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

Abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssb.202100312.

Au‐Free Ohmic Contact for GaN High‐Electron‐Mobility Transistors

“…La/Ti/Al/Ti [12] 15/20/90/20 60s @ 830 C 0.97 GaN uid cap (2 nm) Al 0.25 GaN (20 nm) Si substrate Ti/Al/Ti/TiN [13] 20/100/20/80 30s @ 900 C 0.74 GaN cap (2 nm) Al 0.21 GaN (24 nm) Sapphire substrate included only the first and third steps in Figure 1 (ohmic contact formation and device isolation).…”

“…Several successful approaches are proposed to achieve low-resistive Au-free ohmic contacts without any extra efforts such as recess formation or regrowth of a highly doped area in contact regions. [8][9][10][11][12][13] These ideas are summarized in Table 1. In this study, a Ta/Al/Ta metal stack as proposed by Malmros et al [14] has been selected.…”

(Invited, Digital Presentation) High-κ Gate Oxide Integration and Ohmic Contact Development for AlGaN/GaN MISHEMTs

“…For MISHEMTs fabricated by the high-κ last process, ohmic contacts were realized using an Au-free Ti/Al/Ti/TiN (20/100/20/80 nm) multilayer stack with post deposition rapid thermal annealing (RTA) at 800°C for 30 s in N 2 environment at atmospheric pressure. Fabrication details are published elsewhere [26], [27]. To prevent a Schottky barrier between metallization and the GaN substrate with its very low electron affinity (Χ GaN = 4.1 eV [28]), Ti has established as the preferred contact layer because of its low work function (Φ Ti = 4.33 eV [29]) [11], resulting in a minimized theoretical Schottky barrier of only Φ b = Φ Ti -X GaN = 0.2 eV.…”

“…Furthermore, the formation of AlN at the Al/TiN interface was observed, which increases contact resistance. Thus, an Ti barrier layer was inserted, leading to a Ti/Al/Ti/TiN (20/100/20/80 nm) stack [26]. The TiN capping layer was resistant to RTA and proved its thermodynamic stability and diffusion barrier properties [27].…”

“…However, it was possible to measure the increase of the charge carrier concentration in the sub-contact area during RTA, which was often predicted [6], [11]. The charge carrier concentration was measured using adapted Hall measurements with large area contacts before and after RTA, and a strong increase of ≈300% was shown [26]. It must be emphasized that the Hall method cannot distinguish between the region of the 2DEG channel and the sub-contact region of the device.…”