Charge transport at a molecular GaAs nanoscale junction

Abstract

In recent years, the use of non-metallic electrodes for the fabrication of single-molecule junctions has developed into an elegant way to impart new properties to nanodevices. Integration of molecular junctions in a semiconducting platform would also speed technological deployment, as it would take advantage of established industrial infrastructures. In a previous proof-of-concept paper, we used simple α,ω-dithiol self-assembled monolayers on a gallium arsenide (GaAs) substrate to fabricate molecular Schottky diodes with a STM. In the devices, we were also able to detect the contribution of a single-molecule to the overall charge transport. The prepared devices can also be used as photodiodes, as GaAs is a III-V direct bandgap (1.42 eV at room temperature) semiconductor, and it efficiently absorbs visible light to generate a photocurrent. In this contribution, we demonstrate that fine control can be exerted on the electrical behaviour of a metal-molecule-GaAs junction by systematically altering the nature of the molecular bridge, the type and doping density of the semiconductor and the light intensity and wavelength. Molecular orbital energy alignment dominates the charge transport properties, resulting in strongly rectifying junctions prepared with saturated bridges (e.g. alkanedithiols), with increasingly ohmic characteristics as the degree of saturation is reduced through the introduction of conjugated moieties. The effects we observed are local, and may be observed with electrodes of only a few tens of nanometres in size, hence paving the way to the use of semiconducting nanoelectrodes to probe molecular properties. Perspectives of these new developments for single molecule semiconductor electrochemistry are also discussed.

Metal–Single-Molecule–Semiconductor Junctions Formed by a Radical Reaction Bridging Gold and Silicon Electrodes

“…Even when the molecule is not bridging the two electrodes a shift towards a non-rectifying I-V curves are observed (Figure 7b). for Au-molecule-GaAs junctions by Nichols and co-workers [17][18] in which the degree of rectification was found to depend on the molecules used to bridge the Au and the semiconducting GaAs electrodes: the more conjugated the molecules are (such as bis-diazo) the smaller the band gap and the more likely to have the lowest unoccupied molecular orbitals (LUMO) energy closer to the metal Fermi level enabling states for electron transport from the metal to the semiconductor under reverse bias. Another possible contributor to the high reverse bias current is the introduction of surface states that reduces the barrier height for electron transfer between the metal and the bottom of the conduction band.…”

“…[12][13][14] Although widely used, molecular electronics on gold platforms suffer from major drawbacks, such as the lability of the S-Au bonds, electric field-induced structural changes and the lack of tunability of the electric properties of the gold substrate which limits the scope of these junctions. [15][16] In the last few years there has been an increasing interest in expanding the field of molecular electronics from gold towards semiconducting platforms, particularly GaAs [17][18] and Si. [19][20][21] It is anticipated that combining the vast and tunable range of the electrical properties of semiconductors with the chemical variety of molecules, new technological development can be achieved.…”

Silicon – single molecule – silicon circuits

“…31,36,[47][48][49][50][51] Silicon-molecule-metal junctions can also be envisaged and would have useful properties, by analogy to results found for GaAs-molecule-Au junctions. [52][53][54] Thiol SAMs on gold have dominated previous applications in molecular electronics as they are also easy to prepare and have properties of widespread interest, 32,[55][56][57][58][59][60][61][62] but suffer from drawbacks as the Au-S bonding is weak 63 and dispersion mofits. [64][65][66] To make robust devices of atomic dimensions, structural regularity and stability is important, and hence covalent bonding of molecules to silicon offers new technology directions; related covalent-bonding applications involving, e.g., graphene point contacts are also of modern interest.…”

Covalent Linkages of Molecules and Proteins to Si–H Surfaces Formed by Disulfide Reduction

“…The development of molecular and Si electronics technologies suggest that the time has come to integrate the electrical properties of semiconductors with the chemical diversity of organic molecules to unlock powerful and miniaturized electronic components. [4][5][6][7][8] The outcomes have the potential to revolutionize the Si-based electronics sector, through creating unprecedented capacity for miniaturization and integration of new device properties -the properties of organic molecules. One of the challenges in Si-based molecular electronics is that the naturally grown oxide layer on the surface of crystalline Si produces insulating properties that shadow its semiconducting properties.…”