Low-temperature-annealed Ohmic contacts to ultrathin-AlGaN/GaN heterostructures with no two-dimensional electron gas

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We investigated low-temperature-annealed Ohmic contacts to ultrathin-AlGaN/GaN heterostructures with no two-dimensional electron gas (2DEG), which have extremely high sheet resistances of ∼1012 Ω/□. The Ohmic contacts are formed by metal deposition and low-temperature annealing at <600 °C, where a contact resistance down to ≃0.74 Ω mm can be realized. In order to evaluate the sheet resistance ρs, the sheet electron concentration ns, and the electron mobility μs of the AlGaN/GaN under the Ohmic-metals, we fabricated and characterized multi-probe Hall devices. As a result, we find ρs significantly decreased down to ∼600 Ω/□, ns in the 1012 cm−2 range, and μs>1000 cm2/V-s under the Ohmic-metals, where the relationship between ns and μs suggests that a 2DEG is formed at the AlGaN/GaN heterointerface. The specific contact resistivity weakly depends on both ns and the measurement temperature, being well explained by direct tunneling between the formed 2DEG and the Ohmic-metal. Furthermore, it is found that the sheet resistance after Ohmic-metal removal returns to the initial value of ∼1012 Ω/□. One possible hypothesis to explain this is that polarization doping takes place under the Ohmic-metals.