A 10 Å ultraviolet/ozone grown oxide was used as an atomic diffusion barrier in a Ge/GaAs heterostructure. Good thermal stability of the oxide layer has been demonstrated by the appearance of dendritic crystallization [K. M. Lui, K. P. Chik, and J. B. Xu, J. Appl. Phys. 81, 7757 (1997)], induced by in situ thermal pulse annealing, of the Ge overlayer. In this work, an abrupt heterointerface was revealed by aligned Rutherford backscattering spectroscopy after annealing and compared with a control with no barrier at the interface where considerable diffusion had taken place. Current–voltage measurement indicated good rectifying properties of the oxide barrier heterojunction. © 1998 American Institute of Physics.