Silicon layers grown by molecular beam epitaxy in the interface region of Al/n-GaAs(001) Schottky diodes have been shown to tune the Schottky barrier height in the 0.3-1.1 eV range, provided that high enough excess fluxes of As or Al ate employed during Si deposition. We studied the incorporation of As and Al in the interface layer for Si growth temperatures of 300 and 400 degrees C, and for interlayer thicknesses in the 10-150 Angstrom range. Following deposition under an excess As flux we found an As atomic concentration c(As)=0.4-0.5 at 300 degrees C and c(As)=0.2 at 400 degrees C. The excess Al flux yielded an Al atomic concentration c(Al)=0.7-0.8 at 300 degrees C, and c(Al)=0.6-0.7 at 400 degrees C. Correspondingly, the lattice structure of the interlayers appears dramatically different from that of Si. We conclude that the 10-100 Angstrom interface layers used by some authors to change the barrier height bear little resemblance to degenerate built Si, so that several of the mechanisms proposed to explain Schottky barrier tuning should be reevaluated.