Strained-layer-superlattice technology for vertical-cavity optoelectronic modulators at near-infrared wavelengths

We present recent results on vertical Fabry-Perot cavity reflectance modulators grown using strained-layer epitaxy in the (InAlGa)As material system. Using molecular-beam epitaxy, we have successfully developed devices operating at wavelengths between 1.0 and 1.3 μm. Our approach employs a novel combination of strained and unstrained multilayers grown in a mechanically stable configuration to reach wavelengths longer than possible with lattice-matched (AlGa)As materials. The key to successful device operation is the growth of high-quality strain-relaxed buffer layers to provide an appropriate lattice constant for subsequent growth of the active device structure. For devices operating at 1.3 μm, 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 temperature. We also investigate the dependence on growth temperature of the quality of the devices´ mirror stacks and superlattice active regions. An optimized modulator structure has an rms surface roughness of 8.2 nm, corresponding to a calculated degradation in specular reflectance of only 0.4%. This device, which has a one-wavelength-thick cavity region, 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