Investigation of plasma induced etch damage-changes in AlGaN-GaN HEMTs

In this work, we report on the processing and device characteristics of AlGaN/GaN HEMT devices to investigate the effects of silicon dioxide (SiO₂) etching using Fluoroform (CHF3) gas prior to gate metal deposition. Three different GaN device structures were fabricated: (a) device #1 in which the device passivation (using SiO₂) and gate metallisation are done inone lithography step, (b) device #2 in which the device passivation and gate metallizationare done in 2 separate steps, (c) device #3, in which the gate metallization is deposited priorto passivation. 100 nm of plasma enhanced chemical vapor deposition (PECVD) SiO₂ wasdeposited for surface passivation to the devices. As fabricated, devices #1 and #2 exhibitedvery poor device characteristics with very low output currents which we attribute to surfaceplasma induced damage or changes on the gate region after the SiO₂ etching. A two-steppost gate annealing step was performed on the devices to recover this damage. The highestmaximum drain current of over 1100 mA/mm was observed on device #3 after the firstanneal step compared to other devices which showed higher maximum drain current afterthe second anneal step. All three devices show an improvement in self-heating behavior afterthe second anneal step along with more stable transfer characteristics. The highestmaximum peak transconductance of over 250 mS/mm was observed on devices #2 and #3after the first anneal step. This reduces slightly for all devices but with more stablecharacteristics. The measured threshold voltage values (VTH) are also consistent and stableafter performing the second anneal step. These results indicate that avoiding exposing theactive region of GaN devices is important in achieving expected and stable characteristics. Italso observed that further device improvement can be done by performing a two-step postgate annealing process.