Modeling of electronic states in low-temperature-grown gallium arsenide and conduction properties of tunneling based contact

A set of defect distribution models for un-annealed low-temperature-grown GaAs (LTG:GaAs) are developed, based on experimental observations with scanning tunneling spectroscopy, incorporating the Coulomb gap and Hubbard correlation in the distribution of the density of states. The models are tested against independent experiments on bulk resistivity and surface electric field. With the models as the foundation, the conduction models are developed for nonalloyed ohmic contacts, in micron-scale and nanometer-scale, to n-type GaAs (n-GaAs) which employ a surface layer of LTG:GaAs. For the micron-scale ohmic contact, the conduction model has been used to fit measured ρ_c versus LTG:GaAs layer thickness and versus measurement temperature. These comparisons provide insights into the contact mechanism (electron tunneling between metal states and conduction band states in n-GaAs) and indicate that low potential barrier heights (due to un-pinned surface Fermi level) and the high density of activated donors (∼1020 cm−3) in n++ GaAs have been achieved in these ex situ contacts. For the nanometer-scale ohmic contact, the conduction model is based on the sequential tunneling through two barriers—in xylyl dithiol molecules and in LTG:GaAs/n++GaAs depletion region—with part of LTG:GaAs midgap defect band in between. The model has been used to fit ρ _c versus undoped and Be-doped contacts. It shows quantitatively that the abundance of the available states around the Fermi level in Be-doped case contributes to the superior contact performance.