High Power Blue-Violet Superluminescent Light Emitting Diodes with InGaN Quantum Wells Marco Rossetti  , Julien Dorsaz, Raffaele Rezzonico, Marcus Duelk, Christian Velez, Eric Feltin 1 , Antonino Castiglia 2 , Gatien Cosendey 2 , Jean-Franc ¸ois Carlin 2 , and Nicolas Grandjean 2 EXALOS AG, Wagistrasse 21, CH-8952 Schlieren, Switzerland 1 NOVAGAN s.a.r.l., Chemin de Mornex 5, CH-1003 Lausanne, Switzerland 2 Ecole Polytechnique Fe ´de ´rale de Lausanne, CH-1015 Lausanne, Switzerland Received April 21, 2010; accepted April 29, 2010; published online May 28, 2010 We report on the characteristics of blue superluminescent light emitting diodes based on the emission of InGaN quantum wells. Narrow ridge- waveguide devices realized by standard processing techniques and with extremely low facet reflectivity show single lateral mode emission and continuous-wave output powers >35 mW with a typical spectral bandwidth of 4 – 5 nm. Tuning the composition of the active region, superluminescent light emitting diodes spanning all the spectral range between 410 and 445 nm could be realized. The light output is highly directional and results in a coupling efficiency into single mode fibers >50%. The device temperature behavior is also discussed. # 2010 The Japan Society of Applied Physics DOI: 10.1143/APEX.3.061002 S ince efficient p-type doping was first demonstrated 20 years ago, 1,2) optoelectronic devices based on III–nitride compound semiconductors have been the subject of intense research and development. Short-wave- length laser diodes (LDs) for high-capacity data storage applications have recently entered mass production and are today one of the essential components of the new disc players generation (Blu-ray DiscÔ). At the same time, highly efficient GaN-based light emitting diodes (LEDs) are expected to gradually replace standard light bulbs due to their much lower energy consumption. Besides mainstream applications, short wavelength optoelectronic devices (UV to visible) can be used for a series of different applications like spectroscopy, fluorescence, display lighting and projec- tion. With the III–nitride technology becoming mature, more specific device architectures are now starting to be considered as documented by the recent reports of vertical cavity surface emitting lasers 3–5) and superluminescent light emitting diodes (SLEDs), 6,7) for example. After the demonstration of the first blue SLED based on GaN, 6) in this study we report on the improved character- istics of SLEDs allowing CW operation with optical emission in the blue-violet spectral region (410 – 445 nm) and we analyze their temperature behavior. Superlumines- cent light emitting diodes are edge-emitting devices with a beam-like output offering good coupling to external elements and optical fibers (like LDs) and with a large spectral bandwidth (like LEDs). Their peculiarity is a high spatial and low temporal coherence that makes them suitable light sources in optical coherence tomography (OCT), fiber- optic gyroscopes (FOG), fiber-optic sensors and equipment testing (FOT), and in all the systems where low speckle is desirable. The high spatial coherence is the result of stimulated light emission along an optical waveguide, while the large spectrum typical of solid state emitters can be maintained by suppressing optical feedback — and therefore round-trip gain — at the two ends of the waveguide. The SLEDs described in this letter were grown on c- plane free-standing GaN substrates by metal–organic vapor phase epitaxy and consist of InGaN quantum wells (QWs) embedded in a p–n waveguide with AlGaN claddings. Ridge-waveguide devices with contacts for current injection were then realized by standard fabrication techniques: Dry etching to form the ridge and subsequent thin-film deposi- tion of dielectrics and metals. More details on the epitaxial structure and device fabrication are reported elsewhere. 6) Cleaved output facets with extremely low reflectivity to avoid optical feedback were realized using tilted ridges and applying anti-reflection (AR) coatings. The use of coatings on tilted facets, besides suppressing any residual reflectivity, is useful to recycle the portion of incident light that would otherwise be lost due to back-reflection in the wrong direction inside the chip. Measurements of the L–I characteristics performed on device bars before and after coating showed an improvement of the output power of roughly 20%. This value is consistent with the expected fraction of light back-reflected at the semiconductor-air interface. An appropriate thermal management is essential for a stable continuous-wave (CW) operation. For this reason, single chips were mounted on heat spreaders [chemical vapor deposited (CVD) diamond or copper submounts] and temperature controlled at 25  C during operation. Light output characteristics were first measured under pulsed injection with a constant pulse-width of 1 s. Figure 1 shows the average output power versus duty cycle, measured on a 1 mm long and 2 m wide ridge-waveguide SLED, for a set of different peak currents. At low current (100 {150 mA) the power is a quasi-linear function of the duty cycle. The device can be driven up to CW conditions without signs of thermal degradation and reaches powers in excess of 10 mW. For higher currents device heating starts to be more important: A clear thermal roll-over of the power vs duty cycle appears when the operating current is 250 mA, with a maximum power of 55 mW reached at 350 mA and 80% duty cycle. A fine tuning of the active region growth conditions as well as an improvement of the device I –V characteristics were essential to achieve SLEDs with super- ior characteristics. The L–I curve under CW injection is shown in Fig. 2 (continuous line). The curve shows the exponential trend typical of light amplification due to stimulated emission with the optical power reaching almost 40 mW at 250 mA and not yet saturated. No lasing kink was observed under the present testing conditions and the far-field emission (picture shown in inset) maintained a pure single transversal mode pattern.  E-mail address: rossetti@exalos.com Applied Physics Express 3 (2010) 061002 061002-1 # 2010 The Japan Society of Applied Physics