Band offsets can be modified at semiconductor heterojunctions or created at homojunctions by depositing thin intralayers of group-IV atoms at III-V/III-V polar interfaces. We present here a theoretical study of Si and Ge intralayers deposited along (001) at GaAs and AlAs homojunctions, and at GaAs/AlAs heterojunctions. Our results show that the offset is very sensitive to the coverage and abruptness of the intralayer. A comparison with recent experiments for Si in GaAs/AlAs suggests that Si atoms are confined over two atomic planes for coverages lower than about 0.5 monolayers, whereas for higher coverages Si diffusion occurs.