ABSTRACT
The critical issue of this structure is the aluminum content of the n-contact layer. The aluminum oxide is electrically isolating, which means that the electrons need to be injected laterally. To avoid optical absorption in then-contact layer, AlxGa1_xAs with x:?: 0.5 has to be used. The resistivity and the vertical oxidation rate of this layer are increasing with aluminum content. As a compromise the aluminum content of this layer was fixed to 70%. The lateral oxidation rate of AlxGa1.xAs layers increases exponentially with aluminum content and therefore high aluminum content layers can be selectively oxidized [6]. In this structure the x = 0.7 n-contact layer is on top of the last x = 0.98 layer, which means that, as the oxidation of the x = 0.98 layer proceeds, the x = 0.7layer oxidizes vertically from the interface. Since this layer is quite thin, it will be completely oxidized within a short time, even though its oxidation rate is about two orders of magnitude lower than for the x = 0.98 mirror layers. Therefore the oxidation has to be stopped before the layer is completely oxidized, otherwise no current could be injected. As a result the x = 0.98 layers in the DBR were not completely oxidized in the present experiment. In the center of the diodes a square of about 15-20% of the total surface remains therefore nonoxidized.
