m-Plane GaN-Based Blue Superluminescent Diodes Fabricated Using Selective Chemical Wet Etching Matthew T. Hardy 1 , Kathryn M. Kelchner 2 , You-Da Lin 2 , Po Shan Hsu 1 , Kenji Fujito 3 , Hiroaki Ohta 1 , James S. Speck 1 , Shuji Nakamura 1;2 , and Steven P. DenBaars 1;2 1 Materials Department, University of California, Santa Barbara, CA 93106, U.S.A. 2 Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, U.S.A. 3 Optoelectronics Laboratory, Mitsubishi Chemical Corporation, 1000 Higashi-Mamiana, Ushiku, Ibaraki 300-1295, Japan Received October 30, 2009; accepted November 22, 2009; published online December 11, 2009 An m-plane-GaN based blue superluminescent diode was demonstrated utilizing the asymmetric chemical properties of the c facets. The non-reflecting c plane facet, intended to prevent optical feedback along the c -axis waveguide, was fabricated by KOH wet etching. KOH selectively etched the cleaved c facet leading to the formation of hexagonal pyramids without etching the þc facet. The peak wavelength and full width at half max were 439 and 9 nm at 315 mA, respectively, with an output power of 5 mW measured out of the þc facet. # 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.121004 S uperluminescence diodes (SLDs) have both the high directional output power of laser diodes (LDs) and the relatively broad spectral emission and low coherence of light emitting diodes (LEDs). SLDs make use of stimulated emission along a waveguide to amplify the spontaneous emission, but inhibit its feedback at the facets and prevent net round trip gain that would otherwise lead to lasing. Without lasing, there is no mode selection or highly coherent emission. The intermediate properties between LDs and LEDs make SLDs well suited for various applications. Pico projectors make use of the high power directional emission, while the relatively broad spectral width reduces the risk of eye damage associated with LDs and the low coherence reduces coherence noise or ‘‘speckle’’. SLDs have high fiber coupling efficiencies 1) allowing for applications in fiber coupled lighting and fiber optic gyroscopes. These devices can also be used in optical coherence tomography 2) and retinal scanning displays. 3) Early SLDs based on InP and GaAs were fabricated using various techniques to prevent optical feedback including: an unpumped absorbing region, 4) anti-reflective coatings, 1,5) a bent waveguide 6) and tilted facets. 7) A recent report of a GaN-based SLD also utilized tilted facets. 8) Optoelectronic devices based on GaN have advanced significantly since the first demonstration of InGaN-based LEDs in 1991 9) and LDs in 1996. 10) Although such c-plane devices have been commercialized, they are plagued by the quantum confined stark effect (QCSE), which causes a separation of electron and hole wavefunctions, a concomi- tant decrease in radiative recombination efficiency, and red-shifted emission at low current densities. 11,12) With the advent of low extended defect-density free standing GaN substrates, quantum well (QW) structures grown on semipolar and nonpolar crystal planes have attracted attention because they can suppress or eliminate QCSE. Unbalanced biaxial in-plane strain causes a splitting of the heavy and light hole valence bands, leading to theoretically predicted higher gain along nonpolar and semipolar planes relative to c-plane. 13,14) Nonpolar m-plane LDs have been demonstrated in the violet, 15–17) blue, 18,19) and blue-green spectral regions. 20,21) The absence of the QCSE in m-plane QWs decreases the blueshift with increasing drive current and allows thicker QWs, which increases optical confine- ment without loss in radiative recombination efficiency. 17,22) GaN shows very different chemical properties on the þcð0001Þ and cð000  1Þ faces. This can be seen in chemical- mechanical polishing studies, where the c face can be easily polished while the þc face shows no damage or material removal for identical conditions. 23) In photoelec- tricalchemical (PEC) etching studies, the þc face has shown negligible etch rates under illumination without an electrode, while the c face etched in KOH without any illumination at all. 24–26) KOH wet etching produces a hexagonal pyramidal structure on the c face, where the sides of the pyramids are comprised of f10  1  1g planes. 24) The differing chemical stability is attributed to the opposite surface polarity of c face GaN. In LED technology, this effect has been used to enhance light extraction efficiency. 24) In this work we demonstrate a blue SLD based on m-plane GaN by selectively etching the c facet using KOH, where hexagonal pyramids are formed at the c facet due to crystallographic chemical etching to reduce mirror reflectiv- ity. Detailed processing procedure and device performance will be shown. AlGaN-cladding-free LD structures were grown by standard metal–organic chemical vapor deposition on bulk m-plane substrates manufactured by Mitsubishi Chemical. 19) The structure consisted of a 4-m-thick Si-doped GaN cladding layer, followed by 50 nm of Si-doped n-type InGaN waveguiding layer. The active region consisted of a three period InGaN/InGaN multiple quantum well structure. An unintentionally doped GaN layer was grown on top the active region, followed by a 10-nm-thick Mg-doped Al 0:25 Ga 0:75 N electron blocking layer (EBL). The EBL was followed by a 50 nm Mg-doped p-type InGaN waveguiding layer. The top cladding consisted of about 500-nm-thick Mg-doped p-type GaN, capped with 100 nm Mg-doped p þþ contact layer. A 4 m wide stripe was formed by patterning and dry etching ridges along the c-direction. A standard liftoff process was used for the oxide insulator, followed by Pd/Au metal deposition for cathode electrodes. The facets were formed by cleaving, resulting in cavity lengths of 500 m. Indium was used to from the backside anode electrode. The sample was then mounted face down to protect the top-side electrode and then immersed in nominally 2.2 M KOH solution for 8 h at room temperature to roughen the c facet.  E-mail address: mthardy@engineering.ucsb.edu Applied Physics Express 2 (2009) 121004 121004-1 # 2009 The Japan Society of Applied Physics