Modulation of electron transfer in Si/SiO${}_2$/HfO${}_2$/Graphene by the HfO${}_2$ thickness

We studied by Raman spectroscopy the effect of the variation of the thickness of the HfO${}_2$ layer in Si/SiO${}_2$/HfO${}_2$/Graphene heterostructures on the density of carrier transferred to graphene. Three thicknesses of the HfO${}_2$ layer equal to 30, 20, and 5 nm were used. Analysis of the behavior of peaks G and 2D as well as the Raman intensity ratios I${}_D$/I${}_G$ and I${}_{2D}$/I${}_G$ shows that the density of carriers transferred to graphene increases when the thickness of the HfO${}_2$ layer decreases from 30 to 5 nm. Analysis of the photoluminescence (PL) spectra of Si/SiO${}_2$/HfO${}_2$ structures reveals the presence of bands associated with defects. These are essentially associated with oxygen vacancies in HfO${}_2$ and are responsible for the transfer of carriers to the graphene layer. The measured refractive index of HfO${}_2$ by ellipsometry shows an increase in the density of the defects in HfO${}_2$ with a decrease in its thickness in agreement with Raman results.