Role of Hydration on the Electronic Transport through Molecular Junctions on Silicon

Abstract

ABSTRACT. Molecular electronics is a fascinating area of research with the ability to tune device properties by a chemical tailoring of organic molecules. However, molecular electronics devices often suffer from dispersion and lack of reproducibility of their electrical performances. Here, we show that water molecules introduced during the fabrication process or coming from the environment can strongly modify the electrical transport properties of molecular junctions made on hydrogen-terminated silicon.We report an increase in conductance by up to three orders of magnitude, as well as an induced asymmetry in the current-voltage curves. These observations are correlated with a specific signature of the dielectric response of the monolayer at low frequency. In addition, a random telegraph signal is observed for these junctions with macroscopic area. Electrochemical charge transfer reaction between the semiconductor channel and H + /H2 redox couple is proposed as the underlying phenomenon.Annealing the samples at 150°C is an efficient way to suppress these water-related effects. This study paves the way to a better control of molecular devices and has potential implications when these monolayers are used as hydrophobic layers or incorporated in chemical sensors.

Barrier height distribution and dipolar relaxation in metal-insulator-semiconductor junctions with molecular insulator: Ageing effects

“…33 The reversible increase in dc current density is consistent with the above picture; however, it is not clear if localized interface states are created by some kind of water related complexes, or if the observed increase in low frequency capacitance is simply the result of increased current density, as predicted by recent modeling of MIS tunnel diode applied to hybrid molecular junctions in the forward bias regime. 34 Finally, redox transfer from a semiconductor to a physisorbed water layer has been extensively discussed in another context, namely the high surface conductance of diamond films due to hole accumulation upon exposure to moist air.…”

“…2(b)) would be somewhat arbitrary; in contrast with previous analysis, 33 at least three components (SiO 2 , OH, H 2 O) are expected for an accurate decomposition of O1s spectra of the OML/SiO 2 /Si system. In this context, a comparison of XPS spectra before and after ageing is very helpful.…”

“…71 Similarly, recent studies using a Hg top metal contact 33 have shown a reversible effect of hydration/annealing at 150 C on the conductance and low frequency capacitance of alkyl/n-type Si junctions. This effect is nearly negligible if the molecular monolayer is closely packed.…”

“…50 Modeling of the SC/water interface including the H 2 /2H þ redox couple allows prediction of electron transfer for various semiconductors. 50 The potential of the H 2 O/2H þ redox couple being above the Fermi level of silicon, 33 the thin water layer can act as a surface electron donor for the OML/Si system; hence, the electron exchange from the water layer to silicon, governed by the redox reaction 2 H 2 O þ H 2 ! 2 H 3 O þ þ 2 e À , may contribute to decrease both the positive space charge inside the semiconductor and the upward band bending.…”

“…(mm size) molecular junctions obtained by covalent bonding of a nanometer-thick molecular monolayer to a semiconducting substrate, different sources of fluctuations with possible influence on transport properties have been reviewed. They include: (i) upper electrode non uniformity including formation of conducting filaments through the insulating layer; 17 (ii) non uniformity in OML thickness resulting from low coverage, poor organization, and variable tilt angle of the molecular layer; 19,35 (iii) spatial dependence of some induced density of states in the molecular layer band gap due to the strong covalent bonding with underlying metal or semiconductor; 20,27 (iv) fluctuation of flat-band voltage 24 due to spatial dependence of either top electrode work function (e.g., due to metal oxidation), or Si electron affinity (e.g., due to semiconductor oxidation), or else molecular dipole density; (v) non uniform oxidation of the bottom metal or semiconductor creating interface traps which can be detected by specific trap assisted tunneling signature in noise dynamics or in C-V characteristics; 21,28,[32][33][34] (vi) redox effects consisting in charge transfer 33,50 from the semiconductor to the adventitious water layer which is present on any surface at the ambient, with some preferential water physisorption being expected on oxidized hydrophilic regions.…”