Structure of the adatom electron band of the $\mathrm{Si}\left(111\right)\text{−}7\times 7$ surface

We use variable temperature scanning tunneling spectroscopy to determine the space and energy structure of the adatom electron band of the $\mathrm{Si}\left(111\right)\text{−}7\times 7$ surface. At low temperature $\left(7\mathrm{K}\right)$ we map the wave functions of the electrons in the adatom band in real space. We observe a pronounced splitting of the adatom electron states between corner and center adatoms of the surface reconstruction and identify new spectral features in the unoccupied part of the adatom band. The adatom electron states are subsequently identified in tunneling spectra at room temperature to obtain their energy unaffected by electron transport limitations at low temperature.

Figure 1

(a) STM topography of the $\mathrm{Si}\left(111\right)\text{−}7\times 7$ surface unit cell. (b) Schematic of the surface atoms and orientation of the faulted $\left(F\right)$ and unfaulted $\left(U\right)$ half-unit cells. (c)–(e) STS spectra measured on the surface atoms at $T=7\mathrm{K}$ and assignment of the $\mathrm{Si}\left(111\right)\text{−}7\times 7$ surface electronic features to rest atoms [$S_2$, (c)], corner adatoms [$S_1^{\prime }$, $S_1$, $U_1$, (d)], and center adatoms [$U_1^{\prime }$, $U_1^{″}$, (e)]. Corner adatoms and center adatoms exhibit a distinctly different structure of the unoccupied states. $V_S^0=2\mathrm{V}$, $I_t^0=0.1\mathrm{nA}$, $a=0.4\mathrm{\AA }{\mathrm{V}}^{-1}$, $\Delta V=2\mathrm{V}$.

Figure 2

Wave functions of the surface electrons measured at $T=7\mathrm{K}$ (DOS maps). Maps show the spatial extent of the rest atom state [$S_2$ (a)], the occupied adatom state [$S_1$ (b)], and the unoccupied adatom states [$U_1$ (c) and $U_1^{″}$ (d)]. The states $U_1$ and $U_1^{″}$ belong to corner and center adatoms, respectively. $V_S^0=2\mathrm{V}$, $I_t^0=0.1\mathrm{nA}$. Figure orientation as in Fig. 1b.

Figure 3

(a) Dependence of the STS spectra on $I_t^0$ at $T=7\mathrm{K}$ indicates that a limited transport of electrons influences the measurement. (b)–(d) Transport limitations decrease when the temperature increases [$T=7$, 77, and $297\mathrm{K}$ (RT) in (b), (c), and (d)]. Spectra do not depend on $I_t^0$ at RT allowing the energies of the rest atom state [$S_2$ (b)] and the adatom states [$S_1^{\prime }\text{−}{U}_1^{″}$, from left to right in (b)] to be determined from RT measurement (d), Table .

Figure 4

(a) Energy level schematic of the nonequilibrium tunneling on $\mathrm{Si}\left(111\right)\text{−}7\times 7$. (b) Plots of the true energy of STS peaks $E_S$ as a function of $V_S$. Experimental data for 7 and $77\mathrm{K}$ exhibit a linear behavior $E_S=(1-\beta )e{V}_S$ for $V_S$ where the Si(111) substrate is in depletion.