Microstructure of laterally oxidized Al_xGa_1-xAs layers in vertical-cavity lasers

We have studied the lateral oxidation of Al_xGa_1-xAs (x=0.98 and 0.92) layers contained in vertical-cavity lasers using cross-sectional transmission electron microscopy. We find a fine-grained (∼4 nm) cubic spinel phase of Al₂O₃ in both the 2% Ga- and 8% Ga-oxidized layers. The 8% Ga-oxidized layers contract vertically by 6.7% and not the expected 20% for a fully dense Al₂O₃ layer with the 2% Ga-oxidized layers showing a similar contraction. We observe a ∼17-nm-thick amorphous interface between the oxidized and unoxidized Al_xGa_1-xAs layers, which may account for the excellent electrical properties of these devices. We also observe metastable amorphous cavities associated with the moving reaction front. We infer the reaction proceeds from an initial amorphous phase that then transforms to a porous γ-Al₂O₃ layer.