Oxidation Kinetics and Microstructure of Wet-Oxidized MBE-Grown Short-Period AlGaAs Superlattices
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Oxidation Kinetics and Microstructure of Wet-Oxidized MBE-Grown Short-Period AlGaAs Superlattices René Todt1, Katharine Dovidenko, Alexei Katsnelson, Vadim Tokranov, Michael Yakimov, and Serge Oktyabrsky UAlbany Institute for Materials, University at Albany-SUNY, Albany, NY 12203, U.S.A. 1 currently Walter Schottky Institute, Technical University Munich, 85748 Garching, Germany ABSTRACT The kinetics of the wet oxidation process of MBE-grown high-Al-content AlAs/Al0.6Ga0.4As short-period superlattices (SPSLs) was investigated and compared to AlGaAs alloys and pure AlAs. We found that alloys and superlattices (SLs) have different oxidation characteristics. These differences were attributed to traces of the superlattice structure in the oxidized material. The microstructure and chemistry of SPSLs with an equivalent composition of Al0.98Ga0.02As was studied, using transmission electron microscopy, energy-dispersive x-ray spectroscopy, Rutherford backscattering, and nuclear reaction analysis for hydrogen-profiling. We also report on the mechanical stability of oxidized SPSL layers in optoelectronic device structures. INTRODUCTION Recently, selective wet oxidation of AlGaAs alloys has gained interest for various applications in electronic and optoelectronic devices [1-3]. So far, wet oxidation has had its biggest impact on performance of AlGaAs/GaAs oxide-apertured vertical cavity surface emitting lasers (VCSELs), where devices with the lowest threshold currents and highest wall plug efficiency have been demonstrated [4]. As it is well known by now that VCSELs employing oxidized AlAs layers suffer from delamination and degradation problems [5], high-Al-content AlGaAs layers are generally preferred as wet oxidation layers. Devices employing AlGaAs alloys as wet oxidation layer were reported to be mechanically stable [5]. Also, VCSEL devices with an AlAs/GaAs superlattice (SL) oxidation layer with an equivalent Al content of 98 % were reported to be mechanically stable during post-growth processing, except for minor structural degradations on the edge of the mesa [6]. Due to the high selectivity of the wet oxidation process with respect to the Al content of the layer to be oxidized, a precise control over the composition during molecular beam epitaxy (MBE) growth of the structure is required in order to achieve reproducible oxidations [7]. The growth of high-Al-content alloys is, however, problematic with respect to reproducibility. Shortperiod superlattices (SPSLs), also termed digital-alloys, provide an enhanced control over the composition as compared to alloys. In addition, digital alloys of a wide variety of compositions can be grown without changing the effusion cell temperatures. Moreover, SPSLs are of great technological importance in MBE growth of optoelectronic device structures as they grow more smoothly than alloys, which is especially important when very thick structures like VCSELs with a total thickness in excess of 10 µm are grown. This paper reports on wet oxidation kinetics, structure chemistry and mechani
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