Epitaxial growth and quantum well states study of Sn thin films on Sn induced Si(111)-$\left(2\sqrt{3}\times 2\sqrt{3}\right)$ $R30°$ surface

Surface morphologies and electronic structures of Sn thin films prepared on Si(111)-$\text{Sn}\left(2\sqrt{3}\times 2\sqrt{3}\right)$ $R30°$ substrate are investigated by low temperature scanning tunneling microscopy/scanning tunneling spectroscopy (STS). A typical Stranski–Krastanov growth is observed at various growth temperatures (95–300 K), and the Sn islands above wetting layers exhibit the preferential thicknesses of odd-numbered atomic layers. STS measurement shows the formation of well-defined quantum well states with an oscillation period of 2 ML, which modulates the surface energy and accounts for the observed preferential thicknesses. Due to the interplay between large lattice mismatch and symmetry difference, a transition from $\alpha \text{-Sn}$ to $\beta \text{-Sn}$ occurs at 4 ML, which confirms the previous report. From 4 to 11 ML, the mismatch resulted strain manifests the growth via thickness-dependent striplike modulation structures on the surfaces of all Sn islands. Upon room temperature annealing, the as-deposited Sn islands undergo a metal-insulator transition, while the band gaps of wetting layers increase and oppositely shift with respect to the Fermi level for $n$- and $p$-type substrates. The change in electronic property is attributed to the electron transfer at the Sn-Si interface, which also affects the growth and morphologies of films.

Figure 1

(Color online) STM images of the films after Sn was deposited on the Si(111)-$2\sqrt{3}$ surface at various substrate temperatures. [(a)–(c)] $\left(400\times 400{\text{nm}}^2\right)$ 5 ML Sn deposited at 200, 250, and 300 K, respectively; (d) $\left(400\times 400{\text{nm}}^2\right)$ after the sample shown in (a) was annealed at room temperature for 3 h; (e) $\left(400\times 400{\text{nm}}^2\right)$ after the sample shown in (b) was annealed at 285 K for 1 h; (f) $\left(1000\times 1000{\text{nm}}^2\right)$ 15 ML Sn deposited at 150 K; (g) $\left(100\times 100{\text{nm}}^2\right)$ 30 ML Sn deposited at 95 K; (h) $\left(1000\times 1000{\text{nm}}^2\right)$ after the sample shown in (g) was annealed at room temperature for 1 h. The line profile corresponding to the white line is shown just below each image. The numbers in (h) indicate the thicknesses of islands in the unit of ML.

Figure 2

(Color online) High resolution STM images acquired on islands of various thicknesses: [(a) and (b)] 4 ML, (c) 5 ML, (d) 7 ML, (e) 8 ML, and (f) 10 ML. (e) was acquired at a sample bias of 0.8 V, while others at 0.4 V. Except for (b) $\left(10\times 10{\text{nm}}^2\right)$ which is zoomed from (a) showing the details of the coexisting hexangular and tetragonal structures, other images are $20\times 20{\text{nm}}^2$. The double-arrowed lines indicate the periods of the striplike features caused by strain.

Figure 3

(Color online) High resolution STM images $\left(10\times 10{\text{nm}}^2\right)$ acquired on islands of various thicknesses: (a) 7 ML, (b) $\ge 11\text{ML}$, and [(c) and (d)] 8 ML. (c) was acquired at a sample bias of 0.8 V, while others at 0.4 V. The unit cells are outlined by parallelograms. The arrowed lines in (c) and (d) show the area where the feature distinctly changes with bias.

Figure 4

(Color online) [(a)–(c)] Series $dI/dV\text{−}V$ spectra taken from islands of various thicknesses as deposited at 150 K. For comparison, the STS spectrum for the as-deposited 30 ML film at 95 K is shown in (c). The HOQWSs are marked with down-pointing arrows and the LUQWSs are marked with the up-pointing ones. The dotted line marks the middle position between the HOQWS and the LUQWS. (d) The energy separation from the middle position of the HOQWS and the LUQWS to the Fermi level as a function of island thicknesses. All the spectra were obtained at 0.4 V and 100 pA.

Figure 5

(Color online) A series $dI/dV\text{−}V$ spectra taken from (a) the 5 ML island deposited at 200 K and annealed at room temperature for 3 h on $n$-type and $p$-type Si, (b) the 5 ML islands, and (c) the wetting layers. (d) a $10\times 10{\text{nm}}^2$ STM image of the wetting layer acquired at 1.5 V and (e) a $30\times 30{\text{nm}}^2$ STM image acquired on the 5 ML island at $-1.0\text{V}$. The band gaps are labeled by double-arrowed lines. The spectra were obtained at (a) 0.7 V, 100 pA, (b) 0.5 V, 100 pA, and (c) 1.0 V, 10 pA. No obvious change was observed in the spectra recorded at different setpoints.