AlAs/Si heterostructures grown by atomic layer epitaxy have been evaluated by transmission electron microscopy. The hetero-interface is found to be extremely abrupt as compared with the case of GaAs/Si heterostructures grown by metalorganic vapor phase epitaxy. It is also noted that atomic steps are clearly observed at the interface and that no amorphous or alloyed regions are found. At the very initial stage of growth, a three dimensional island growth mode is dominant. However, two-dimensional growth takes place after depositing only 36 atomic layers of AlAs. Two types of defects are observed in the AlAs layer: stacking faults and microtwins. Although the density of these defects is high, they tend to self-annihilate at the interface between the AlAs layer and the upper GaAs thick layer. These results lead us to conclude that using an AlAs layer grown by atomic layer epitaxy as a buffer layer is very promising for achieving high quality GaAs/Si heterostructures with low defect density.