Impact of Ti/Al atomic ratio on the formation mechanism of non-recessed Au-free Ohmic contacts on AlGaN/GaN heterostructures

Abstract

The formation mechanism of non-recessed Au–free Ohmic contacts on the AlGaN/GaN heterostructures is investigated for various Ti/Al atomic ratios (Al–rich versus Ti–rich) and annealing temperatures ranging from 500 to 950 °C. It is shown that Ti/Al atomic ratio is the key parameter defining the optimum annealing temperature for Ohmic contact formation. Ti–rich contacts processed at high temperature result in low contact resistance ∼0.7 Ω mm, better to those obtained at low temperature or with Al–rich metal stacks. The variation of the contact resistance with Ti/Al atomic ratio and annealing temperature is correlated with the intermetallic phase changes and interfacial reaction. Depending on the Ti/Al atomic ratio, two distinct mechanisms can be distinguished. For a small quantity of Ti (e.g., Al–rich contacts), Ohmic contact formation is done through a weak interfacial reaction which is nonexistent at high temperature due to the degradation of the metal morphology. However, for a quantity of Ti higher than 25 at. % (e.g., Ti–rich contacts), the agglomeration is delayed by 200 °C as compared to Al–rich contacts, and optimal contacts are formed at high temperature through a strong interfacial reaction.

Metal/Semiconductor Barrier Properties of Non-Recessed Ti/Al/Ti and Ta/Al/Ta Ohmic Contacts on AlGaN/GaN Heterostructures

“…unreacted Al from the Al3Ti phase can occur at higher annealing temperatures [14], which is consistent with our experimental observation. An analogous behavior has been observed in the annealed Ta/Al/Ta contact, where the Al3Ta was the main phase detected by XRD, with unreacted Al still present in the system.…”

“…An analogous behavior has been observed in the annealed Ta/Al/Ta contact, where the Al3Ta was the main phase detected by XRD, with unreacted Al still present in the system. In fact, the formation of the Al3Ta phase is expected in a temperature range between 600 and 1500 °C [14,16]. For the annealing temperature considered in our case, Al3Ta and Al3Ti are the most thermodynamically favored phases [25].…”

“…The easiest approach to fabricate ohmic contacts without gold ("Au-free"), is to remove the uppermost Au layer in the stack. In particular, 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 Φ B with AlGaN and, hence, a low value of specific contact resistance ρ c [Ωcm 2 ]. All these works revealed the importance of several parameters, such as the thermal annealing processes, the metal's thickness or the GaN material quality [15,16,19,20].…”

“…In this context, although many papers have reported on Ti-or Ta-based ohmic contact [4,13,14,16,18,19,21], these systems deserve further investigations for a better comprehension of their electrical behavior. Indeed, in the literature, there is a lack of information on the barrier height properties and on the carrier transport mechanisms at these interfaces.…”

Improved Ohmic Contact by Pre‐Metallization Annealing Process in Quaternary In0.04Al0.65Ga0.31N/GaN HEMTs

“…In order to enhance the power and high frequency performance of GaN HEMTs, high performance ohmic contacts are required. To achieve the better ohmic contact performance than that of the conventional Ti/Al‐based contacts, various approaches including Au‐free contacts, recessed ohmic contacts, pre‐metallization surface treatments, and regrown contacts have been investigated.…”