Monitoring thickness changes in GaAs/AlAs partial VCSEL Bragg reflector stacks using optical spectroscopic, x-ray and electron microscopic methods

Vertical-cavity surface-emitting lasers (VCSELs) are complex multi-layer st ructures whose operating characteristics are highly sensitive to variations in layer thickness and composition. We have made non-destructive measureme nts of the thickness of GaAs/AlAs layers in three wafer-sized VCSEL substru ctures, consisting of partial distributed Bragg reflectors, grown by metal- organic vapour-phase epitaxy. A range of techniques were investigated and c ompared, including reflectance, photo-modulated reflectance, spectroscopic ellipsometry and high-resolution x-ray diffraction. The results obtained fr om each technique for the layer thicknesses are equal to each other to with in +/- similar to1%. The precision of the optical techniques is as good as, and sometimes better than, direct measurements made using destructive cros s-sectional transmission electron microscopy.