Investigation of IrO2 and RuO2 Schottky contacts on AlGaN/GaN heterostructure

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Abstract
Electrical properties of rare-earth metal contacts on AlGaN/GaN heterostructure were interpreted in terms of the changes in microstructure and chemical bonding state. When the contacts were annealed under oxygen ambient at 500 °C, the Schottky barrier height increased from 0.56 to 1.10 eV for the Ru and from 0.68 to 1.07 eV for the Ir contact. Moreover, the reverse leakage current at −10 V dramatically reduced by 4 orders of magnitude by oxidation annealing. Such an improvement originated from the formation of RuO2 and IrO2, playing a key role in increasing the solubility of group-III atoms, namely, Ga and Al atoms. As a result, the surface Fermi level shifted toward the energy levels of group-III vacancies, resulting in the increase of Schottky barrier height. The electrical properties of heterostructure field effect transistor (HFET) applying the Ru gate contact significantly degraded when the device was annealed at 500 °C under O2 ambient. This was due to the indiffusion of the Ru atoms into the AlGaN layer during the oxidation annealing. However, no electrical degradation was found in the HFET using the Ir gate contact. The maximum drain current density of 714 mA/mm and transconductance of 171 mS/mm were kept even after annealing at 500 °C.