Ultrathin cross-linked polymer thin films are highly desirable materials because of their important roles in many applications. However, they are difficult and challenging to fabricate. Here we report a one-step process for depositing cross-linked polyurea thin films using a vapor-phase molecular layer deposition (MLD) technique. 1,4-Diisocyanatobutane and a series of different multiamines, including diethylenetriamine, triethylenetetramine, and tris(2-aminoethyl)amine, were used to grow polyurea MLD films via urea-coupling reactions. The deposited cross-linked polyurea films exhibit characteristic MLD film growth behaviors, constant growth rates, infrared absorption by expected urea modes, and stoichiometric chemical compositions. More importantly, the cross-linking is shown to be capable of improving the film properties. Based on cross-linking, the thin film density can be increased by approximately 50%. In addition, the film decomposition temperature is increased by about 30 degrees C, suggesting an enhanced thermal stability of the cross-linked ultrathin polyurea films. such as