Selective Chemical Etching of GaN for Non Linear Optics

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					    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.