Silicon overlayer growth on clean and hydrogen-terminated two-dimensional holmium silicide

The growth of silicon overlayers on both two-dimensional (2D) holmium silicide and hydrogen-terminated 2D holmium silicide grown on Si(111) has been investigated using scanning tunneling microscopy (STM). In the nonhydrogen-terminated case, the surface is heavily islanded, exhibiting the $7\times 7$ and $2\times 1$ silicon reconstructions. Growth on the hydrogen-terminated surface differs substantially, being better ordered and less heavily islanded. Growth mechanisms are proposed to explain these observed differences. STM data are also presented from the hydrogen-terminated 2D holmium silicide surface.

Figure 1

(a) Typical STM image of 2D Ho silicide, no overlayer. $2\mathrm{V}$, $2\mathrm{nA}$, $300\mathrm{nm}\times 300\mathrm{nm}$ (b) STM image of 1 ML of Si deposited on a 2D Ho silicide at room temperature, without any annealing. A step edge is visible in the top right. $2\mathrm{V}$, $2\mathrm{nA}$, $200\mathrm{nm}\times 200\mathrm{nm}$. (c) Schematic of (b). A—2D $\mathrm{Ho}{\mathrm{Si}}_2$ with silicon clusters, B—defect in film (d) Closeup STM image of (b); defect in film, $2\mathrm{V}$, $2\mathrm{nA}$, $35\mathrm{nm}\times 35\mathrm{nm}$.

Figure 2

2 ML of Si deposited on a 2D Ho silicide at room temperature, followed by a $15\min$ $400°\mathrm{C}$ anneal. (a) STM image, $2\mathrm{V}$, $2\mathrm{nA}$, $100\mathrm{nm}\times 100\mathrm{nm}$ (b) STM image, $2\mathrm{V}$, $2\mathrm{nA}$, $30\mathrm{nm}\times 30\mathrm{nm}$ (c) Schematic of (b). A—2D $\mathrm{Ho}{\mathrm{Si}}_2$, B—“$1\times 1$,” C—$2\times 1$, D—fragment of $7\times 7$.

Figure 3

STM images of 4 ML of Si deposited on a 2D Ho silicide at room temperature, followed by a $30\min$ $400°\mathrm{C}$ anneal. (a) Overview of surface. $200\mathrm{nm}\times 200\mathrm{nm}$, $1.5\mathrm{V}$, $2\mathrm{nA}$. (b) Line diagram of (a) A—2D $\mathrm{Ho}{\mathrm{Si}}_2$, B—“$1\times 1$” C—mix of $2\times 1$ and $7\times 7$ (c) $7\times 7$ region. Some $9\times 9$ is also visible. $2\mathrm{V}$, $2\mathrm{nA}$, $28\mathrm{nm}\times 19\mathrm{nm}$ (d) $2\times 1$ region $2\mathrm{V}$, $2\mathrm{nA}$, $30\mathrm{nm}\times 30\mathrm{nm}$.

Figure 4

8 ML of Si deposited on a 2D Ho silicide at $400°\mathrm{C}$, followed by a $30\mathrm{min.}$ anneal at the same temperature. (a) Typical morphology of surface. $200\mathrm{nm}\times 200\mathrm{nm}$, $2\mathrm{V}$, $2\mathrm{nA}$. (b) and (c) Two fields of STM data taken simultaneously from mixed region of $2\times 1$ and $7\times 7$ $20\mathrm{nm}\times 9.4\mathrm{nm}$. (b) $1.4\mathrm{V}$, $2\mathrm{nA}$, (c) $-3\mathrm{V}$, $2\mathrm{nA}$.

Figure 5

(Color online) Block diagram of silicon growth on $\mathrm{Ho}{\mathrm{Si}}_2$. (a) Initial deposition (b) $1\times 1$ island formation (c) $7\times 7$ island formation (d) $2\times 1$ regions form on cooling.

Figure 6

All images are of different samples, 1 ML of Ho deposited on $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$, followed by a $400°\mathrm{C}$ anneal. The defect density is roughly constant between samples. (a) No hydrogen exposure, $50\mathrm{nm}\times 50\mathrm{nm}$, $2\mathrm{V}$, $0.2\mathrm{nA}$. (b) $10\mathrm{min.}$ hydrogen exposure at room temperature, $50\mathrm{nm}\times 50\mathrm{nm}$, $2\mathrm{V}$, $0.2\mathrm{nA}$. (c) $20\mathrm{min.}$ hydrogen exposure at room temperature, $41\mathrm{nm}\times 41\mathrm{nm}$, $2\mathrm{V}$, $2\mathrm{nA}$.

Figure 7

STM image of 1 ML of Si deposited on a hydrogen-terminated 2D Ho silicide, followed by an anneal at $400°\mathrm{C}$. $2\mathrm{V}$, $2\mathrm{nA}$, $44\mathrm{nm}\times 44\mathrm{nm}$.

Figure 8

STM image of 1.5 ML of Si deposited on a hydrogen-terminated 2D Ho silicide, followed by an anneal at $400°\mathrm{C}$. $-2\mathrm{V}$, $2\mathrm{nA}$, $7.8\mathrm{nm}\times 7.8\mathrm{nm}$.