phys. stat. sol. (a) 201, No. 12, 2736–2739 (2004) / DOI 10.1002/pssa.200405119

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Feature Article

High brightness LEDs for general lighting applications Using the new ThinGaN™-Technology

V. Haerle*, B. Hahn, S. Kaiser, A. Weimar, S. Bader, F. Eberhard, A. Plössl, and D. Eisert

OSRAM Opto Semiconductors, Wernerwerkstr. 2, 93049 Regensburg, Germany

Received 6 May 2004, accepted 8 July 2004 Published online 2 September 2004

PACS 78.60.Fi, 85.60.Jb

During the last years GaN-technology has proven to fulfill the requirements of solid state lighting. Light-ing requirements are mainly driven by brightness, operation voltage and lifetime. Brigthness is determined by internal efficiency as well as extraction efficiency whereas the ohmic losses determining the operating voltage are dominated by series resistance and contact resistance. Both, brightness and voltage, strongly depend on the device structure as well as the chip design. SiC based [1, 2] as well as Sapphire based LEDs [3] have proven their capability for high brightness devices, still suffering from various compromi-ses such as cost, ESD-stability, high series resistance etc. Recently OSRAM-OS has demonstrated its newly developed product line based on the so called ThinGaN™ technology, a true thinfilm approach that overcomes most of the compromises mentioned. The technology allows highest brightness levels at lowest operating voltage, is scalable and supports all wavelengths. The devices act as true surface emitters with a lambertian emission pattern.

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Improvements in chip technology

Starting some years ago, GaInN LED-technology improvements were strongly driven by modifications of the epi structure, as tuning the bandstructure, mainly to enlarge the internal efficiency. Various vertical device structures from single to multi quantum well devices have been developed, not touching the chip design [4]. These first generation chip designs were based on cubic shapes. In the late 90-th, OSRAM was one of the first companies starting to modify the chip structure in order to further improve extraction efficiency, and the first organization to utilize such techniques for GaInN-based LEDs. To do so ray tracing technologies were utilized to optimize the device structure, resulting in the so called “ATON”-Technology. This approach doubled extraction efficiency compared to the widely utilized cubic device, reaching 50% efficiency [5]. Further improvements were achieved by fliping the device improving the extraction efficiency by another 20% to an total of 60% extraction efficiency. This technology is called “NOTA”-technology, an approach inverting polarity and using soldering techniques for packaging [6]. The very resent improvements were achieved by following a principal approach already pub-lished by Yablonovich for GaAs based devices [7]. The basic idea is dominated by three prin-cipals: 1. Reduction of internal absorption; 2. Highly reflective mirror layers; 3. Ergodic angular redistribution of photons withing the device structure.

* Corresponding author: e-mail: volker.haerle@osram-os.com, Phone: +49-941-850-1423, Fax: +49-941-850-3314

phys. stat. sol. (a) 201, No. 12 (2004) / www.pss-a.com 2737

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 1 ThinGaN-LED brightness as a function of operating current.

So far no technology has been developed that was capable to fulfill such requirements on a large vol-ume technology basis. This newly developed technology starts with the epitaxial growth on Sapphire substrates, making use of internal structures already developed on various substrates, mainly SiC and Sapphire, such as single as well as multi quantum well structures. After preparation of the epitaxial GaN-surface, the structure is eutectically bonded to a carrier wafer such as GaAs or Ge. The bonding metalization layer package contains the metal mirror as well as the electrical contact. Reflectivities of more than 80% have been reached using Ag-mirror layer. To separate the eptaxial layer from the initial Sapphire substrate a new key technology had to be developed, the so called “Laser lift off” technology. This technology relies on OSRAM patents, jointly developed in coop-eration with the Schottky Institut in Munich [8]. A laser beam operating at a wavelength well below the GaN emission wavelength is used to thermally decompose the GaN buffer layer into nitrogen and metalic Ga. After removing the Sapphire substrate from the composit wafer, the final chip processing defining the mesa is taking place. A second key step is done by using a variaty of etching techniques to define the extraction facet. The structure achieved is a multi-facet surface structure. To get optimized extraction a surface texturing is necessary, that on one hand allows high extraction of the incident pho-tons. On the other hand, the photons reflected need to be redirected in all angular directions, achieving a so called ergodic distribution. This principle gives photons a multiple chance to find an escape cone. Finally the upside n-contact is being placed. As a last process step chip dicing is done in order to sepa-rate the individual LED-chips. Using this technologies record extraction efficiencies of up to 75% have been realized. To our knowl-edge, this is the highest value ever reported in the material system of Ga(In, Al)N. Typical brightness

Fig. 2 Lambertian far field pattern of ThinGaN-LEDs.

2738 V. Haerle et al.: High brightness LEDs for general lighting applications

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

0.04 - 0.1 mm2 1 mm2

values of 12 mW are realized and record values of up to 16.1 mW were demonstrated. According to the device structure with area bonding contacts on the p-side and n-bondpad contacts on the n-side, utilizing the high current spreading capability of Si doped GaN, typical operating voltage of 3.15 V at 20 mA were achieved (Fig. 1). Lowest operating voltage at highest brightness of 2.8 V have been demonstrated. Due to the low operating voltage at 16.1 mW, quantum efficiencies of almost 30% and wall plug effi- ciencies of 26% were realized. Record quantum efficiencies of up to 35% and wall plug efficiencies of 30% were demonstrated at low operating current. The emission pattern of such devices is truly lambertian as shown in Fig. 2. At this point, it is worth while to look back at the definitions of luminous flux and luminance. Luminous flux is the optical power, weighted by the sensitivity of the human eye, whereas luminance is given by the luminous flux per unit solid angle and emitting area. Since there is a high need to increase total luminous flux, it becomes obvi- ous to enlarge the emitting surface area (Fig. 3). When doing this, the emission pattern becomes domi- nant. For devices with side emission the total optical flux does not scale with the chip area. Therefrom the efficiency drops and the luminance is reduced. Both resulting in less efficient devices. Using on the other hand OSRAM’s ThinGaN™-approach, having a pure surface emitter with lambertian characteris- tics, large area devices scale in luminance flux as well as brigthness with the emitting area, as shown in Fig. 4. The scaling effect of the luminous flux and therewith the total brigthness with the emitting area results in power LEDs with 182 mW or 42 lm packaged in OSRAM’s Dragon housing, when operated at 350 mA. By comparing those data with data taken from standard size chips packaged in comparable packages and operated at 20 mA, the luminous flux directly scales with the operating current density, going from 20 mA to 350 mA for standard 260 µm to 1 mm devices. Beside the scaling capability, ThinGaN has already demonstrated that the technology is also usable for all wavelength reachable with AlGaInN.

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Fig. 3 Increasing optical flux by in-creasing chip size or emitting area.

phys. stat. sol. (a) 201, No. 12 (2004) / www.pss-a.com 2739

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Summary

By using the new at OSRAM’s newly developed ThinGaN-Technology, record extraction efficiencies of up to 75% were demonstrated. Maximum light output of up to 16.1 mW were demonstrated indicating the high performance of this new technology. Since the devices are Lamertian emitters, the technology has demonstrate its capability of scaling. It was shown that both, luminous flux and brightness, directly scale with the emitting area. Records in luminous flux of 42 lm were demonstrated.

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