  We study the physisorption of organic oligomers on the ZnO($10\bar{1}0$)
surface using first-principles density-functional theory and non-empirical
embedding methods. We find that both in-plane location and orientation of the
molecules are completely determined by the coupling of their quadrupole moments
to the periodic dipolar electric field present at the semiconductor surface.
The adsorption is associated with the formation of a molecular dipole moment
perpendicular to the surface, which bears an unexpected linear relation to the
molecule-substrate interaction energy. Long oligomers such as sexiphenyl become
well-aligned with stabilization energies of several 100 meV along rows of
positive electric field, in full agreement with recent experiments. These
findings define a new route towards the realization of highly-ordered
self-assembled arrays of oligomers/polymers on ZnO($10\bar{1}0$) and similar
surfaces.
