Molecular control over semiconductor surface electronic properties: Dicarboxylic acids on CdTe, CdSe, GaAs, and InP

We present "design rules" for the selection of molecules to achieve electro nic control over semiconductor surfaces, using a simple molecular orbital m odel. The performance of most electronic devices depends critically on thei r surface electronic properties, i.e., surface band-bending and surface rec ombination velocity. For semiconductors, these properties depend on the den sity and energy distribution of surface states. The model is based on a sur face state-molecule, HOMO-LUMO-Like interaction between molecule and semico nductor. We test it by using a combination of contact potential difference, surface photovoltage spectroscopy, and time- and intensity-resolved photol uminescence measurements. With these, we characterize the interaction of tw o types of bifunctional dicarboxylic acids, the frontier orbital energy lev els of which can be changed systematically, with air-exposed CdTe, CdSe, In P, and GaAs surfaces. The molecules are chemisorbed as monolayers onto the semiconductors. This model explains the widely varying electronic consequen ces of such interaction and shows them to be determined by the surface stat e energy position and the strength of the molecule-surface state coupling. The present findings can thus be used as guidelines for molecule-aided surf ace engineering of semiconductors.