Effects of biased irradiation on charge trapping in HfO₂ dielectric thin films

This paper reports the low-dose-rate radiation response of Al-HfO₂/SiO₂-Si MOS devices, in which the gate dielectric was formed by atomic layer deposition (ALD) with 5-nm equivalent oxide thickness. The degradation of the devices was characterized by a pulse capacitance-voltage (CV) and on-site radiation response technique under continuous gamma (γ) ray exposure at a relatively low dose rate of 0.116 rad (HfO₂)/s. Compared with conventional CV measurements, the proposed measurements extract significant variations of flat-band voltage shift of the hafnium based MOS devices. The large flat-band voltage shift is mainly attributed to the radiation-induced oxide trapped charges, which are not readily compensated by bias-induced charges produced over the measurement timescales (for timescales less than 5 ms). A negative flat-band voltage shift up to -1.02 V was observed under a positive biased irradiation with the total dose up to 40 krad (HfO₂) and with the electric field of 0.5 MV/cm. This is attributed to net positive charge generation in the HfO₂ oxide layer. The generated charges are transported towards the HfO₂/SiO₂ interface, and then form effective trapped holes in the HfO₂. Similarly, a positive flat-band voltage shift up to 1.1 V was observed from irradiation under negative bias with an electric field of -0.5 MV/cm. The positive shift is mainly due to the accumulation of trapped electrons. Analyses of the experimental results suggest that both hole and electron trapping can dominate the radiation response performance of the HfO₂-based MOS devices depending upon the applied bias. It was also found there was no distinct border traps with irradiation in all cases.