Calculations of pathways of precise P incorporation into chlorinated Si(100) surface

TL;DR

The study addresses how to achieve atomically precise phosphorus incorporation into Si(100) by biasing the P–Si exchange toward a single neighboring Si atom. It employs spin-polarized density functional theory with NEB on Si(100)-2x1-Cl slabs containing three Cl vacancies to map adsorption sites and exchange pathways. The results show that an asymmetric Cl-vacancy arrangement favors a dominant P–Si exchange pathway with a barrier of $E_a=1.16$ eV (and a diffusion barrier of $E_a\approx1.21$ eV), enabling controlled incorporation; a detailed STM-based protocol involving Cl bivacancies and PBr3 adsorption is proposed to realize this site-selective exchange, with annealing completing the substitution. This approach offers a route to deterministic single-impurity devices, contingent on experimental precision in halogen manipulation and subsequent epitaxial silicon growth to preserve P positions.

Abstract

The precise incorporation of a phosphorus atom into a silicon surface is essential for the fabrication of nanoelectronic devices in which the active area is formed from single impurities. The most accurate approach employs scanning tunneling microscopy (STM) lithography, which may be done with atomic precision. However, the accuracy decreases when phosphorus is incorporated into the surface because P substitutes one of two neighboring Si atoms with equal probability. Here, the P-Si exchange mechanism was studied theoretically on a chlorinated Si(100) surface with an asymmetric configuration of Cl vacancies surrounding the P atom. Density functional theory was used to estimate the activation barriers and exchange rates between a P atom and neighboring Si atoms on a Si(100)-2$\times$1-Cl surface with three Cl vacancies. The calculation of various P-Si exchange pathways revealed that phosphorus has a higher probability of substituting one Si atom than the others due to the asymmetric configuration of Cl vacancies. Based on the theoretical study of the P-Si exchange mechanism and experimental results from previous works, a scheme for controlled P incorporation into the silicon surface without uncertainty is proposed.

Figures (6)

• Figure 1: Model of the Si(100)-2$\times$1-Cl surface with three Cl atoms removed and an adsorbed P atom, shown as an example in the end-bridge position. The Si atoms in the top layer are represented by big gray circles, Si in the second layer by small gray circles, Cl by green, and P by blue. The dotted lines indicate the supercell for which the calculations were done.
• Figure 2: Different adsorption positions of the P atom on the Si(100)-2$\times$1-Cl surface with three Cl vacancies. The Si atoms in the top layer are shown as large gray circles, Si in the second layer as small gray circles, Cl in green, and P in blue.
• Figure 3: The P-Si exchange pathways with phosphorus located in the end-bridge (a) and bridge (b) adsorption positions. The Si atoms in the top layer are represented by big gray circles, Si in the second layer by small gray circles, Cl by green, and P by blue. The blue, black, and green arrows show the paths of the P, Si, and Cl atoms, respectively.
• Figure 4: Energy barrier diagram of the P-Si exchange. Only the most favorable pathways for P exchange with Si1, Si2, and Si3 are shown. All energies are given in electronvolts relative to the energy of the initial position P1. The energies of initial and final states are shown in black, and activation barriers in red. The structures of initial and final states are shown at the bottom, and the structures of transition states in the upper part of the diagram. The Si atoms in the top layer are shown as large gray circles, Si in the second layer as small gray circles, Cl in green, and P in blue.
• Figure 5: Proposal for atomically precise substitution of the Si atom by phosphorus. An STM tip is utilized to create a Cl bivacancy on Si(100)-2$\times$1-Cl (structure S1), into which a PBr$_3$ molecule is adsorbed. The dissociated molecule in the Cl bivacancy forms structures S2--S5 with phosphorus 2024Pavlova. Structure S3 can be converted to S2 by scanning or to S4 by removing a Br atom from PBr$_2$ with a tip. The removal of one Cl and all Br atoms transforms S2 and S4 to S6. Structure S5 converts to S7, from which the P atom can diffuse to more favorable positions in structures S6 or S8. After annealing, S6 and S8 convert to S9. The Si atoms in the top layer are shown as large gray circles, Si in the second layer as small gray circles, Cl in green, Br in red, and P in blue.