[en] Electrical breakdown in the oxidized barrier of hydrogen-terminated and thus surface-conductive diamond devices is a limiting factor in the fabrication of nanoscale structures. Possible applications as a free electron source as well as the electron transport across the potential barrier are investigated in this work. We report on a reduced effective barrier height due to an inhomogeneous potential landscape and a new field-dependent optical absorption feature at this interface. A new defect level is proposed that is formed at the hydrogen-/oxygen-terminated interface.