W 2 B based High Thermal Stability Ohmic Contacts to n-GaN
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W2B based High Thermal Stability Ohmic Contacts to n-GaN Rohit Khanna(1), S.J.Pearton(1), C.J.Kao(2), I.Kravchenko(3), F.Ren(3), G.C.Chi(2) , A.Dabiran(4) and A.Osinsky(4) (1) Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 USA (2) Department of Electrical Engineering, National Central University, Chung-Li 32054,Taiwan (3) Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 USA (4) SVT Associates, Eden Prairie, MN 55344 ABSTRACT A novel metallization scheme for Ohmic contact (Ti/Al/ W2B /Ti/Au) to n-GaN using high temperature boride was studied using contact resistance, scanning electron microscopy and Auger Electron Spectroscopy measurements. A minimum contact resistance of 7x10-6 Ω.cm2 was achieved for W 2B based scheme at an annealing temperature of 800 °C. Contact resistances were found to be essentially independent of measurement temperature, indicating that tunneling plays a dominant role in the current transport. The outdiffusion of Ti to the surface at temperatures of ~500°C, and at 800°C the onset of intermixing of Al within the contact was found to occur. By 1000°C, the contact showed a reacted appearance and AES showed almost complete intermixing of the metallization. The reliability measurements for the contact resistance of W 2B based contact showed excellent stability for extended periods at 200°C, which simulates the type of device operating temperature that might be expected for operation of GaN-based power electronic devices. INTRODUCTION There is a strong interest in the development of more reliable and thermally stable Ohmic contacts on GaN-based electronic devices such as high electron mobility transistors (HEMTs)(1-33) ,which show outstanding potential in advanced power amplifiers for radar and communication systems over a broad frequency range from S-band to V-band(1-3). A key aspect of operation of nitride-based HEMTs at high powers is the need for temperature-stable high quality Ohmic contacts. There is increasing interest in the application of AlGaN/GaN HEMTs to microwave power amplifiers capable of uncooled or high temperature operation where thermal stability of the contact metallurgy is paramount. The most common Ohmic metallization for AlGaN/GaN HEMTs is based on Ti/Al. This bilayer must be deposited with over-layers of Ni, Ti or Pt, followed by Au to reduce sheet resistance and decrease oxidation during the high temperature anneal needed to achieve the lowest specific contact resistivity (4-24). For improving the thermal stability of Ohmic contacts, there is interest in high temperature metals such as W (25,26,28) ,WSiX(25-27),Mo (4),V(27) and Ir(28) and potential boride-based contacts such as
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CrB2 ,ZrB2,TiB2 and W 2B5.These metallization systems are expected to be less reactive with GaN than the conventional Ti/Al. Previous work has shown good contact resistance obtained with Ti/Al/Mo/Au, Ti/Al/Ir/Au, Ti/Al/Pt/WSi/Ti/Au and Ti/Al/Pt/W/Ti/Au on n-GaN(4,27) ,but there is a broad r
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