Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

We compare the charge transport characteristics of heavy-doped p${}^{++}$- and n${}^{++}$-Si-alkyl chain/Hg junctions. Based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction we suggest that for both p${}^{++}$- and n${}^{++}$-type junctions, the energy difference between the Fermi level and lowest unoccupied molecular orbital (LUMO), i.e., electron tunneling, controls charge transport. This conclusion is supported by results from photoelectron spectroscopy (ultraviolet photoemission spectroscopy, inverse photoelectron spectroscopy, and x-ray photoemission spectroscopy) for the molecule-Si band alignment at equilibrium, which clearly indicate that the energy difference between the Fermi level and the LUMO is much smaller than that between the Fermi level and the highest occupied molecular orbital (HOMO). Furthermore, the experimentally determined Fermi level - LUMO energy difference, agrees with the non-resonant tunneling barrier height, deduced from the exponential length attenuation of the current.

Figure 1

Normalized $G$-$V$ curves for p${}^{++}$- (solid line) and n${}^{++}$- (dashed line) Si-C${}_{16}$H${}_{33}$/Hg junctions. The conductance of the n${}^{++}$ junction is parabolic around 0 V, while for the p${}^{++}$ junction at negative bias on the metal, there is a voltage range, where the conductance increases slowly (shaded area), followed by a sharp increase. The transition bias between the above-mentioned regions is $\sim 1.1$ V, very close to the Si band gap ($\sim 1.12$ eV). Regions a, b, and c refer to voltage ranges, shown in Fig. 2 and discussed in the text, for the p${}^{++}$ junction only.

Figure 2

Schematic band energy diagrams for MIS structures with p${}^{++}$ semiconductor substrates and LUMO-dominated transport (top) and semilog $J$-$V$ curve of a typical p${}^{++}$-Si-C${}_{16}$H${}_{33}$/Hg junction (averaged over 12 scans) (bottom) with the a, b, and c labels corresponding to the diagrams above. Three major bias regimes are considered in the different energy schemes (top) and separated in the $J-V$ plot by vertical lines (bottom), corresponding to region (a) metal Fermi above (i.e., closer to vacuum level) the SC gap [$V<-(E_{\mathrm{g}}+\zeta$)]; region (b) metal Fermi within the SC gap [$-(E_{\mathrm{g}}+\zeta )<V<-\zeta$]; and region (c) metal Fermi below the SC gap ($V>-\zeta$), where $E_{\mathrm{g}}$ and $\zeta$ are the forbidden gap and the energy difference between the SC Fermi level and the valence-band (VB) edge. Top: ${\phi }_t$ is the energy difference between the tunneling electrons (marked by a horizontal arrow) and the molecular level (spatially averaged), which is taken as the estimated effective tunneling barrier height. The insulator level is assumed to vary across the insulator. Electron tunneling into the VB (as in b) is practically identical to holes tunneling in the opposite direction. Surface states, band bending, and image forces are neglected for simplicity.

Figure 3

(a) Combined UPS (left of $E_{\mathrm{F}}$ line, blue) and IPES (right to $E_{\mathrm{F}}$ line, red) spectra of C${}_{16}$H${}_{33}$-Si$\left(111\right)$ surfaces of heavy-doped p${}^{++}$- (upper) and n${}^{++}$- (lower) Si. The vertical black line marks the experimentally determined Fermi level of both samples. The crossings of the tangent lines determine the edges of the monolayer edge-to-edge gap. While the UPS measurements yield a single unambiguous edge, which corresponds with the position of the theoretically calculated HOMO (Ref. 40), the IPES spectra show two transitions, marking two possible band edges, which are noted as #1 and #2, for both p${}^{++}$ and n${}^{++}$. The black vertical arrow marks the position of the estimated tunneling barrier relative to the Fermi level, extracted from length-dependent $J-V$ measurements of the p${}^{++}$-Si-alkyl/Hg junction. (b), (c) Proposed band diagrams of p${}^{++}$- and n${}^{++}$-Si- C${}_{16}$H${}_{33}$ surfaces, respectively, which are extracted from the edges of the UPS and IPES results. The Si band lines (solid line) are slightly bent to reflect the experimentally determined $0.2$-eV band bending in both samples, and the dashed line marks the postion of Fermi level in the Si band gap. The gray shaded areas mark the estimated uncertainties for the positions of the actual HOSO and LUSO (see text).

Figure 4

$J-V$ measurements of p${}^{++}$- and n${}^{++}$-Si-C${}_2$H${}_5$/Hg junctions (solid and dashed curves, respectively) compared to the $J$-$V$ behavior of p${}^{++}$- and n${}^{++}$-Si-H/Hg junctions (dotted curve; p${}^{++}$ and n${}^{++}$ curves are identical). The error bars shown here only for the n${}^{++}$Si-C${}_2$H${}_5$ represent the standard deviation of multiple measurements on eight different junctions.

Figure 5

Length dependence of current density vs voltage curves for for p${}^{++}$-Si-C${}_n$H${}_{2n+1}$/Hg junctions ($n=14,16,$ and 18). Data for the Si-H/Hg junctions (dashed line) are given as reference. The current density exponentially decreases with the molecular length. Error bars represent the standard deviation of at least eight different junctions.