Impact of postdeposition annealing upon film properties of atomic layer deposition-grown Al₂O₃ on GaN

Atomic layer deposition-grown Al₂O₃ thin films are grown on n-type GaN and annealed at 300 or 500 °C in various atmospheres. Metal–insulator–semiconductor capacitors (MISCAPs) are used as simplified test structures for AlGaN/GaN heterostructure field effect transistors with an Al₂O₃ gate dielectric. Electrical characterization of the unannealed MISCAPs reveals a low leakage current density of ∼1.4 × 10⁻⁹ A/cm² at −2 MV/cm. Annealing at 500 °C in N₂ or a forming gas results in a degradation of this leakage level by more than one order of magnitude, whereas the leakage current of the Al₂O₃ films annealed at 500 °C in O₂ is increased to ∼5.2 × 10⁻⁹ A/cm² at −2 MV/cm. The photoassisted capacitance–voltage technique, the conductance method, and border trap analysis are used to study the influence of the annealing ambient atmosphere upon the Al₂O₃/GaN interface. For all atmospheres, thermal treatments at 500 °C marginally affects the border oxide trap density, but the forming gas anneal at 500 °C passivates the interface traps most efficiently. While the O₂ thermal treatment reduces the interface trap density in the Al₂O₃/GaN system, the N₂ anneal creates interface trap states, indicating the formation of an oxygen deficient defect level at the Al₂O₃/GaN interface during N₂ annealing.