Charge trapping and recovery in ALD HfO2/β-Ga2O3 (010) MOS capacitors
The reliability of atomic layer deposited HfO2 β-Ga2O3 (010) MOS capacitors (MOSCAPs) was tested using constant voltage stress measurements. The HfO2/β-Ga2O3 MOSCAPs showed significant positive flatband voltage shifts (ΔV fb) of up to +1.3 V after 4.3 × 103 s stress, likely due to electron trapping in the dielectric in border traps. Results indicate there may exist an electric field threshold between 0.98 and 1.23 MV cm−1 for electron trapping to occur in these MOSCAPs. Deep ultra-violet (DUV) illumination enables full recovery from the charge trapping. In contrast, HfO2/Si MOSCAPs do not recover under DUV illumination and exhibit smaller voltage shifts upon initial testing. We thus surmise that the type-II band alignment between HfO2 and β-Ga2O3 enables full recovery from stress-induced charge trapping via photogeneration of electron–hole pairs and hole transport across the negative valence band offset. This hypothesis is borne out by the observation of photo-current in the HfO2/β-Ga2O3 MOSCAPs when biased in depletion. In sum, HfO2/β-Ga2O3 MOS structures appear to be susceptible to flatband voltage shifts during stress bias due to the availability and accessibility of border traps. This work highlights the unique challenges in forming reliable ultra-wide band gap MOS devices, due to the significant role that border traps in the lower half of the insulator band gap play in bias stress response.