Comprehensive study of AuMn p-type ohmic contact for GaAs/GaAlAs heterojunction bipolar transistors

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Abstract
A comprehensive study of AuMn ohmic contact for p‐type GaAs is presented using electrical testing, scanning electron microscopy, Auger electron spectroscopy, and secondary ion‐mass spectroscopy. The contact fabrication—Mn proportion against Au, alloying cycle—was optimized taking into account the results of the electrical tests. Contact resistivity values are presented for different doping levels. Unalloyed AuMn contact was found to give a good contact resistivity on samples with an ultrahigh doping level of 2 × 1020 cm−3. The reliability of the contact was also studied, the reproducibility was found to be perfect and the surface morphology smooth. The contact metallurgy has also been investigated. The Mn diffusion was shown to be a shallow one. The contact formation has been demonstrated combining the classical mechanism of Ga vacancies filled with Mn acceptors and the out‐diffusion of the dopant (Be) from the epitaxial layer toward the metal/GaAs interface due to the presence of Mn at this interface.