Depth uniformity of electrical properties and doping limitation in neutron-transmutation-doped semi-insulating GaAs

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Abstract
Depth uniformity of electrical properties has been evaluated for neutron-transmutation-doped (NTD), semi-insulating GaAs irradiated with thermal neutrons of 1.5×1018 cm−2 by the van der Pauw method combined with iterative etching of the surface. In NTD-GaAs wafers (thickness ∼410 μm) annealed for 30 min at 700 °C, the depth profiles of the resistivity, the carrier concentration, and the Hall mobility show constant values of 1×10−2 Ω cm, 2.0×1017 cm−3, and 3100 cm2/V s, respectively, within an experimental error of 5%. In an annealing process, the redistribution and/or the segregation of NTD impurities is not observed. We also discuss the limitations of low-level NTD in semi-insulating GaAs. It is suggested that the activation of the NTD-impurities below ∼1×1016 cm−3 is mainly restricted by the presence of the midgap electron trap (EL2).