Molecular control over Au/GaAs diodes

The use of molecules to control electron transport is an interesting possib ility, not least because of the anticipated role of molecules in future ele ctronic devices(1). But physical implementations using discrete molecules a re neither conceptually(2,3) simple nor technically straightforward (diffic ulties arise in connecting the molecules to the macroscopic environment). B ut the use of molecules in electronic devices is not limited to single mole cules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal-semicondu ctor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecu les onto single crystals of n-type GaAs semiconductor. Gold contacts were d eposited onto the modified surface, using a 'soft' method to avoid damaging the molecules(4). By using a series of multifunctional molecules whose dip ole is varied systematically, we produce diodes with an effective barrier h eight that is tuned by the molecule's dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after m olecular modification. This behaviour is consistent with that of unmodified metal-semiconductor diodes, in which the barrier height can depend on the metal's work function.