Selective Chemical Etching of GaN for Photonic Crystal System Fabrication for Biomolecule Detection R.P. Tompkins, O. C. N Myers, J. R. Nightingale*, C. R. Vemuri*, X. Cao*, D. Korakakis*, L. A. Hornak* and T.H. Myers Multifunctional Materials Laboratory, Department of Physics *Lane Department of Computer Science and Electrical Engineering West Virginia University, Morgantown, WV 26506 Photonic crystals are periodic nano-scale structures used to control the propagation of electromagnetic waves. Their fabrication requires processing that includes highly anisotropic etching to produce deep, straightwalled structures without introducing significant damage. Gallium Nitride (GaN) is a wide bandgap semiconductor suitable for fabrication of photonic crystal structures operating in the visible spectrum and allows incorporation into optoelectronic structures. GaN exists in the wurtzite structure. Due to a lack of inversion symmetry, there are two inequivalent faces, the (0001) Ga-polar and (000-1) N-polar face. The (0001) Ga-polar GaN is chemically inert, whereas the (000-1) N-polar face is easily etched. Etch rates perpendicular to the c-axis lie in between. Furthermore, through Mg doping of GaN, it has well been established that there exists a critical surface concentration of Mg in which the polarity inverts from Ga-polar to N-polar GaN during growth. In this study, KOH in water or ethylene glycol solution was investigated as an anisotropic etchant for GaN. Vertical etch rates were measured on N-polar GaN samples grown by molecular beam epitaxy (MBE). A horizontal etch rate was measured using MOCVD (0001) GaN where the Ga-polar face acted as a mask. A ratio of the N-polar vertical etch rate to the horizontal (10-10) face etch rate was measured. This value was determined to be 50-75:1. Thus, highly anisotropic etching could be obtained. Multiple samples were grown using MBE and the polarity was successfully inverted through Mg doping. These samples were subsequently etched. SEM images will be presented indicating sidewall quality of these structures. These current results open the door for re- growth of the opposite polarity on these structures, resulting in the periodic variation of N and Ga-polar regions. These subsequent structures have applications in non-linear optics in general and can serve as structures to be etched to fabricate photonic crystals. This work was supported in part by a grant from the West Virginia Graduate Student Fellowships in Science, Technology, Engineering, and Math (STEM) program to R.P. Tompkins.
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