The effect of oxygen plasma on activation efficiency of implanted silicon in gallium arsenide

The effect of oxygen (O-2) plasma on the threshold voltage (V-th) shift in gallium arsenide (GaAs) metal semiconductor field effect transistors (MESFETs), which were fabricated using ion implantation, was investigated. An O-2 plasma process in barrel type reactor was found to cause a large V-th difference in MESFETs between wafers that were processed in the same batch. Numerical calculations were performed to understand the phenomena quantitatively. Experimental analyses, using secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS) and capacitance-voltage (C-V) measurement, were employed to confirm the key factor in the V-th shift mechanism. As a result, it was demonstrated that the shift was caused not by the etching of the FET channel but by the degradation of the activation efficiency. The degradation was related to the excessive oxidation, which proceeded by pulling up the substrate atoms and increased vacancy type defects in the channel.