Ta-based ohmic contacts to gallium nitride high electron mobility transistor (GaN HEMT) epitaxial structures were investigated. Two metallization schemes were considered: Ta/Al/Ni(Ta)/Au and Ta/Al/Ta. The latter was superior in terms of lower contact resistance (R c ) and wider process window. The metallizations were applied to two different heterostructures (GaN/Al 0.14 Ga 0.86 N/GaN and Al 0.25 Ga 0.75 N/GaN). The lowest measured R c was 0.06 and 0.28 mm, respectively. The main advantage of the Ta-based ohmic contacts over conventional Ti-based contacts was the low anneal temperature. The optimum temperature of annealing was found to be 550-575 • C. From optical and scanning electron microscopy, it was clear that excellent surface morphology and edge acuity were obtained at these low temperatures. This facilitates lateral scaling of the GaN HEMT. TEM images were taken of the contact cross sections onto which EDX measurements were performed. The aim was to investigate the microstructure and the contact mechanism. Storage tests at 300 • C for more than 400 h in air ambient showed no deterioration of R c .

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The Schottky contact on GaN HEMT power devices is a key element for high power performance and its long term reliability behavior. Up to now such GaN devices are still suffering very often by a leakage current increase during operation resulting in deteriorated transistor behavior. One reason for this problem is owing to insufficiently stable Schottky contacts based on NiAu metalisation. We analysed GaN HEMTs with different aging behaviors by means of HR‐TEM and EDX investigations. It could be shown that on transistors with increasing leakage current Au was found directly on the Schottky interface which may be responsible for the increase of the leakage current. Furthermore, there are strong indications that such Au is diffusing via the gate foot side wall Ni‐SiN interface towards the Schottky barrier. It is proposed that an additional metallic diffusion barrier may solve this aging problem. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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