Modulation of electron transfer in Si/SiO2/HfO2/Graphene by the HfO2 thickness

We studied by Raman spectroscopy the effect of the variation of the thickness of the HfO2 layer in Si/SiO2/HfO2/Graphene heterostructures on the density of carrier transferred to graphene. Three thicknesses of the HfO2 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 ID/IG and I2D/IG shows that the density of carriers transferred to graphene increases when the thickness of the HfO2 layer decreases from 30 to 5 nm. Analysis of the photoluminescence (PL) spectra of Si/SiO2/HfO2 structures reveals the presence of bands associated with defects. These are essentially associated with oxygen vacancies in HfO2 and are responsible for the transfer of carriers to the graphene layer. The measured refractive index of HfO2 by ellipsometry shows an increase in the density of the defects in HfO2 with a decrease in its thickness in agreement with Raman results.