Initial stages of oxygen adsorption on $\text{In/Si(111)-}4\times 1$

The In/Si(111)4$\times 1$ surface with In wires is a prototypical one-dimensional electron system showing a temperature-induced structural phase transition. While most impurities are known to decrease the transition temperature $\left(T_c\right)$, oxygen was reported to increase the $T_c$. In order to understand the exceptional influence of oxygen, we have examined the initial stages of oxygen adsorption on the surface by scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. Oxygen molecules were found to dissociate and adsorb as atoms on the surface. STM revealed three distinct O adsorption features as well as occasional transformations from one to another. The high-resolution images showed that all three adsorbed O features were located at the center of the In trimer at both edges of the In wire. This registry is in contradiction with the previous theoretical predictions [S. Wippermann et al., Phys. Rev. Lett. 100, 106802 (2008)]. The present DFT calculations identified two of the features as O atoms incorporated in the interstitial region between the In wire and subsurface Si layer.

Figure 1

(a) Representative STM image of the In/Si(111)-4$\times 1$ surface exposed to ${\mathrm{O}}_2$ gas (4.7 L) at RT. Two types of adsorption features, bright (B) and dark (D), are clearly visible. (b)–(d) STM images taken successively over time at approximately two-minute intervals during the ${\mathrm{O}}_2$ dosing with 1$\times {10}^{-9}$ Torr. During dosing, new adsorption features appeared [B (circle) and D (rectangle)] and the transformation between the adsorption features [B-to-D (solid arrow) and D-to-B (dashed arrow)] were observed. The images were filled-state images taken at $V_s=-1.0$ V.

Figure 2

STM images taken with opposite-polarity bias voltages over the same area: (a) filled state $(V_s=-1.0$ V) and (b) empty state $(V_s=+1.0$ V). The three distinct features are labeled as B, ${\mathrm{D}}_1$, and ${\mathrm{D}}_2$. One atomic H adsorption feature (cracked from residual gas by ion gauge filament) was also observed (ellipse). The atomic H feature was distinguished from the bright oxygen feature B by its a distinct “seagull” shape with two side lobes in the empty-state image (see Ref. [22]).

Figure 3

(a) Experimental (top) and simulated (bottom) STM images at $V_s=+0.2$ V, and (b) atomic structure of the clean In/Si(111)-4$\times 1$ surface. (c)–(e) High-resolution STM images $(V_s=+0.2$ V) of the three O adsorption features, B, ${\mathrm{D}}_1$, and ${\mathrm{D}}_2$. The O adsorption sites are indicated by solid circles ($•)$. A theoretical adsorption site from the previous study (Ref. [31]) is also indicated ($\times )$ for comparison.

Figure 4

Bias dependence of the appearances of the O-induced STM features (B, ${\mathrm{D}}_1$, and ${\mathrm{D}}_2)$ on the preferred side in the STM images. The sample bias voltages representing for the filled $\left(V_s<0\right)$ and empty $\left(V_s>0\right)$ states are given.

Figure 5

Series of STM images $(V_s=+1.24$ V) taken successively with time over the same area, showing the mutual transformations between the O-induced adsorption features. The B-to-${\mathrm{D}}_1$, ${\mathrm{D}}_1$-to-B, ${\mathrm{D}}_1$-to-${\mathrm{D}}_2$, and ${\mathrm{D}}_2$-to-${\mathrm{D}}_1$ transformations are indicated by the arrows.

Figure 6

(a) Pristine In/Si(111)-4$\times 1$ surface with six different adsorption sites marked by $a$–$f$. The 4$\times 1$ surface unit cell is denoted by a dashed rectangle. (b) and (c) Two low-energy interstitial O structures below the $f$ site: (b) $I_{f1}$ and (c) $I_{f2}$. The green and cyan circles are unreacted and reacted In atoms, respectively. The Seiwatz Si atoms are represented by yellow circles, and the substrate Si atoms denoted by grey and white circles. The substrate Si atom that reacted with O is denoted by the orange circle. The arrows indicate the reacted In atom, which shows a characteristic structural difference between $I_{f1}$ and $I_{f2}$ (see the text).

Figure 7

Simulated STM images of the two low-energy structures, $I_{f1}$ and $I_{f2}$, compared to the experimental features, B and ${\mathrm{D}}_1$. (Left) $I_{f1}$ vs the B feature. (Right) $I_{f2}$ vs the ${\mathrm{D}}_1$ feature [40].