Ordered, quasiepitaxial growth of an organic thin film on Se-passivated GaAs(100)

Films of the organic semiconductor 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) are deposited at room temperature in ultrahigh vacuum on the (2×4)-c-(2×8) As-terminated, and the (2×1) Se-passivated (100) GaAs surfaces. The PTCDA deposition on the (2×4)-c(2×8) surface produces domains randomly oriented in the plane parallel to the interface, giving rise to diffuse ringlike low energy electron diffraction (LEED) patterns. A marked improvement in the PTCDA molecular order is observed for the growth on the Se-passivated substrate. The resulting LEED patterns are sharp, and indicate that the interface molecular unit cells are azimuthally oriented with respect to the Se-dimers. The improvement in the crystallinity of the PTCDA layer is attributed to the termination of the chemically active sites by Se and to the smoothness of the Se-passivated surface. This work provides information as to the conditions under which the quasiepitaxial growth of a lattice mismatched van der Waals film oriented to the substrate can be achieved.