Reliable Schottky contact for AlGaN/GaN high electron mobility transistor (HEMT) should use low resistivity, large work function metal. Moreover, the metallisation for this application should withstand high-temperature treatments during device processing (realisation of self-aligned HEMT) without change of structure, electrical conductivity and composition, and be stable in contact with the semiconductor. Recently, several studies have demonstrated the applicability of RuO₂ as Schottky contact to n-GaN. Ru and RuO₂ are high melting point materials, characterised by low resistivity and work function > 5 eV.
With an eye on high temperature applications we have chosen to study thermally stable, electrically conducting, amorphous Ru-Si-O films being a combination of conducting RuO₂ and SiO₂. We compare Ru, RuO₂ and Ru-Si-O Schottky contacts fabricated on n-GaN and AlGaN/GaN HEMT heterostructure. Ti/Al ohmic contacts were prepared for this study. Ru, RuO₂ and Ru-Si-O layers were deposited by sputtering, using either Ru or RuSi target, in a mixture of Ar and O₂. Heat treatments were done at 400-1000⁰C in either O₂ or N₂ flow. The electrical properties of contacts were extracted from I-V and C-V characteristics. Contact microstructure was analysed by high-resolution XRD, SIMS and RBS.
Crucial point during the deposition was to optimise the O₂/Ar ratio and the working pressure with regard to thin film resistivity and stress.
Of three investigated metallisations, Ru films yielded the lowest potential barriers. Although barrier heights of RuO2 and Ru-Si-O Schottky contacts were very similar, amorphous Ru-Si-O contact exhibited significantly lower reverse leakage current and superior thermal stability. We will discuss in detail the behaviour of Ru-based contacts in terms of electronic interactions at either crystalline or amorphous metallisation/semiconductor interface.
Work supported by the grant from the State Committee for Scientific Res.3 T11B 008 026.

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