AlON gate dielectric and gate trench cleaning for improved reliability of vertical GaN MOSFETs

Abstract:
This work evaluates the threshold voltage stability of GaN MOSFETs with an AlON interface dielectric with different nitrogen contents and deposition sequence by means of threshold voltage hysteresis and Positive Bias Temperature Instability (PBTI) measurements. The effect of the choice of the interface dielectric and gate trench cleaning prior to dielectric deposition on long-term reliability is addressed by Time Dependent Dielectric Breakdown (TDDB) measurements. Atomic Layer Deposition (ALD) of AlON with either an AlO cycle first, or an AlN cycle first and with different nitrogen percentages are compared to the more commonly used Al2O3. It is found that the threshold voltage hysteresis is significantly smaller, around 0.4 V less at 4 V gate overdrive, for AlON dielectrics with an AlO cycle deposited first in comparison to AlON with an AlN cycle first, and Al2O3, regardless of the nitrogen content. PBTI measurements further confirm this trend, in which the AlON dielectrics with AlO first present 0.2 V and 0.85 V smaller threshold voltage shift after 104 s of stress at 13 V gate overdrive voltage compared to AlON, AlN first and Al2O3, respectively. TDDB analysis shows that using AlON as an interface dielectric does not impact the dielectric wear out, while an optimized gate trench cleaning can increase the maximum electric field for a 1% failure rate in 10 years of operation from 6 MV/cm to 6.5 MV/cm.