Ab Initio Theoretical Study of Substituted Dicarboxylic Acids Adsorbed on GaAs Surfaces:  Correlation between Microscopic Properties and Observed Electrical Behavior

Abstract

Five substituted tartaric acid derivatives are studied using density functional theory, both isolated and adsorbed onto an oxidized GaAs cluster, to model molecular layers on semiconductor surfaces. The structures, energies, and electronic properties are computed to clarify the interactions responsible for the electric behavior of the modified surfaces, used in semiconductor/metal junction devices. The chemical structure of the molecule/GaAs adducts is optimized ab initio and discussed for the first time. A strong binding scheme is found, providing useful insights about the microscopic structure of the molecular layer. A widely used model based on molecular dipole layers is discussed and verified, by computing the dipole moment for the isolated systems and estimating the charge separation in the adducts; moreover the molecular orbitals energies are analyzed and correlated to the experimental measures of the modified surface electron affinity.

Controlled Modulation of Conductance in Silicon Devices by Molecular Monolayers

“…All of this leads to unique charge redistributions and the resulting molecular effects that depend on the relative structures of the grafted molecules and their resultant monolayers. Hence, the charge-transfer process is closely related to the alignment of the molecular and surface energy levels in the devices. ,− …”

Cooperative effects and dipole formation at semiconductor and self-assembled-monolayer interfaces

“…This is important, because first principles calculations of substrate ͑SAM͒ structures often use a model of a single molecule adsorbed on a cluster such that the local chemical environment of the moleculesubstrate bonding is correct. 27,28 Naturally, such calculations cannot account for cooperative molecular effects. To examine the extent to which this could be a problem in practice, we examined the extent of depolarization for a finite number of point dipoles by using Eq.…”

Controlling Semiconductor/Metal Junction Barriers by Incomplete, Nonideal Molecular Monolayers

“…The presence of molecular states within ∼0.4 eV of the CBM of GaAs could lead to resonant transmission in the present experiment, because photoexcited electrons, with a maximum photon energy of 1.2 eV, have significant distribution in this energy range. It is unlikely that the molecules presently used have such low-lying molecular states because of the large LUMO−HOMO gaps . Furthermore, when resonant states contribute to the carrier transport, the IPE spectrum is expected to reveal “plateaus”.…”

“…We use here as the relevant dipole moment that of the molecule bound to the GaAs surface, as calculated in ref , rather than the free molecule's dipole moment, as we did in earlier work. We note that, as noted also in ref , the general trends are very similar, reflecting the changes within the series of molecules with identical binding group.…”

“…A layer of electrical dipoles on a surface, with a net layer dipole moment perpendicular to the substrate, can produce a substantial shift in surface potentials, i.e., in the work function, φ m , − of a metal and in the electron affinity, χ sc , and work function, φ sc , ,, of a semiconductor. To a first-order approximation, the shift arises from how the distribution of the dipoles at the interface perturbs the potential of the electrons in the bulk phase.…”