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

摘要

The transition from thermally-grown SiO₂ to alternativegate dielectrics is proceeding in two steps, initially to Si oxynitridealloys, and then high-k dielectrics. The author defines a classificationscheme based on Pauling bond-ionicity that defines three differentamorphous morphologies for non-crystallization gate oxide gatedielectric materials. This approach to local bonding identifies animportant relationship between oxygen atom bonding coordination and i)dielectric constant, ii) stability against chemical phase separation andcrystallization, and iii) stability against hydrophobic chemicaldegradation. A molecular orbital ab initio approach for obtaining thelocal electronic structure at T-M and R-E earth atoms that bond toO-atoms of the elemental or alloy oxide is described. This approachgenerates a universal energy band scheme that is applicable tonon-crystalline as well as crystalline dielectrics. The results of thesecalculations are compared with local density function (LDF) calculationsemploying the local density approximation (LDA). The agreement betweenthese two different methods confirms that the lowest band-gap for T-Mand R-E oxides and oxide alloys are determined by the atomic energystates of the T-M or R-E atom, and its immediate O-atom neighbors,yielding important scaling relations for band-gaps and band-offsetenergies. The results of spectroscopic studies of transition metaloxides, silicates and aluminates are presented. These results establishthe validity of the electronic structure calculations and provide abasis for interpretation of electrical data on device structures. Deviceperformance issues are addressed, and the relationship betweenelectronic structure and ultimate performance limitations of the high-kgate dielectrics in aggressively-scaled silicon devices is discussed