Literature Review: High-power and high-efficiency InGaN-based light ...

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 1, JANUARY 2010

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High-Power and High-Efficiency InGaN-Based Light Emitters Ansgar Laubsch, Matthias Sabathil, Johannes Baur, Matthias Peter, and Berthold Hahn

(Invited Paper)

Abstract—In this paper, we report on the latest advancements in improving AlGaInN-based visible-light-emitting-diode (LED) efficiency in epitaxy, chip, and package designs. We investigate the fundamental origin of the typical high current “droop” of efficiency observed in such LEDs. We show that this effect is most likely not caused by incomplete carrier injection or carrier escape but that it is rather a fundamental material property of InGaN/GaN-heterostructure-based light emitters. The droop can be reduced in improved epitaxial LED active-layer designs. We show how this can be achieved by lowering InGaN volume carrier density in multiple quantum wells (MQWs) and thick InGaN layers. Improved epitaxial MQW structures are then combined with a new advanced chip concept. It is optimized for high efficiency at high current operation and arbitrary scalability and can be manufactured at low cost. This is accomplished by improving lightextraction efficiency, homogenizing the emission pattern, reducing forward voltage, and lowering thermal resistance. The improved high current efficiency can be fully exploited by mounting the chip in the highly versatile new OSLON SSL package. It features very stable package materials, a small footprint, and an electrically isolated design decoupling electrical and thermal contacts. Index Terms—Droop, GaN, InGaN, internal quantum efficiency (IQE), LED-chip technology, LED degradation, LED package technology, light-emitting diode (LED), nitride.

I. I NTRODUCTION HE LAST decade has seen tremendous progress in the epitaxial growth and in advanced chip designs of InGaNbased light-emitting diodes (LEDs). The light-emitting region of these devices consists of InGaN/GaN quantum-well (QW) heterostructures. Recently, a record wall-plug efficiency (WPE) (for conversion of electrical to optical power) of about 60% for a blue ThinGaN LED at operation current density was achieved [1]. Using OSRAM Opto Semiconductors GmbH’s ThinGaN technology, the chip-level light-extraction efficiency of such LED devices could be increased beyond 80%. This chip concept also provides excellent chip-size scalability and is the basis for high-performance chips ranging from

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Manuscript received June 2, 2009; revised October 19, 2009. Current version published December 23, 2009. This work was supported by the German Federal Ministry of Education and Research (BMBF) under Contract 13N9400. The review of this paper was arranged by Editor G. Meneghesso. The authors are with the OSRAM Opto Semiconductors GmbH, 93055 Regensburg, Germany (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2009.2035538

100 mW up to several watts per chip [1]–[3]. Later on in this paper, we will see which even further improvements can be made in chip design. A second field for improvements is the internal quantum efficiency (IQE) for light generation by electron-hole carrier recombination. In InGaN-based light emitters, it remains at values considerably below 100% at operation current density and decreases with increasing current density as well as with increasing emission wavelength. On the one hand, the reduced IQE for longer wavelength InGaN-based LEDs is often called “green gap,” and its origin is still debated [16]. On the other hand, when the IQE is plotted over injection current density, it usually peaks regardless of emission wavelength at current densities much below the operating current and then decreases monotonously toward higher currents. This phenomenon is frequently called “droop.” Understanding and reducing this mechanism is crucial to reach the ultimate limits in InGaNbased LED epitaxy efficiency. Various causes have been discussed in the literature as the origin of this effect. Among these are carrier escape [4]–[6], losses due to dislocations [7]– [10], piezoelectric fields [11]–[13], as well as the Auger effect [14]–[16]. We introduce the experimental details of our samples and measurements in Section II of this paper. The succeeding Section III is devoted to elucidating the physical origin of the high current droop. We give evidence that it is caused by an Auger-like QW internal high-density loss process. This conclusion is derived from the observation that temperatureand excitation-power-density-dependent resonant photoluminescence (PL) and electroluminescence (EL) measurements show the same behavior. We can reproduce the dependence of internal efficiency on current density in our measurements using a simple rate equation model. It comprises an Augerlike loss term that becomes dominant at high volume carrier densities and thus is responsible for the droop. Consequently, we investigate structures with reduced volume carrier density. Their active layer comprises thicker QWs (Section III-A) and multiple QWs (MQWs) (Section III-B). Afterward, Section IV shifts focus to the efficiency of the integral LED device. We review the different contributions to the overall efficiency. We derive figures for a device with optimum efficiency that may be the ultimate achievable limit. One step toward this is the advanced chip design that we present in Section V. By changing the design concept, we could improve

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light-extraction efficiency, reduce forward voltage, and lower thermal resistance. This fits perfectly with the new OSLON SSL package described in Section VI. Finally, Section VII demonstrates very long lifetimes for our InGaN-based LED chips. II. E XPERIMENTAL P ROCEDURE The GaN-based LED structures analyzed in this paper were grown on c-plane sapphire using metal–organic vapor phase epitaxy. The structures consist of a 5-µm GaN:Si buffer layer, an active region containing one or more InGaN/GaN doubleheterostructure QWs, a 30-nm p-type AlGaN:Mg electron blocking layer, and a 120-nm GaN:Mg contact layer. The epitaxial wafers have been processed using OSRAM Opto Semiconductors GmbH’ ThinGaN technology or the advanced LED process described in Section V. LEDs with a standard chip size of 290 × 290 µm2 as well as square-millimeter power chips have been fabricated. EL was measured in dc operation for small current densities up to a few amperes per square centimeter and with 20-ms or 400-ns pulses (0.8% duty cycle) above to avoid heating and thermal rollover of the device [17]. Spectra were taken using a standard compact array spectrometer. PL measurements are performed using a 405-nm semiconductor diode laser with a CW output power of 50 mW for excitation. A confocal optical setup is used for excitation and detection. The laser is focused to a minimum spot size of ∼10 µm to achieve high excitation power densities. For temperature-dependent EL and PL measurements, the LED samples were mounted on the cold finger of a helium-cooled continuous flow optical cryostat. The chips were contacted electrically via HF vacuum feedthroughs. This enables measurements with short pulses also at low temperatures. Additionally, high temperature measurements were performed in a setup where the sample is placed in an air stream of variable temperature, thus producing device temperatures between 230 K and 450 K. III. O RIGIN OF InGaN LED D ROOP In this section, we first concentrate on measurements performed on a green emitting single QW (SQW) LED structure. Choosing an SQW LED guarantees the comparability of EL and PL measurements. The PL excitation of an MQW structure below the GaN band gap would generate electron-hole pairs in all QWs. For EL, however, we have shown elsewhere [18] that the carrier distribution in an MQW is, in general, inhomogeneous and temperature as well as current dependent. This is in contrast to the results of David et al. [19], who report that the dominant recombination center is the QW closest to the p-side. The usage of an SQW structure avoids this conflict. Additionally, in a green emitting SQW structure, the band-gap discontinuity between InGaN QW and GaN barriers is more than 1 eV. This is favorable due to better carrier confinement, and thermionic escape is rendered unlikely. Using a comparison between EL and PL, we validate that the high current losses are QW internal. We compare the temperature- and excitation-power-density-dependent resonant Authorized licensd use limted to: IE Xplore. Downlade on May 13,20 at 1:486 UTC from IE Xplore. Restricon aply.

Fig. 1. (Open symbols) Internal efficiency of a green emitting SQW LED measured by EL is compared to (full symbols) the efficiency measured in a resonant PL experiment. Both experiments were performed at (black) 300 K and (red) 4 K. Carrier generation and recombination in EL and PL are shown schematically in the conduction- and valence-band diagrams in the insets.

PL and EL measurements of the green emitting SQW structure. We measure PL and EL on the same ThinGaN chip. For PL excitation, the 405-nm laser optically excites electron-hole pairs well below the band-gap energy of the GaN barrier [20]. This procedure thus allows to reliably isolate QW internal losses from parasitic carrier recombination outside the QW. Fig. 1 shows a comparison of EL/PL efficiency as a function of current/excitation-power density for temperatures of 300 K and 4 K. All significant trends of the EL efficiency are perfectly followed by the resonant PL efficiency: Both efficiencies increase at small current/excitation power densities (below several A · cm−2 /102 W · mm−2 ) and decrease again at higher densities. The position of the maximum is similar for both curves. Additionally, the decrease of the internal efficiency with rising temperature, resulting in a shift of the entire curve toward lower efficiency, is quantitatively reproduced by the PL measurement. Our experimental data thus strongly support the theory of a QW internal loss process. We can reproduce the dependence of internal efficiency on current density J using a simple rate equation model of the form J ∼ A · n + B · n2 + C · n3 .

(1)

In this equation, n is the QW carrier density and A, B, and C are parameters for nonradiative, radiative, and (nonradiative) Auger-like recombination, respectively. Fig. 2 schematically shows that this model can describe the emission characteristics of a green emitting InGaN MQW-based LED over a wide current-density range. We thus identify a high-density QW internal loss process with Auger-like characteristics as the culprit for the high current losses in InGaN-based light emitters [21]. The EL data shown in Fig. 1 for 300 K can be reproduced using a parameter set of A = 4.7 · 106 s−1 , B = 1.2 · 10−12 cm3 · s−1 , and C = 3.5 · 10−31 cm6 · s−1 . The same measurement at 4 K yields A = 2.5 · 106 s−1 , B = 2 · 10−12 cm3 · s−1 , and C = 3.5 · 10−31 cm6 · s−1 . Since the C-parameter does not change with temperature, the high current decrease in efficiency


LAUBSCH et al.: HIGH-POWER AND HIGH-EFFICIENCY InGaN-BASED LIGHT EMITTERS

Fig. 2. Double logarithmic plot of emission power over current density for a green emitting MQW LED structure. Different slopes in the curve can be attributed to different terms dominating the recombination rate equation (cf. solid blue, green, and orange lines). The corresponding recombination processes are shown in the band-diagram inset in the same colors.

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of Auger recombination in rate equation (1), at high current densities, could thus be reduced, and the fraction of radiative recombination ∼ Bn2 on the total recombination could be enhanced. One solution to reduce the QW volume carrier density is the use of thick InGaN QWs [28]–[30]. However, due to the rapidly decreasing material quality of thick Indium-rich InGaN films, no improvement over state-of-the-art MQW LEDs could be shown yet. We investigate the effect of a QW-thickness increase in the following section. Alternatively, the active-layer carrier density can be reduced by enabling MQW operation. The applicability of this concept has been shown for devices in the whole spectral range, from UV to green, being accessible to InGaN-based light emitters [18]. InGaN/GaN MQW operation will be investigated in Section III-B. A. Thick InGaN QWs

is unlikely to be influenced by thermally activated losses like carrier escape. Additionally, we can derive from these fits that the maximum of the IQE, occurring at a current density of a few amperes per square centimeter at 300 K, corresponds to a carrier density of approximately 5 · 1018 cm−3 . As said before, the C coefficient was determined to be C = 3.5 · 10−31 cm−6 · s−1 . However, the direct band-to-band Auger effect as physical origin of the C · n3 term is rendered unlikely according to theoretical many-particle calculations [15]. Due to the comparably large band gap of the InGaN QWs studied in this paper, this is consistent with expectations from the classical Auger theory [22]. It predicts Auger recombination in wide band-gap materials to be less likely, because the need for conservation of carrier momentum in the course of the recombination process renders these transitions increasingly difficult with increasing band gap. However, it has been shown [23] that the total Auger recombination rate in wide-band-gap materials does not necessarily need to decrease as fast as expected for band-to-band Auger with increasing gap. For example, if momentum conservation can be relaxed by the coupling of electronic transitions to the emission of phonons, Auger rates can increase drastically [24]. Since it is known that electron–phonon coupling is strong in InGaN [25], [26], a phonon-assisted Auger effect may be the origin of the IQE droop. Theoretical investigations of phononassisted Auger recombination in InGaN/GaN heterostructures are currently under way. Additionally, spatial carrier localization in InGaN [27] may further contribute to a relaxed momentum conservation as well as increase the electron–phonon coupling [26]. Furthermore, the possibility for further enhancement of Auger rates due to defect-assisted Auger recombination [24] in InGaN QWs also needs to be investigated in the future. Finally, resonances due to interband Auger recombination may even enable direct Auger recombination in blue-green InGaNbased light emitters [16]. According to our measurements, the droop of high current efficiency is related to a C · n3 term which dominates the total recombination rate at high current density and thus at high carrier volume density. Accordingly, a decrease in InGaN active-layer carrier density is central to improve the high current efficiency of InGaN-based light emitters. The dominance Authorized licensd use limted to: IE Xplore. Downlade on May 13,20 at 1:486 UTC from IE Xplore. Restricon aply.

The LED structures analyzed in this section have been designed to emit at a wavelength of a few nanometers within 400 nm. To analyze the physics of emission in QW double heterostructures of different thicknesses, a series of samples with 2-, 5-, and 20-nm InGaN SQWs on c-plane sapphire has been grown as described previously. The thickness of the InGaN QWs was confirmed by X-ray diffraction and transmission electron microscopy (TEM) measurements. The threading dislocation density in the samples was determined to be a few 108 cm−2 . For comparison, an eightfold MQW structure with integral InGaN active-layer thickness comparable to the 20-nmthick SQW has been grown and optimized for optimal MQW operation. It has been shown separately that, actually, MQWs are emitting in this LED (compare Section III-B and [18] and [31] for the corresponding experimental procedure). To assess the benefit of thick QWs and to compare their performance to MQW structures, the optical emission power at high current densities has been measured in pulsed operation, as described in Section II, and is shown in Fig. 3 (upper plot). The structure with a 2-nm SQW exhibits stronger saturation than the MQW and the 20-nm-wide SQW, which both exhibit improved high current saturation. This is reflected in the external quantum efficiency (EQE), which is shown in the lower plot in Fig. 3. In the group of the three SQW structures, the 2-nm-thin QW shows the highest efficiency at small current densities. The EQE of the thicker QWs exhibits crossover points at 3 and 40 A · cm−2 , respectively. At these current densities, the 5-nm QW surpasses the 3-nm QW in efficiency, and respectively, the 20-nm QW surpasses the 5-nm QW. In both cases, this can be attributed to a reduced QW carrier density in thick QWs. The reduced small current efficiency of the thicker QWs can be attributed to two mechanisms. On the one hand, nonradiative recombination is increased because of the reduced material quality of thick InGaN layers [32]. TEM images of the samples reveal alloy fluctuations in the QWs that may be responsible for point defects acting as nonradiative recombination centers. On the other hand, the remaining carriers suffer from inefficient recombination from the QW ground states, which exhibit less electron-hole overlap with increasing well thickness [29]. Both the 20-nm-thick InGaN well and the optimized MQW reference sample exhibit less saturation of optical emission

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Fig. 4. (Left axis) Optical power and (right axis) peak wavelength of a green SQW and a sevenfold MQW LED (chip size of 290 µm × 290 µm, mounted in a TO18 package). Measurements were performed under pulse operation (0.4-µs pulses, 0.8% duty cycle). The current density was varied between 0.5 and 1000 A · cm−2 .

Fig. 3. (Above) Current-density-dependent optical emission power and (below) EQE of different LED structures emitting around 400 nm.

power at large current densities. As a drawback, both samples exhibit an additional barrier of approximately 100 mV in the voltage–current characteristics compared with the thin SQW structures. The observed trends regarding the IQE are consistent with simulations that we have shown elsewhere [20], [21], [33]. The results presented in this section thus lead to the conclusion that decreasing the InGaN active-layer carrier density is the key to reduce the saturation of emission power in InGaN-based light-emitting devices. Thick InGaN QWs are one way to do this. However, this option is only feasible in the short wavelength range. For larger In content, material quality is decreasing quickly, resulting in poor small current efficiency. MQW structures, in contrast, exhibit better crystal quality and can be realized up to the green spectral range. Such structures are investigated in the following section.

B. InGaN/GaN MQW LEDs David et al. [19] report that, for typical InGaN/GaN MQW LEDs, only the topmost QW adjacent to the p-side contributes to the light emission. This is generally attributed to low hole mobility in GaN, hindering uniform carrier distribution in an MQW structure. Since piezoelectric transport barriers are increasing with longer emission wavelength, this effect is exAuthorized licensd use limted to: IE Xplore. Downlade on May 13,20 at 1:486 UTC from IE Xplore. Restricon aply.

pected to be even more pronounced for green LEDs; hence, particularly green MQW LEDs would act as SQW LEDs. To optimize MQW operation, attempting to reduce the droop in InGaN-based light emitters, we extensively analyzed the carrier distribution in green emitting InGaN MQW-based LEDs. We compared the emission characteristics of green SQW, monochromatic MQW, and color-coded MQW LEDs [18], [31]. In this section, we sum up our results on the fabrication of optimized green LED structures with real MQW operation. The LED structures were grown, processed as 290 × 290 µm2 standard chips, and measured as described before in Section II. At ∼ 50 A · cm−2 , the samples emit around 525 nm. In the active layer, the InGaN QW thickness was varied between 2 and 4 nm, the MQW GaN barrier width was varied between 10 and 20 nm, and the number of green QWs was varied between four and nine. Among these, results on three different types of sample structures are discussed in detail as follows: 1) green SQW structures; 2) green sevenfold QW structures; and 3) blue and green color-coded MQW structures. The first color-coded MQW structure consists of six blue QWs and one green QW adjacent to the p-side (“bbbbbbg”), and the second color-coded MQW structure consists of four blue QWs and three green QWs next to the p-side (“bbbbggg”). Fig. 4 shows the optical output power and the wavelength shift of an SQW and an optimized sevenfold MQW LED in comparison. The current density was varied from 0.01 to 1000 A · cm−2 . A combination of dc and pulsed measurements as described in Section II was used as before to prevent the heating of the device. The MQW LED exhibits a much better high current behavior (less droop) than the SQW LED, and the blueshift of the peak wavelength is significantly smaller for the MQW structure. Both LEDs emit at a peak wavelength of 525 nm at 50 A · cm−2 . The reduced blueshift is a sign for reduced volume carrier density in the MQW structure compared with the SQW LED. Fig. 5 shows the corresponding IQE curves plotted versus driving current on a logarithmic scale. The high current tale of the SQW and the MQW IQE curves show a similar slope. However, for all current densities, the MQW



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Fig. 7. (Left) Top view on a five-current-spreader PowerThinGaN chip and (right) a schematic cross-sectional view of a ThinGaN-LED design.

and room temperature, in agreement with the results obtained before. IV. ThinGaN-BASED LED E FFICIENCY Fig. 5. IQE of SQW and MQW LEDs plotted over current density. For information about samples, preparation, and measurement conditions, see caption of Fig. 4.

Fig. 6. Logarithmic plot of EL spectra of two color-coded sevenfold MQW structures (“bbbbbbg” and “bbbbggg”) and a green MQW structure (“ggggggg”) taken at 300 K and 50 A · cm−2 . The spectra of the two colorcoded LEDs exhibit a double peak, whereas the green MQW reference shows only a single emission peak.

IQE is significantly higher than the SQW IQE. By increasing the current density for the MQW by a factor of three, thus simulating the transition from real threefold MQW to SQW operation, the two IQE curves roughly match. This indicates that the current density within each QW in the MQW LED is about a factor of three smaller than in the SQW; therefore, effectively, three QWs emit in our green InGaN MQW LED structure. This is confirmed in Fig. 6, which shows logarithmically plotted EL spectra of three different MQW LEDs at 300 K and 25 A · cm−2 . The EL spectra of the two color-coded MQW structures (“bbbbbbg” and “bbbbggg”) exhibit two peaks, i.e., at about 450 and 520 nm, whereas the monochromatic green MQW LED (“ggggggg”) shows only one single peak at 525 nm. For the first color-coded LED (“bbbbbbg”), the blue peak intensity reaches 34%, and for the second structure (“bbbbggg”), it is still 2% of the green emission. The blue emission of the second color-coded MQW structure (“bbbbggg”) indicates that even at least four QWs contribute to the light emission at 25 A · cm−2 Authorized licensd use limted to: IE Xplore. Downlade on May 13,20 at 1:486 UTC from IE Xplore. Restricon aply.

The results discussed in the preceeding Section III demonstrate how the internal efficiency of InGaN-based LEDs can be optimized for better high current performance on the level of epitaxial active-layer design. Realizing optimal high-power and high-efficiency InGaN-based LEDs however requires improvements in all areas of device design. The overall efficiency (ηtot ) of a LED device is the product of the contributions of many LED components: 1) ηel : the electrical contacts and series resistances; 2) ηIQE : the IQE of the active layer; 3) ηLEC : the light-extraction efficiency from the chip (LEC); 4) ηconv : the phosphor conversion efficiency for white LEDS; 5) ηLEP : the light-extraction efficiency from the package. The IQE ηIQE and the linearity improvement (droop optimization) have been discussed in Section III. We investigated the approaches described there for our power LEDs and found the MQW approach with several active QWs to be the superior solution. The light-extraction efficiency ηLEC from the chip is determined by chip design, and the refractive index of the surrounding material, which is typically silicone for state-ofthe-art packages. For good light extraction, the InGaN-LED thin-film approach was pioneered by OSRAM Opto Semiconductors GmbH in 2003 [2], [34] and runs productive since 2004. Fig. 7 shows the structure of a ThinGaN-PowerLED chip. The basic idea of the ThinGaN concept is to confine the light to the GaN layer by a reflective mirror on the carrier side and extract the light via a structured surface at the LED topside. The key factors for good light extraction are high mirror reflectivity (the photons hit the mirror several times), optimized surface structures, and minimized absorbers within or on the GaN layer. For power chips, these absorbers are mainly defined by the current spreader metallization, which is necessary for homogenous current spreading over the full chip area. The degree of metallization depends on the typical operation current of the LED and particularly on the current-spreading ability of the epitaxial layer between the n-contact and the active region. We investigated n-doped GaN/AlGaN superlattice structures and optimized Si-doping profiles to reduce the number of current spreaders from five to two (compare Fig. 7), which yields about 35% absorber reduction. Additional improvements were achieved by further absorber reduction, reflectivity

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Fig. 9. Comparison of the cross-sectional views of the (left) ThinGaN chip structure and (right) advanced chip design. The light extraction can be remarkably enhanced by moving the n-side current distribution layers below the p-contact. Fig. 8. (Top) Power flow of optical emission power and heat generation for OSRAM Opto Semiconductors GmbH Dragon power device with 3.24-V forward voltage at 350 mA. The resulting WPE is 53.3%. (Bottom) Power flow in a realistically estimated optimization limit.

improvement of the Ag-based mirror, and optimization of the light-extraction surface structures. By these measures, we increased the LEC from 75% to > 85%. These numbers were derived from ray-tracing calculations, which were calibrated by experimental data of test structures and also full LED-chip structures. In view of a nearly linear brightness increase of InGaN-based LEDs over the years [1], the question on the optimization limits arises. We derive these limits based on an analysis of the current situation for each single efficiency parameter and later estimate achievable optimum values. The power flow for a current R&D Dragon device is shown in Fig. 8 (top) with LEC and light extraction from package (LEP) derived from calibrated raytracing analysis models. We obtain values of 95% for LEP, 86% for light extraction from chip (LEC), and 75% for IQE. As an optimization limit, on the basis of detailed electrooptic simulations, we assume a voltage reduction to 2.9 V. This is already very close to the band gap. From the calibrated raytracing simulation model, we extrapolate realistic limits of 95% and 97% for LEC and LEP, respectively. For the IQE, detailed simulations (compare [20]) suggest a limit of 85%, which is essentially due to the apparent inevitability of InGaNbased LED droop. Using these numbers, we expect future R&D values up to 740 mW for 1-mm2 power LEDs, which is about 25% above the present status [see Fig. 8 (bottom)]. Assuming the same blue to white conversion factor as observed for the advanced device, the 740-mW output power would result in about 180 lm white light with an efficacy of 180 lm/W. V. A DVANCED InGaN LED-C HIP D ESIGN The ThinGaN approach described in Section IV has proven its capabilities, quality, and long-term stability in, e.g., backlighting, SSL, and automotive applications. However, this design still has some room for improvement. In the used vertical chip geometry, the current has to be distributed on the n-side of the chip via a metallized n-contact grid on the light-extraction surface. This metallization is necessary for current spreading in high power operation, as the lateral conductivity of the less than 5-µm-thick n-GaN is limited. The n-contact, however, reduces the active light-emitting area and also provides light absorption, thus reducing the light-extraction efficiency. Another challenge is the compromise of low light absorption. This means using as little metallization as possible. Still, sufficient current spreading Authorized licensd use limted to: IE Xplore. Downlade on May 13,20 at 1:486 UTC from IE Xplore. Restricon aply.

capability for high current operation is necessary. Depending on the operation current density that the chip is designed for, this defines the minimum possible amount of n-contact metallization. This challenge was overcome by moving the n-side current distribution from the top contact to a buried n-contact layer under the p-contact. Fig. 9 shows the comparison of the advanced design to a ThinGaN LED. The p-contact is formed as a mirror by a proprietary Ag-based highly reflective contact. The p-metallization also serves as a current distribution layer for the cathode. The p-contact is completely covered by an insulation layer, which separates the n-electrode from the pelectrode. The reflecting n-contact fingers are produced using a standard metallization scheme. The n-contact fingers are connected to the buried n-current distribution layer by insulated vias through the insulation and the p-metallization. The whole structure is wafer bonded to a carrier substrate, and the sapphire is subsequentially removed via laser liftoff. In this design approach, the 5 µm GaN epilayer transferred from the host substrate is fully supported by a conductive thermally matched carrier system like Gallium Arsenide or Germanium [35]. The design and control of the mechanical carrier and the interconnect are crucial to ensure homogenous heat spreading and mechanical stability of the device. The design has already passed reliability testing according to automotive standards in sufficient numbers. In addition to the optimization of EQE by reducing the absorbing contact structures, the WPE is increased by a reduced serial resistance of the device. In particular, at high current operation, a significant reduction in forward voltage can be observed. The corresponding lower ohmic losses enable high current operation up to 3 A for a 1-mm2 device. Through the combination of an optimized epitaxial MQW design (compare Section III) with the excellent current distribution of the advanced chip design, which originates in the utilization of the mechanical carrier for current spreading, the chip shows excellent high current characteristics. The efficacy and light output of the 1-mm2 chips are shown in Fig. 10. At 350 mA, the device produces 643 mW, and at 3000 mA, 3200 mW is achieved. With suitable phosphor coating, a peak luminous efficiency of 136 lm/W was observed. The device delivered up to 830 lm from a single chip when it was driven at 3 A. Even at these high driving current densities, the emission pattern kept constant, as can be seen in Fig. 11. This is a key requirement for automotive headlamp systems, projection, and illumination. As a true surface emitter, the technology is



Fig. 10. I–V characteristics of a blue emitting ThinGaN ux3 chip in a Golden Dragon Plus package.

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Fig. 13. New OSRAM OSLON SSL LED package featuring a small 3 mm × 3 mm footprint and only 7-K/W thermal resistance.

first generation. This feature enables an upgrade of existing products without changing the optical design parameters in secondary optics. VI. A DVANCED LED PACKAGE

Fig. 11. Emission patterns of (left) a standard ThinGaN chip structure and (right) the advanced chip design optimized for high current operation. The new technology shows even at 2, 8 A · mm−2 , a homogeneous emission pattern. Current distribution is no longer limiting the device performance.

Fig. 12. I–V characteristics of a green emitting 1-mm2 chip in a Golden Dragon Plus package.

not only fully scalable but also able to produce homogeneous emitting multichip arrays with high luminance. The device was also fit with green epitaxial structures (see Fig. 12). In this case, a maximum efficacy of > 200 lm · W−1 was achieved. At 350 mA, 117 lm (100 lm/W) was measured, and at 1 A, we obtained 224 lm. The corresponding high luminance for green is an enabling technology for highperformance projection systems. In order to simplify the introduction of the new chip design, the dimensions and main optical characteristics of the design were kept compatible with the OSRAM ThinGaN chips of the Authorized licensd use limted to: IE Xplore. Downlade on May 13,20 at 1:486 UTC from IE Xplore. Restricon aply.

To better exploit the unique features of the latest chip technology described in Section V, compatible LED packages are to be used. They preserve the chip’s high current linearity in the application and also enable sophisticated optical designs. Although higher in efficiency and producing less heat, the good linearity of this new chip enables very high optical densities that need to be supported by a good thermal package design as well. Additionally, the long lifetime of the chip needs to be maintained in the application. This is achieved by the choice of highly stable package materials. From a customer ease-of-use standpoint, a small footprint and a versatile optical interface are demanded as well. Finally, new packages offer electrical decoupling between the electrical and the thermal path—chip polarity changes do not impact customers’ PCB layout. One example of such a new package is the OSLON SSL (see Fig. 13). Its low thermal resistance of 7 K/W simplifies thermal management and enables high-power-density applications. The use of ceramic and silicone ensures high reliability and enables decoupling of electrical and thermal interfaces. OSLON’s small footprint of only 3 mm × 3 mm combined with a well-defined beam shape for easy coupling to secondary optics (80◦ FWHM) provides designers increased flexibility to create extremely sophisticated lighting solutions. Moreover, if a particularly strong light or color mixing is required, several light sources can be easily combined into clusters. VII. InGaN-BASED LED L IFETIME P ROGNOSIS With the increasing market share of general lighting applications, LED lifetimes between 50 000 and 100 000 h became an important performance criterion to avoid the need of LED replacement during a luminaire’s life. A lifetime of 100 000 h is a period of more than 11 years. Therefore, accelerated lifetime tests were developed to allow a reliable prognosis with reasonable test duration. We estimate the lifetime of our LEDs using a

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degradation model with an exponential time dependence of the LED intensity. The decay parameter and an activation energy Ea were determined by temperature-dependent steady-state lifetime tests of our chips. With these parameters, we derive an L70B50 lifetime of 100 000 h (i.e., 50% of the chips have 70% or more of the original brightness after 100 000-h dc operation) at junction temperatures up to 135 ◦ C.

[9] [10]

[11]

VIII. C ONCLUSION In this paper, we have shown that, regardless of the employed concept, a decrease in active-layer carrier density is central to improve the high current IQE of InGaN-based light emitters. To achieve this goal, the optimization of MQW emission is very promising since it reduces the high current saturation of efficiency in InGaN-based LEDs in the whole wavelength range from UV to green without sacrificing material quality. Such structures tuned for optimal MQW operation have been combined with an advanced chip design optimized for high current density operation and high light-extraction efficiency. At 350 mA, such a device produces 643-mW optical output power at 440-nm emission wavelength, and at 3000 mA, we get 3199 mW. With suitable phosphor coating, this translates into 830 lm from a single chip driven at 3 A. For a green emitter, 117 lm and 100 lm/W could be shown at 350 mA. Combined with the new OSLON SSL package with only 3 mm × 3 mm footprint and 7-K/W thermal resistance, this creates extremely flexible and sophisticated high-power and highefficiency InGaN-based light sources.

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Ansgar Laubsch received the Diploma degree in physics from RWTH Aachen Technical University, Aachen, Germany, in 2005. From 2004 to 2005, he was with the Institute of Solid State Research, Research Center Jülich, Jülich, Germany. Using scanning tunneling microscopy, he investigated the influence of defects on the electronic structure of compound semiconductors on an atomic scale. He joined OSRAM Opto Semiconductors GmbH, Regensburg, Germany, in 2005 for a Ph.D. project elucidating the mechanisms limiting GaN-based visible-light-emitting-diode efficiency, in cooperation with the university and the Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany. He is currently with Development/Analytics, OSRAM Opto Semiconductors GmbH.

Matthias Sabathil was born in Hamburg, Germany, in 1974. He received the Ph.D. degree in physics from the Technical University of Munich, Munich, Germany. Since 2004, he has been with OSRAM Opto Semiconductors GmbH, Regensburg, Germany, focusing on the modeling of device properties as well as the internal quantum efficiency of high-performance InGaN LEDs.

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Johannes Baur received the Ph.D. degree from the Albert Ludwigs University, Freiburg, Germany, in 1998, working as a Ph.D. student at the Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany, from 1995 to 1998. He joined OSRAM Opto Semiconductors GmbH, Regensburg, Germany, in 1998, where he headed several InGaN chip and power-LED development projects. Since 2008, he has been the Head of InGaN LED-chip development.

Matthias Peter received the Ph.D. degree from the Albert Ludwigs University, Freiburg, Germany, in 1999, working as a Ph.D. student at the Fraunhofer Institute for Applied Solid State Physics (IAF), Freiburg, Germany, from 1996 to 1999. After working as a Research Fellow with IAF, he joined OSRAM Opto Semiconductors GmbH, Regensburg, Germany, in 2000, where he is a Senior Research Engineer responsible for nitride MOCVD development and coordinates epitaxy-based efforts to improve device brightness.

Berthold Hahn received the Ph.D. degree in physics from Regensburg University, Regensburg, Germany, in 1998. He joined OSRAM Opto Semiconductors GmbH, Regensburg, in 1998, where he was heading several epitaxy and product development projects for InGaN high-power LEDs. Since 2008, he has been heading the LED-chip development.