  The step edge barrier of a prototypical organic semiconductor molecule,
3,4,9,10-perylene-tetracaboxylic-dianhydride (PTCDA) has been analysed by means
of calculations based on emperical potentials. The minimum energy path (MEP)
has been calculated for a single molecule on a substrate of three molecular
layers between equivalent minimum energy positions within two neighboring unit
cells. To determine the step edge barrier, we have calculated the MEP over a
step to a fourth layer of molecules. We found energy barriers of E_D= 80 meV
for in-layer diffusion and E_S = 750 meV for step crossing, indicating a strong
Ehrlich-Schwoebel effect for PTCDA.
