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Base metallization stability in InP/InGaAs heterojunction bipolar transistors and its influence on leakage currents

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The defect structure in lattice-mismatched Al0.25Ga0.75As/InxGa1−xAs/GaAs heterojunction bipolar transistors (HBTs) was investigated using cathodoluminescence (CL), planar and cross-sectional transmission electron microscopy (TEM), and electron-beam-induced current images of the emitter-base and base-collector junctions. It was found that the individual techniques listed above cannot lead to a complete understanding of the defect structure; however, by combining techniques, we are able to derive a detailed knowledge of the dislocation structure, formation, and nonradiative recombination properties in the HBT structures. We find that misfit dislocations predominately form at the mismatched interface located most deeply in the structure. For noninverted HBT structures, which have the AlGaAs emitter/InGaAs base junction on top, we find the majority of misfit defects near the base-collector junction at the bottom InGaAs/GaAs interface. At low dislocation density interfaces, dark line defects observed in CL and misfit dislocations seen in TEM micrographs correlate one to one, but at high densities the dark lines correspond to groups of misfit dislocations. At high In concentrations, a dense network of defects forms at the bottom InGaAs/GaAs interface. The dislocations that are expelled by interdislocation forces from this interface propagate into the GaAs side of the interface, leaving the base region and the emitter-base interface relatively free of misfit dislocations. The relaxation of the strained base by misfit dislocation formation at the InGaAs/GaAs interface is much slower than that predicted using mechanical equilibrium theory. Utilizing the above results, we describe a method for obtaining dislocation-free interfaces in mismatched multilayer structures. This method can be incorporated in lattice-mismatched HBT structures to obtain dislocation-free emitter-base junctions.

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