Direct green LED development in nano-patterned epitaxy

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1 Invited Paper Direct green LED development in nano-patterned epitaxy Christian Wetzel and Theeradetch Detchprohm Future Chips Constellation, Rensselaer Polytechnic Institute, Troy NY U.S.A. Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy NY U.S.A. ABSTRACT Blue LED progress has laid the ground works of nitride technology to tackle the higher challenge of longer wavelength direct emitters of green, yellow, and orange. Use of bulk GaN substrate allows leapfrogging epitaxy development and offers crystallographic planes that allow higher crystal perfection and a control over piezoelectric polarization. Their combination allows stabilization of emission wavelength with current. Further improvement is found in substrate patterning on the micro and nano-meter length scale where we find roughly equal performance enhancement due to both, enhancement in light extraction and enhanced crystalline perfection. Keywords: solid state lighting, energy efficiency, direct emitting LED, green LED, yellow LED, nanopatterning, GaInN-GaN, MOVPE 1. ENERGY EFFICIENCY BY SOLID STATE LIGHTING Little in our life does not relate to light. Our language is full of light referring allegories. As such it is just human that in a phase of technology development driven by end consumer appeal technology will learn to please humanity with light. Primarily, light is associated with safety. Safety from intruders of the wild --- be that by camp fires in the West African Savannah or by mercury vapor lamps in an alley way of an upscale Manhattan apartment complex. So, light is both, desired and needed. For a society of ever growing energy demand providing more light with less energy is increasingly crucial.[1] Recognizing the limited spectral width of human vision, generating photons only within that window is the primary approach to energy efficient lighting. Next, however, ensues the discussion to what extent, the offered spectrum can further be reduced without unacceptable losses in vision provided by such light.[2] At that point already it becomes obvious just what level would be acceptable or insufficient for the variety of human tasks at hand. Apparently, a diversification of lighting products will be likely but in parallel necessitates a broad education of the consumer about just what qualities in lighting match up with which specific tasks. While all that quickly rises to a socio-economic debate between provider and consumer in the market place, it is up to science and technology to provide the means of light sources of all colors and a range of line widths. Beyond that, those light sources need to be perfected to the point of highest efficiency in terms of cost of ownership and operation. Else, their deployment can hardly be justified. In this cost is not just purchase and energy cost, but includes all aspects of the socialized expense of their carbon foot print. Historically, the very broad and continuous spectrum of a heated tungsten filament in incandescent lamps and the discrete spectrum of narrow atomic transitions in metal halide lamps are familiar to the consumer. For spectral line widths in between, i.e., in the 50 to 100 nm range, semiconductor light emitting diodes (LEDs) are the preferred emitter. Opting for either more narrow lines, semiconductor laser diodes would be available,[2] while for broader spectra, materials of variable strong photon-lattice coupling in a wide range of phosphors is considered.[3] Laser diodes are well known for their highly directed emission pattern, yet due to their rather complex device structures they are currently not deemed cost effective for the high volume production demanded for general lighting. Phosphors, on the other hand, achieve their broad emission spectrum by internal down conversion of high energy photons, whereby the unwanted differential photon energy is deposited as low temperature heat. The resulting heating of the phosphor conversion LED light source has at least two aspects. Firstly, it is an indication of wasted energy and secondly, most aspects of LED efficiency turn worse with an increase of the operation temperature. Since all known phosphors' efficiency is higher at low lattice temperature, such heat has to be extracted by thermal conduction. Due to the low temperature differential to the environment, cooling by radiation, as known for incandescent lamps is not a Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XVII,edited by Klaus P. Streubel, Heonsu Jeon, Li-Wei Tu, Martin Strassburg, Proc. of SPIE Vol. 8641, SPIE CCC code: X/13/$18 doi: / Proc. of SPIE Vol

2 relevant process here. Therefore, tremendous effort has been paid passive, and in some cases active, cooling elements as part of the LED lamp's casing. Development of the wide bandgap semiconductors, most prominently GaN[1] and its alloys with GaInN and AlGaN has allowed for extremely efficient LEDs emitting in the nm range. LED performance over a wider range of peak wavelengths, e.g., the UV and the green spectral range has also improved tremendously.[4] This to the most part, however, comes from collateral benefits in the development of the blue LEDs, i.e., by the shared benefit of improved epitaxy equipment and refined techniques of electrical contacts and light extraction techniques. The effort specifically aiming at those other wavelengths, however, has been rather minor perpetuating the concept of a self enforcing development boost for blue LEDs. In consideration of this imbalance, the inherent progress in those other wavelength ranges has indeed been extremely good and their ultimate utilization in the spectral illumination market will ultimately dependent on customer demand and performance of competing technologies. The currently primary approach for white light LEDs in solid state lighting therefore comprises the combination of a blue or near UV emitting GaN based LED with a phosphor optically excited by the same to fill in the spectrum with lower energy photons. The photon energy conversion with the phosphor most naturally is a lossy process even if a 100% quantum efficiency could be maintained. This simply lies in the fact of a lower quantized photon energy of at, say λout = 550 nm photon versus that of a λin = 450 nm photon. So, dependent on the actual quantum efficiency of the phosphor, a minimum Stokes conversion loss of 1 - λin / λout = 18% are lost as heat in the phosphor. 2. DIRECT EMITTING LEDS The only way to avoid such huge phosphor induced Stokes losses is the combination of LEDs of various peak emission wavelengths and colors that each contributes a certain portion to the visible spectrum. When fully developed, such LEDs would inject electrons at only a slight overvoltage (< 0.1 V) over the emitted photon's energy avoiding all those detrimentally large Stokes conversion losses inherent to the working principle of phosphors. In the consequence, to make use of their energy efficiency advantage, such a combination of such different color LEDs would require various distinctively different input voltages. They could not be driven in parallel from a shared power supply or controller. This is in contrast to single type LED excitation with phosphor conversion. Therefore, the need for multiple drivers may add additional cost but the added color tunability would be a desirable aspect of product differentiation that could justify the added expense. Moreover, in the long run, stabilized serial connection of proper balanced LEDs for higher voltage operation is likely to obsolete the problem. At the present state of development, serial connections would be performed by metal contact formation to the individual pn-junction LEDs and external wire bond daisy chaining. In further development stages, the bonding would be replaced by internal metal layers connecting separate junctions. Ideally, however, tunnel junctions would be formed directly between the epitaxial layers of several consecutively grown pn-junction layers. Various progress has already been achieved in the implementation of such reverse biased Zener diodes in the group-iii nitrides in direct epitaxial growth. In the ideal case, junctions of various emission energies could therefore be stacked and vertically integrated provided the proper sequence of layers avoids the re-absorption of generated photons in sub-sequent layers. In principle, the stack could have similarities with multi-junction solar cells, operating under high current densities, such as in concentrator photovoltaics. The AlGaInN group-iii nitrides have demonstrated suitability for such performance up to some 200 suns.[5] Due to their rather narrower emission spectrum such light emitters should always be able to carry higher photon fluxes than the broader spectrum photovoltaic cells. This also is due to lower high photon energy thermalization losses, familiar from the phosphor-based emitters. The process inherent thermalization losses incurred in phosphor converted light emitters and associated heating, leads to a thermal instability of the device. Any increased temperature in the phosphor will reduce its conversion efficiency and thereby further increase the thermal load. The system therefore is inherently unstable and requires higher order control mechanisms to avoid burn-out failure. Therefore, any increase in overall efficiency does not just lower the cooling requirements but furthermore helps stabilizing the device in operation. One approach to better cool the conversion phosphor is its remote placement, spatially separated from the excitation source. Such separation allows for a thermal decoupling yet introduces additional interfaces of additional reflection losses and stability concerns. According to Snell's law, light prefers propagation in the higher index medium, Proc. of SPIE Vol

3 the highest of which typically is the semiconductor of the excitation source followed by that of the conversion phosphor. To achieve highest possible light transfer between light source and phosphor, the medium in between would be of a refractive index between that of the source and the phosphor, while being thermally insulating yet matched under thermal expansion cycles. No suitable such material has yet been identified. In practice, as the best thermal insulator, an air gap is introduced between the LED source and the phosphor necessitating full adaptation of refractive index from the source to air and then back to that of the phosphor. Surface roughening to achieve a high scattering rate in the interface and, surface shaping to achieve higher light extraction by funneling and variable surface porosities to achieve a gradual variation of refractive indices have been employed to this extend. The benefit of such approaches lies in its universal applicability, since extraction to air is the most common of all LED light extraction challenges. Naturally, the same efforts will have to be applied for the phosphor, both for light coupling into and out of it. Phosphors in the form of a powder afford a very high rate of scattering that could be part in the consideration of such light coupling. Drawbacks, however, are higher trapping losses in optically thick layers and the generally reduced thermal conductivity over crystalline material of longer range lattice order. Efforts are therefore underway to develop crystalline non-scattering phosphor materials, e.g., by sintering. For such, of course, surface preparation similar to those of the LED extraction planes must be added. Unlike the capability to direct LED light into a specific direction of propagation, e.g., by means of reflective mirror layers, the phosphor conversion process is random in propagation and unrelated to the beam of excitation. The same holds for the non-scattering type of phosphors. All of the above light guiding concerns could indeed be avoided, if the light of desired wavelength were directly generated in the location of the pn-junction. In this case, all thermalization losses were to occur in the material of highest index of refraction and a single light extraction effort would suffice. Such is the principle of direct emitting LEDs and the proclaimed benefit of the Ga 1-x In x N bandgap that by varying of the InN to GaN alloy fractions x covers the entire visible spectral region.[6] Development effort has enabled the ever higher efficiency deployment of GaInN/GaN quantum well active regions emitting in the 450 nm blue spectral region and progressively at longer and shorter wavelengths including the 540 nm green spectral region. Yellow and orange LEDs have also been achieved including by this group and red ones are possible as well. 3. OPPORTUNITIES FOR DEVELOPMENT From our extensive research into the epitaxial growth of appropriate GaInN/GaN structures, the generation of structural defects as witnesses in transmission electron microscopy is a major limiting factor in light output performance and efficiency as the wavelength extends from the blue over green to the yellow.[7-9] Huge piezoelectric polarization effects, scaling with the InN incorporation in regular polar c-axis growth have been successfully reduced by the growth along semipolar and non-polar crystallographic directions.[6] While the provisioning of substrates suitable for epitaxial growth along those direction is a lingering challenge, results on small piece bulk GaN has demonstrated substantial promise for such work.[10] In particular, the issue of wavelength stability under variation of current density has now essentially been eliminated in the cyan to green regions.[6] Furthermore, work on crystallographic equivalent planes but opposite surface directions have provided strong evidence, that the growth chemistry itself is strongly affected by the surface termination of the material of polar nature. Such distinctive behavior cannot be explained by the volume effects of polarization alone, where time inversion along the growth axis should results in identical results, in substantial disagreement with experimental observation. Reproducibility is an important aspect of the epitaxial process dial-in. The availability of mere small pieces of various substrate orientations, i.e., of the order of (1 mm) 2, is a substantial detriment. While bulk GaN based results so far have provided the encouraging evidence of alleviated performance limitations, it will require relevant waver size substrates, i.e., (1 cm) 2, to achieve sufficient process dial-in to establish the overall performance margins. The development of larger scale bulk GaN form which a wide range of prospective surface orientations can be prepared is therefore a necessary investment step. In parallel, development of alternate approaches to achieve specific promising crystal planes of larger size, e.g., (5 cm) 2, is necessary to lead into productions scale development. From a commercialization aspect, all those aspects of substrate orientation and their respective size availability dominate in importance over the much discussed performance droop and green gap issues. Both aspects are inherently scientific questions the answers to which could enable an entire overhaul of LED and solid state light source design.[11] Proc. of SPIE Vol

4 Non-radiative Auger recombination[12] and electron overshot [13] are the currently most favored explanations for efficiency droop, i.e., the reduction of light output efficiency with increasing current density. In a very recent theoretical analysis, a model was proposed by which the effect could be related to the observation of intensity blinking in nanocrystals, e.g., in CdSe.[14, [15]] That in turn was explained by an elimination of the momentum conservation rule in heterostructures layers leading to a vastly enhanced probability of Auger processes, extending their relevance to atypical low carrier injection densities. In the consequence electron overshot would be particularly enhanced by help of this low threshold Auger process, essentially combining both long held explanation into a Solomonic joint explanation. The further analysis of the process predicts an alleviation of those limitations by the replacing energetically abrupt transitions from QWs to their barriers by gradually increasing confining potentials. Evidence of the success of such approaches in CdSe-based nanocrystal systems [16] has been suggested and the implementation in the GaInN system is therefore of utmost immediacy. [[15]] Implementation of soft confining potentials into Ga 1-x In x N/GaN QW systems [[15]] is therefore an opportunity to alleviate the unfortunate separation of light generation layers from the spectrum forming layers of an external phosphor. Their inherent integration will enable the single light extraction approach and their progressively increased efficiency will spur solid state lighting development based primarily on the enabler side of the low cost materials development rather than the high cost aspect of deficiency amelioration by peripheral cooling elements. A range of epitaxial techniques are likely to enable such development, most notably the opportunities afforded by nano-patterned heteroepitaxy. We recently demonstrated the virtually defect free transition from sapphire substrate to GaN, a very heavily mismatched transition that forms the basis of most GaN-based devices as of today.[17] 4. CONCLUSIONS In conclusion, the creation of a pn-junction in wide bandgap GaN has enabled profoundly different opportunities for electronic and optoelectronic devices. The addition of GaInN alloys has now lead to the development of high efficiency blue LEDs that, in combination with external phosphors allow the commercialization of white light LEDs on the grounds of higher energy efficiency and vastly extended operation lifetime. Opportunities emerging in form of nano-patterning technology as a means to overcome large heteroepitaxial lattice mismatch now enables the implementation of soft potential transitions in advanced GaInN light emission layers to extend the spectral range of high efficiency LEDs into the green, yellow and orange spectral regions. In direct response to recent theoretical findings, such structures should overcome both, widely discussed efficiency droop and green gap limitations in second generation, phosphor free direct emitting all color LEDs. ACKNOWLEDGEMENTS This work was supported by a DOE/NETL Solid-State Lighting Contract of Directed Research under DE-EE This work was supported in part by the Engineering Research Centers Program of the National Science Foundation under NSF Cooperative Agreement No. EEC and in part by New York State under NYSTAR contract C REFERENCES [1] Akasaki, I. and Wetzel, C., Future Challenges and Directions for Nitride Materials and Light Emitters, Proc. IEEE 85(11), 1750 (1997), DOI: / [2] Neumann, S., Wierer, J., Davis, W., Ohno, Y., Brueck S., and Tsao, J., Four-color laser white illuminant demonstrating high color-rendering quality, Optics Express 19, A (2011), DOI: /OE.19.00A982. [3] Daicho, H.; Iwasaki, T.; Enomoto, K.; Sasaki, Y.; Maeno, Y.; Shinomiya, Y.; Aoyagi, S.; Nishibori, E.; Sakata, M.; Sawa, H.; Matsuishi, S.; and Hosono, H.; "A novel phosphor for glareless white light-emitting diodes", Nature Comm., 3, 1132 (2012), DOI: /ncomms2138 [4] Akasaki, I. and Amano, H. Breakthroughs in Improving Crystal Quality of GaN and Invention of the p n Junction Blue-Light-Emitting Diode, Jpn. J. Appl. Phys. 45, 9001 (2006), DOI: /JJAP Proc. of SPIE Vol

5 [5] Yamamoto, S.; Mori, M.; Kuwahara, Y.; Fujii, T.; Nakao, T.; Kondo, S.; Iwaya, M.; Takeuchi, T.; Kamiyama, S.; Akasaki, I.; and Amano, H.; "Properties of nitride-based photovoltaic cells under concentrated light illumination", Phys. Status Solidi RRL 6, 145 (2012). DOI: /pssr [6] Wetzel, C. and Detchprohm, T., Wavelength-Stable Rare Earth-Free Green Light-Emitting Diodes for Energy Efficiency, Optics Express 19(S4), A962 (2011), DOI: /OE.19.00A962 [7] Zhu, M., You, S., Detchprohm, T., Paskova, T., Preble, E., A., Hanser, D., and Wetzel, C., Inclined Dislocation Pair Relaxation Mechanism in Homoepitaxial Green GaInN/GaN Light Emitting Diodes, Phys. Rev. B 81, (2010), DOI: /PhysRevB [8] Li., T., Fischer, A., M., Wei, Q., Y., Ponce, F., A., Detchprohm, T., and Wetzel, C., Carrier localization and nonradiative recombination in yellow emitting InGaN quantum wells, Appl. Phys. Lett. 96, (2010), DOI: / [9] Zhu, M., You, S., Detchprohm, T., Paskova, T., Preble, E., A., and Wetzel, C., Various Misfit Dislocations in Green and Yellow GaInN GaN Light Emitting Diodes, Phys. Stat. Sol. A 207, 1305 (2010). DOI: /pssa [10] Detchprohm, T.; Zhu, M.; Li, Y.; Xia, Y.; Wetzel, C.; Preble, E.A.; Liu, L.; Paskova, T.; and Hanser, D.; Green Light Emitting Diodes on a-plane GaN Bulk Substrates, Appl. Phys. Lett. 92(24), (2008). doi: /1. [11] U.S. Department of Energy Solid State Lighting, [12] Shen, Y.C.; Mueller, G.O.; Watanabe, S.; Gardner, N.F.; Munkholm, A.; and Krames, M.R.; "Auger recombination in InGaN measured by photoluminescence", Appl. Phys. Lett. 91, (2007). DOI: / [13] Kim, M. H.; Schubert, M. F.; Dai, Q.; Kim, J. K.; Schubert, E. F.; Piprek, J.; and Park, Y.; "Origin of efficiency droop in GaN-based light-emitting diodes", Appl. Phys. Lett. 91, (2007). [14] Wang, X.; Ren, X.; Kahen, K.; Hahn, M.A.; Rajeswaran, M.; Maccagnano-Zacher, S.; Silcox, J.; Cragg, G.E.; Efros, Al.L.; and Krauss, T.D.; "Non-blinking semiconductor nanocrystals", Nature 459 (2009) DOI: /nature08072 [15]] Vaxenburg, R.; Lifshitz, E.; and Efros, Al.L.; "Suppression of Auger-stimulated efficiency droop in nitride-based light emitting diodes", Appl. Phys. Lett. 102, (2013); DOI: / [16] Javaux, C.; Mahler, B.; Dubertret, B. ; Shabaev, A.; Rodina, A.V.; Efros, Al.L.; Yakovlev, D.R.; Liu, F.; Bayer, M.; Camps, G.; Biadala, L.; Buil, S.; Quelin, X.; and Hermier, J-P.; "Thermal activation of non-radiative Auger recombination in charged colloidal nanocrystals", Nature Nanotechnol. (February 2013) DOI: /NNANO [17] Li, Y.; You, S.; Zhu, M.; Zhao, L.; Hou, W.; Detchprohm, T.; Taniguchi, Y.; Tamura, N.; Tanaka, S.; and Wetzel, C.; Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire," Appl. Phys. Lett, 98(15), (2011). DOI: / Proc. of SPIE Vol

Structural Analysis in Low-V-defect Blue and Green GaInN/GaN Light Emitting Diodes

Mater. Res. Soc. Symp. Proc. Vol. 1040 2008 Materials Research Society 1040-Q03-02 Structural Analysis in Low-V-defect Blue and Green GaInN/GaN Light Emitting Diodes Mingwei Zhu 1,2, Theeradetch Detchprohm