Doping Molecular Monolayers: Effects on Electrical Transport Through Alkyl Chains on Silicon

Abstract

Abstractn‐Si/CnH2n + 1/Hg junctions (n = 12, 14, 16 and 18) can be prepared with sufficient quality to assure that the transport characteristics are not anymore dominated by defects in the molecular monolayers. With such organic monolayers we can, using electron, UV and X‐ray irradiation, alter the charge transport through the molecular junctions on n‐ as well as on p‐type Si. Remarkably, the quality of the self‐assembled molecular monolayers following irradiation remains sufficiently high to provide the same very good protection of Si from oxidation in ambient atmosphere as provided by the pristine films. Combining spectroscopic (UV photoemission spectroscopy (UPS), X‐ray photoelectron spectroscopy (XPS), Auger, near edge‐X‐ray absorption fine structure (NEXAFS)) and electrical transport measurements, we show that irradiation induces defects in the alkyl films, most likely CC bonds and CC crosslinks, and that the density of defects can be controlled by irradiation dose. These altered intra‐ and intermolecular bonds introduce new electronic states in the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of the alkyl chains and, in the process, dope the organic film. We demonstrate an enhancement of 1–2 orders of magnitude in current. This change is clearly distinguishable from the previous observed difference between transport through high quality and defective monolayers. A detailed analysis of the electrical transport at different temperatures shows that the dopants modify the transport mechanism from tunnelling to hopping. This study suggests a way to extend significantly the use of monolayers in molecular electronics.

Molecules on Si: Electronics with Chemistry

“…Only at higher forward bias do the currents vary according to the thickness of the insulator, i.e., the molecular layer width here. Such length-independence of current at low forward bias was observed for several moderately-doped n-type MOMS junctions with n-Si [29,56,88,134,135,138,139] and n-GaAs, [35,36,140] contacted with medium-high work-function metals such as Hg.…”

“…[34,55,96] In MIM junctions, a pseudo-metallic temperature response is generally attributed to conductance via metallic filaments penetrating the monolayer. [34,171] This is very unlikely here, because if we change the insulator by electron irradiation [164] (or ''dope'' it, [139] ) the metal-like dependence is replaced by a thermally activated behavior, as shown in Figure 7b (dotted lines). The $100 eV electron irradiation used here to induce defects should not dissolve metallic filaments, if they ever existed.…”

“…Figure 7 shows the effect of temperature on the transport characteristics for Hg/C 18 Àp-Si and Hg/C 14 Àn-Si junctions, including the effect of electron irradiation. [139,164] …”

“…[139] For comparison, if E a is extracted from room temperature J-V data, i.e., from J sat using the geometric contact area and literature Richardson constant, a value of 0.9 eV is found. [139] From Table 2 we see that the low E a values from the temperature-dependent data indicate transport by recombination (or generation at forward bias). [20] The fact that both the (thermal) E a (Fig.…”

“…[20] Irradiation changes the forward bias ''activation energy'' from negative to positive, suggesting a change in transport from tunneling, possibly to hopping. [139] In summary, temperature dependent current measurements allow insights into transport mechanisms that are hard to obtain otherwise.…”