HfO₂ dielectric thickness dependence of electrical properties in graphene field effect transistors with double conductance minima

We investigate the electrical properties in back-gated graphene field effect transistors (GFETs) with SiO₂ dielectric and different thickness of high-k HfO₂ dielectric. The results show that transform characteristic (I_ds-V_gs) curves of GFETs are uniquely W-shaped with two charge neutrality point (left and right) in both SiO₂ and HfO₂ dielectric (SiO₂-GFETs and HfO₂-GFETs). The gate voltage reduces drastically in HfO₂-GFETs compared with that in SiO₂-GFETs, and it becomes much smaller with the decline of HfO₂ thickness. The left charge neutrality point in I_d-V_g curves of all HfO₂-GFETs is negative, compared to the positive ones in SiO₂-GFETs, which means that there exists n-doping in graphene with HfO₂ as bottom dielectric. We speculate that this n-doping comes from the HfO₂ layer, which brings fixed charged impurities in close proximity to graphene. The carrier mobility is also researched, demonstrating a decreasing trend of hole mobility in HfO₂-GFETs contrast to that in SiO₂-GFETs. In a series of HfO₂-GFETs with different HfO₂ dielectric thickness, the hole mobility shows a tendency of rise when the thickness decreases to 7 nm. The possible reason might be due to the introduced impurities into HfO₂ film from atomic layer deposition process, the concentration of which varies from the thickness of HfO₂ layer.