Date of Graduation

2009

Document Type

Thesis

Abstract

Photonic crystals are periodic nanostructures used to control the propagation of electromagnetic waves. Depending on geometry and refractive index contrast between adjacent regions, periodic variation of the refractive index can result in a photonic band gap or non-allowed set of frequencies that cannot propagate through the crystal. Defects can be introduced at photonic crystal lattice sites, resulting in localized modes that lie within the photonic gap. Such defect cavities can be tuned to resonant frequencies of a defect mode and whole planes or lines of defects can be fabricated in photonic crystals resulting in optical confinement of defect modes. These properties of photonic crystals make them useful in a wide variety of applications such as chemical and biological sensors, high Q lasers, and optical wave guiding. With its transparency in the visible wavelength regime of the electromagnetic spectrum, GaN is a candidate for photonic crystal structures with photonic band gaps corresponding to visible wavelengths. GaN is a wide, direct band gap semiconductor which exists primarily in the wurtzite crystal structure. The wurtzite crystal structure lacks inversion symmetry, resulting in two distinct crystal polarities or crystal growth directions, the Ga-polar or [0001] and N-polar or [0001¯]. Through choice of substrate or growth conditions, GaN can be grown with either polarity. An unusual, but potentially useful, result is that by generation of near-monolayer surface coverage of Mg, the crystal polarity can be inverted during growth from gallium polar to nitrogen polar without introducing any additional defects at the domain boundary. Subsequent patterning and etching of the inversion layer, followed by re-growth, results in periodically poled GaN. This changes the nonlinear optical response of the material and such a structure can be used in a variety of applications. This study uses a subsequent highly anisotropic wet etch of polarity inverted GaN to selectively etch N-polar regions, where Ga-polar regions remain unaffected without introducing any additional structural damage. This wet etching technique for fabricating nanostructures has potential advantages over other dry etching techniques, such as inductively coupled plasma and reactive ion etching. The specific aim of this work is to develop the knowledge and techniques to allow fabrication of GaN photonic crystals via wet etching of periodically poled GaN. Growth conditions for polarity inversion by Mg doping during Molecular Beam Epitaxy growth of GaN, as well as process development for fabrication of photonic crystal structures on both the micron and nanometer scales are investigated. This study also involves theoretical modeling using MIT photonic bands software to determine photonic crystal geometries for the fabrication of GaN photonic crystals with photonic band gaps in the visible as well as the infrared wavelength regimes for future optical characterization. This work is part of a larger collaborative effort at West Virginia University for the design, fabrication, and testing of a flow-though, resonant florescence based GaN photonic crystal biosensor.

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