STRAINED-LAYER-SUPERLATTICE TECHNOLOGY FOR VERTICAL-CAVITY OPTOELECTRONIC MODULATORS AT NEAR-INFRARED WAVELENGTHS

We present recent results on vertical Fabry-Perot cavity reflectance m odulators grown using strained-layer epitaxy in the (InAlGa)As materia l system. Using molecular-beam epitaxy, we have successfully developed devices operating at wavelengths between 1.0 and 13 mum. Our approach employs a novel combination of strained and unstrained multilayers gr own in a mechanically stable configuration to reach wavelengths longer than possible with lattice-matched (AlGa)As materials. The key to suc cessful device operation is the growth of high-quality strain-relaxed buffer layers to provide an appropriate lattice constant for subsequen t growth of the active device structure. For devices operating at 1.3 mum, we use buffer compositions graded to a final mismatch to the GaAs substrates of 2.4%. We discuss the optimization of surface smoothness of these relaxed buffers with respect to composition and growth tempe rature. We also investigate the dependence on growth temperature of th e quality of the devices' mirror stacks and superlattice active region s. An optimized modulator structure has an rms surface roughness of 8. 2 nm, corresponding to a calculated degradation in specular reflectanc e of only 0.4%. This device, which has a one-wavelength-thick cavity r egion, was designed for free-space communications applications. It has a 4 : 1 contrast ratio, exhibits a 4-dB insertion loss, and operates at a 5.5-V applied bias.