Study of Ti-Rich and Al-Rich Contact Metallization for AlGaN/GaN HEMT Power Devices

AlGaN/GaN heterostructures have great potential for high-power applications due to their material properties: wide band-gap, high electron mobility in a 2D electron gas and high breakdown field. Low resistance ohmic contacts are required to benefit from the full advantages of the high-electron-mobility-transistor (HEMT) structure. A choice of metallization in agreement with the CMOS-industry standards (i.e. without Au) is required to facilitate the industrialization of such devices. Hence, a Ti/Al stack is a viable solution. Realization of low resistive ohmic contacts is very sensitive to Ti/Al composition and annealing temperatures [1-2]. Understanding metallurgical mechanisms is crucial to optimize the contact.

This work is focused on the study of the Ti/Al metallization and the reactions occurring during a two-steps annealing (300s at 600°C then 60s at 900°C in N₂ ambient) in both Ti-rich and Al-rich cases. Two Ti/Al stacks, Al-rich (75/225 nm) and Ti-rich (95/200 nm) have been studied first on Si₃N₄/Si substrates in order to observe only the Ti+Al reactions and eventual nitridation. Then, the best promising stack (the Ti-rich one) has been transferred on AlGaN substrate. Characterizations consist in AFM, sheet resistance (R), XRD (Fig.1) and cross-section SEM (Fig.2.a) & STEM-EDX measurements (Fig.3). Thermodynamics study has been performed to follow the material behavior.

Regarding the Al-rich stack, XRD peaks (Fig.1.a) prove the formation of TiAl₃ at 600°C. This is confirmed by the R value (570 mW) corresponding to a resistivity close to the TiAl₃ one (respectively ~20 and 17 µW.cm). At 900°C, R reaches a very resistive value (>8 W) and voids can be identified on SEM images above the remaining TiAl₃ (Fig.2.a). XRD reveals the presence of AlN (Fig.1.b) probably formed by the nitridation of Al in excess during the annealing by the N₂. This is consistent with the ternary diagram of the Ti-Al-N system (Fig.2.b).

Regarding the Ti-rich stack, XRD graph (Fig.1.c) demonstrates the formation of TiAl₃ and a Ti-rich phase at 600°C. At 900°C, XRD reveals the presence of a titanium nitride phase in the contact. This probably results from the reaction of Ti-Al stack with the Si₃N₄substrate.

The Al-rich metallization creates a thick and resistive layer on top of the stack which is not suitable for achieving low resistive contacts. At the opposite, the Ti-rich metallization allows formation of a titanium nitride layer involving the substrate, a key-reaction for achieving ohmic contacts. Hence, we transferred on AlGaN substrate the Ti/Al 95/200nm stack. Same methodology using also cross-section STEM+EDX (Fig.3) has been applied to complete the scenario of reactions between the Ti/Al stack and AlGaN substrate.

References

[1] B. Jacobs et al., J. Cryst. Growth 241, 15 (2002)

[2] H-S. Lee et al., IEEE Electron Device Lett. 32, 623 (2011)

[3] Q. Chen, B. Sundman, J. Phase Equil. 19, 146 (1998)