Probing the Electronic Structure of HfO2 polymorphs via Electron Energy Loss Spectroscopy

Hafnia is one of the most widely used wide gap materials in the microelectronic industry. Stemming from the need to miniaturize electronic devices, high κ materials such as hafnia-based compounds replace SiO2 in gate oxides, thereby decreasing the equivalent oxide thickness of the gate dielectric. On the other hand, scaling down deteriorates the physical properties of the latter viz., leakage current and carrier mobility. By increasing κ or the dielectric constant of the material, these losses can nevertheless be compensated for. Doping of HfO2 is known to increase its κ; these dopants include mostly divalent elements such as Mg or trivalent elements such as Y, Al and La. Multivalent elements such as V and Ta are also used as dopants but their oxidation state depends upon the growth conditions. Doping via elements with +2 and +3 oxidation states, has a propensity to introduce oxygen vacancies and stabilize higher symmetry phases of HfO2 i.e. cubic, tetragonal and even orthorhombic which present higher κ compared to the monoclinic phase. In all cases, the electronic structure of the material at the atomic level, varies as a function of crystallinity, crystal structure, oxygen vacancies and dopants; all of which affect bond lengths and in turn the atomic bonding and thereby the hybridization. Moreover, the electronic properties also affect the physical properties of the gate oxide and therefore, understanding the oxidation state of the dopants which affect the electronic structure of HfO2 by probing the O-K edge Energy Loss Near Edge Structure (ELNES) of the material via Electron Energy Loss Spectroscopy (EELS) in the Scanning/Transmission Electron Microscope (STEM), is indispensable. In fact, by using techniques such as Spectral Imaging (SI) and Energy Filtered Transmission Electron Microscopy (EFTEM), it is now tractable to study the diffusion of dopants within HfO2. When EFTEM is coupled with (ELNES), local changes in the electronic structure as a function of the concentration of dopants are revealed.