Tuning of Au/n-GaAs diodes with highly conjugated molecules

Bifunctional conjugated molecules, consisting of electron donating or accepting groups that are connected, via a conjugated bridge, to a carboxylic acid group, were adsorbed as monomolecular carboxylate films on n-GaAs (100) and characterized by reflection FTIR, ellipsometry, and contact angle techniques. The way the donors and acceptors affected the electronic properties of the semiconductor was investigated. In agreement with theory, we find a linear relation between the calculated dipole moment of the molecules and the change in electron affinity of the moleculary modified surface, as well as between the barrier height of Au/molecule on n-GaAs junctions, extracted from their current-voltage characteristics and the dipole moment. The experimental results show little effect of the nature of the conjugated bridge in the molecules. Comparison with earlier work shows a clear decrease in the effect of the dipole of the free molecule on the semiconductor surface and interface behavior, notwithstanding the strongly conjugated link between the donor or acceptor groups of the molecule and the semiconductor surface. The simplest way to understand this is to consider the higher polarizability of the intervening bonds. Such effect needs to be considered in designing molecules for molecular control over devices.