Performance of High-Power AlInGaN Light Emitting Diodes

Transcription

1 phys. stat. sol. (a) 188, No. 1, (2001) Performance of High-Power AlInGaN Light Emitting Diodes A.Y. Kim, W. Götz 1 ), D.A. Steigerwald, J.J. Wierer, N.F. Gardner, J. Sun, S.A. Stockman, P.S. Martin, M.R. Krames, R.S. Kern, and F.M. Steranka LumiLeds Lighting, 370 W. Trimble Road, San Jose, California 95131, USA (Received July 10, 2001; accepted August 4, 2001) Subject classification: Fd; Jb; S7.14 The performance of high-power AlInGaN light emitting diodes (LEDs) is characterized by light output current voltage (L I V) measurements for devices with peak emission wavelengths ranging from 428 to 545 nm. The highest external quantum efficiency (EQE) is measured for short wavelength LEDs (428 nm) at 29%. EQE decreases with increasing wavelength, reaching 13% at 527nm. With low forward voltages ranging from 3.3 to 2.9 V at a drive current density of 50 A/cm 2, these LEDs exhibit power conversion efficiencies ranging from 26% (428 nm) to 10% (527nm). Introduction With recent improvements in AlInGaN materials and device quality, visible blue and green light emitting diodes (LEDs) based on the AlInGaN material system have reached power conversion efficiencies that have surpassed those of conventional light sources. These LEDs are approaching the efficiencies of modern halogen and compact fluorescent lamps [1]. Together with yellow and red AlInGaP-based LEDs, for which luminous efficiencies >100 lm/w (610 nm) have been reported [2], AlInGaN LEDs have become the light source of choice for applications where low energy consumption and long life are required. General illumination based on white LEDs that use either AlInGaN LED-based phosphor conversion or a combination of red, green and blue (RGB) LED chips is rapidly emerging. The brightness or quality of an LED is often characterized by quoting the external quantum efficiency (EQE), which is given by EQE ¼ h int h inj h extr ; ð1þ where h int, h inj, and h extr are the internal, injection, and extraction efficiency, respectively. However, to determine whether an LED is a viable light source, the power conversion efficiency must be considered. The efficiency of an LED as a light source is best characterized by wall-plug efficiency (WPE) or luminous efficiency (LE), referring to radiometric or photometric reference systems, respectively. The WPE and LE are defined as WPE ¼ P opt and LE ¼ F L ; ð2þ; ð3þ P el P el respectively. P opt is total optical power, F L is the total luminous flux emitted by the LED, and P el is the electrical power input to the LED. WPE is related to EQE by the 1 ) Corresponding author; Phone: ; Fax: ; werner.goetz@lumileds.com # WILEY-VCH Verlag Berlin GmbH, Berlin, /01/ $ 17.50þ.50/0

2 16 A.Y. Kim et al.: Performance of High-Power AlInGaN Light Emitting Diodes following equation: WPE ¼ EQE E ph ; ð4þ V f q where E ph is the photon energy (in ev) at peak wavelength, V f the forward voltage, and q one elemental charge. LEDs that efficiently convert electrical power into light (of a given wavelength) not only possess high EQE but also require a low forward voltage. In general, EQE, WPE, and LE are functions of forward current density (J) and should be quoted as such. Besides efficiency, total emitted flux is an important criterion for an LED in lighting applications. Conventional 5 mm LEDs typically emit less than one or at best a few lumens of light whereas a light bulb or a halogen lamp can emit hundreds or even thousands of lumens. For applications where high flux is required, hundreds of conventional LEDs are combined to generate the required amount of light. This is possible, for example, in traffic signals since the LEDs can be spread over the entire area of an 8 or 12 inch-diameter traffic signal. However, for applications that require a high-flux point light source, conventional 5 mm LEDs are often not suitable because their power or flux density is too low. In this study, we report on the performance of high-power (H-P) LEDs that circumvent this flux density limitation. H-P LEDs use LED chips with junction area about ten times that of conventional 5 mm LEDs. Further, H-P LEDs are typically inserted in LED packages that allow for both high current and high temperature operation. Such LEDs are now commercially available and single devices emitting more than 100 lm of light (530 nm) have been demonstrated. We report on the characterization of H-P LEDs by flux measurements, further demonstrating their benefits over conventional LEDs for lighting applications. Their performance as a function of wavelength and current density will also be discussed. Experimental The LED structures were grown by metalorganic chemical vapor deposition (MOCVD) at low pressure (100 to 400 mbar) on c-plane sapphire substrates. The structures employ a low-temperature, In-containing nucleation layer and a thick, highly n-type conductive, Si-doped base layer. The light-emitting region comprises multiple In- GaN quantum wells separated by Si-doped GaN barriers. A Mg-doped, p-type confinement layer containing Al is grown on top of the quantum well stack, followed by Mgdoped, p-type GaN layers. LEDs with different emission wavelength were created by adjusting the InN content of the QWs. For the LEDs reported in this study the InN composition ranged from less than 10% (428 nm) to 20% (545 nm). H-P LED chips were prepared in flip-chip configuration, utilizing a highly reflective metal layer that reflects downward propagating light and forms an Ohmic contact with the final Mg-doped GaN layer. Light is therefore extracted through the transparent sapphire substrate [3]. Figure 1a 2 ) shows a top view (through the substrate) and Fig. 1b shows a schematic cross section view of an H-P LED chip. The dashed line in Fig. 1a represents the cross sectional cut shown in Fig. 1b. Also depicted in Fig. 1b is the submount onto which the flip-chip LED is soldered to allow for electrical connection. The junction area of the H-P LED chip is 0.007cm 2 and the nominal drive current is 0.35 A (J = 50 A/cm 2 ). 2 ) Colour figure is published online (

3 phys. stat. sol. (a) 188, No. 1 (2001) 17 Fig. 1 (color). a) Top view through the substrate and b) a cross section view of high power LED. The dashed line in a) represents the cross-sectional cut shown in b). The LEDs were characterized with calibrated, integrating-sphere flux measurement systems. Measurements were conducted with pulsed drive currents (1% duty factor at 1 khz repetition rate) at room temperature. Results External quantum efficiency versus emission wavelength is shown for four different H-P LEDs in Fig. 2. The LEDs shown in Fig. 2 span a peak wavelength range from 428 to 527nm. The data were taken at a forward current of 0.35 A translating to a current density of 50 A/cm 2. The EQE is highest for the shortest wavelength LED (29.2% at 428 nm) and decreases with increasing emission wavelength. The EQE data shown in Fig. 2 are summarized in Table 1. Also presented in Table 1 are V f, WPE, P opt, LE, and F L for each of the H-P LEDs. Figure 3 shows the WPE for the same four H-P LEDs shown in Fig. 2 as a function of forward current. The peak wavelength for each LED at the nominal drive current of 350 ma is depicted in the figure. For the forward current range shown, and for each LED, the WPE efficiency drops with increasing forward current. Further, longer wavelength H-P LEDs exhibit lower WPE than shorter wavelength LEDs over the entire range of current densities measured (10 to 150 A/cm 2 ). The emission wavelength of AlInGaN LEDs has been reported to blue-shift with increasing drive current [4]. Such behavior is also observed for H-P LEDs and is demonstrated for a blue (454 nm), blue-green (501 nm), and deep green (545 nm) LED in Fig. 4. In Fig. 4 the wavelength shift is shown as l l 0 with l 0 being the emission wavelength at 0.35 A. The observed wavelength shift depends on LED color. The longer the emission wavelength of Fig. 2. External quantum efficiency vs. peak wavelength for high-power AlInGaN LEDs

4 18 A.Y. Kim et al.: Performance of High-Power AlInGaN Light Emitting Diodes Table 1 Forward voltage (V f ), external quantum efficiency (EQE), wall-plug efficiency (WPE), optical power output (P opt ), luminous efficiency (LE), and luminous flux (F L ) for four different high power LEDs with different peak wavelengths (WL). Data are given for a forward current density of 50 A/cm 2 at room temperature. peak WL (nm) V f (V) EQE (%) WPE (%) P opt (mw) LE (lm/w) F L (lm) the LED, the larger the blue shift. The wavelength shift is only 1 nm for the blue LED, while the wavelength shift is 15 nm for the deep-green LED. Figure 5 illustrates the luminous flux as a function of input power for an H-P LED and for a conventional 5 mm LED. The LED chips for these LEDs were fabricated from similar AlInGaN wafers. The junction area of the conventional LED is cm 2 and the typical drive current is 20 ma (J = 33 A/cm 2 ). In Fig. 5, circles indicate the operating conditions of both LEDs. Typically, conventional LEDs are operated at an input power of 0.1 W, while H-P LEDs are operated at 1 W. Consequently, the generated light measured as luminous flux is significantly higher for H-P LEDs. While the conventional LED depicted in Fig. 5 generates 2 lm of green light, the H-P LED emits 51 lm. This difference is partly due to the difference in operating conditions, but also is due to the fact that the WPE of the H-P LED is 60% higher than the WPE of the conventional LED (10.3% vs. 6.4%). Discussion H-P LEDs possess significant advantages over conventional 5 mm LEDs for applications that require high flux and high flux density. These advantages result from their device design as flip-chips and from their larger junction area (Fig. 1). Most conventional AlInGaN LEDs are top-emitting and employ thin metal layers (e.g., Ni/Au) as the p-contact metalization, which covers the entire device mesa [5]. Such metal layers are required, because current spreading in Mg-doped GaN is insufficient due to its high sheet resistance. However, most of the light generated by the LED must Fig. 3. Wall-plug efficiency vs. forward current for AlInGaN power LEDs. Shown are data for four LEDs with the peak wavelengths at nominal drive current of 0.35 A as depicted in the figure. For each LED, the nominal operation point of the LEDs is indicated by a circle

5 phys. stat. sol. (a) 188, No. 1 (2001) 19 Fig. 4. Peak wavelength shift l l 0 (l 0 the peak wavelength at 0.35 A) vs. forward current for three high power AlInGaN LEDs. The peak wavelengths of the LEDs at nominal drive current of 0.35 A are as depicted in the figure also travel through the p-metalization. As a consequence, there is a trade-off between maximizing current spreading and minimizing light absorption in the p-metalization. Typical thicknesses for such p-metalization layers range between 10 and 40 nm providing insufficient current spreading at high drive currents and leading to absorption of a significant portion of the emitted light, especially in the blue and UV wavelength range [6]. H-P LEDs with the flip-chip design avoid this trade-off by employing a thick (>100 nm), highly reflective metal layer that provides excellent current spreading. Light generated in the active layer and emitted towards the p-metal will be reflected at the highly reflective metal/gan interface upwards and leave the LED chip through the transparent sapphire substrate (Fig. 1b). As a consequence, the efficiency of H-P LEDs increases as the emission wavelength is reduced (Fig. 2). The external quantum efficiency of standard AlInGaN LEDs generally increases with decreasing wavelength; however, an optimum efficiency occurs at about 450 nm and then decreases at shorter wavelengths [7]. Such behavior is absent for H-P LEDs because light absorption in the semi-transparent p-metalization is nearly eliminated. Further, due to the improved current spreading, current crowding at high operating current is generally absent and H-P LEDs typically exhibit lower V f than conventional LEDs at comparable current density. For example, the H-P LED shown in Fig. 5 has a V f of 2.9 V, while a conventional LED fabricated from a similar wafer exhibits a forward voltage of 3.1 V at a current density of 50 A/cm 2. Another advantage of the flip-chip design is that the heat generated in the LED flows directly from the device layers to the Si submount. This avoids heat extraction Fig. 5. Luminous flux vs. input power for a conventional 5 mm LED and for a high-power LED both emitting at 527nm. Circles indicate the operating points defined by the nominal drive currents. The nominal operating current densities (J) and luminous efficiencies (LE) are depicted in the figure

6 20 A.Y. Kim et al.: Performance of High-Power AlInGaN Light Emitting Diodes through the thermally resistive sapphire substrate, as required in conventional top-emitting AlInGaN LEDs. In this way, the H-P LEDs are well prepared for high-current and high-temperature operation. With these improvements in extraction efficiency and ability to drive the H-P LEDs at high operating currents, it is important to further improve the internal and injection efficiency of these devices. While the current generation of H-P LEDs exhibits high WPE especially at low current, the LED efficiency rapidly decreases with increasing drive current (Fig. 3). For example, LED #3 (Fig. 3, 501 nm) exhibits a WPE of 20.5% at a forward current of 75 ma (J 10 A/cm 2 ), but the WPE drops to 13.7% at 350 ma and to 9.2% at 1 A (J 145 A/cm 2 ). Such behavior is observed for most AlInGaN LEDs. The presence of InN-composition modulation within the InGaN quantum wells has been proposed to explain the reduction of efficiency with increasing drive current [7]. In-enriched regions ostensibly act as quantum confined dot-like structures at low current densities, resulting in enhanced efficiency due to the increased oscillator strength of the lowest level radiative transitions or because carriers are confined away from nonradiative centers associated with dislocations [8]. With increasing current density, more carriers escape the confinement within the InN-enriched regions and overall efficiency decreases. A low density of states near the band edge could also explain enhanced color shift with current at low current density. This dependence is present in H-P LEDs as well and is illustrated in Fig. 4. The presence of an InN compositional inhomogeneity could be due to inherent InGaN phase instability or could be driven by the large lattice mismatch between InGaN and underlying GaN (typically 1% to 2% strain). In either case, In-composition modulation should increase with increasing InN content. Since longer peak emission wavelength is achieved by increasing InN content in the H-P LEDs, the In-composition modulation model appears to be consistent with the larger color shifts (Fig. 3) and the changes in efficiency versus current density observed with increasing wavelength (Fig. 4). Further improvement of the WPE, Eq. (4), is expected by lowering the forward voltage of H-P LEDs. In general, the V f of AlInGaN LEDs only shows a weak dependence on color (Table 1), while V f is strongly correlated with the emission wavelength for LEDs fabricated from AlInGaP or AlInGaAs, i.e., the V f scales with the bandgap energy of the light emitting active region. This behavior can be explained by the presence of strong polarization fields at the heterointerfaces in the LED device structure. These fields tend to introduce energy barriers for electron and hole injection into the active region and thereby increase V f [9]. While active regions with higher InN composition have a lower bandgap, the stronger spontaneous and piezoelectric polarization fields due to higher InN content in the InGaN alloy and the larger lattice mismatch between the QW and barrier material, respectively, lead to more pronounced energy barriers at the QW/barrier layer interfaces. However, proper device design leading to low series resistance in the n- and p-type device layers, formation of low contact resistance Ohmic contacts, and proper doping of the active region can result in AlInGaN LEDs with low V f [10]. The lower the V f, the higher the WPE of an LED with a given EQE (Eq. (4)). Another challenge for H-P LEDs is the reduction of efficiency for longer wavelength devices (Fig. 2). The EQE of the 527nm LED is less than half the EQE of the 428 nm LED (Table 1). The decrease in efficiency with increasing wavelength observed in In- GaN LEDs is qualitatively explained by increasingly poor microstructure with increased InN content in the InGaN quantum wells. Strained layers in all other conventional

7 phys. stat. sol. (a) 188, No. 1 (2001) 21 semiconductors can only be grown with much less than 1% lattice mismatch, otherwise rough surfaces, phase separation, dislocations, and stacking faults result. InGaN quantum wells are typically grown with 1% to 3% lattice mismatch to GaN, so some decrease in microstructural quality is expected with increasing InN content. In addition, producing QWs with higher InN composition requires growth conditions that typically lead to poorer materials quality. Although there remains a substantial potential for improvement, significant progress has been achieved towards making InGaN LEDs viable light sources for general illumination. The combination of recent advances in epitaxial and device structure have resulted in single LED packages that emit 103 lm (521 nm) under an operating current of 1 A (Fig. 5). Figure 5 clearly demonstrates that the performance level of H-P LEDs far exceeds what has been possible with conventional AlInGaN LEDs. The H-P LED features higher light extraction efficiency due to the flip-chip configuration and a vastly increased operating power range due to large die size and efficient heat extraction compared to conventional InGaN LEDs. Thus, H-P LEDs have significantly advanced LED technology, enabling their penetration into lighting applications as highly efficient, high flux light sources. Summary H-P LEDs with MQW InGaN active layers and employing flip-chip device structures circumvent significant limitations of conventional, small-junction, topemitting AlInGaN LEDs. They are typically operated at input power levels >1 W, while conventional LEDs are operated at 0.1 W. While still exhibiting efficiency and color dependencies that are characteristic of the AlInGaN materials system, H-P AlInGaN LEDs possess high power-conversion efficiency enabling solid state light emitters with high flux and high flux density. References [1] M. Holcomb, P. Grillot, G. Höfler, M. Krames, and S. Stockman, Compound Semicond. 7, 59 (2001). [2] M.R. Krames, M. Ochiai-Holcomb, G.E. Höfler, C. Carter-Coman, E.I. Chen, I.-H. Tan, P. Grillot, N.F. Gardner, H.C. Chui, J.-W. Huang, S.A. Stockman, F.A. Kish, and M.G. Craford, Appl. Phys. Lett. 75, 2365 (1999). [3] J.J. Wierer, D.A. Steigerwald, M.R. Krames, J.J. O Shea, M.J. Ludowise, G. Christenson, Y.-C. Shen, C. Lowery, P.S. Martin, S. Subramanya, W. Götz, N.F. Gardner, R.S. Kern, S. Stockman, H.C. Chui, J.-W. Huang, S.A. Stockman, F.A. Kish, and M.G. Craford, Appl. Phys. Lett. 78, 3379 (2001). [4] J.S. Im, H. Kollmer, J. Off, F. Scholz, and A. Hangleiter, Mater. Sci. Eng. B 59, 315 (1999). [5] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, T. Yamada, and T. Mukai, Jpn. J. Appl. Phys. 34, L1332 (1995). [6] D. Steigerwald, S. Rudaz, Heng Liu, R. S. Kern, W. Götz, and R. Fletcher, JOM 49, 18 (1997). [7] S. Chichibu, T. Azuhata, T. Soda, and S. Nakamura, Appl. Phys. Lett. 69, 4188 (1996). [8] S. Chichibu, H. Marchand, M.S. Minsky, S. Keller, P.T. Fini, J.P. Ibbetson, S.B. Fleischer, J.S. Speck, J.E. Bowers, E. Hu, U.K. Mishra, S.P. DenBaars, T. Deguchi, T. Sota, and S. Nakamura, Appl. Phys. Lett. 70, 981 (1997). [9] T. Takeuchi, S. Sota, M. Katsuragawa, M. Komori, H. Takeuchi, H. Amano, and I. Akasaki, Jpn. J. Appl. Phys. 36, 382 (1997). [10] R.S. Kern, W. Götz, C.H. Chen, H. Liu, R.M. Fletcher, and C.P. Kuo, in: Semiconductors and Semimetals, Vol. 64: Electroluminescence I, Ed. G. Mueller, Academic Press, San Diego 2000 (Chap. 3, p. 129).

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