Electronic structure of high-k transition-metal and rare-earth gate dielectrics for aggressively-scaled silicon devices

The transition from thermally-grown SiO/sub 2/ to alternative gate dielectrics is proceeding in two steps, initially to Si oxynitride alloys, and then high-k dielectrics. The author defines a classification scheme based on Pauling bond-ionicity that defines three different amorphous morphologies for non-crystallization gate oxide gate dielectric materials. This approach to local bonding identifies an important relationship between oxygen atom bonding coordination and i) dielectric constant, ii) stability against chemical phase separation and crystallization, and iii) stability against hydrophobic chemical degradation. A molecular orbital ab initio approach for obtaining the local electronic structure at T-M and R-E earth atoms that bond to O-atoms of the elemental or alloy oxide is described. This approach generates a universal energy band scheme that is applicable to non-crystalline as well as crystalline dielectrics. The results of these calculations are compared with local density function (LDF) calculations employing the local density approximation (LDA). The agreement between these two different methods confirms that the lowest band-gap for T-M and R-E oxides and oxide alloys are determined by the atomic energy states of the T-M or R-E atom, and its immediate O-atom neighbors, yielding important scaling relations for band-gaps and band-offset energies. The results of spectroscopic studies of transition metal oxides, silicates and aluminates are presented. These results establish the validity of the electronic structure calculations and provide a basis for interpretation of electrical data on device structures. Device performance issues are addressed, and the relationship between electronic structure and ultimate performance limitations of the high-k gate dielectrics in aggressively-scaled silicon devices is discussed.