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Ga As∕ Al As∕ Ge (100) samples grown at 650 C with AlAs interfacial layer thickness of 0, 10, 20, and 30 nm were characterized using transmission electron microscopy, secondary ion mass spectrometry (SIMS), and photoluminescence (PL) techniques. SIMS results indicate that the presence of an ultrathin AlAs interfacial layer at the Ga As∕ Ge interface has dramatically blocked the cross diffusion of Ge, Ga, and As atoms, attributed to the higher Al–As bonding energy. The optical quality of the GaAs epitaxy with a thin AlAs interfacial layer is found to be improved with complete elimination of PL originated from Ge-based complexes, in corroboration with SIMS results.