Relaxation dynamics in covalently bonded organic monolayers on silicon

Abstract

We study the dynamic electrical response of a silicon-molecular monolayer-metal junctions and we observe two contributions in the admittance spectroscopy data. These contributions are related to dipolar relaxation and molecular organization in the monolayer in one hand, and the presence of defects at the silicon/molecule interface in the other hand. We propose a small signal equivalent circuit suitable for the simulations of these molecular devices in commercial device simulators. Our results concern monolayers of alkyl chains considered as a model system but can be extended to other molecular monolayers. These results open door to a better control and optimization of molecular devices.Comment: 1 pdf file including text, figures and tables. Phys. Rev. B, in pres

Effect of high current density on the admittance response of interface states in ultrathin MIS tunnel junctions

“…This low frequency contribution (0.5 nF.cm -2 ), attributed to Si-C interface dipoles of the fresh Hg // C 12 H 25 -Si device, is much smaller than the dipolar capacitance values (30-40 nF.cm -2 ) reported in a recent admittance study of alkyl (C 18 -n + Si) molecular assemblies. 14 In MIS tunnel junctions with molecular insulator, low dipolar contribution is an important condition to detect a small surface states response, since the latter may be masked by any dipolar relaxation of interface or embedded polar moieties. As an illustration, in a study of long term stability of molecular devices (ageing at the ambient for eighteen months), a significant increase has been observed in the dipolar relaxation capacitance (x 50) and in the interface states response (x 7) of the Hg // C 12 H 25 -Si junction; besides the appearance of an interface silicon oxide layer revealed by XPS, some presence of adsorbed water vapour cannot be excluded.…”

“…Due to sterical constraints, the density of surface Si-H sites which are not grafted with alkyl molecules is larger than 4×10 14 cm -2 (≈60% of surface Si atom density); however, the density of electrically active defects, S D , remains relatively small, at least immediately after grafting, in the 1×10 10 -3×10 11 eV -1 .cm -2 range. [10][11][12][13][14][15] However, besides the problem of engineering a homogeneous and reliable top electrode in contact with a nanometer-thick molecular monolayer, a major issue in hybrid molecular-silicon junctions is the structural disorder which results from the irreversible grafting mechanism related to the strong covalent bonding ; hence structural disorder probably induces some distribution of barrier heights for electrical transport and may explain the wide range of conductance values observed experimentally. 16,17 In metal / semiconductor (SC) junctions, the presence of interface states and of a thin insulating layer have been recognized as non idealizing factors in the current-voltage (I-V) characteristics.…”

“…[18][19][20][21] A number of methods based on non-stationary measurements have been developed for interface defect characterization in thick MIS diodes 22,23 and tentatively extended to MIS junctions with interfacial layers, 24,25 ultrathin SiO 2 , 19,26-29 and molecular insulator. 3,14,15,30 In the latter devices, frequency and temperature dependence of molecular MIS admittance brings valuable information to discriminate interface states response and dipolar relaxation effects. 14,[31][32][33] MIS tunnel diodes are non equilibrium devices due to the large current density flowing through the thin insulating layer (Figure 1).…”

“…3,14,15,30 In the latter devices, frequency and temperature dependence of molecular MIS admittance brings valuable information to discriminate interface states response and dipolar relaxation effects. 14,[31][32][33] MIS tunnel diodes are non equilibrium devices due to the large current density flowing through the thin insulating layer (Figure 1). It has been shown that, between 1 and 2 nm thick SiO 2 , the quasistatic hypothesis progressively fails and a kinetic approach should be developed; 34 this effect is expected to occur at larger thickness values for an insulator with a lower tunnel barrier height.…”

Molecular relaxation dynamics in organic monolayer junctions

“…The temperature has been varied but the range of frequencies centered about 1 kHz was very limited. 22 We have done complex admittance measurements for usual planar geometry: 23 the alkyl chains are grafted on planar surfaces and contacted on top with metallic electrodes. A small ac signal ͑10 mV͒, with frequency ranging between 20 and 10 6 Hz, plus a dc bias varying from 0 to 1 V are applied perpendicular to the surface electrodes.…”