Model structures of substoichiometric germanium oxide generated by ab initio molecular dynamics reveal a rich variety of bonding configurations, including threefold coordinated Ge and O atoms which correspond to valence alternation pairs. A sizable fraction of twofold Ge atoms carrying unoccupied dangling bonds is also found. The present structures are prone to electron capture. The electrons are trapped in additionally formed Ge-Ge bonds. The associated defect levels fall within the Ge band gap and could explain the poor performance of n-type Ge devices. (C) 2010 American Institute of Physics. [doi : 10.1063/1.3486175]