Surface diffusion of phosphorus on Si(100) after PBr3 adsorption

TL;DR

This study investigates phosphorus diffusion on the Si(100) surface after dissociative adsorption of PBr3 at two temperatures (77 K and 300 K) using scanning tunneling microscopy and density functional theory. The experiments reveal multiple diffusion pathways along and across Si dimer rows, with diffusion barriers ranging from about $0.79$ to $1.81$ eV depending on the pathway and bromine presence; DFT calculations (VASP, PBE) with NEB support these pathways and provide activation energies. Bromine near phosphorus markedly inhibits diffusion, while phosphorus proximity to oxidized dimers or oxygen defects stabilizes P, highlighting environment-dependent diffusion. These results enhance the understanding of phosphorus–silicon surface interactions and provide guidance for achieving atomically precise phosphorus incorporation in silicon devices.

Abstract

Phosphorus diffusion on a Si(100) surface was studied using scanning tunneling microscopy (STM) at temperatures of 77 and 300 K. The phosphorus source utilized was the PBr$_3$ molecule, which fully dissociates on the surface at 77 K. We observed diffusion of P atoms both along and across the rows of Si dimers. To support the observation of different diffusion pathways of phosphorus, activation energy calculations were performed using density functional theory. At 77 K, phosphorus diffusion started and (or) finished mostly in bridge positions. At 300 K, phosphorus diffuses predominantly between end-bridge positions, accompanied by bromine diffusion. The presence of Br near phosphorus significantly restricts its mobility. Additionally, phosphorus was found to diffuse to an oxygen atom that appeared on the surface as a result of water adsorption. This diffusion occurs because the P site near the oxidized dimer is more stable compared to that on the clean surface. The obtained results complement the knowledge about the interaction of phosphorus with the silicon surface, specifically the phosphorus diffusion pathways on the Si(100) surface.

Figures (8)

• Figure 1: Model, simulated and experimental empty state STM images of structure S1 (a) and S2 (b) after PBr$_3$ dissociative adsorption on the Si(100) surface. Si atoms are shown in gray, Br in red, and P in blue. The dashed line denotes the middle of the dimer row. In the most stable structure, S1 (a), the P atom occupies an end-bridge position between two adjacent dimers and above the Si atom of the previous layer. In structure S2 (b), the P atom occupies a bridge position on the dimer, slightly displaced from its center. The Br atoms are located above the Si atoms slightly away from the center of the dimer. Experimental empty state STM images ($U_s =+2.3$ V, I$_t$ = 2.0 nA) were recorded at 77 K. The voltage ($U_s$) was applied to the sample.
• Figure 2: Phosphorus diffusion from the end-bridge to bridge position on Si(100). (a--c) Empty state STM images ($U_s =+2.5$ V, I$_t$ = 1.0 nA, 77 K) of structure S1 (a), S1 with a line scan break above the P atom indicating transition from the end-bridge to the bridge position (b), and structure S2 (c). Scanning was performed from the bottom to top. In the empty state STM images, the dark stripes along the dimer rows pass through the center of the dimer. (d) Energy barrier diagram of the P diffusion from the end-bridge to the bridge site. All values are given in electronvolts, the red number denotes the activation barrier. Si atoms are marked in gray, Br atoms in red, and the P atom in blue.
• Figure 3: Phosphorus diffusion between end-bridge positions on the opposite sides of a dimer row. Empty state STM images ($U_s =+1.5$ V, I$_t$ = 2.5 nA, 300 K) of the initial (a) and final surface structure (b). (c) Energy barrier diagram for the P diffusion between two end-bridge positions across the dimer row. All values are given in electronvolts, the red numbers denote the activation barriers. Si atoms are marked in gray, Br atoms in red, and the P atom in blue.
• Figure 4: Phosphorus diffusion between end-bridge positions on the same side of a dimer row. Empty state STM images ($U_s =+1.5$ V, I$_t$ = 2.5 nA, 300 K) of the initial (a) and final surface structure (b). (c) Energy barrier diagram for the P diffusion between two end-bridge positions within the dimer row through the bridge position on the central dimer. All values are given in electronvolts, the red numbers denote the activation barriers. Si atoms are marked in gray, Br atoms in red, and the P atom in blue.
• Figure 5: (a--d) Empty state STM images ($U_s =+3.0$ V, I$_t$ = 2.0 nA, 77 K) of P diffusion between the bridge positions. Initially P was located in structure S2 (a), then jumped to the adjacent dimer row (b) and moved within the row (c,d). Arrows show the scanning direction. (e) Energy barrier diagram of the P diffusion between dimer rows. (f) Energy barrier diagram of the P diffusion within the dimer row. All values are given in electronvolts, the red numbers denote the activation barriers. Si atoms are marked in gray, Br atoms in red, and the P atom in blue.