Ta-based interface ohmic contacts to AlGaN/GaN heterostructures

Al/Ti based metallization is commonly used for ohmic contacts to n-GaN and related compounds. We have previously reported an ohmic contact scheme spec ially designed for AlGaN/GaN heterostructure field-effect transistors (HFET s) [D. Qiao , Appl. Phys. Lett. 74, 2652 (1999)]. This scheme, referred to as the "advancing interface" contact, takes advantage of the interfacial re actions between the metal layers and the AlGaN barrier layer in the HFET st ructure. These reactions consume a portion of the barrier, thus facilitatin g carrier tunneling from the source/drain regions to the channel region. Th e advancing interface approach has led to consistently low contact resistan ce on Al0.25Ga0.75N/GaN HFETs. There are two drawbacks of the Al/Ti based a dvancing interface scheme, (i) it requires a capping layer for the ohmic fo rmation annealing since Ti is too reactive and is easily oxidized when anne aling is performed in pure N-2 or even in forming gas, and (ii) the atomic number of Al and that of Ti are too low to yield efficient backscattered el ectron emission for e-beam lithographic alignment purposes. In this work, w e investigated a Ta based advancing interface contact scheme for the HFET s tructures. We found that the presence of Ta in this ohmic scheme leads to ( 1) a specific contact resistivity as low as 5x10(-7) Omega cm(2), (2) effic ient electron emission for e-beam lithographic alignment, and (3) eliminati on of the capping layer for the ohmic annealing. (C) 2001 American Institut e of Physics.