A thorough study of the selective wet oxidation in digital AlxGa1–xAs alloys is presented. We report experimental results and physical interpretation on the oxidation kinetics within those ranges of the AlGaAs composition (x = 0.95 to 1) and layer thickness (20 to 50 nm) of interest for oxide-aperture vertical-cavity surface-emitting laser (VCSEL) application. We demonstrate the high controllability of the oxidation reaction between different Al compositions; made different thanks to the use of digital alloys. Unlike standard alloys, we measured an invariability of the oxidation rates in the studied thickness range (20–50 nm), implying a better control of the fabrication process. The dependence of the reaction rate with the temperature is expressed as an Arrhenius law. Two activation energies (1.2 and 0.55 eV) have been derived for composition ranges of x = 0.95–0.98 and x = 0.99–1, respectively, revealing that two different mechanisms are involved depending on the Al content and the superlattice structure of the digitally-grown AlGaAs.
AbstractThe oxidation of high Al content AlxGa1-xAs has received much attention due to its use in oxide-aperture, vertical-cavity surface emitting lasers (VCSELs) and for passivating AlAs against environmental degradation. We have recently identified the spinel, gamma phase of Al2O3 in layers laterally oxidized in steam at 450°C for =0.98 & 0.92 and have seen evidence for an amorphous precursor to the gamma phase. At the interface with the unoxidized AlxGa1-xAs , an ~17nm amorphous phase remains which could account for the excellent electrical properties of oxide-confined VCSELs and help reduce stress concentrations at the oxide terminus.
Single-Mode and Polarization-Stable VCSELs with Diamond-Shaped Oxide Aperture