An investigation of CH4/H2 reactive ion etching damage to thin heavily doped GaAs metal–semiconductor field effect transistor layers during gate recessing

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Abstract
Damage during the reactive ion etching in CH4/H2 of thin, heavily silicon doped, molecular beam epitaxy grown GaAs layers has been investigated. Hall effect and CV measurements showed near complete loss of the free carrier concentration on etching due to passivation by hydrogen of the silicon donor activity. Annealing for a few minutes at 400 °C was sufficient to recover the sheet concentration of the material, measured by Hall effect, but only to a limit determined by the dc bias employed during etching. At 800 V only 5% of the carriers were restored, while reducing the dc bias to 80 V allowed 70% to 80% recovery. CV measurements showed that increased surface depletion accounted for the loss of sheet concentration, while the doping profile in the undepleted channel was unaffected. At 80 V dc bias, the increased surface depletion was only 2.6 nm. Additionally Hall mobility was restored by annealing, to close to the original value of the unetched material, indicating minimal reactive ion etching damage to the depassivated material. Etching of the gate recess of 120-nm gate length metal–semiconductor field effect transistors, at low dc bias, allowed devices with excellent dc and rf performance to be fabricated; dc transconductance of gm=400 mS/mm and unity current gain cutoff frequency of fT=70 GHz were achieved.

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