Au-free ohmic Ti/Al/TiN contacts to UID n-GaN fabricated by sputter deposition

Abstract

The fabrication and characterization of an Au-free Ti/Al/TiN (20/100/100 nm) contact stack to unintentionally doped n-GaN with TiN serving as the diffusion barrier is presented. Sputter deposition and lift-off in combination with post deposition annealing at 850 °C are used for contact formation. After annealing, contact shows ohmic behavior to n-GaN and a specific contact resistivity of 1.60 × 10−3 Ω cm2. To understand the contact formation on the microscopic scale, the contact was characterized by current–voltage measurements, linear transmission line method, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results show the formation of Ti-N bonds at the GaN/Ti interface in the as-deposited stack. Annealing leads to diffusion of Ti, Al, Ga, and N, and the remaining metallic Ti is fully consumed by the formation of the intermetallic tetragonal Al3Ti phase. Native oxide from the GaN surface is trapped during annealing and accumulated in the Al interlayer. The TiN capping layer, however, was chemically stable during annealing. It prevented oxidation of the Ti/Al contact bilayer successfully and thus proved to be a well suitable diffusion barrier with ideal compatibility to the Ti/Al contact metallization.

Au‐Free Ohmic Contacts and Their Impact on Sub‐Contact Charge Carrier Concentration in AlGaN/GaN Heterostructures

“…As can be seen, there is a local minimum at T RTA ¼ 650 C, which seems to correlate to the melting point of Al at T m (Al) ¼ 660 C, which is with a thickness of 100 nm the main component of the used contact metallization, forming Al 3 Ti alloys as shown in our previous work. [14] Furthermore, Al diffuses down into the (Al)GaN region below the contact area during RTA treatment. [14] At RTA temperatures above 700 C, the total resistance decreases again (except for the point at 900 C).…”

“…[14] Furthermore, Al diffuses down into the (Al)GaN region below the contact area during RTA treatment. [14] At RTA temperatures above 700 C, the total resistance decreases again (except for the point at 900 C). To determine how the contact processing is degrading the 2DEG sheet resistance R sh of the channel, the initial R sh of the heterostructure substrate was measured by Eddy current before any processing.…”

“…The Ti/Al/Ti/TiN (20/100/20/80 nm) multilayer metal deposition process was done using magnetron sputtering at 400 W DC (Ti and TiN) and 600 W RF (Al), with reactive sputtering for TiN, as described in our previous publication. [14] The metal layer deposition was followed by a RTA process up to 900 C for 30 s in an N 2 environment at atmospheric pressure.…”

“…[13] If Ti is deposited on (Al)GaN using sputtering, TiN-formation is already triggered at room temperature, as shown in our previous work. [14] If oxygen from the GaN surface substitutes these vacancies, the region below the contacts consequently becomes heavily n-doped. [15,16] This leads to pinning of the Fermi level [12,17] and thus to sharp semiconductor band bending, [1,13,18] allowing current transport in form of quantum mechanical tunneling with a high probability (Figure 2).…”

Formation and preferential orientation of Au-free Al/Ti-based ohmic contacts on different hexagonal nitride-based heterostructures

“…As such, the third condition seems to be critical to form a low contact resistance in the Al-rich regime. We propose that some reaction with Ti is required in combination with the presence of epitaxial Al 61 . We speculate that the reaction of Ti can result in to 600 • C. As a fibre alignment does not result in a systematic geometry at the interface between semiconductor and contact, in contrast to epitaxy, one can expect that a fibre-aligned contact has poorer contact resistivity values.…”