ALD TiN Schottky Gates for Improved Electrical and Thermal Stability in III-N Devices
Authors: Wheeler Virginia D. , Anderson Travis J , Ahn Shihyun , Shahin David I , Tadjer Marko J , Koehler Andrew D. , Hobart Karl D , Ren Fan , Kub Francis J , Christou Aris , Eddy Charles R. 
Published: February 2020

AlGaN/GaN high electron mobility transistors (HEMTs) are useful devices for next-generation RF and power electronics systems1,2. Traditional Ni-based Schottky gates in these devices have been shown to degrade when subjected to electrical stress, thermal stress, and radiation due to Ni migration into adjacent metal or semiconductor layers3,4. The instability of these Ni-based gates limits device reliability, rendering the search for replacement gate materials that are electrically- and thermally-stable a topic of tremendous importance. Of the transition metal nitrides, TiN is a particularly promising material, due to its near-metallic conductivity, suitable Schottky barrier heights and ideality factors on GaN and AlGaN, and high temperature stability. This work investigates the performance of atomic layer deposited (ALD) TiN gates and directly compares them to traditional Ni/Au gates.
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.

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