How Do Electronic Carriers Cross Si-Bound Alkyl Monolayers?

Electron transport through Si-C bound alkyl chains, sandwiched between $n\mathrm{\text{−}}\mathrm{Si}$ and Hg, is characterized by two distinct types of barriers, each dominating in a different voltage range. At low voltage, the current depends strongly on temperature but not on molecular length, suggesting transport by thermionic emission over a barrier in the Si. At higher voltage, the current decreases exponentially with molecular length, suggesting transport limited by tunneling through the molecules. The tunnel barrier is estimated, from transport and photoemission data, to be $\sim 1.5\mathrm{eV}$ with a $0.25m_e$ effective mass.

Figure 1

Experimental $J\mathrm{\text{−}}V$ curves for $n\mathrm{\text{−}}\mathrm{Si}\mathrm{\text{−}}{\mathrm{C}}_n{\mathrm{H}}_{2n+1}\parallel \mathrm{Hg}$ ($n=12,14,16,18$) junctions at forward bias. Dashed line: $n\mathrm{\text{−}}\mathrm{Si}:\mathrm{H}\parallel {\mathrm{H}}_{29}{\mathrm{C}}_{14}\mathrm{\text{−}}\mathrm{S}\mathrm{\text{−}}\mathrm{Hg}$ junction. Contact area: $0.1{\mathrm{mm}}^2$. Curves are the average for $\sim 20$ junctions. The error is 10%. Inset: Temperature dependence of $I\mathrm{\text{−}}V$ curves for $n\mathrm{\text{−}}\mathrm{Si}\mathrm{\text{−}}{\mathrm{C}}_{14}{\mathrm{H}}_{29}\parallel \mathrm{Hg}$ over the same voltage span [36, 37].

Figure 2

${\beta }^2$ vs forward bias at high voltages, derived from the data shown in Fig. 1 and Eqs. (2, 3). The solid line shows the best linear fit.

Figure 3

Proposed band diagram for the $n\mathrm{\text{−}}\mathrm{Si}\mathrm{\text{−}}{\mathrm{C}}_{14}{\mathrm{H}}_{29}\parallel \mathrm{Hg}$ system from UPS, IPES, and $J\mathrm{\text{−}}V$ data. $(E_{\mathrm{CB}}^{\mathrm{bulk}}\mathrm{\text{−}}{E}_F)=0.25\mathrm{eV}$. Experimental uncertainties in HOMO and LUMO values are indicated. The work functions are 4.3 eV (Si) and 4.5 eV (Hg). From UPS and XPS, we find $\sim 0.4\mathrm{eV}$ band bending before making Hg contact. The $\sim 0.8\mathrm{eV}$ Schottky barrier implies that band bending increases to $\sim 0.5\mathrm{eV}$. As the LUMO was found to be $\sim 2.2\pm 0.3\mathrm{eV}$ above $E_F$, it is $1.4\pm 0.3\mathrm{eV}$ above the Si $E_{\mathrm{CB}}$.