Electrochemical etching in wet-chemical gate recess for InAlAs/InGaAs heterojunction FETs
When resist openings are employed to monitor the drain current of InP-based heterojunction FETs during wet-chemical gate recess, etching rates for InGaAs and InAlAs can be significantly modified by the exposure of the surface metal on the non-alloy ohmic electrodes to citric-acid-based etchants. Surface metal of Ni enhances the recess etching rate to a degree that is much higher than that in its absence. With this non-selective citric-acid-based etchant, the presence of Pt surface metal, however, leads to a preferential etching of InGaAs over InAlAs. This behaviour of selective etching is attributed to the excess oxidation of InAlAs induced by the high electrode potential of Pt via electrochemical effects. This investigation discloses that the selection of the surface metal that lies beneath the resist openings can be very important if gate recess grooves with desired shapes are to be fabricated.