Density functional theory is used to calculate the adsorption and diffusion behavior of Al atoms on clean, H-terminate, Cl-terminate Si(100) and Si(111) surfaces. The most stable position of Al atom adsorption and the diffusion path are different on Si(100) surface terminated by different methods. On the surface of clean Si(100), the Tr site is the most stable site for Al atom with an adsorption energy of 4.01 eV, and the H and M sites are the sub-stable stable sites with the adsorption energies of 3.51 eV, and 3.63 eV, respectively. When the Al atom is adsorbed at the Tr site on the clean Si(100) surface, it bonds with the Si atom to destroy the Si—Si bond in the dimer. Therefore Al is easily adsorbed at the Tr site of the trench and diffuses in a zigzag pattern along the trench. On the H-terminate and Cl-terminate Si(100) surface, Si—Si bonds in the dimer column are changed from cross to parallel. Al is easily adsorbed at the H position at the top of the dimer column, and diffuses along the line at the top of the dimer. The differential charge density shows that the Al atom transfers electrons to the Si atoms on the surface, and the surface H-terminate and Cl-terminate weaken the interaction between Al atoms and Si, and reduces the diffusion energy barrier of Al atoms. The Si(111) surface terminated by different methods has the same stable position (T₄ site) for the adsorption of Al atoms. When Al atom adsorbs at the T₄ site on the clean Si(100) surface, it bonds to Si atom, which located at the three T₁ site, then Al atom is firmly fixed by the three Al—Si bonds with a bond length of 2.55 ?. Thus Al atom can has the largest adsorption energy and form the most stable state at the T₁ site. With the diffusion and migration of Al atom, the bond between Al atom and the T₁ site in the opposite direction appears to be broken. When Al atom migrating to the saddle point position is the most unstable. Here Al atom bonds to the Si atoms of the two adjacent T₁ sites to form a bond with a length of 2.49 ?, which is 0.06 ? shorter than the initial Al—Si bond (2.55 ?). What’s more, the diffusion energy barrier of Al atom at this position is 0.65 eV, which impede Al atom to diffuse and migrate. When Al atom migrates to the H site, it rebonds to the three Si atoms on the adjacent surface and forms a bond with a length of 2.52 ?, which is 0.03 ? shorter than the Al—Si bond (2.55 ?) at the initial position. On the H-terminate and Cl-terminate Si(111) surface, Al atom doesn’t bond with Si atom for the H or Cl saturates the dangling bonds on the Si surface. The Si(111) surface terminated by different methods has the same stable position for adsorption of Al atoms. The diffusion paths of Al atoms are similar, and they are easy to be adsorbed to the top position (T₄ site) of the second Si atom, and the path along T₄ to H₃ is diffused. Similarly, the H-terminate or Cl-terminate of Si(111) surface weakens the electron transfer between Al and Si atoms and reduces the diffusion energy barrier of Al atoms. Regardless of the Si(100) or Si(111) surface, the H-terminate and Cl-terminate Si surfaces are effective in reducing the diffusion barrier of Al atoms.