Abstract
This paper compares the metal/semiconductor barrier height properties of non-recessed Ti/Al/Ti and Ta/Al/Ta contacts on AlGaN/GaN heterostructures. Both contacts exhibited a rectifying behavior after deposition and after annealing at temperatures up to 550 °C. The ohmic behavior was reached after annealing at 600 °C. High-resolution morphological and electrical mapping by conductive atomic force microscopy showed a flat surface for both contacts, with the presence of isolated hillocks, which had no significant impact on the contact resistance. Structural analyses indicated the formation of the Al3Ti and Al3Ta phases upon annealing. Furthermore, a thin interfacial TiN layer was observed in the Ti/Al/Ti samples, which is likely responsible for a lower barrier and a better specific contact resistance (c = 1.6 10−4 Ωcm2) with respect to the Ta/Al/Ta samples (c = 4.0 10−4 Ωcm2). The temperature dependence of the specific contact resistance was described by a thermionic field emission mechanism, determining barrier height values in the range of 0.58–0.63 eV. These results were discussed in terms of the different microstructures of the interfaces in the two systems.
Highlights
Gallium Nitride (GaN) and relative alloys have been considered as very good materials for modern high-power and high-frequency devices to replace existing silicon (Si) technology [1,2]
The temperature dependence of the specific contact resistance was described by a thermionic field emission mechanism, determining barrier height values in the range of
The existence of a two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures is the key feature for the fabrication of high electron mobility transistors (HEMTs) [3]
Summary
Gallium Nitride (GaN) and relative alloys have been considered as very good materials for modern high-power and high-frequency devices to replace existing silicon (Si) technology [1,2]. The easiest approach to fabricate ohmic contacts without gold (“Au-free”), is to remove the uppermost Au layer in the stack To this aim, many authors proposed the use of low work-function Ti-based or Ta-based systems (Ti/Al, Ta/Al, Ti/Al/TiN) [12,13,14,15,16,17,18], to obtain a low Schottky barrier height ΦB with AlGaN and, a low value of specific contact resistance ρc [Ωcm2 ]. The electrical results have been linked to the structural and morphological modifications occurring after the annealing process
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