Ultrathin Bi(110) films on $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ substrates

We studied the structure of ultrathin Bi(110) films on $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ substrates using scanning tunneling microscopy. Atomically flat Bi islands were nucleated on the substrates at room temperature. The edges of these islands were parallel to the short side of the Bi(110) rectangular unit cell. The islands extended along six specific orientations because the rectangular Bi(110) and rhombus $\sqrt{3}\times \sqrt{3}$ unit cells were commensurate at the interface. Bi(110) domains along different orientations coexisted and formed various domain boundary structures on the wide terraces of the islands. In particular, the domains along $\pm {87}^{\circ }$ from the $\left\{1\overline{1}0\right\}$ direction were connected perfectly on an atomic scale by changing the direction of the $p$-like bond of the in-plane zigzag chains locally at the straight domain boundary. No exclusive preference for the black phosphorus structure was observed for the Bi(110) ultrathin films, in contrast to the islands grown on the $\mathrm{Si}\left(111\right)7\times 7$ substrate.

Figure 1

Structural models of the Bi(110) island with $A7$ (a) and BP (b) structures. Top: Three-dimensional (3D) image. Middle: top view. Bottom: side view. The Bi atoms are indicated by filled circles. The bonds between Bi atoms are indicated by solid lines. Each Bi atom has three $p$-like bonds: one out-of-plane and two in-plane bonds. Note that the in-plane bonding in the top and the second layer are distinguished by thick and thin lines in the top view. The rectangular unit cell is also indicated by black dashed lines in the top view.

Figure 2

STM image of Bi films after (a) 1 ML and (b) 2 ML Bi deposition at room temperature on the $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ substrates. The size of the image is (a) $200\times 200$ nm and (b) $217\times 217$ nm. The images were taken with a bias voltage $\left(V\right)$ of +2.0 V and a tunneling current $\left(I\right)$ of 0.1 nA at the temperature of liquid nitrogen. (c) Distribution of the in-plane orientation of the needlelike islands obtained for the 1 ML Bi deposition. The rotational angle was measured from the $\left[1\overline{1}0\right]$ direction of the $\mathrm{Si}\left(111\right)1\times 1$ substrate lattice. The preferred orientations are referred to as $A_i\mathrm{s}$ and $B_i\mathrm{s}$ in the following description. (d) Polar diagram of the preferred orientations of the Bi islands.

Figure 3

STM images of (a) $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ substrate, and the Bi(110) islands with a height of 6 MLs extending in the (b) $A_1$ and (c) $B_1$ orientations. The direction of the islands and the $\left\{1\overline{1}0\right\}$ orientation are indicated by yellow and white arrows in the STM images, respectively. The image size is $14.5 \times 14.5$ nm in (a), and $3.5 \times 3.5$ nm in (b) and (c). $V=+2.0$ V in (a), $-0.2$ V in (b), and $-0.3$ V in (c). $I=0.1$ nA in (a) and 0.3 nA in (b) and (c). (d) Unit cell of the Bi atom arrangement. Bi atoms are arranged with a centered rectangular unit cell regardless of the orientation. The experimentally obtained sizes of the long and short sides of the unit cell are shown in the schematic. The direction of the Bi(110) islands along the short side of the unit cell is indicated by the black arrow.

Figure 4

(a) Possible ways to place the diagonal of the Bi(110) rectangular unit cell on the $\left\{01\overline{1}\right\}$ axes. For each $\left\{01\overline{1}\right\}$ axis, there are two ways to place the diagonal of the rectangular unit cell $(A$ in red and $B$ in blue). The rectangular unit cells are distorted to illustrate the placement more clearly. The rectangular unit cells with the actual length ratio are shown on the lower left. (b) STM image of a terrace on the 5-ML-high Bi(110) islands. The unit cell is indicated by the red rectangle with yellow dots. $A_2$ and $B_2$ Bi(110) domains are bounded at the straight boundary along the $\left[01\overline{1}\right]$ direction (yellow dashed line). The directions in which the two domains extend are indicated by yellow arrows. The image sizes are $10 \times$ 10 nm. $V=-0.1$ V, $I=0.3$ nA. (c) FFT pattern of the STM image in (b). (d) Sketch of the protrusions in the STM image in (b). The unit cell is indicated by the red rectangle. (e) Reciprocal points that were calculated for the two domains in (d).

Figure 5

STM images of the boundaries between different types of domain ($10\times 10$ nm, $V=-0.1$ V, $I=0.3$ nA). Boundary between (a) $A_2\text{−}{B}_2$ domains, (b) $A_1\text{−}{A}_3$ domains, and (c) $A_1\text{−}{B}_2$ domains. The boundaries are indicated by yellow dashed lines. The unit cell is indicated by the red rectangle with yellow dots (Bi atoms) in each domain. The directions in which the islands extend are indicated by yellow arrows.

Figure 6

Magnified STM images of the domain boundaries in Fig. 5 ($3.5\times 3.5$ nm, $V=-0.1$ V, $I=0.3$ nA). (a) $A_2\text{−}{B}_2$ domain boundary. (b) $A_1\text{−}{A}_3$ domain boundary. (c) $A_1\text{−}{B}_2$ domain boundary. The schematics below are sketches of the protrusions in the STM images. The domain boundaries are indicated by yellow and red dashed lines. The unit cells are indicated by red rectangles. In the STM images, the in-plane zigzag bonds are indicated by solid lines. The Bi atoms in the zigzag chain are overlaid with blue dots and lines. The bonds connect the adjacent atoms to maintain the zigzag chains where possible for the atoms at the boundaries.

Figure 7

(a) Height distribution of the Bi(110) islands obtained from the 2-ML Bi deposition on the $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ substrate. (b),(c) Wide-range STM images of the Bi(110) islands with heights of (b) 6 and (c) 5 MLs, $20\times 20$ nm, $V=-0.6$ V, $I=0.3$ nA. The long-range modulation of the contrast is overlaid on the periodic arrangement of Bi atoms on the 6-ML Bi(110) islands (b). The distances between the bright regions in the contrast modulation are displayed in the histogram in the inset in (b).

Figure 8

Structural model of the atomic arrangement at the edge of the Bi(110) islands with $A7$ (a),(b) and BP (c),(d) structures. Upper panel: top view. Lower panel: side view. The edge, indicated by the red line, is along the short side of the unit cell in (a) and (c), and along the long side of the unit cell in (b) and (d). The Bi atoms and their bonds are indicated by circles and short lines. The zigzag chains are parallel to the short side.