Fabrication and modeling distributed Bragg reflectors

Abstract

Distributed Bragg reflectors (DBRs) are dielectric mirrors composed of alternating high and low refractive index materials, which use interference effects from partial reflections within the stack to reflect a tunable range of wavelengths. This property makes DBRs valuable in photonic applications such as lasers, telecommunications, optical filters, and photovoltaics. To achieve a high and uniform reflection band, DBRs require high-quality films, traditionally fabricated using time- and energy-intensive physical vapor deposition methods such as sputtering. Typically, achieving near-100% reflectivity in DBRs requires increasing the number of layer pairs, often exceeding 10 pairs, which in turn demands greater fabrication resources.

In this thesis the main sputtering parameters affecting deposition time for Silicon dioxide (SiO2) and Tantalum pentoxide (Ta2O5) are identified and optimized for DBR purposes. The process of designing and modeling DBRs with established software (WVASE®) is introduced and a novel MATLAB code is developed for simulating the reflectivity spectrums of DBRs. The MATLAB code is intended to be more concise and easier to understand compared to existing codes by focusing purely on the reflectivity modeling by utilizing Fresnel coefficients in a transfer matrix formalism.

The acquired optimized deposition parameters are presented along with the MATLAB code. Resulting reflectivity spectrums from established WVASE® modeling software, the MATLAB code and measured samples are compared, and it is found that the built MATLAB code matches closely. An optimal stack configuration for DBRs is also reported, which achieves close to 100% peak reflectivity with under seven-layer pairs. Notably, a 175% increase in the sputtering rate of Ta2O5 is also reported.