Gallium Nitride Ultraviolet Optical Modulators

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Title: Gallium Nitride Ultraviolet Optical Modulators
Author: Oberhofer, Andrew Edward
Advisors: Dr. John F. Muth, Committee Chair
Dr. Richard T. Kuehn, Committee Member
Dr. Salah M. Bedair, Committee Member
Dr. Dennis M. Maher, Committee Member
Dr. Mark Johnson, Committee Member
Abstract: In narrower band gap semiconductors researchers have exploited the ability to manipulate the exciton resonance via the Quantum Confined Stark Effect to make a variety of different types of optical modulators at infrared wavelengths. In this thesis, the large exciton binding energy of Gallium Nitride is used as the basis for ultraviolet optical modulators without the need for quantum confinement. A 5x5 array of UV optical modulators at 360 nm was fabricated. The modulators operated in a transverse geometry and consisted of a GaN active layer surrounded by transparent AlGaN insulating and electrical contact layers. The typical thickness of the GaN layer was 0.4 um so the effects of the electric field on the exciton resonance could be directly observed. A hydrogenic model for the bulk exciton was assumed. The applied electric field opposed the attractive coulomb potential between the electron and hole and broadens the exciton resonance. This results in more or less light through the device depending on the spectral position. To understand the magnitude of the applied field within the device structure a 1D Poisson Solver was used. Spontaneous polarization and piezoelectric effects due to lattice strain between the AlGaN and GaN layers were included in the model and were found to have influence on the device at lower operating voltages. In the electric field modulated devices a contrast ratio of about 20 percent was obtained. In thermally modulated devices, at low frequencies less than 200 Hz large shifts in the band edge led to large contrast ratios as expected. The temperature dependence of the device followed the Varshni relationship and allowed the magnitude of the temperature shift to be quantified. At higher frequencies from 1kHz to 120 kHz an optical modulation of ~ 5 percent was readily observed and was attributed to electronic effects. The limitation of 100 kHz was equipment related and it is conjectured that the modulation bandwidth would extend into the MHz.
Date: 2005-02-28
Degree: PhD
Discipline: Electrical Engineering

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