Phosphine Dissociation on the Si(001) Surface

Density functional calculations are performed to identify features observed in STM experiments after phosphine (${\mathrm{P}\mathrm{H}}_3$) dosing of the Si(001) surface. On the basis of a comprehensive survey of possible structures, energetics, and simulated STM images, three prominent STM features are assigned to structures containing surface bound ${\mathrm{P}\mathrm{H}}_2$, PH, and P, respectively. Collectively, the assigned features outline for the first time a detailed mechanism of ${\mathrm{P}\mathrm{H}}_3$ dissociation and P incorporation on Si(001).

Figure 1

(a) Filled-state STM image of Si(001) after room-temperature dosing with ${\mathrm{P}\mathrm{H}}_3$. (b)–(d) High-resolution filled-state images of the asymmetric, centered, and U-shaped features. (e)–(g) Empty-state images of the same. STM images acquired with $-1.6\mathrm{V}/+1.2\mathrm{V}$ and 0.1 nA [5, 10, 11].

Figure 2

Overview of computed energies for structures considered in our survey of stable ${\mathrm{P}\mathrm{H}}_3$ adsorption and dissociation products on the Si(001) surface. Structures are ordered on the horizontal axis according to ${\mathrm{P}\mathrm{H}}_x$ fragment ($x=3$, 2, 1, 0), indicating the degree of dissociation. Low energy configurations along the dissociation series from left to right are labeled; the corresponding structures are shown in Fig. 3.

Figure 3

Schematic plan view of structures discussed in text. P, H, and Si atoms are indicated by gray, white, and black circles, respectively. Silicon dimers are indicated by solid black bars; the solid gray half bar in structure (E1) indicates the position of the dimer incorporated P atom.

Figure 4

Simulated filled- and empty-state STM images of (a) structure (B1), (b) structure (C1), and (c) structure (D2), which we assign to the experimentally observed asymmetric, centered, and U-shaped STM features seen in Fig. 1. Because of the absence of pinning, the images for (B1) and (C1) are averaged over both buckling isomers for the bare dimers [19]. Images were calculated for a ($2\times 4$) slab model in the Tersoff-Hamann approximation [20] summing over the four states nearest the Fermi energy.

Figure 5

3D view and schematic bonding diagram of structure (D2). P, H, and Si atoms are indicated by gray, white, and black spheres, respectively. Bond lengths (given in Å) illustrate the lengthening of a Si-Si backbond.

Figure 6

STM images showing the progressive dissociation of $\mathrm{P}{\mathrm{H}}_3$ via (a) the asymmetric feature (B1), (b) the centered feature (C1), and (c) the U-shaped feature (D2). All images were acquired with $-2.0\mathrm{V}$ sample bias and 0.2 nA. The schematic outlines the observed mechanism of dissociation.