ALD TiN gates (15-75nm thick) were deposited on AlGaN/GaN HEMTs and diodes at 150-350°C using Tetrakis(dimethyamido)titanium (TDMA-Ti) and a 300W N2/H2 plasma as precursors with a growth rate of 0.6 Å/cy. Increasing the growth temperature, while remaining in a linear ALD regime, reduced the oxygen content by a factor of two and eliminated carbon impurities within the film resulting in 7x lower film resistivity. The plasma gas chemistry and pressure (8-80 mTorr) had an even greater impact on the electrical properties of these films. An exponential decrease in resistivity was achieved by increasing the N2/H2 ratio from 0.1-1.6 at a constant temperature. The lowest resistivity (120 µΩ-cm) was attained using a N2-only plasma at 7 mTorr at 350°C. While this resistivity is higher than typical bulk values (10-30 µΩ-cm), it is similar to PVD (30-100 µΩ-cm) and lower than many CVD films, which can vary widely (200-10,000 µΩ-cm). Using this growth parameter knowledge, a series of TiN gates were deposited on GaN diodes with varying oxygen content from 3-12 at%. TiN films with lower oxygen content also exhibited lower barrier heights in Schottky barrier diodes, suggesting a way to tailor the properties for different applications.
ALD TiN gates (75nm) also exhibited improved static and dynamic on-state characteristics when directly compared to identical Ni/Au-gated AlGaN/GaN HEMTs. Reverse bias gate stressing indicated a higher critical voltage (VgsTiN = -210V, VgsNi/Au = -120V) and a higher breakdown voltage (VgsTiN = -270 ± 10 V, VgsNi/Au = -240 ± 30V) for the TiN gates. Furthermore, the TiN gates exhibited a decrease in reverse leakage current after stressing suggesting enhanced stability. Gate thermal stability was assessed through sequential device annealing from 400-800°C in 100°C increments. The TiN gated devices exhibited stable DC operation up to 800°C, while the Ni/Au gates showed significant degradation after annealing above 500°C and failed above 700°C. This suggests that ALD TiN gates are a strong candidate for reliable HEMT gate metallization and other applications where increased stability is required at higher temperatures.
1 R.S. Pengelly, et al., IEEE Trans. Microwave Theory Tech. 60 , 1764 (2012).
2 S.J. Pearton, et al., J. Vac. Sci. Technol. A 31 , 050801 (2013).
3 Y.H. Choi, et al., Materi. Res. Soc. Symp. Proc. 1167, 1167-O05-06 (2009).
4 A.D. Koehler, et al., IEEE Elect. Dev. Lett. 35 , 1194 (2014).