Improving the Optical Properties of Self-Catalyzed GaN Microrods toward Whispering Gallery Mode Lasing Christian Tessarek,* ,†,‡ Robert Rö der, § Tom Michalsky, ∥ Sebastian Geburt, § Helena Franke, ∥ Rü diger Schmidt-Grund, ∥ Martin Heilmann, † Bjö rn Hoffmann, † Carsten Ronning, § Marius Grundmann, ∥ and Silke Christiansen †,⊥ † Max Planck Institute for the Science of Light, Gü nther-Scharowsky-Str. 1, 91058 Erlangen, Germany § Institut fü r Festkö rperphysik, Friedrich-Schiller-Universitä t Jena, Max-Wien-Platz 1, 07743 Jena, Germany ∥ Institut fü r Experimentelle Physik II, Universitä t Leipzig, Linne ́ str. 5, 04103 Leipzig, Germany ‡ University Erlangen-Nuremberg, Institute of Optics, Information and Photonics, Staudtstr. 7/B2, 91058 Erlangen, Germany ⊥ Helmholtz Centre Berlin for Materials and Energy, Hahn-Meitner Platz 1, 14109 Berlin, Germany ABSTRACT: GaN microrods were grown self-catalyzed by a fast and simple metal−organic vapor phase epitaxy method without any processing before or after deposition. The prismatic microrods with a regular hexagonal cross-section, sharp edges, straight, and smooth sidewall facets act as a microresonator, as seen by the appearance of whispering gallery modes in the yellow defect band range. To improve their optical properties, a reduced Ga precursor flow is required during growth. However, their hexagonal microrod morphology is not maintained under these growth conditions. The approach to start growth with a high Ga precursor flow and applying a ramp to a reduced precursor flow yield in significant enhancement of the near band edge emission in the upper part of the microrods. Whispering gallery modes in superposition with the near band edge emission can now be detected by cathodoluminescence measurements. These improvements lead to stimulated emission of a single whispering gallery mode up to ∼2 MW/cm 2 and multimode lasing with a threshold of 2.86 MW/ cm 2 from an as-grown microrod under optical excitation at room temperature. KEYWORDS: GaN, microrods, growth, MOVPE, whispering gallery modes, lasing G aN laser diodes (LDs) are expected to be an alternative to light emitting diodes (LEDs) for solid state lighting since LDs are less affected by the “efficiency droop” at high current densities. 1 LDs can be based on GaN rod and wire structures, which have several advantages compared to two-dimensional (2D) GaN layers. High quality GaN material can be achieved in heteroepitaxy such as bottom-up growth of GaN nano- and microrods/-wires on inexpensive sapphire substrates. 2,3 Free- standing rods only suffer from strain-generated defects at the sapphire−GaN interface. 3,4 The relaxed upper part of the rods is nearly defect-free. A coalescence process typically leading to grain boundaries in 2D layers is also not required. 5 GaN rod structures are already applied in optoelectronics such as LEDs for the visible range but also as a basis for UV emission. 6−8 Stimulated emission under optical excitation has been observed in GaN nanowires; however, the fabrication is either time- consuming, needs complex processing before/after growth, and requires additional plasmonic or photonic crystal support. 9 GaN microrods can be used as microcavity resonators and whispering gallery modes (WGMs) with high quality (Q-) factors up to 4000 have already been demonstrated. 10 These Q- factors are comparable to complex fabricated GaN microdisk structures in which WGM lasing activity has been reported. 11−15 However, the drawback of circular-shaped microdisk structures is the nondirectional light emission, whereas a hexagonal resonator benefits from directional out- coupling of light at the corners. 16 Compared to microdisks, the great technological advantage of the microrods is the short growth time of just 5−20 min without the need of additional growth and processing before or after microrod growth. 3,17,18 However, no lasing activity has been demonstrated in these kind of self-catalyzed GaN microrods. The potential of such microrods to be used as laser devices was hinted by another research group by the observation of whispering gallery modes in the strong and weak coupling regime. 19 For hexagonally shaped microrods based on the ZnO material system electrically driven WGM lasing has been demonstrated in combination with a p-type GaN layer. 20 It is expected that GaN microrods will also be a candidate to be used as whispering gallery mode laser diodes. The GaN microrods presented in our previous work suffer from low efficiency of the GaN near band edge (NBE) emission and WGMs are not observed in the NBE emission range. 10 It has already been presented that a reduced Received: June 19, 2014 Article pubs.acs.org/journal/apchd5 © XXXX American Chemical Society A dx.doi.org/10.1021/ph500220v | ACS Photonics XXXX, XXX, XXX−XXX