Growth of the Organic Molecular Semiconductor PTCDA on Se-Passivated GaAs(100): An STM Study

We investigate the monolayer and multilayer growth of the organic molecular semiconductor (OMS) 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) on the Se-passivated GaAs(100) (2 x 1) surface using STM. Deposition of similar to 2 ML PTCDA at room temperature results in the formation of clusters, implying good chemical passivation of the substrate. However, we also find a significant number of molecules pinned at high energy defect sites, some of which induce molecular ordering. At higher PTCDA coverage we find that the film invariably orients to the substrate revealing a critical, though weak, molecule-substrate interaction. We present the first molecularly resolved STM images obtained from a thick PTCDA film (similar to 60 Angstrom) and show unit cell dimensions and orientation in excellent agreement with our previous LEED study.