High-k oxides with properties interesting for gate stacks

In order to satisfy the demands for lower power consumption in modern transistor design, research focus moved from gate oxides of SiO2 to oxides based on metals from the transition and rare-earth series of the periodic table. The elements of major interest for the purpose are marked with dotted frames in the periodic table, shown in Fig. 3.1. The transition metals, Sc, Ti, Y, Zr, La, Hf and Ta, belonging to periods 4, 5 and 6, all have outer electron shell populated as d-orbitals. The rare-earth metals, starting at Ce and ending at Lu, arise as one of the irregularities in the design of the elements as given by Nature (Grimes and Grimes, 1998; Xue et al., 2000). They are inserted as exceptions between La and Hf. Because of the complicated energy scheme occurring when adding the second d-electron in period 6, 14 f-electrons with lower energy levels are added to provide the rare-earth metals, before the 72nd electron is added as a d-orbital to produce Hf. When reacting with oxygen, these metals give rise to insulators with high dielectric constants, which to some degree are related to their higher ionicity, but also to their atomic structures (Busani and Devine, 2005; Perevalov et al., 2007). This, in turn, links to their electronic configurations. The ionicity concept (Phillips, 1968; Van Vechten, 1969), which measures the degree of electron displacement in a chemical compound, is related to the difference in electronegativity between the reacting atoms as further discussed in Sections 3.2, 3.5 and 3.6 and will be shown to have an important meaning for the dielectric constant and the energy offset values at oxide/silicon interfaces (Engström et al., 2007).