## 3 Results and Discussion  
### Optimization of CF\({}_{4}\)/O\({}_{2}\) Plasma  
The maximum G\({}_{\rm m}\) and I\({}_{\rm d}\) as a function of different RF power (CF\({}_{4}\)/O\({}_{2}\) plasma treatment time: 3 min) are shown in Figure 6 (W/L = 600/20 \(\mu\)m, at V\({}_{\rm d}\) = 50 mV and 0.5 V). The maximum G\({}_{\rm m}\) and I\({}_{\rm d}\) of the control sample are 3.1 mS/mm and 1.3 mA/mm (V\({}_{\rm d}\) = 50 mV), and 20.3 mS/mm and 12 mA/mm (V\({}_{\rm d}\) = 0.5 V), respectively. With F plasma treatment of 30W, the maximum G\({}_{\rm m}\) and I\({}_{\rm d}\) reach 3.7 mS/mm and 1.7 mA/mm (V\({}_{\rm d}\) = 50 mV), and 26 mS/mm and 16.2 mA/mm (V\({}_{\rm d}\) = 0.5 V), respectively. However, the maximum G\({}_{\rm m}\) and I\({}_{\rm d}\) roll back with power larger than 30 W indicative of possible plasma damage. SS data with different RF power are shown in Figure 7. In\({}_{0.53}\)Ga\({}_{0.47}\)As MOSFETs have similar EOT (\(\sim\)1.4 nm, data not shown) with different RF power treatment. With F incorporation, SS has been improved from 127 to 118.1 mV/dec (as shown in the inset of Figure 7), which suggests that the interface quality has been improved. For the RF power of 40W, SS increases to 127.5 mV/dec, indicating that excessive CF\({}_{4}\)/O\({}_{2}\) plasma treatment degrades the interface quality. Figure 5: Equivalent oxide thickness (EOT) _versus_ physical thickness for the samples with and without F incorporation. Inset: the gate leakage current comparison of samples with and without F incorporation. Figure 8 compares the maximum G\({}_{\rm m}\) and I\({}_{\rm d}\) as a function of plasma treatment times ranging from 1 to 7 min with a fixed RF power of 30W (W/L = 600 \(\rm\upmu\)m/20 \(\rm\upmu\)m at V\({}_{\rm d}\) = 50 mV and 0.5 V). 3 min plasma treatment reaches the peak values of the maximum G\({}_{\rm m}\) and I\({}_{\rm d}\), whereas 5 min plasma treatment achieves the lowest SS value (Figure 9). The inset of Figure 8 compares the I\({}_{\rm d}\)-V\({}_{\rm g}\) curves (in log-linear scale) of the control sample and the sample with F treatment 30 W/5 min. A steeper SS slope is clearly observed. Figure 6: Maximum G\({}_{\rm m}\) and I\({}_{\rm d}\) as a function of CF\({}_{\rm a}\)/O\({}_{2}\) plasma RF power (W/L = 600 \(\rm\upmu\)m/20 \(\rm\upmu\)m at V\({}_{\rm d}\) = 50 mV and 0.5 V). CF\({}_{\rm a}\)/O\({}_{2}\) plasma treatment time: 3 min. Figure 7: Subthreshold swing (SS) as a function of CF\({}_{\rm a}\)/O\({}_{2}\) plasma RF power. CF\({}_{\rm a}\)/O\({}_{2}\) plasma treatment time: 3 min. Inset: I\({}_{\rm d}\)-V\({}_{\rm g}\) comparison of the control sample and the sample with CF\({}_{\rm a}\)/O\({}_{2}\) plasma treatment for 30 W/3 min. Effective channel mobility (\(\mu_{\text{eff}}\)) of In\({}_{0.53}\)Ga\({}_{0.47}\)As MOSFETs with different plasma treatment times are plotted in Figure 10. The peak \(\mu_{\text{eff}}\) of 30 W/3 min reaches 1,144 cm\({}^{2}\)/Vs, which is 38% improvement compared to the control samples (826 cm\({}^{2}\)/Vs). The improvements in the I\({}_{\text{d}}\), G\({}_{\text{m}}\), SS and \(\mu_{\text{eff}}\) are believed to be due to the improved interface quality by an appropriate amount of CF\({}_{4}\)/O\({}_{2}\) plasma post-HfO\({}_{2}\) treatment. Figure 8: Maximum G\({}_{\text{m}}\) and I\({}_{\text{d}}\) as a function of CF\({}_{4}\)/O\({}_{2}\) plasma treatment time (W/L = 600 \(\mu\)m/20 \(\mu\)m at V\({}_{\text{d}}\) = 50 mV and 0.5 V). RF power: 30 W.  
Figure 9: SS as a function of CF\({}_{4}\)/O\({}_{2}\) plasma treatment time. RF power: 30 W. Inset: I\({}_{\text{d}}\)-V\({}_{\text{g}}\) comparison of the control sample and the sample with CF\({}_{4}\) plasma treatment for 30 W/5 min. We noticed that the plasma damage occurred if excessive plasma (either plasma wattage or treatment time) was applied. The plasma damage could come from disordering, surface roughening, and fluorine contamination [13].