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.