The Schottky barrier height in Al/Si/Ga-As(001) junctions grown by molecular-beam epitaxy was determined in situ by means of x-ray photoemission spectroscopy and ex situ through current-voltage and capacitance-voltage measurements. We found that the barrier height can be tuned from a minimum value of 0.2 eV to a maximum of 1.1 eV provided that a sufficiently high As or Al flux is employed during the growth of the Si interface layer. The minimum and maximum values of the barrier are already established for Si layer thicknesses in the 1-2 monolayer range. We propose that the changes in barrier height derive from the establishment of a Si-induced local interface dipole. The magnitude and orientation of the dipole reflects the detail of the atomic reconstruction achieved at the interface in the different growth conditions.