Low Threshold Single Mode ZnO Nanowire Nanolasers

With a wide bandgap of 3.36 eV at room temperature and large exciton binding energy of 60 meV (excitons being stable up to room temperature), ZnO holds promise for use in blue and ultraviolet optical devices [1], including ultraviolet microlasers. Lasing has been demonstrated with epitaxial and microcrystalline thin films [2], and in arrays of ZnO nanorods [3] and nanowires [4], for example. Usually, the feedback mechanism in nanorod and nanowire-based lasers is related to the formation of Fabry–Perot cavities for longitudinal modes [5], or to the structures ability to couple and facilitate guided modes [6]. Discriminating between the feedback mechanisms in ZnO nanostructures is not an easy issue, especially in microstructures consisting of nanorods with various orientations. Additionally, nanoscale systems also suffer from impure spectral characteristics and multimodal emission. Strategies to improve emission characteristics have seen limited success due to the sensitivity of nanoscale resonator cavities to optical properties dependant on structural shape and details of the crystallinity. Whispering gallery modes and complicated multimodal emission with a spread in gain factors compound the issue of spectrally pure emission in nanoscale systems. High crystallinity is a prerequisite for short-cavity ZnO nanorod lasers due to expected coexistence of heavy mirror losses and scattering losses caused by the intrinsic structure and surface defects typically affecting ZnO. Here, arrays of vertically aligned ZnO nanorods were fabricated by CVD, with very high structural purity and quality. Characterization of aluminum deposition as surrounding layers on nanorods of 2-5 μm in length and 150-200 nm in width will be presented, and we detail the effect of selectively coated individual nanorods on the waveguiding and emission quality of the nanorod laser. The aluminum deposition alters the hexagonal cavity properties by reducing the guided modes and lowering the gain of the higher order modes so that a better, more spectrally pure emission is possible. [1] U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, M. Morkoc, J. Appl. Phys. 98, 041301 (2005). [2] Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72, 3270 (1998). [3] G. Visimberga, C. C. Faulkner, M. Boese and C. O'Dwyer, ECS Trans., 45, 51 (2012). [4] S. P. Lau, H. Y. Yang, S. F. Yu, H. D. Li, M. Tanemura, T. Okita, H. Hatano, H. H. Hng, Appl. Phys. Lett. 87, 013104 (2005). [5] Y. Zhang, R. E. Russo, S. S. Mao, Appl. Phys. Lett. 87, 043106 (2005). [6] H. Zhou, M. Wissinger, J. Fallert, R. Hauschild, F. Stelzl, C. Klingshirn, H. Kalt, Appl. Phys. Lett. 91, 181112 (2007).