Fig

Light Emitting Diodes

E. Fred Schubert(1), Jaehee Cho(2), and Jong Kyu Kim(3)

(1)Future Chips Constellation, Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy NY 12180, USA

Email: efschubert@rpi.edu; Phone: +1-518-253-3762

(2)School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea

Email: jcho@chonbuk.ac.kr; Phone: +82-63-270-3973

(3)Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea;

Email: kimjk@postech.ac.kr; Phone: +82-54-279-2149

Keywords: Light-emitting diodes; III-V semiconductor device; p-n junction; spontaneous recombination; LED emission spectrum; internal quantum efficiency; light-extraction efficiency; light-escape cone; surface roughening; external quantum efficiency; White LEDs; UV LEDs; phosphor-converted LED; color temperature; solid-state lighting; LED packaging

Abstract

Inorganic semiconductor light-emitting diodes (LEDs) have found widespread use in small-area mobile displays, large-area displays, signaling, signage, and general lighting. The entire visible spectrum can be covered by light-emitting semiconductors: AlGaInP and AlGaInN compound semiconductors are capable of emission in the red-to-yellow and violet-to-green wavelength range, respectively. For white light sources based on LEDs, the most common approach is the combination of a blue LED chip with a yellow phosphor. White LEDs are currently used to replace incandescent and fluorescent sources. In the present review, the properties of inorganic LEDs will be presented, including emission spectra, electrical characteristics, and current-flow patterns. Structures LED structures providing high internal quantum efficiency, namely particularly heterostructures and multiple quantum well structures, will be discussed. Advanced tTechniques enhancing the external quantum efficiency will be reviewed, including chip shaping and surface roughening. Different approaches to white LEDs will be presented and figures-of-merit such as the color rendering index and luminous efficacy will be explained. Besides visible LEDs, the technical challenges in newly evolving deep ultraviolet (deep UV) LEDs will be introduced. Finally, the packaging of low-power and high-power LED chips will be discussed.

Introduction

During the past 100 years, light-emitting diodes (LEDs) have undergone a significant evolution. The first LED emitting in the visible wavelength region was based on SiC compound semiconductor. The device had an external quantum efficiency of much less than 1.0 % (the “external quantum efficiency” is defined as the ratio of (i) the number of photons emitted into free space per unit time divided by (ii) the number of electrons injected into the device per unit time). TodayAt the present time, the external efficiencies of red LEDs based on AlGaInP can exceed 50 %; this semiconductor is capable of emitting in the orange, amber, and yellow wavelength range. AlGaInN compounds can emit efficiently in the near ultraviolet (near UV), violet, blue, cyan, and green wavelength range. Thus, all colors of the visible spectrum are covered by semiconductor materials. This has opened up the use of LEDs in areas as diverse as indicator, signage, display, and lighting applications. In particular, as device output powers continuously increase, LED lamps have reached luminous flux levels of, e.g., 800 lm, comparable to those of conventional incandescent and fluorescent lamps. Furthermore, LED lifetimes exceeding 20 000 hours compare favorably with those of incandescent sources (~ 500 hrs) and fluorescent sources (~ 5 000 hrs) thereby contributing to the attractiveness of LEDs.

Inorganic LEDs are generally based on p-n junctions. However, in order to achieve high internal quantum efficiency, free carriers need to be spatially confined. The “internal quantum efficiency” is defined as the ratio of (i) the number of photons emitted by the active region per unit time divided by (ii) the number of electrons injected into the device per unit time. Furthermore to reduce re-absorption effects, the bandgap energy of the confinement layers should be greater than the bandgap energy of the active region. That is, the greater bandgap energy of the confinement layers not only confines carriers to the active region, but also reduces reabsorption effects. These requirements led to the development of heterostructure LEDs that employ different semiconductor materials (i.e. alloy compositions) for the light-emitting active region and the confinement regions. Particularly popular are multiple quantum wells embedded into the(MQW) active regions. The light-extraction efficiency, which measures is the fraction of photons leavingemitted out from the semiconductor chip into the surrounding free space (and these photonsthus become are the useful, i.e. visible or detectable photons), is strongly depends onaffected by the device LED chip’s geometric shape and surface structure. For devices with high internal quantum efficiency, the maximizingation of the light-extraction efficiency is a key formidable challenge.

This review concerns inorganic LEDs and introduces the basic concepts of optical emission. Band diagrams of direct-gap and indirect-gap semiconductors and the spectral shape of spontaneous emission will be discussed along with electrical properties and current flow. Subsequently, strategies to improve light extraction out offrom the LED chip are presented including surface roughening and chip shaping. Due to total internal reflection at the surfaces of an LED chip, the light-extraction efficiency in standard devices is well below 100 %. A particular challenge in achieving efficient emission is the minimizingation of optical absorption processes inside the semiconductor. This can be achieved by, for example, covering absorbing regions, such as absorbing substrates, with highly reflective mirrors. The development of white LEDs, including phosphor-based approaches and multiple-LED approaches, is will be presented, and including their color rendering properties, such as color rendering and luminous efficacy, are discussed. A short section will address the accomplishments and challenges in deep ultraviolet (deep UV) LEDs. Finally, the current state of the art in LED packaging including packages with low thermal resistance, will be discussed.

Recombination in direct-gap and indirect-gap semiconductors

The probability that electrons and holes recombine radiatively is proportional to the product of electron and hole concentration, that is, R np. The recombination rate per unit time per unit volume can be written as

(1)

where B is a proportionality constant, the bimolecular recombination coefficient, with a typical value of 10–10 cm3 s–1 for direct-gap III–V semiconductors. Fundamental types of band structures and associated recombination processes are shown in Figure 1.

Figure 1: Semiconductor band structure for an (a) direct-gap semiconductor, (b) indirect-gap semiconductor, and (c) indirect-gap semiconductors with a deep isoelectronic impurity level. Indirect radiative transitions have a much smaller probability because due to the requirement of a phonon is required in the recombination process.

During the recombination process, the electron momentum (p = (2m*E)1/2) cannot change significantly because momentum must be conserved and the photon momentum (p = h/) is negligibly small. Thus optical transitions must be ‘‘vertical’’ in nature, that is, electrons recombine only with holes that have the same k value as shown in Figure 1. Efficient recombination occurs in direct-gap semiconductors shown in Figure 1a. However, the recombination probability is much lower in indirect-gap semiconductors because a phonon is required to satisfy momentum conservation, as shown in Figure 1b. The “radiative efficiency” is defined as the fraction of electron-hole pairs in the active region that recombine radiatively (note that a small fraction of electron-hole pairs recombine non-radiatively, e.g. by Shockley-Read-Hall recombination). The radiative efficiency of indirect-gap semiconductors can be increased by isoelectronic impurities, for example, N in GaPGaAs1–xPx. Isoelectronic impurities can form an optically active deep level that is localized in real space (small dx) but, according to the uncertainty relation, delocalized in k space (large dk), so that the impurity can satisfy theenables momentum conservation, as indicated in Figure 1c. During nonradiative recombination, the electron energy is converted to vibrational energy of lattice atoms, that is, phonons. There are several physical mechanisms by which nonradiative recombination can occur with the most common ones being nonradiative recombination at point defects (impurities, vacancies, interstitials, anti-site defects, and impurity complexes) and spatially extended defects (screw and edge dislocations, cluster defects). It is quite common for such defects to form one or several energy levels within the forbidden gap. The defects act as efficient recombination centers (Shockley–Read–Hall or SRH recombination centers), particularly if the center’s energy level is close to the middle of the gap.

Figure 2: Theoretical emission spectrum of an LED. The full-width at half-maximum (FWHM) of the emission line is 1.8 kT.

Optical emission spectrum

The spectral line-width of spontaneous emission can be calculated to be 1.8 kT (see, for example, Schubert, 2006). The theoretical emission spectrum is shown in Figure 2. The line-width of 1.8 kT is caused by the thermal energy of carriers and is therefore referred to as the “thermal broadening”. It indicates that the line-width broadens with increasing temperature. It also indicates that the emission line-width becomes narrower at cryogenic temperatures. For example, at room-temperature, the theoretical line-width of a GaAs LED emitting at 870 nm is E = 46 meV or  = 28 nm.

The spectral line-width of LED emission is important in several respects. Firstly, the line-width of an LED emitting in the visible range is relatively narrow compared with the range of the entire visible spectrum. Furthermore, the LED emission line-width is generally equal to or narrower than the spectral width of a single color as perceived by the human eye. For example, red colors range from 625 to 730 nm ( = 105 nm), which is much wider than the typical emission spectrum of a red LED (  25 nm). Therefore, LED emission is perceived (by the human eye) as monochromatic. Secondly, optical fibers are dispersive, which leads to a range of propagation velocities for a light pulse comprising a range of wavelengths. The material dispersion in optical fibers limits the “bit rate distance product” achievable with LEDs. The spontaneous lifetime of carriers in LEDs in direct-gap semiconductors typically is on the order of 1 – 100 ns depending on the active region doping concentration (or carrier concentrations) and the material quality. ThusAccordingly, modulation speeds of at least 10 Mbit/s and up to 1 Gbit/s are attainable with LEDs.

In practice, the emission line-width is usually broader than the theoretical value of 1.8 kT. A spectral width of 1.8 kT is expected for the thermally broadened emission. However, due to other broadening mechanisms, the line-width can exceed the theoretical value. A prominent broadening mechanism among ternary and quaternary III–V alloy semiconductors is alloy broadening, i.e. the statistical fluctuation of the active region’s alloy composition. Alloy broadening is independent of temperature and can broaden the line-width to by a factor of 2 to 5 times of its beyond the theoretical 1.8 kT thermally broadened value. For example, at room temperature, GaInN blue and green LEDs can have a line-width of 3 kT – 10 kT. In addition to alloy broadening, other broadening mechanisms, such as “alloy clustering” and “phase separation” have been proposed, particularly for Ga1–xInxN alloys with indium mole fractions exceeding 25%.

Figure 3 shows typical LED emission spectra at room temperature. The emission spectra include an AlGaInP/GaAs red, GaInN/GaN green, GaInN/GaN blue, GaInN/GaN UV, and an AlGaN/AlGaN deep-UV LEDs. The active region of visible-spectrum LEDs is typically comprised of a ternary or quaternary alloy, for example, Ga1–xInxN or (AlxGa1–x)0.5In0.5P. An idealized alloy-broadened emission spectrum has a gaussian lineshape. It may be noted that deep UV LEDs are not yet as mature as visible-spectrum devices. For example, the emission spectrum of the 290 nm device shown in Figure 3 has a low-energy shoulder that is due to an undesired parasitic emission, most likely a defect-related emission.

Figure 3: Emission spectra, at room temperature, of AlGaInP/GaAs red, GaInN/GaN green, GaInN/GaN blue, GaInN/GaN UV, and AlGaN/AlGaN deep UV LEDs.

Homostructures and heterostructures

In the vicinity of the unbiased p-n junction plane, electrons originating from donors on the n-type side diffuse to the p-type side where they recombine, as shown in the band diagram of Figure 4. A corresponding process occurs with holes. As a result, a region near the p-n junction, the depletion region, is depleted of free carriers. In the absence of free carriers, the types of charges in the depletion region are ionized donor and acceptor charges. The charges in the depletion region produce a potential, the built-in potential, Vbi, given by

(2)

where NA and ND is the acceptor and donor concentration, respectively, and ni is the intrinsic carrier concentration of the semiconductor. The built-in voltage represents a barrier that free carriers must overcome in order to reach the neutral region of opposite conductivity type.

The width of the depletion region is given by

(3)

where = r 0 is the dielectric permittivity of the semiconductor and V is the diode bias voltage.

Figure 4: Free carrier distribution in (a) homostructure under equilibrium conditions, (b) forward-biased homostructure, and (c) forward-biased double heterostructure (DH). Ln/p and n/p are the minority carrier diffusion lengths and lifetimes, respectively. WD is the width of the depletion region. In the forward-biased homostructure, carriers are distributed over the diffusion length (Ln + Lp); in a DH, they are distributed over the thickness of the DH (WDH). The carrier confinement provided by the DH reduces SRH recombination.

An external bias applied to a p-n junction will drop across the depletion region, which is resistive due to the lack of free carriers. A forward bias decreases the p-n junction barrier causing electrons and holes to be injected into the neutral regions with opposite conductivity type. As the current increases, carriers diffuse into the regions of opposite conductivity type and recombine, thereby emitting photons.

The theory of current transport in p-n junctions was first developed by William Shockley in the early 1950s and the equation describing the I–V characteristic is known as the Shockley equation

(4)

where A is the junction area and Dn,p and n,p are the electron and hole diffusion constants and minority carrier lifetimes, respectively.

Under typical forward bias conditions, the diode voltage is V >> kT / e, and thus [exp (eV / kT) –1] exp (e V / kT). For such forward bias conditions, the Shockley equation can be rewritten as

.(5)

The exponent in the equation illustrates that the current strongly increases as the diode voltage approaches a threshold, which is about equal to the built-in voltage, i.e. Vth  Vbi.

In an ideal diode, every electron injected into the active region will generate a photon. Thus conservation of energy requires that the electron energy eV equals the photon energy h, i.e.

e V  =  h .(6)

Beyond turn-on, the diode becomes highly conductive and the diode forward voltage V is about the same as the threshold voltage Vth. The As indicated in Figure 2, the energy of photons emitted from a semiconductor with energy gap Eg is given by

.(7)

Thus equation (6) can be rewritten as

.(8)

For example, a GaAs IR LED emitting at 870 nm has a threshold voltage of about 1.4 V (the bandgap energy of GaAs is Eg = 1.42 eV); similarly, a GaInN blue LED (460 nm) has a threshold voltage of about 2.8 V (the bandgap energy of Ga0.85In0.15N is Eg = 2.7 eV).

Homojunction LEDs have significant drawbacks and hence are no longer used. At the present time, virtually all commercial devices are heterojunction LEDs. Heterojunction active regions have several advantages that will be discussed below.

The effect of a double heterostructure (DH) on the carrier concentration is illustrated in Figure 4. Under forward bias, carriers diffuse across the p-n junction. In the case of a homostructure under forward bias, minority carriers are distributed over the electron and hole diffusion lengths (Ln and Lp) as illustrated in Figure 4b. In III–V semiconductors, diffusion lengths can be 10 µm or longer. The wide distribution of carriers and the correspondingly low carrier concentration (particularly towards the end of the diffusion tail) can be avoided by the employment of double heterostructures. Carriers are confined to the active region of width WDH, as shown in Figure 4c. To attain good carrier confinement, the barrier heights should be much greater than the thermal energy kT.

Confining carriers to a thin active region will result in high carrier concentrations. Note that in the limit of high carrier concentrations, the radiative efficiency (RE), which can be expressed as RE = B n2 / (ASRH n + B n2), approaches 100%. Accordingly, the confinement of carriers to a thin active region is beneficial for attaining a high radiative efficiency.

An important The advantage of the DH design is further illustrated in Figure 5. The DH structure has a much smaller number of defects within the recombination region so that non-radiative SRH recombination via deep levels is much less significant in a DH and in quantum well (QW) active regions. As a result, DH or QW LEDs have a much higher radiative efficiency than homostructure LEDs.

Figure 5: In a double heterostructure (or QW structure), the recombination region, and thus the number of defects contained in this region, is much smaller than in a homostructure. Therefore, non-radiative SRH recombination will play a much smaller role in DH active regions (as well as QW active regions).

The term “double heterostructures” is frequently used for active layers with thickness of 100 to 500 nm. The term “quantum well” active region is used for active layers with thickness of 2 to 100 nm. Single quantum well (SQW) and multi-quantum well (MQW) active regions provide additional carrier confinement, which can further improve the internal quantum efficiency. If a MQW active region is used, the barriers between the wells may impede the flow of carriers between adjacent wells. Thus the barriers in a MQW active region need to be sufficiently “transparent” (i.e. low and / or thin quantum barriers) in order to allow for efficient carrier transport between the adjacent wells (via quantum-mechanical tunneling) and to avoid the an inhomogeneous distribution of carriers within the active region.

Another advantage of DH and QW structures is the optical transparency of the barrier cladding layers (also called confinement layers). That is, the two cladding layers (cladding the active region) have a wider bandgap than the active region. That is, light generated in the active region cannot be absorbed in the cladding layers because their bandgap energy is greater than the photon energy (Eg > h).

Light-extraction in light-emitting diodes

Owing to the high refractive index of semiconductors, light incident on a planar semiconductor–air interface is totally internally reflected, if the angle of incidence is sufficiently large. Snell’s law gives the critical angle of total internal reflection. As a result of total internal reflection, light can be “trapped” inside the semiconductor chip. Light trapped in the chip will eventually be reabsorbed, e.g. by the substrate, active region, cladding layer, or by a metallic contact.

If the light is absorbed by the substrate, the electron–hole pair will most likely recombine non-radiatively due to the inherently low efficiency of substrates. If the light is absorbed by the active region, the electron–hole pair may re-emit a photon or recombine non-radiatively. For active regions with internal quantum efficiencies of less than 100%, any reabsorption event will reduce the efficiency of the LED. For this reason, light should be coupled out of the LED chip with the shorted possible path length inside the chip.

If the angle of incidence of a light ray is close to normal incidence, light can escape from the semiconductor. However, total internal reflection occurs for light rays with oblique and grazing-angle incidence. Total internal reflection reduces the external quantum efficiency significantly, in particularly for LEDs consisting of high-refractive-index materials.

Assume that the angle of incidence in the semiconductor at the semiconductor–air interface is given by (see Figure 6). Then the critical angle of total internal reflection, c, can be inferred from Snell’s law and is given by

.(9)

The refractive indices of semiconductors are usually quite high. For example, GaAs has a refractive index of 3.4. Thus, according to Eqn. (9), the critical angle for total internal reflection is quite small. In this case, we can use the approximation sin c c. The critical angle for total internal reflection is then given by

.(10)

The angle of total internal reflection defines the light escape cone, as shown in Figure 6a. Light emitted into the escape cone can escape from the semiconductor, whereas light emitted outside the cone is subject to total internal reflection.

Figure 6: (a) Illustration of refraction and total internal reflection at a semiconductor–air interface. (b) Definition of the escape cone by the critical angle c. Area of calotte-shaped section of the sphere defined by radius r and angle c.

Calculating the spherical surface area defined by a cone with radius r allows one to determine the total fraction of light that is emitted into the light escape cone. The surface area of the calotte-shaped surface is shown in Figures 6b. The fraction of the light emitted inside a semiconductor can escape from the semiconductor is given by

.(11)

where Psource is the total power emitted by the source and Pescape is the power that can escape. The light-escape problem is significant: In most semiconductors, the refractive index is quite high (  2.5 ) and thus only a small percentage of the light generated in the semiconductor can escape from a planar LED. The problem is less significant in semiconductors with a small refractive index such as polymer LEDs (PLEDs) and small-organic-molecule LEDs (OLEDs); PLEDs and OLEDs have refractive indices on the order of 1.5.

The light-escape problem, illustrated in Figure 7a, has been known since the infancy of LED technology in the 1960s. It has also been known that the geometrical shape of the LED chip plays a critical role. The optimum LED would be spherical in shape with a point-like light-emitting region in the center of the LED. Unfortunately, spherical LEDs with a point-like light source in the center of the LED are impractical. Semiconductor fabrication technology is, in view of the flat substrate wafers used in epitaxial growth, a planar technology. Thus spherical LEDs would be exceedingly difficult to fabricate using conventional processing technologies.

More realistic alternatives include the resonant-cavity LED (see Figure 7b), surface-textured LEDs (see Figure 7c), and chip-shaped LEDs (see Figure 7d).

Figure 7: (a) Light rays emanating from a point-like emitter are trapped inside the semiconductor due to total internal reflection. Only light rays with propagation directions falling within the escape cone can escape from the semiconductor. Strategies increasing the extraction efficiency include the (b) resonant-cavity LED, (c) surface-textured LED, and (d) chip-shaped LED.

A popular method to increase the light-extraction efficiency is the use of roughened or textured semiconductor surfaces, first introduced by Bergh and Saul (1973) for GaP-based devices. The authors showed that at the first instance light impinges at the semiconductor-air interface, the same amount of light escapes from a rough surface as does from a smooth surface. However, since the reflections occur at random angles, a similar proportion of light will escape at the second and subsequent instances the light ray impinges on the roughened surface. This is in contrast to a smooth surface plane which preserves reflection angles and thus traps the reflected light. Figure 8a illustrates the enhanced out-coupling of a light ray through a roughened surface.

GaN textured surfaces created by crystallographic etching (Stocker et al., 1998) are very well suited to enhance light extraction. Several publications reported an increase in light extraction in GaN-based LEDs with surface texturing achieved by crystallographic wet chemical etching (Gao et al., 2004; Fujii et al., 2004; Jung et al., 2010; Horng et al., 2011). The strongly textured GaN surfaces shown in Figure 8b reveal the exposure of a variety of crystal planes that are characteristic for a crystallographic etch (Haerle, 2004; Jung et al., 2010).

Figure 8: (a) LED chip with roughened surface enabling out-coupling of light (after Bergh and Saul, 1973). (b) Scanning electron micrograph (SEM) of N-face GaN surface after crystallographic wet chemical etching using a KOH-solution-based photo-electrochemical process (after Jung et al., 2010).

White light-emitting diodes

High-efficiency red, orange, and yellow devices (AlGaInP LEDs) had been developed in the 1980s. High-efficiency violet, blue, and green devices (GaInN LEDs) had been developed in the 1990s. Therefore, in the mid-1990s monochromatic high-efficiency devices, covering the entire visible spectrum, were available. As a result, the generation of white light by LEDs had been enabled. There are several approaches for white LEDs:

A first approach is a multi-LED-chip approach in which the light from multiple LED chips emitting different colors is mixed to create white light. This approach became feasible with the demonstration of highly efficient blue and green LEDs (Nakamura et al., 1994; 1995). Nakamura et al. (1995) stated: “By combining high-power and high-brightness blue InGaN SQW LED, green InGaN SQW LED and red GaAlAs LED, many kinds of applications, such as LED full-color displays and LED white lamps for use in place of light bulbs or fluorescent lamps, are now possible with characteristics of high reliability, high durability and low energy consumption.”

A second approach is a UV-LED-plus-multi-color-phosphor combination (Baretz and Tischler, 2003). However, this approach suffers from a lower efficiency compared with the multi-LED approach since UV light is converted to visible light whereby energy is lost. We also note that this approach is similar to a conventional fluorescent lamp; in both cases UV light is converted into visible light by means of a phosphor.

A third approach is a blue-LED-plus-yellow-phosphor combination. This approach is most efficient and most successful. Due to the complementary nature of blue and yellow light, white light can be created by the mixture of blue light from an LED chip, and yellow light from a yellow phosphor. Cerium-doped yttrium-aluminum garnet phosphor (Ce-doped YAG), a stable and highly efficient yellow phosphor, was first used for a white LED by Bando et al. (1996, 1998), as further described by Nakamura and Fasol (1997) and Shimizu et al. (1999).

The third approach evolved into a pervasive commercial success, in part due to its efficiency, simplicity, and the requirement of only one LED chip driven by only one power supply.

Indeed, on September 13, 1996, a new era in lighting began: An article in the Japanese newspaper Nikkei (1996) announced a new type of white light LED, based on a blue LED chip and a yellow Ce-doped YAG phosphor, as shown in Figure 9. The white LED was reported to be efficient (5 lm / W), low cost, and have a predicted lifespan of 50 000 hours. What was announced in a relatively short newspaper article was a novel light source that was set to revolutionize the world of lighting.

Figure 9: (a) White LED lamp consisting of a GaInN blue LED chip and a yellow YAG:Ce phosphor. (b) Blue luminescence and wavelength-converted yellow fluorescence (after Nakamura and Fasol, 1997).

In October 1996, full-scale production of the phosphor-converted white LED started at the Nichia Company in Anan, Japan. On November 29, 1996, technical details of Nichia’s white LED were presented at a technical meeting of the Institute of Phosphor Society (Japan) and the associated 264th Proceedings of the Institute of Phosphor Society (Japan). The emission spectrum of the first phosphor-converted white LED is shown in Figure 10 (Bando et al., 1996). The emission spectrum is continuous covering nearly the entire visible wavelength range. Compared with the “spiky” emission spectrum of the then-common compact fluorescent lamps (CFLs), the quality of the white light emitted by the LED was superior. Likewise, compared with the power consumption of the then-common incandescent light bulbs, the prospect of white LEDs, in terms of power consumption, was far superior. Thus, the foundation of a new LED white-light source, that was poised to replace conventional white-light sources, had been established.

Figure 10: Emission spectrum of first phosphor-converted white LED. The LED consists of a GaInN blue LED chip and a YAG:Ce yellow phosphor (after Bando et al., 1996).

Whereas organic phosphors (dyes having carbon ring molecules) had been initially used to generate white light, these organic phosphors simply were not sufficiently stable: When subjected to the high optical radiation density of an LED, the double bonds of the organic phosphor molecules tend to break thereby degrading the phosphor molecule (frequently containing aromatic rings). The first successful and commercially viable white LED was based on a yellow garnet phosphor, Y3Al5O12 doped with the optically active element Ce. Such YAG:Ce phosphors are characterized by (i) and optical absorption band in the blue spectral range, (ii) an emission band that ranges from green to red and has a peak wavelength of about 550 – 580 nm, (iii) high chemical stability, (iv) high stability under high optical irradiance as encountered in proximity of a high-brightness blue LED, (v) short radiative lifetime (about 50 – 80 ns), and (vi) tunability of the optical emission spectrum by chemical modifications of the YAG host material, e.g. the substitution of Y atoms by Gd atoms and of Al atoms by Ga atoms yielding a phosphor with the general formula (Y1–xGdx)3(Al1–xGax)5O12:Ce. These favorable characteristics have made YAG:Ce the phosphor of choice for white LEDs.

At the present time, “phosphor blends” are frequently employed. Such phosphor blends are a mixture of multiple phosphors. Common phosphors include the YAG:Ce family of phosphors, Eu-doped silicate phosphors emitting in the green spectral range and Eu-doped nitride phosphors emitting in the red spectral range. Figure 11 shows typical emission spectra of white LEDs with low, medium, and high correlated color temperatures. As the correlated color temperature of the light source is decreased, (i) the relative phosphor emission increases and (ii) a red phosphor with a peak wavelength of about 600 – 650 nm is added.

Figure 11: Emission spectra of phosphor-converted white LEDs having a low, medium, and high correlated color temperature (CCT). The relative intensities of the LED-chip emission and the phosphor emission allow one to tune the correlated color temperature within the ranges indicated.

The temporal evolution of the luminous efficiency of light sources is shown in Figure 12. The “luminous efficiency of a light source” is defined as the luminous flux emitted by the source (measured in lumen) divided by the electrical input power supplied to the source (measured in W). The figure shows the following light sources: Incandescent lamp with C filament demonstrated in 1879 by Thomas A. Edison; Incandescent lamp with ductile W filament demonstrated in 1911 by William D. Coolidge; Linear fluorescent lamp introduced in 1937 at the New York World’s Fair; Compact fluorescent lamp introduced in 1985 by Osram Company (after Kane and Sell, 2001). These conventional light sources and their typical efficiencies are as follows:

· Incandescent light bulb with C (carbon) filament: 1.2 – 2lm/W

· Incandescent light bulb with W (tungsten) filament: 10 7.5 – 18lm/W

· Incandescent halogen light bulb with W (tungsten) filament: 16 – 24lm/W

· Linear fluorescent lamp (LFL): 65 – 95lm/W

· Compact fluorescent lamp (CFL): 50 – 70lm/W

In 2010, reported efficiencies of white LEDs were in the 150 – 250 lm/W range. Very high efficiencies were reported by several companies including the following: Nichia Company’s Takashi Mukai announced a laboratory-result efficiency of 249 lm/W for a white LED injected with a very low current (Mukai, 2009). The Cree Company announced a laboratory-result efficiency of 208 lm/W for a white LED with a correlated color temperature of about 4600 K at an injection current of 350 mA (Cree, 2010). On March 26, 2014, the Cree Company announced a laboratory-result efficiency of 303 lm/W for a white LED lamp (excluding power supply) with a correlated color temperature of 5150 K at an injection current of 350 mA (Cree, 2014). On March 28, 2014, the Osram Company announced a lamp efficacy of 215 lm/W and a system efficiency (including power supply) of 205 lm/W for a white LED lamp system with a color temperature of 3000 K (Osram, 2014).

Figure 12: Temporal development of the luminous efficiency of different types of lamps. The 2014 points represent: (1) White LED device performance (Cree Company, 2014; 303 lm/W; CCT = 5150 K), and (2) White LED device and system performance (Osram Company, 2014; 215 lm/W (device); 205 lm/W (system); CCT = 3000 K).

Deep-ultraviolet light-emitting diodes

III-Nitrides compound semiconductors such as AlN, GaN, and InN, and their alloys are known to produce light in a wide range of wavelengths from the near infrared, through the visible, down to the ultraviolet spectral range. The Ga1xInxN-based blue-LED-plus-yellow-phosphor combination has evolved into a commercial success for highly efficient, environmentally friendly white light sources for general illumination. AlxGa1xN-based LEDs, covering the wide range of deep-ultraviolet (DUVdeep UV) wavelengths from about 360 nm (GaN) down to 210 nm (AlN), become increasingly attractive due to their promising potential in applications such as purification of air and water, sterilization in food processing, disinfection in hospitals, and medical applications (Khan et al., 2007, Shur et al., 2010). The promise of AlxGa1xN DUV deep UV LEDs goes far beyond just replacing the mercury lamps in existing applications and includes the creation of new market opportunities.

Figure 13: Maximum external quantum efficiencies (EQEs) attained by various research groups as compiled by Kneissl (2015), and acceptor activation energy and corresponding hole concentrations, and reported values of degree of polarization (Northrup et al., 2012)

However, the reported external quantum efficiency (EQE) values of AlxGa1xN DUV deep UV LEDs are low, and decrease almost exponentially rapidly with increasing Al molar fraction x, as shown in Figure 13. The reasons for the decrease are related to intrinsic material properties of AlGaN: Firstly, increasing the Al composition generally leads to a high density of extended defects such as dislocations, grain boundaries, and cracks in the AlxGa1xN epilayer, which results in poor internal quantum efficiency. Secondly, doping, especially p-type doping, of AlxGa1xN films becomes difficult because the ionization energy of acceptor dopants (Mg) increases with Al composition (~170 meV for GaN, ~630 meV for AlN), which causes multiple additional problems including (i) poor ohmic contact properties for both n- and p-type AlGaN thereby causing a high operating voltage, (ii) poor hole-injection efficiency into the active region, and (iii) high resistivity of the AlGaN layer causing non-uniform current injection into the active region, also referred to as “current crowding” (Guo and Schubert, 2001). The “hole-injection efficiency” is defined as the fraction of the LED’s hole current that is injected into the active region. Although a p-type GaN layer is typically used as a contact and a hole-supplying layer to overcome (i) and (ii), it causes poor light-extraction efficiency due to a strong absorption of UV photons by the p-type GaN. Thirdly, the light-extraction efficiency in AlGaN DUV deep UV LEDs grown on c-plane sapphire substrates is severely limited by strong TM-polarized anisotropic emission due to the unique valence band structure of AlxGa1xN for x  0.25 in which the topmost valence band is the crystal field split-off hole band having a PZ-orbital-like property symmetry (Taniyasu and Kasu, 2010). Conventional light-extracting-enhancing techniques, such as surface texturing and substrate pattering, favor extracting TE-polarized light, and thus have a negligible effect on AlGaN DUV deep UV LEDs. Accordingly, this calls for a totally new approach to deliver a real breakthrough in light extraction.

Despite the challenges originating from intrinsic material properties of AlGaN, impressive research efforts in the development of AlGaN DUV deep UV LEDs have advanced the EQE external quantum efficiency from less than 0.1% to about 1~10% (depending on the emission wavelength) over the past 10 years. Various growth techniques including pulsed atomic-layer epitaxy and high-temperature growth, and stress-managing techniques such as the use of AlN/AlGaN superlattices (SLs) resulted in a strongly reduced dislocations density densities and crack formation, and thus enhanced internal quantum efficiency.

Another critical area that is in need of a major improvement is the p-type doping of AlGaN. One alternative way to achieve a highly conductive p-type AlGaN is to grow SLs superlattices that have been shown to enhance the free hole concentration in AlGaN by a factor of 10 (Goepfert et al., 2000). The band modulation of SLs superlattices along with the modulation caused by the polarization fields enables a higher acceptor ionization ratio and thus a higher hole concentration. Another proposed alternative to overcome the p-type doping problem is the injection of holes by means of a polarization-charge-enhanced tunnel junction (Schubert, 2010). According to this concept, polarization-charge-enhanced tunnel junctions can improve the hole injection efficiency by enabling a p-side-down structure and even reduce optical absorption in the p-type GaN contact layers. Finally, a new light-extraction-enhancing approach has been proposed to utilize the inherently strong TM polarized light emitted from the AlGaN active region (Kim et al., 2015). The side-emission-enhanced DUV deep UV LEDs show better electrical properties as well as much enhanced light output power with a strongly upward-directed emission due to an exposed sidewall of the active region and Al-coated selective-area-grown n-type GaN micro-reflectors (Kim et al., 2015).

Oto et al. (2010) proposed an alternative way to realize a highly efficient DUV deep UV emitter based on electron-hole generation by means of electron-beam excitation. In contrast to p–n junction devices (like an LED), electron-beam excitation uses highly accelerated electrons to generate electron–hole pairs inside the target material. At an acceleration voltage of 8 kV, a maximum power efficiency of 40% was reported for 240 nm emission (the “power efficiency” is the optical output power divided by the electrical input power); this is remarkable given the much lower efficiencies attained by conventional p-n junction DUV deep UV LEDs emitting at similar wavelengths.

LED Packaging

Virtually all LEDs are mounted in a package that provides two electrical leads, a transparent optical window for the light to escape, and, in power packages, a thermal path for heat dissipation. The chip-encapsulating material advantageously possesses high optical transparency, a high refractive index, chemical inertness, high-temperature stability, and hermeticity. The refractive index contrast between the semiconductor and air is reduced by including an encapsulant thereby increasing the light-extraction efficiency. Virtually all encapsulants are polymers with a typical refractive index of about 1.5 to 1.8. The reduced index contrast at the semiconductor-to-encapsulant interface increases the critical angle for total internal reflection thereby enlarging the light escape cone and the light-extraction efficiency.

A low-power package is shown in Figure 14a. The LED chip is die-bonded (e.g. attached by means of an adhesive) to the bottom of a cup-like depression (“reflector cup”) located in one of the lead terminals (usually the cathode terminal). A bond wire connects the LED top contact to the other lead terminal (usually the anode terminal). The LED package shown in the figure is frequently referred to as a “radial”, a “5 mm” or “T1-3/4” package.

Figure 14: (a) Traditional LED package having radial miniature lamp configuration. (b) High-power surface-mount LED package having a dedicated heat path by means of a heat-sink slug.

In low-power LEDs, the encapsulant has the shape of a hemisphere, as shown in Figure 14a, so that the angle of incidence at the encapsulant–air interface is always normal. As a result, total internal reflection does not occur at the encapsulant–air interface. There are types of LEDs that do not have a hemispherical shape for the encapsulant. Some LEDs have a rectangular or cylindrical shape with a planar front surface.

A high-power package is shown in Figure 14b. Power packages have a direct, thermally conductive path from the LED chip, through a heat-sink, to, e.g., a printed circuit board with a metal core. The power package shown in the figure has several advanced features. Firstly, the package contains an Al or Cu heat-sink slug with low thermal resistance to which the LED submount is attached by means of a metal-based adhesive material. Secondly, the chip is encapsulated with silicone. Because standard silicone retains mechanical softness when cured, the silicone encapsulant is covered with a plastic cover that also serves as lens. Thirdly, the chip is directly mounted on a Si submount that includes electrostatic discharge (ESD) protection.

Generally, packaging processes are associated with high costs. During the front-end epitaxial-wafer processing stage, many LED chips are processed at the same time (there are thousands of LED chips per epitaxial wafer). However, during the back-end packaging process, packages may be processed one-at-a-time. Accordingly, the packaging process is a serial process and thus expensive. To reduce costs, the LED packaging process is “parallelized”, that is, arrays of packages are processed, e.g. in a 10 20 15 array. As one of the final steps in the process, the array of LED packages is “singulated”, i.e. separated into single devices.

A singulated LED package is shown in Figure 15 (Luxeon Rebel blue and white LED of the Philips Lumileds Company; Philips, 2014). Inspection of the figure shows that the chip is mounted on a ceramic substrate. The ceramic substrate was originally an element of an array of devices that was singulated, e.g. by sawing the ceramic board on which the array of devices is located. Ceramics have a high thermal conductivity that is close to that of metals. For example, AlN and Al have a thermal conductivity of 170 – 190 and 205 – 250 W m–1 K–1, respectively.

Figure 15: High-power LED package having a ceramic substrate for improved heat sinking and a dedicated thermal pad that is electrically insulated from the anode and cathode contact (adapted from Luxeon Rebel blue and white LED of the Philips Lumileds Company; Philips, 2014).

References

Bergh A. A. and Saul R. H. (issued on1973) US Pat. 3,739,217 “Surface roughening of electroluminescent diodes”

Bando K., Noguchi Y., Sakano K., and Shimizu Y. (1996) “Development and application of high-brightness white LEDs” (in Japanese) Technical Digest, Phosphor Research Society, 264th Meeting, November 29

Bando K., Sakano K., Noguchi Y., and Shimizu Y. (1998) “Development of high-bright and pure-white LED lamps” Journal of Light and Visual Environments 22, 2

Baretz B. and Tischler M. A. (2003) US Pat. 6,600,175 B1 having a priority date of March 26, 1996 “Solid-state white light emitter and display using the same”

Cree Company (press release) “Cree breaks 200 lumen per watt efficacy barrier” (February 3, 2010)

Cree Company (press release) “Cree first to break 300 lumens-per-watt barrier” Internet page: < http://www.cree.com/News-and-Events/Cree-News/Press-Releases/2014/March/300LPW-LED-barrier > (March 26, 2014)

Fujii T., Gao Y., Sharma R., Hu E. L., DenBaars S. P., and Nakamura S. (2004) “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening” Applied Physics Letters 84, 855

Gao Y., Fujii T., Sharma R., Fujito K., DenBaars S. P., Nakamura S., and Hu E. L. (2004) “Roughening hexagonal surface morphology on laser lift-off (LLO) N-face GaN with simple photo-enhanced chemical wet etching” Japanese Journal of Applied Physics 43, L 637

Goepfert, I. D., Schubert, E. F., Osinsky, A., Norris, P. E., and Faleev, N. N. (2000) “Experimental and theoretical study of acceptor activation and transport properties in p-type AlxGa1xN/GaN superlattices” Journal of Applied Physics 88, 2030

Guo X. and Schubert E. F. (2001) “Current crowding and optical saturation effects in GaInN/GaN light-emitting diodes grown on insulating substrates” Applied Physics Letters 78, 3337

Haerle V. “Naturally textured GaN surface” China Hi-Tech Fair (CHTF) Shenzhen, China, October 12–17 (2004)

Horng R.-H., Lu Y.-A., and Wuu D.-S. (2011) “Light extraction investigation for thin-film GaN light-emitting diodes with imbedded electrodes” IEEE Photonics Technology Letters 23, 54

Jung Y., Baik K. H., Ren F., Pearton S. J., and Kima J. (2010) “Effects of photo-electrochemical etching of N-polar and Ga-polar gallium nitride on sapphire substrates” Journal of the Electrochemical Society 157, H676

Kane R. and Sell H. (eds.) (2001) “Revolution in lamps: A chronicle of 50 years of progress” (2nd edition) Fairmont Press, Lilburn, Georgia

Khan A., Balakrishnan K., and Katona T. (2007) “Ultraviolet light-emitting diodes based on group three nitrides” Nature Photonics 2, 77

Kim D. Y., Park J. H., Lee J. W., Hwang S. Y., Oh S. J., Kim J. S., Sone C., Schubert E. F., and Kim J. K. (2015) “Overcoming the fundamental limitation in light-extraction efficiency of Deep-UV LEDs by utilizing transverse-magnetic-dominant emission” Accepted for publication in Light: Science & Applications 4, e263; doi:10.1038/lsa.2015.36

Kneissl M. (press release) “UV LED Efficiency 2015” Internet page: < http://www. Researchgate.net/publication/271302876_UV_LED_Efficiency_2015 > (January 25, 2015)

Mukai T. presentation at SPIE Photonics West (January 29, 2009); see also Compound Semiconductor “Nichia’s 249 lm/W LEDs mark tech shift” page 5 (March 2009)

Nakamura S., Mukai T., and Senoh M. (1994) “Candela‐class high‐brightness InGaN/AlGaN double‐heterostructure blue‐light‐emitting diodes” Applied Physics Letters 64, 1687

Nakamura S., Senor M., Iwasa N., Nagarama S.-I., Yamada T., and Mukai T. (1995) “Super-bright green InGaN single-quantum-well-structure light-emitting diodes” Japanese Journal of Applied Physics 34, 1332

Nakamura S., Fasol G. (1997) “The blue laser diode: GaN based light emitters and lasers” Springer-Verlag, Berlin, Germany

Nikkei, Japanese newspaper ” (September 13, 1996) “White LED lamp: Light emission with high luminous efficiency halving production costs

Northrup J. E., Chua C. L., Yang Z., Wunderer T., Kneissl M., Johnson N. M., and Kolbe T. (2012) “Effect of strain and barrier composition on the polarization of light emission from AlGaN/AlN quantum wells” Applied Physics Letters 100, 021101

Osram Company (press release) “Osram constructs the world's most efficient LED lamp [with 205 lm/W system efficiency]” Internet page: http://www.osram.com/osram_com/press/press-releases/ _trade_press/2014/osram-constructs-the-worlds-most-efficient-led-lamp/index.jsp (March 28, 2014)

Oto T., Banal R.G., Kataoka K, Funato M., and Kawakami Y. (2010) “100mW deep-ultraviolet emission from aluminum-nitride-based quantum wells pumped by an electron beam” Nature Photonics 4, 767-770

Philips, Luxeon Rebel blue and white LED of the Philips Lumileds Company (2014)

Schubert E. F. (2006)“Light-Emitting Diodes” 2nd edition, Cambridge University Press, Cambridge UK

Schubert M. F. (2010) “Polarization-charge tunnel junctions for ultraviolet light-emitters without p-type contact” Applied Physics Letters 96, 031102

Shimizu Y., Sakano K., Noguchi Y., and Moriguchi T. (1999) US Pat. 5,998,925 “Light-emitting device having a nitride compound semiconductor and a phosphor containing a garnet fluorescent material”

Shur M. S. and Gaska R. (2010) “Deep-Ultraviolet Light-Emitting Diodes” IEEE Transactions on Electron Devices 57, 12

Stocker D. A., Schubert E. F., and Redwing J. M. (1998) “Crystallographic wet chemical etching of GaN” Applied Physics Letters 73, 2654

Taniyasu Y. and Kasu M. (2010) “Improved emission efficiency of 210nm Deep-ultraviolet aluminum nitride light-emitting diode” NTT Technical Review 8, 1

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