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Vacancy diffusion on a brominated Si(100) surface: Critical effect of the dangling bond charge state

T. V. Pavlova, V. M. Shevlyuga

TL;DR

This work addresses how the charge state of silicon dangling bonds governs vacancy diffusion on the Si(100)-2×1-Br surface under STM. It combines low-temperature STM experiments with DFT/NEB calculations to link diffusion to charge-state dependent activation barriers and electronic structure, particularly the antibonding Si–Br orbital. The results show that diffusion occurs only for positively charged vacancies (V+), with lower diffusion barriers for V+ and both two-electron and one-electron diffusion pathways depending on the applied voltage. These findings have practical implications for imaging and fabricating ordered, charged-DB structures and for understanding diffusion of other adsorbates on Si(100).

Abstract

Silicon dangling bonds (DBs) on an adsorbate-covered Si(100) surface can be created in a scanning tunneling microscope (STM) with high precision required for a number of applications. However, vacancies containing DBs can diffuse, disrupting precisely created structures. In this work, we study the diffusion of Br vacancies on a Si(100)-2$\times$1-Br surface in an STM under typical imaging conditions. In agreement with previous work, Br vacancies diffuse at a positive sample bias voltage. Here, we demonstrated that only vacancies containing a positively charged DB hop across the two atoms of a single Si dimer, while vacancies containing neutral and negatively charged DBs do not. Calculations based on the density functional theory confirmed that positively charged Br (and Cl) vacancies have a minimum activation barrier. We propose that diffusion operates by both one-electron and two-electron mechanisms depending on the applied voltage. Our results show that the DB charge has a critical effect on the vacancy diffusion. This effect should be taken into account when imaging surface structures with charged DBs, as well as when studying the diffusion of other atoms and molecules on the Si(100) surface with vacancies in an adsorbate layer.

Vacancy diffusion on a brominated Si(100) surface: Critical effect of the dangling bond charge state

TL;DR

This work addresses how the charge state of silicon dangling bonds governs vacancy diffusion on the Si(100)-2×1-Br surface under STM. It combines low-temperature STM experiments with DFT/NEB calculations to link diffusion to charge-state dependent activation barriers and electronic structure, particularly the antibonding Si–Br orbital. The results show that diffusion occurs only for positively charged vacancies (V+), with lower diffusion barriers for V+ and both two-electron and one-electron diffusion pathways depending on the applied voltage. These findings have practical implications for imaging and fabricating ordered, charged-DB structures and for understanding diffusion of other adsorbates on Si(100).

Abstract

Silicon dangling bonds (DBs) on an adsorbate-covered Si(100) surface can be created in a scanning tunneling microscope (STM) with high precision required for a number of applications. However, vacancies containing DBs can diffuse, disrupting precisely created structures. In this work, we study the diffusion of Br vacancies on a Si(100)-21-Br surface in an STM under typical imaging conditions. In agreement with previous work, Br vacancies diffuse at a positive sample bias voltage. Here, we demonstrated that only vacancies containing a positively charged DB hop across the two atoms of a single Si dimer, while vacancies containing neutral and negatively charged DBs do not. Calculations based on the density functional theory confirmed that positively charged Br (and Cl) vacancies have a minimum activation barrier. We propose that diffusion operates by both one-electron and two-electron mechanisms depending on the applied voltage. Our results show that the DB charge has a critical effect on the vacancy diffusion. This effect should be taken into account when imaging surface structures with charged DBs, as well as when studying the diffusion of other atoms and molecules on the Si(100) surface with vacancies in an adsorbate layer.
Paper Structure (7 sections, 6 figures, 1 table)

This paper contains 7 sections, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experimental and computational details
  3. Results
  4. Experimental results
  5. Computational results
  6. Discussion
  7. Conclusions

Figures (6)

  • Figure 1: Experimental (a--c) and simulated (d--f) empty state STM images of the V$^+$, V$^0$, and V$^-$ on the Si(100)-2$\times$1-Br surface. Experimental images were obtained at I$_t$ = 1.7 nA; $U_s =+2.8$ V (a), I$_t$ = 1.7 nA; $+2.5$ V (b), I$_t$ = 2.0 nA; $+2.0$ V (c). Theoretical images were simulated at $U_s =+2.8$ V (d), $U_s =+2.5$ V (e), and $U_s =+2.0$ V (f). Silicon dimer is marked by a dumbbell in (a).
  • Figure 2: STM images (3$\times$3 nm$^2$, $U_s =+2.5$ V) of the hopping motion of the positively charged vacancy V$^+$ on Si(100)-2$\times$1-Br. The tunneling current is indicated above each STM frame. The neutral vacancy V$^0$ does not hop at any current. The recording time of each scan was 13 sec, the slow scan direction proceeded from bottom to top. An atomic step and a point defect are present in the upper part of the STM images. Silicon dimer is marked by a dumbbell in (a). The switching event is indicated by the arrow in (b). The surface area used to calculate the hopping rate is shown by the rectangle in (b).
  • Figure 3: Hopping rate of three different V$^+$s on Si(100)-2$\times$1-Br as a function of the voltage at $I_t = 1$ nA. The lines are guides to the eye. The statistical error based on the number of hopping events is indicated by error bars.
  • Figure 4: Hopping rate of different V$^+$s on the Si(100)-2$\times$1-Br surface as a function of the tunneling current in the range from 1.0 to 3.5 nA at $U_s =+2.5$ V (a) and $+3.5$ V (b). The lines fit the points by a power law $R \propto I_t^N$. The statistical error based on the number of hopping events is indicated by error bars.
  • Figure 5: Diffusion of a positively charged vacancy on the Si(100)-2$\times$1-Cl surface. STM images (3.8$\times$5.0 nm$^2$, U$_s =+3.5$ V, I$_t$ = 2.0 nA) were recorded sequentially with a time interval of one minute. The vacancy in the lower position of the Si dimer (a) passes through the bridge configuration (b) to the upper position (c). Silicon dimer is marked by a dumbbell. The inset to (b) shows the simulated STM image of Cl in the bridge configuration at U$_s =+3.0$ V.
  • ...and 1 more figures