Microstructural and Electrical Analyses of Oxygen Diffusion into Iridium Metal Gates

Full text: Iridium is used as gate metal for GaN-based In0,17Al0,83N/AlN barrier High Electron Mobility Transistors (HEMTs). Before annealing, a high oxygen concentration confined at the Ir/InAlN interface is detected, but neither an aluminium nor an indium oxide interfacial layer has been formed. In this work we investigate the annealing-induced diffusion of the oxygen-rich interlayer and its electrical effects on the device. No stable iridium oxide forms - instead oxygen is able to diffuse out of the interlayer into the iridium gate metal, thus the interlayer is reduced. Above 700oC a homogeneous oxygen concentration is observed in the iridium layer, whereas at 500oC oxygen is distributed inhomogeneously. This behaviour is also verified electrically. The diffusion length of oxygen in evaporated iridium is only in the order of 1 nm for 2 minutes annealing at 500oC. Therefore, oxygen cannot diffuse efficiently in dense iridium at that temperature, so that oxygen diffusion, enhanced by crystal defects and grain boundaries, is assumed. Annealing at 700oC increases the diffusion length to about 100 nm and allows homogenous diffusion of oxygen into iridium, leading to the most complete removal of the oxygen interlayer, as seen from the C-V-measurements. Additionally, the analysis of the microstructure proves that the thickness of the InAlN/AlN barrier does not change after annealing and metal does not diffuse into the barrier. That confirms the already proven robustness of InAlN. The rapid thermal annealing was performed for 2 minutes at 400oC, 500oC and 700oC. Two samples with different gate metals (iridium and nickel) are analysed in order to investigate the dependence on the metal. High-Resolution Transmission Electron Microscopy (HRTEM) gives detailed analysis of the microstructure at the different interfaces. Electron Energy Loss Spectroscopy (EELS) shows the two-dimensional element distribution in the samples. I-V- and C-V-measurements are used to determine the electrical properties. (author)