Reflectivity spectroscopy of the effects of successive etching of AlAs/AlGaAs Distributed Bragg Reflectors
The effects on the reflectivity spectra of successive etching of an AlAs/AlGaAs Distributed Bragg Reflector are studied. Etching was done using base piranha solution (NH4OH + H2O2 + H2O) and characterization was done using a normal reflectivity set-up. The central reflectivity of the DBR was observed at 9100Å, with full-width at half maximum of 875Å. With the design wavelength of 8500Å, the disparity was attributed to the error in either layer thickness or molar composition of Al in AlxGa1-xAs. The error was found improbable for Al mole fraction alone. When error caused by layer thickness was investigated, it was found that the layer thickness for AlAs was 767Å with 8.3% error, and AlGaAs 676Å with 10.6% error. Each successive etching reduced the peak reflectance and increased the full-width at half maximum of the reflectance spectra. A big drop in reflectivity was also observed after the 6th etching, and the peak reflectivity of the DBR were less than the expected values. These observations were attributed to the changes in the etchant and the surface morphology produced by etching.