Optical Properties of Bulk and Epitaxial ZnO for Waveguide Applications
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Optical Properties of Bulk and Epitaxial ZnO for Waveguide Applications S. Ganesan1, Z. C. Feng2, D. Mehta3, S. Kandoor4, E. J. Wornyo3, J. Nause1, and I. Ferguson3∗ 1) Cermet, Inc., 1019 Collier Road, Atlanta GA 30318 2) Graduate Institute of Electro-optical Engineering, National Taiwan University, Taiwan 3) Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 4) Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ABSTRACT This paper investigates the optical properties of bulk and epitaxial ZnO layers. High quality undoped and doped bulk ZnO crystals have been produced by melt growth techniques in addition to ZnO thin films grown by Metalorganic Chemical Vapor Deposition (MOCVD) on silicon, sapphire and the bulk ZnO substrates. This work focuses on investigating the suitability of bulk and epitaxial ZnO for waveguide applications using various spectroscopic techniques. The photoluminescence showed the dominance of strong and narrow band due to the band edge emissions for undoped ZnO. Ultraviolet-visible transmission data revealed the variation of the bandgap with different doping elements. Raman spectra showed a narrow and strong peak, corresponding to the E2 mode at 438 cm-1, characteristic of the ZnO crystallinity. A broad 2LO peak appeared near 1150 cm-1 due to the coupling between LO phonons and free carriers. A clear variation in refractive index with doping was observed by spectroscopic ellipsometery suggesting that ZnO could be used for waveguide applications.
1. INTRODUCTION Zinc oxide (ZnO) has a room temperature band gap of 3.37 eV and an exciton binding energy of 60 meV [1]. The large band gap makes ZnO an excellent candidate for short wavelength emitters and the exciton binding energy could enable practical semiconductor lasers with low thresholds, even above room temperatures [1]. ZnO has a number of advantages for this application: it is transparent, can be made insulating, is relatively easy to produce high conductivity n-type material, and, furthermore, it can be grown in high quality, bulk form for homoepitaxial substrates. Furthermore, ZnO can be grown at relatively low growth temperatures by thin film deposition techniques at temperatures
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