Electrically Active Defects and Traps and Their Relation to Stoichiometry and Chemical Environment in HfO2/Al2O3 Dielectric Stacks as Revealed by XPS

Charge-trapping memory (CTM) is a viable contender to supersede the floating gate technology in high-density flash memory applications. To this end, very reliable charge storage in CTM should be secured. This requires optimization of trap density, their energy and spatial location as well as a deep understanding of their origin. In this work, we used X-ray photoelectron spectroscopy (XPS) to investigate chemical bonds in nanolaminated and doped HfO₂/Al₂O₃ stacks in an effort to gain insight into the nature of defects in the electron/hole trapping processes. The impact of Al incorporation into the HfO₂ and rapid thermal annealing (RTA) in O₂ on the composition, stoichiometry and bonding configurations was studied. Incorporation of Al into HfO₂ leads to an increased concentration of Hf-suboxides. Subsequent RTA effectively reduces suboxides, enhances the stoichiometry of the HfO₂/Al₂O₃ stacks and facilitates intermixing at the dielectric interface, resulting in the formation of Hf-Al-O bonds. The valence band spectra indicate that both Al incorporation and RTA change the dielectric/Si band alignment in a similar way, lowering the valence band offset. The observed changes were considered in relation to the electrically active defects and traps in the structures.