, K. Fujiwara, J.K. Shue, U. Jahn, H. Kostial, H.T. Grahn, ‘Enhanced radiative efficiency in blue (In,Ga)N multiple-quantum-well light-emitting diodes with an electron reservoir layer’, p. 876, 2004, with permission from Elsevier).
electroluminescent structure is attributed to the improvement of electron capture process by radiative recombination centres in the MQW layers. 3.4.5. Quantum dots LEDs QD demonstrates a number of unique properties as compared to bulk materials and QW structures. The electronic states in a QD are spatially localized and the energy is fully quantized, therefore, this QD structure is more stable against any thermal perturbation. Moreover, the electron localization may dramatically reduce the scattering of electrons by bulk defects and reduce the non-radiative recombination rate [23]. InGaN/GaN blue LEDs and InAs/ GaAs infrared LED with multiple quantum dots (MQD) were successfully fabricated [24,25]. Fig. 6 shows the schematic structure of the MQD blue LED. Although, high
Fig. 6. Schematic structure of the blue LED with InGaN/GaN MQD. (after Ref. [24]. Reprinted from J. Crystal Growth, vol. 263, L.-W. Ji, Y.K. Su, S.J. Chang, C.S. Chang, L.W. Wu, W.C. Lai, X.L. Du, H. Chen, ‘ InGaN/GaN multi-quantum dot light-emitting diodes’, p. 114, 2004, with permission from Elsevier.)
3.4.6. LEDs with flip–chip structure Conventional LED structure possesses several shortcomings which reduce the efficiency of the LEDs. The light extraction efficiency is enhanced substantially for LEDs built in flip–chip geometry. Since flip–chip structure offers light extraction from substrate, therefore, no attenuation of light by the semi-transparent metal electrode. Absorption of wave-guided light is minimized significantly by the adoption of highly reflective metallization. Fig. 7 shows the schematic structure of a commercially available GaN-based flip–chip LED. As compared to thin currentspreading layer in conventional LEDs, the use of thick metallization contacts in flip–chip structure allows LEDs operate with higher current densities, and the obscuration of light is reduced substantially with the removal of the wirebonds on the top of the device [26,27]. The structure of this device also offers the heat generated to be dissipated from the semiconductor through the thick metallization contacts. 3.4.7. Non-resonant cavity LEDs The maximization of light extraction is one of the primary goals in the fabrication of LEDs. The external efficiency of conventional LEDs is restricted by total internal reflection of the light generated which occurs at the interface of the semiconductor and air. This is due to
Fig. 7. Schematic structure of a commercially available GaN-based flip– chip LED. (After Ref. [26]. Reprinted from D.A. Steigerwald, J.C. Bhat, D. Collins, R.M. Fletcher, M.O. Holcomb, M.J. Ludowise, P.S. Martin, S.L. Rudaz, ‘Illumination with Solid State Lighting Technology’, IEEE, Journal on Selected Topics in Quantum Electronics 8 (2) (2002) 310, q 2004 IEEE).
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Fig. 8. Schematic structure of the NRC-LED (After Ref. [33]. Reprinted from R. Windisch, P. Heremans, B. Dutta, M. Kuijk, S. Schoberth, P. Kiesel, G.H. Dohler, G. Borghs, ‘High-efficiency non-resonant cavity lightemitting diodes’, IEEE Electronics Letters 34 (11) (1998) 1153, q 2004 IEEE.)
the large difference in the refractive index between the semiconductor and air [28]. A number of methods have been developed to enhance the external efficiency, one of the approaches is to design chip with a geometry that facilitates the escape of the photons onto the surface of the chip, for example, the introduction of the hemispherical LED [29]. Other methods include the use of transparent substrate to replace the absorbing substrate [30,31], however, transparent substrate could be difficult to implement in some of the material/substrate systems, and therefore, another option is to incorporate the distributed Bragg reflector (DBR) in the device structure [31]. An alternative successful method is the adoption of the surface-texture thin-film LED [28,32, 33], which has a textured top surface and a back mirror. The back mirror offers photons multiple chances to escape from the semiconductor medium. The device consists of GaAs active layer sandwiched by AlGaAs layers. A current aperture made of oxidized AlGaAs is introduced in order to reduce the generation of light below the top contact. Fig. 8 shows schematic structure of the GaAs non-resonant cavity (NRC) LED. High external quantum efficiency (68%) of the NRC-LED measured at 90 K was reported [32].
3.4.8. Resonant cavity and photonic crystal LEDs Spontaneous emission in the LED is a phenomenon where the recombination process occurs spontaneously. Spontaneous emission was thought to be uncontrollable; nevertheless, recent research and development of the nanotechnologies in the semiconductor industry suggests that there are possibilities of controlling the spontaneous emission properties by the adoption of microcavity and photonic crystal structures in the optoelectronic devices. The optical mode density could be modified by the interference due to the optical environment. The main advantages of these structures compared to conventional LEDs are higher emission intensities, better spectral purity and more directional emission patterns [34]. The first GaInAs/GaAs resonant cavity LEDs (RC LEDs) emitting at 930 nm was realized in early 1990s by Schubert et al. [35]. However, the overall external efficiency remained low. The high efficient RC LED was made by De Neve et al. [36] with external quantum efficiency, hextw23%. The idea of resonant cavity is being further studied and eventually extended to two and three dimensions, which lead to the concept of photonic crystal LEDs and photonic bandgap enhanced LEDs [5]. Photonic crystal (PC) is an one-, two, or three-dimensional (1D, 2D, 3D) periodic corrugated medium. Its wavelength-scale period and high refractive index contrast can strongly influence the optical mode density [6]. Baba et al. [37] showed that 2D photonic crystal emitter exhibited a more than 10-fold increase in photoluminescence extraction compared to a planar wafer by creating a honeycomb array of InGaAsP/InP micropillars in the device. Most recently, the demonstration of PC on III-nitride PC with the intention to exploit the green, blue and UV emitter has been reported. Under optical pumping, InGaN/GaN quantum well LEDs with PC produced an unprecedented 20-fold increase in the intensity of the emitted light at 475 nm [38,39]. Fig. 9 shows the SEM image and near-field
Fig. 9. SEM and NSOM intensity images collected above the patterned PC region. 20 times enhancement is observed under optical pumping. (After Ref. [39]. Reprinted from III–Vs Review: The advanced Semiconductor Magazine, vol. 17 (1), Alan Mills ‘First time III-nitride photonic crystals’, p.40 (Feb. 2004), with permission from Elsevier.)
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scanning optical microscopy (NSOM) intensity image collected above the patterned PC region.
4. White LEDs The rapid progress of the LED technology, especially with the development of the high efficient blue and UV LEDs from the AlGaInN material system has great impact on the lighting technology. Since then white light LEDs could be realized by the introduction of these high performance blue and UV LEDs [40]. Basically, there are some approaches for white light generation: (a) Phosphor Conversion (PC) LEDs [1] Dichromatic PC LEDs: blue LED with phosphor converters. [2] Polychromatic PC LEDs: UV LED with phosphor converters. (b) RGB LEDs. The mixture of three primary light colors to form white light. Each of these approaches has potential advantages and shortcomings. 4.1. Phosphor conversion LEDs Currently, most of the commercial available white LEDs are dichromatic PC LEDs which are made from blue GaN-based chip and phosphor wavelength converter. The common phosphor converter is Ce-doped yttrium aluminum garnets (YAG), with chemical formula of (Y1KaGda)3 (Al1KbGab)5O12:Ce [41]. White LED usually consists of an AlInGaN blue LED chip and a phosphor-containing epoxy [41]. When current is applied to the LED, the chip starts emitting blue light; part of light is absorbed by phosphor and reemitted to amber light. The amber colored light is the complimentary color of the blue light emitted by the LED, thereby producing white emission. However, the ‘halo effect’ and low level of absorption of blue light by the phosphor are the two major problems [42]. The halo effect or bleed-through effect occurs because the light from the blue LED is directional while the amber light from the phosphor radiates over a 2p solid angle. Therefore, the light appears in multicolor when an observer looks from the side. Polychromatic PC LEDs employ a GaN-based UV chip to pump a combination of red, green and blue phosphors; this method offers high color rendering indices, similar to fluorescent lamps. However, the use of UV pump source will lead to a reduced radiant efficiency due to the Stokes shift characteristic of the photonic down conversion process of the phosphors [26].
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4.2. RGB LEDs The mix of three single primary color (red, green, blue) LEDs is another technique used to generate white light RGB mixing is the most efficient way to produce white light, this approach offers excellent color rendering of white light. By changing the relative intensity of the different color LEDs it is relatively easy to change the hue of this light source for different applications, and no quantum deficit arising from the Stokes shift loss. However, a relatively complicated external detector and feedback system are required to control the intensity of the light. Since each LED will degrade at different rate over a period of time, therefore, certain light intensity ratio emitted by each of these LED must be maintained. 4.3. Other approaches In addition to the above mentioned methods, there are few novel approaches for producing white light LEDs. The poor reproducibility, complexity of fabrication, and low performance of these LEDs are among the major technical problems, which need to be overcome; therefore, they are still in the research and development stage. A brief overview on these methods is presented here. The first approach explores the nanotechnology. It is well known that the fabrication of different sizes of lowdimensional structures (i.e. QDs or QWs) in the device could lead to the variation of the wavelengths. The shift of energy due to the change of the size or thickness is ascribed to the quantum confined Stark effect which is induced by the presence of large piezoelectric field [43,44]. By combining two or more suitable wavelengths emitted from different sizes or thickness of the low-dimensional structures, white light could be generated. The generation of white light by stacking the GaN/AlGaN QDs with different sizes, as well as the incorporation of InGaN/GaN QWs with different thicknesses in the active region had been intensively investigated by Damilano et al. [44–47].
Fig. 10. Schematic structure of the PR-LED. (After Ref. [48]. Reprinted from X. Guo, J. Graff, E.F. Schubert, ‘Photon Recysling Semiconductor Light Emitting Diode’, IEEE IEDM Technical Digest, IEDM-99, (1999) 600, q 2004 IEEE.)
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Photon recycling semiconductor (PRS) LEDs is another potential approach used to generate white light. The principle of this PRS-LED is similar to the phosphor conversion LEDs. Fig. 10 depicts the schematic structure of the PRS-LED. The device consists of InGaN/GaN LED, the primary light source which emits light in blue spectral region. The secondary light source, PRS is made of AlGaInP, and bonded to the InGaN/GaN LED. In this method, PRS layer absorbs the light produced by the primary light source and re-emits with a complementary light color [48].
[10] [11]
[12]
[13]
5. Conclusion [14]
The performance of the high brightness LEDs is determined by the internal and external efficiency, as well as the operating voltage. All these parameters are fundamentally dependent on the device structure and the chip design. An overview of rapid progress in the development of high brightness LEDs has been presented. The semiconductor material systems for high brightness LEDs; the LED structures from simple pn junction, to advanced nano scale low-dimensional structures as well as the recent development of the innovative and novel structures for new generation LEDs are briefly discussed. In this article, conventional and innovative ways of producing white LEDs are also reported. With the advances and breakthroughs of new technologies, the performance and lifetimes of LEDs could be further improved and enhanced. The potential of LEDs in future looks bright.
[15] [16] [17]
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Acknowledgements [24]
The authors would like to thank Universiti Sains Malaysia for the technical support.
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