The effects of the Al mole fraction of a thin (4nm) AlxGa1-xAs cap on the barrier height and the thermal stability of Al/AlxGa1-xAs/Al0.25Ga0.75As Schottky barriers prepared in situ by MBE have been investigated. The behaviour of the barrier heights and the ideality factors of diodes after annealing up to 435-degrees-C are studied.