Hg/Molecular Monolayer−Si Junctions: Electrical Interplay between Monolayer Properties and Semiconductor Doping Density

Abstract

Metal-organic molecule-semiconductor junctions are controlled not only by the molecular properties, as in metal-organic molecule-metal junctions, but also by effects of the molecular dipole, the dipolar molecule-semiconductor link, and molecule-semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current-voltage, capacitance-voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled junction is not sensitive to molecular changes in the organic monolayer at reverse and low forward bias and is controlled by series resistance at higher forward bias. However, in the case of highly doped Si, the internal barrier is smaller, and as a result, the charge transport properties of the junction are affected by changing from an alkyl to an alkenyl monolayer. We propose that the double bond near the surface primarily increases the coupling between the organic monolayer and the Si, which increases the current density at a given bias by increasing the contact conductance.

Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

“…18,20 By contrast, values of J 0 ≈ 10 8 A·cm −2 are reported for large-area junctions using flat metal substrates and flat (e.g., graphene 11 ) or conformal (e.g., evaporated Au 12 ) top-electro- ). For junctions using Hg-drops on top of alkyl chains anchored to heavily doped Si surfaces, Cahen and co-workers reported a value of J 0 ≈ 10 6.5 A·cm −2 for p-Si substrates, 4 and a value of J 0 ≈ 10 4 A·cm −2 for n-Si substrates 48 (we estimated these values of J 0 from data reported by the authors 4,48 ). For the system with J 0 ≈ 10 4 A·cm −2 , however, fitting the Simmons equation to experimental J−V curves for individual n-alkanes required a correction of the contact area by a factor of ∼10 −4 in order to get meaningful fitting parameters 3 (30 μm 2 contact area required by the fitting, over a geometrical contact area of 5 × "may lack physical relevance".…”

“…Using mercury drops as top-electrodes, measurements of rates of tunneling across n-alkanes anchored to heavily doped silicon surfaces led to β = 0.9 ± 0.2 nC −1 , similar to the values observed for nalkanethiolates on Au and Ag substrates. 3,4 By contrast, values of the injection current J 0 (V = +0.5 V) the limiting value of current for an ideal junction with no hydrocarbon present (d = 0), but with all the interfaces and characteristics of junctions containing the SAMsvary from…”

Introducing Ionic and/or Hydrogen Bonds into the SAM//Ga2O3 Top-Interface of AgTS/S(CH2)nT//Ga2O3/EGaIn Junctions

“…8,34,35 There are, however, unresolved questions in most of these experiments about the interpretation of the parameters J 0 , β, and d. The attenuation parameter β describes the falloff in tunneling current with distance, and is remarkably consistent (β = 0.73−0.89 Å −1 or 0.9−1.1 nC…”

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

“…(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.…”