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

Abstract

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).

Electrochemical impedance spectroscopy of carboxylic-acid terminal alkanethiol self assembled monolayers on GaAs substrates

“…We note that since the monolayers were maintained under dark conditions, it is unlikely that illumination is the cause for poor monolayer quality on n GaAs. We also note that the EIS spectra of monolayers formed at 50 • C for 8 h [12,39,41,42], closely resembled those formed at room temperature for 24 h under N 2 ambient [20], suggesting an equivalent quality of SAMs synthesized by either method, as inferred from EIS data.…”

“…As expected, OCP values become marginally more positive upon SAM-modification. Susceptibility to corrosion and etching of bare GaAs (both n-and p-type) increases with doping level [41,51,52], due to a thinning of the potential barrier at the interface with increasing doping density, and our results suggest a good degree of SAM-induced surface passivation of n+ and p+ GaAs at OCP. Lower GaAs doping density, on the other hand, favors uniform oxidation.…”

“…The deposition procedure for SAMs on GaAs was based on prior work [12,[39][40][41]. Prior to deposition, wafers were cleaned with acetone for 10 min, followed by etching with 30% NH 4 OH for 1 min to remove the oxide.…”

Molecular Transport Junctions: Clearing Mists

“…Even though these pose some of their own difficulties (band bending, impurity states), the covalent bonding associated with the C-C, [38] C-Si [34][35][36] , or C-Ga [37] structures at semiconductor interfaces is highly attractive both for stability and for reproducibility. The first decade of molecular junction transport measurement and modeling is almost over, and major advances have been made.…”

The physics and chemistry of the Schottky barrier height

“…Generally, systematic shifts of the SBH in the direction consistent with the polarity of the molecules have been frequently observed in experiments, 284 even though the magnitudes of such shifts were typically found to be less than those observed for the free surface. 247,284,[292][293][294][295][296][297] Interaction of the molecules with the metal layer, structural change in the molecules, change in the molecule-semiconductor bond as a result of the metal, and other collective effects could lead to the observed reduction in dipole. [298][299][300] Such an expected systematic trend in SBH is shown in Fig.…”