Identification of Si and Ge atoms by atomic force microscopy

We successfully identify individual Ge and Si atoms on intermixed $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(7\times 7)$ surfaces by force spectroscopy using atomic force microscopy at room temperature. Tips with high chemical reactivity show distinct peaks derived from Ge and Si in the histograms of maximum attractive forces. The ratio of the maximum attractive force on Ge to that on Si takes a constant value of 0.84 independently of the tips. We apply the present method to $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(5\times 5)$ surfaces to elucidate the elemental composition of the topmost layer.

Figure 1

(a) AFM image of the $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(7\times 7)$ surface prepared with 0.25 ML of Ge. (b) Schematic diagram for the $\left(7\times 7\right)$ “DAS” model. The shaded region is made up of one unit cell where the faulted half unit cell is the darker shaded region on the right. (c) The $\mathrm{\Delta }f\left(z\right)$ curves were acquired on an FH corner adatom (black curve) and a corner hole (green curve) site, which are indicated by the black and green crosses in (a). (d) The converted short-range $F\left(z\right)$ curve on a FH corner adatom. The acquisition parameters are the resonance frequency $\left(f_0\right)=152.913$ kHz, the cantilever-oscillation amplitude $\left(A\right)=125\AA$, the spring constant $\left(k\right)=28.2$ N/m, the sample bias $\left(V_S\right)=+70$ mV, and the frequency shift $\left(\mathrm{\Delta }f\right)=-7.3$ Hz.

Figure 2

(a)–(c) The histograms of maximum attractive forces $\left(F_{\max }\right)$ on the 0.25 ML Ge-deposited $\mathrm{Si}\left(111\right)-(7\times 7)$ surface obtained by changing the tip reactivity. Individual histograms were obtained on the same sample but at different scanning regions. The acquisition parameters are $f_0=152.913$ kHz, $A=125\AA ,k=28.2$ N/m, $V_S=+70$ mV.

Figure 3

Histograms of topographic heights and maximum attractive forces measured at FH corner adatom sites on the intermixed $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(7\times 7)$ surfaces prepared with Ge deposition amounts of 0.13 ML [(a) and (b)], 0.25 ML[(c) and (d)], and 0.38 ML [(e) and (f)]. The data sets related to a Ge deposition amount of 0.25 ML are the same as in Fig. 2. Insets of (a) and (b), (c) and (d), and (e) and (f) show the AFM topographic images of the measured area before and after the chemical identification, where Ge and Si adatoms are indicated with blue and red circles, respectively. The acquisition parameters of (a) and (b) are $f_0=151.729$ kHz, $A=133\AA ,k=27.5$ N/m, $V_S=+500$ mV, and $\mathrm{\Delta }f=-6.5$ Hz. The acquisition parameters of (c) and (d) are $f_0=152.913$ kHz, $A=125\AA ,k=28.2$ N/m, $V_S=+70$ mV, and $\mathrm{\Delta }f=-10.6$ Hz. The acquisition parameters of (e) and (f) are $f_0=151.730$ kHz, $A=147\AA ,k=27.5$ N/m, $V_S=+500$ mV, and $\mathrm{\Delta }f=-7.0$ Hz.

Figure 4

AFM topographic images of the Ge/Si(111) surface prepared with Ge deposition amounts of about 3 ML showing (a) the coexistence of $\left(5\times 5\right)$ and $\left(7\times 7\right)$ reconstructed surface phases and (b) the largely formed $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(5\times 5)$ surface (wetting layer). (c) Schematic diagram for the $\left(5\times 5\right)$ “DAS” model. The shaded region is made up of one unit cell where the faulted half unit cell is the darker shaded region on the right. Histograms of (d) topographic heights and (e) maximum attractive forces measured at FH adatom sites on the intermixed $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(5\times 5)$ surface with a less reactive tip. Insets of (d) and (e) show the AFM topographic images of the measured area before and after the chemical identification, where Ge and Si adatoms are indicated with blue and red circles, respectively. The acquisition parameters are $f_0=151.736$ kHz, $A=137\AA ,k=27.5$ N/m, $V_S=+500$ mV, $\mathrm{\Delta }f=-6.5$ Hz.