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

Abstract

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 (Ids–Vgs) 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 Id–Vg 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.