Effect of contact properties on current transport in metal/molecule/GaAs devices

Previous reports on metal/molecule/semiconductor (MMS) devices have investigated the effects of molecular species, including dipoles, but have not considered the semiconductor contact properties in detail. In this paper we report on a study of the effects of variations in the semiconductor contact on the conduction properties of MMS devices. Metal/molecule/gallium arsenide (GaAs) devices were fabricated using various semiconductor contact layers, electrically characterized versus temperature and analyzed using an electrostatic model. The various semiconductor contacts included heavily doped n-type and p-type GaAs layers, as well as n-doped and p-doped surface layers of low-temperature-grown GaAs (LTG:GaAs), which provide a high density of midgap defect states near the semiconductor surface. The impact of changing the work function of the top metal contact has also been studied. An electrostatic model that incorporates information on the molecular dipole moment, defect states in GaAs surface layers, and the work function of the metal contact, has been developed in order to understand the band diagrams corresponding to the various device types, and to explain the current-voltage behavior observed in the devices. It is shown by controlling the properties of the semiconductor contact that the device characteristics can be tuned from being dominated by the GaAs barrier (heavily doped n-type GaAs) to being dominated by the molecular states that are strongly coupled to the GaAs contact (LTG:GaAs and heavily doped p-type GaAs).