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.