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In this work, we address the charge trapping properties of Ge dangling bond (DB) defects - GePb1 centers as typified by electron spin resonance spectroscopy (ESR) - found at the interfaces between condensation-grown Si1− xGe x (0.28 < x < 0.8) alloys and insulating SiO2. The ESR observation of singly-occupied paramagnetic GePb1 centers, carried out at 4.3 K, is complemented by temperature-dependent positron annihilation spectroscopy (PAS) in the Doppler broadening mode, which enables observation of the neutral-to-negative defect transitions as the temperature increases from 50 to 300 K. Through correlation of this re-charging behavior with the temperature-induced shift of the Fermi energy, the energy of the Ge DB −/0 transition in the Si0.27Ge0.73 alloy is inferred to be energetically distributed in a ∼0.1 eV interval above the top of the semiconductor valence band. This result refines previous estimates from capacitance-voltage measurements, thus providing independent affirmation that the energy levels of Ge DBs lie inside the band gap of the Si1− xGe x alloys. [ABSTRACT FROM AUTHOR]