Forming a good Ge/dielectric interface is important to improve the electron mobility of a Ge metal oxide semiconductor field-effect transistor. A thin yttrium germanate capping layer can improve the properties of the Ge/GeO(2) system. We employ electronic structure calculations to investigate the effect of oxygen vacancies in yttrium-doped GeO(2) and the yttrium germanates Y(2)Ge(2)O(7) and Y(2)GeO(5). The calculated densities of states indicate that dangling bonds from oxygen vacancies introduce in-gap states, but the system remains insulating. However, yttrium-doped GeO(2) becomes metallic under oxygen deficiency. Y-doped GeO(2), Y(2)Ge(2)O(7) and Y(2)GeO(5) are calculated to be oxygen substoichiometric under low Fermi energy conditions. The use of yttrium germanates is proposed as a way to effectively passivate the Ge/dielectric interface.