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(Invited) Low-Resistance Ohmic Contacts to Al0.45Ga0.55n/ Al0.3Ga0.7n HEMTS

Monday, 2 October 2017: 14:50
Chesapeake A (Gaylord National Resort and Convention Center)
B. Klein, A. G. Baca, A. M. Armstrong, A. A. Allerman, E. A. Douglas, C. Sanchez, P. Kotula, M. Miller, and S. Reza (Sandia National Laboratories)
RF amplifiers and power electronics transistors in III-N semiconductors are increasingly important in emerging applications, as are photo-transistors for the purpose of UV light detection. Increasing the Al fraction in AlGaN-based HEMT devices enables transistors with higher breakdown voltages and a potentially favorable breakdown - specific on-resistance tradeoff. However, the advantages of higher aluminum content comes with a major challenge: Ohmic contacts with acceptable contact resistance become increasingly problematic with increasing Al content in the channel and barrier layer. We report sub 10-4 W-cm2 specific contact resistivity (rc) for a 30% channel, improving on the best prior result of approximately 2x10-4 W-cm2for AlGaN channels of at least 30% Al.

We investigated five different Ohmic contact metallizations consisting of X/Al/Y/Au where X = Ti, Nb/Ti, Zr, or V, and Y = Ni, Mo, or V (Table 1) as function of alloying conditions. The MOCVD-grown structure (Figure 1) is on sapphire substrates and consists of a 1.6 µm AlN buffer layer, a 4.15 µm thick Al0.3Ga0.7N channel layer and a 50 nm thick Al0.45Ga0.55N barrier with Rsheet of 3500 Ω/□ and ns= 5x1012/cm2. Representative current-voltage plots measured on circular TLM structures are shown in Figure 2. Ohmic contacts with average specific contact resistance (pc) of 7x10-5 Ω-cm2 (3.3x10-5 Ω-cm2) extracted from device (TLM) data were achieved with a conventional Ti/Al/Ni/Au metal stack and a multi-temperature stepped-anneal process. Structural characterization was carried out on the Ti/Al/Ni/Au contacts, to compare Ohmic and Schottky conditions. TEM was used to characterize the interaction between the contact metals and the AlGaN, revealing an interface consisting primarily of Ti-Au with limited indication of spiking behavior. Using the optimized Ohmic contact process, HEMTs with 10 µm source-to-drain spacing and 2 µm gate lengths were fabricated; current densities of 78 mA/mm were achieved, and parasitic loading of RON was reduced.

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.