9 research outputs found
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 CîťC bond of the decadiene serves for
further in situ reaction, without harming the surface passivation,
to âOH- or âBr-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<sub>C</sub><sup>â1</sup> across <i>n</i>-alkanethiolate (SC<sub><i>n</i></sub>) monolayer based tunneling junctions determined over a multitude
of test beds, the origins of the large spread of injection current
densitiesî¸the hypothetical current density, <i>J</i><sub>0</sub> (in A/cm<sup>2</sup>), that flows across the junction
when <i>n</i> = 0î¸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><sub>0</sub>. We controlled
the number of defects in Ag<sup>TS</sup>-SC<sub><i>n</i></sub>//GaO<sub><i>x</i></sub>/EGaIn junctions by varying
the geometrical contact area (<i>A</i><sub>geo</sub>) of
the junction. The value of <i>J</i><sub>0</sub> (âź10<sup>2</sup> A/cm<sup>2</sup>) is independent of the junction size when <i>A</i><sub>geo</sub> is small (<9.6 Ă 10<sup>2</sup> Îźm<sup>2</sup>) but increased by 3 orders of magnitude (from 10<sup>2</sup> to 10<sup>5</sup> A/cm<sup>2</sup>) when <i>A</i><sub>geo</sub> increased from 9.6 Ă 10<sup>2</sup> to 1.8 Ă
10<sup>4</sup> Îźm<sup>2</sup>. With increasing <i>J</i><sub>0</sub> values the yields in nonshorting junctions decreased
(from 78 to 44%) and β increased (from 1.0 to 1.2 n<sub>C</sub><sup>â1</sup>). 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 positiveî¸associated with tunnelingî¸to
negativeî¸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><sub>eff</sub>. 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 AlâAlOx/AlkylâPhosphonate/Hg Junctions
A combined electronic transportâstructure characterization of self-assembled monolayers (MLs) of alkylâphosphonate (AP) chains on AlâAlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that âlongâ APs (C14 and C16) form much denser MLs than do âshortâ APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed âshortâ AP MLs attenuate the current relatively more efficiently than those with densely packed, âlongâ ones. Furthermore, âlongâ AP ML junctions showed strong bias variation of the length decay coefficient, β, while for âshortâ 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 (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> Films
Halide perovskite-based solar cells
still have limited reproducibility,
stability, and incomplete understanding of how they work. We track
electronic processes in [CH<sub>3</sub>NH<sub>3</sub>]ÂPbI<sub>3</sub>(Cl) (âperovskiteâ) films <i>in vacuo</i>, and in N<sub>2</sub>, air, and O<sub>2</sub>, 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> N<sub>2</sub> 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 O<sub>2</sub>-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 materialâs electrical
properties and electrochemical reactivity, which can also affect material
stability
Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the MoleculeâSilicon 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 SiâC 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 MoleculeâSurface 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<sub>N</sub>) and radical chain reaction (RCR), depending on adsorption
conditions. Moderate heating leads to the S<sub>N</sub> reaction,
whereas with UV irradiation RCR dominates, with S<sub>N</sub> as a
secondary path. We show that the site-sensitive S<sub>N</sub> 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<sub>N</sub> 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
OddâEven Effect in Molecular Electronic Transport via an Aromatic Ring
A distinct oddâeven effect
on the electrical properties,
induced by monolayers of alkyl-phenyl molecules directly bound to
Si(111), is reported. Monomers of H<sub>2</sub>CîťCHâ(CH<sub>2</sub>)<sub><i>n</i></sub>âphenyl, with <i>n</i> = 2â5, were adsorbed onto SiâH and formed
high-quality monolayers with a binding density of 50â60% Si(111)
surface atoms. Molecular dynamics simulations suggest that the binding
proximity is close enough to allow efficient ĎâĎ
interactions and therefore distinctly different packing and ring orientations
for monomers with odd or even numbers of methylenes in their alkyl
spacers. The oddâeven 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 GrapheneâFerritin 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
biomoleculeâelectrode 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 amineâgraphene interactions confirmed with Raman
spectroscopy. In contrast to junctions with metal electrodes, graphene
has a vanishing density of states toward its intrinsic Fermi level
(âDirac pointâ), 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
interfaceâs 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 CH<sub>2</sub> groupî¸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. Currentâvoltage
and capacitanceâvoltage 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 semiconductorâs
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