Alternative Quantum Well Systems for Asymmetric Fabry-Perot Modulators

Electroabsorption in three quantum well structures, the rectangular quantum well, asymmetric triangular quantum well and coupled quantum well, was explored to determine if a suitable alternative to the use of the standard rectangular quantum well in an asymmetric Fabry-Perot modulator could be found. The absorption coefficient was measured as a function of wavelength and applied electric field in the two new types of quantum well and compared to that of the rectangular well. It was found that a new method using simultaneous reflectance and transmittance measurements with an approximate model to extract the absorption coefficient on appropriately designed test structures had good accuracy. The asymmetric triangular quantum well was found to have a much lower absorption coefficient than the equivalent rectangular well, due its much larger width, making it a poor replacement. The coupled well displayed a stronger absorption with applied field, but its absorption edge did not shift as easily as that of the rectangular quantum well. The reflectance performance of modulators based upon the rectangular and coupled wells was simulated using the measured absorption coefficients. Modulator structures were then grown by molecular beam epitaxy and devices fabricated. Both types of modulators were found to have very good performance compared to previously published results, with some devices having the largest contrast ratios ever reported. Measurements of six performance parameters on the two types of modulators were compared and it was found that the rectangular and coupled well types were of roughly equal performance.