QUASIEPITAXIAL GROWTH OF THE ORGANIC MOLECULAR SEMICONDUCTOR 3,4,9,10-PERYLENETETRACARBOXYLIC DIANHYDRIDE

Thin films of the organic molecular semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were grown by ultrahigh vacuum sublimation on graphite, and on As-saturated c(4X4), As-rich (2X4)-c(2X8), and Se-passivated (2X1) GaAs (100) surfaces. The Se-passivated GaAs and graphite surfaces resulted in PTCDA films with good crystallinity, whereas deposition under similar conditions on the (2X4)-c(2X8) GaAs surface produced films randomly oriented in the plane parallel to the surface. The films grown on the c(4X4) GaAs surface showed intermediate molecular ordering. These differences are analyzed in terms of interface bonding between the molecules and the substrate, specifically at defects and dangling bonds which pin the molecules at random sites. These interactions are considerably reduced on the inert surfaces of graphite and Se-passivated GaAs. The results presented here provide information on the conditions under which the quasiepitaxy of organic molecular crystals can be achieved on highly mismatched inorganic substrates.