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