Surface mass spectrometric analysis of SiCl4/SiF4/O2 dry etch gate recessed 120Â nm T-gate GaAs pHEMTs

In addition to the importance of the geometry of the gate recess trench in high performance 120 nm gate length GaAs pseudomorphic high electron mobility transistors (pHEMTs), the recess etch surface condition is also a key parameter. Selective dry etching of the gate recess in a SiCl4/SiF4/O2 chemistry uniformly and controllably removes the GaAs cap layer of the device stopping on the AlGaAs etch stop layer. In this work, the etched surface has been investigated by surface SIMS analysis. It was found that there were much higher concentrations of species of C, O, Cl, F, GaO, GaH, GaCl, Al, and Si on the surface just after the RIE etching than before etching or after short HF dipping. This indicated that the etched surface included significant traces of chloride, fluoride, oxide, and polymer. Depth profiles showed that the surface contaminants were restricted to only the top few nanometres. The post etching but pre-metallisation surface treatment through a short HF solution dip has been used to optimise the performance of GaAs pHEMT-based Schottky diodes and demonstrated that significant improvements in both DC and RF performance of Schottky diode can be achieved.