Controlling Space Charge of Oxide-Free Si by in Situ Modification of Dipolar Alkyl Monolayers

Good passivation of Si, both electrically and chemically, is achieved by monolayers of 1,9-decadiene, directly bound to an oxide-free Si surface. The terminal CC bond of the decadiene serves for further in situ reaction, without harming the surface passivation, to −OH- or −Br-terminated monolayers that have different dipole moments. Such a two-step procedure meets the conflicting requirements of binding mutually repelling dipolar groups to a surface, while chemically blocking all surface reactive sites. We demonstrate a change of 0.15 eV in the Si surface potential, which translates into a 0.4 eV variation in the Schottky barrier height of a Hg junction to those molecularly modified n-Si surfaces. Charge transport across such junctions is controlled both by tunneling across the molecular monolayer and by the Si space charge. For reliable insight into transport details, we resorted to detailed numerical simulations, which reveal that the Si space charge and the molecular tunneling barriers are coupled. As a result, attenuation due to the molecular tunneling is much weaker than in metal/molecule/metal molecular junctions. Simulation shows also that some interface states are present but that they have a negligible effect on Fermi level pinning. These states are efficiently decoupled from the metal (Hg) and interact mostly with the Si

Defect Scaling with Contact Area in EGaIn-Based Junctions: Impact on Quality, Joule Heating, and Apparent Injection Current

Although the tunneling rates decrease exponentially with a decay coefficient β close to 1.0 n_C^–1 across <i>n</i>-alkanethiolate (SC_<i>n</i>) monolayer based tunneling junctions determined over a multitude of test beds, the origins of the large spread of injection current densitiesthe hypothetical current density, <i>J</i>₀ (in A/cm²), that flows across the junction when <i>n</i> = 0of up to 12 orders of magnitude are unclear. Every type of junction contains a certain distribution of defects induced by, for example, defects in the electrode materials or impurities. This paper describes that the presence of defects in the junctions is one of the key factors that cause an increase in the observed values of <i>J</i>₀. We controlled the number of defects in Ag^TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions by varying the geometrical contact area (<i>A</i>_geo) of the junction. The value of <i>J</i>₀ (∼10² A/cm²) is independent of the junction size when <i>A</i>_geo is small (<9.6 × 10² μm²) but increased by 3 orders of magnitude (from 10² to 10⁵ A/cm²) when <i>A</i>_geo increased from 9.6 × 10² to 1.8 × 10⁴ μm². With increasing <i>J</i>₀ values the yields in nonshorting junctions decreased (from 78 to 44%) and β increased (from 1.0 to 1.2 n_C^–1). We show that the quality of the junctions can be qualitatively determined by examining the curvature of the d<i>J</i>/d<i>V</i> curves (defects change the sign of the curvature from positiveassociated with tunnelingto negativeassociated with Joule heating) and fitting the <i>J</i>(V) curves to the full Simmons equation to (crudely) estimate the effective separation of the top- and bottom-electrode <i>d</i>_eff. This analysis confirmed that the electrical characteristics of large junctions are dominated by thin-area defects, while small junctions are dominated by the molecular structure

Molecular Length, Monolayer Density, and Charge Transport: Lessons from Al–AlOx/Alkyl–Phosphonate/Hg Junctions

A combined electronic transport–structure characterization of self-assembled monolayers (MLs) of alkyl–phosphonate (AP) chains on Al–AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that “long” APs (C14 and C16) form much denser MLs than do “short” APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed “short” AP MLs attenuate the current relatively more efficiently than those with densely packed, “long” ones. Furthermore, “long” AP ML junctions showed strong bias variation of the length decay coefficient, β, while for “short” AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed

Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH₃NH₃)PbI₃ Films

Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH₃NH₃]­PbI₃(Cl) (“perovskite”) films <i>in vacuo</i>, and in N₂, air, and O₂, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance <i>in vacuo</i> or post<i>-vacuo</i> N₂ exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O₂-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material’s electrical properties and electrochemical reactivity, which can also affect material stability

Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the Molecule–Silicon Interface

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si–C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface

Effect of Molecule–Surface Reaction Mechanism on the Electronic Characteristics and Photovoltaic Performance of Molecularly Modified Si

We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (S_N) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S_N reaction, whereas with UV irradiation RCR dominates, with S_N as a secondary path. We show that the site-sensitive S_N reaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both S_N and RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. <i>These results show how chemical kinetics and reaction paths impact electronic properties at the device level</i>. It further suggests an approach for effective passivation of other semiconductors

Odd–Even Effect in Molecular Electronic Transport via an Aromatic Ring

A distinct odd–even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H₂CCH–(CH₂)_<i>n</i>–phenyl, with <i>n</i> = 2–5, were adsorbed onto Si–H and formed high-quality monolayers with a binding density of 50–60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient π–π interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of methylenes in their alkyl spacers. The odd−even alternation in molecular tilt was experimentally confirmed by contact angle, ellipsometry, FT-IR, and XPS with a close quantitative match to the simulation results. The orientations of both the ring plane and the long axis of the alkyl spacer are more perpendicular to the substrate plane for molecules with an even number of methylenes than for those with an odd number of methylenes. Interestingly, those with an even number conduct better than the effectively thinner monolayers of the molecules with the odd number of methylenes. We attribute this to a change in the orientation of the electron density on the aromatic rings with respect to the shortest tunneling path, which increases the barrier for electron transport through the odd monolayers. The high sensitivity of molecular charge transport to the orientation of an aromatic moiety might be relevant to better control over the electronic properties of interfaces in organic electronics

Temperature-Dependent Coherent Tunneling across Graphene–Ferritin Biomolecular Junctions

Understanding the mechanisms of charge transport (CT) across biomolecules in solid-state devices is imperative to realize biomolecular electronic devices in a predictive manner. Although it is well-accepted that biomolecule–electrode interactions play an essential role, it is often overlooked. This paper reveals the prominent role of graphene interfaces with Fe-storing proteins in the net CT across their tunnel junctions. Here, ferritin (AfFtn-AA) is adsorbed on the graphene by noncovalent amine–graphene interactions confirmed with Raman spectroscopy. In contrast to junctions with metal electrodes, graphene has a vanishing density of states toward its intrinsic Fermi level (“Dirac point”), which increases away from the Fermi level. Therefore, the amount of charge carriers is highly sensitive to temperature and electrostatic charging (induced doping), as deduced from a detailed analysis of CT as a function of temperature and iron loading. Remarkably, the temperature dependence can be fully explained within the coherent tunneling regime due to excitation of hot carriers. Graphene is not only demonstrated as an alternative platform to study CT across biomolecular tunnel junctions, but it also opens rich possibilities in employing interface electrostatics in tuning CT behavior

Effect of Internal Heteroatoms on Level Alignment at Metal/Molecular Monolayer/Si Interfaces

Molecular monolayers at metal/semiconductor heterointerfaces affect electronic energy level alignment at the interface by modifying the interface’s electrical dipole. On a free surface, the molecular dipole is usually manipulated by means of substitution at its external end. However, at an interface such outer substituents are in close proximity to the top contact, making the distinction between molecular and interfacial effects difficult. To examine how the interface dipole would be influenced by a single atom, internal to the molecule, we used a series of three molecules of identical binding and tail groups, differing only in the inner atom: aryl vinyl ether (<b>PhO</b>), aryl vinyl sulfide (<b>PhS</b>), and the corresponding molecule with a CH₂ groupallyl benzene (<b>PhC</b>). Molecular monolayers based on all three molecules have been adsorbed on a flat, oxide-free Si surface. Extensive surface characterization, supported by density functional theory calculations, revealed high-quality, well-aligned monolayers exhibiting excellent chemical and electrical passivation of the silicon substrate, in all three cases. Current–voltage and capacitance–voltage analysis of Hg/PhX (X = C, O, S)/Si interfaces established that the type of internal atom has a significant effect on the Schottky barrier height at the interface, i.e., on the energy level alignment. Surprisingly, despite the formal chemical separation of the internal atom and the metallic electrode, Schottky barrier heights were not correlated to changes in the semiconductor’s effective work function, deduced from Kelvin probe and ultraviolet photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather, these changes correlated well with the ionization potential of the surface-adsorbed molecules. This is interpreted in terms of additional polarization at the molecule/metal interface, driven by potential equilibration considerations even in the absence of a formal chemical bond to the top Hg contact