Transitions between positive and negative charge states of dangling bonds on a halogenated Si(100) surface

TL;DR

This work addresses how individual dangling bonds on halogenated Si(100)-2×1 surfaces switch between charge states under STM, revealing a non-equilibrium $(+/-)$ transition that occurs without involving the neutral state. It combines low-temperature STM experiments with DFT-STM simulations to show that the transition is governed by the local charge neutrality level (CNL) and the electrostatic environment, and that the two-electron process can be reproduced by superposing $DB^+$ and $DB^-$ contributions. The findings extend to DBs in the third silicon layer and on brominated surfaces, suggesting a general non-equilibrium charging mechanism for DBs on halogenated Si(100). These results have implications for precise charge manipulation in DB-based nanoelectronic devices and for understanding surface reactivity in halogen-terminated silicon systems.

Abstract

Dangling bonds (DBs) are common defects in silicon that affect its electronic performance by trapping carriers at the in-gap levels. For probing the electrical properties of individual DBs, a scanning tunneling microscope (STM) is an effective instrument. Here we study transitions between charge states of a single DB on chlorinated and brominated Si(100)-2$\times$1 surfaces in an STM. We observed transitions between positively and negatively charged states of the DB, without the participation of the neutral state. We demonstrated that the $(+/-)$ transition occurs when the DB and substrate states are out of equilibrium. This transition is related to the charge neutrality level (CNL), which indicates a change in the DB's character from donor-like to acceptor-like. The STM voltage at which the $(+/-)$ transition took place varied depending to the electrostatic environment of the DB. Our results complement the understanding of the electronic properties of the DBs, and they should be taken into account in applications that use charge manipulation on the DBs.

Figures (6)

• Figure 1: (a) Structure of the Si(100)-2$\times$1-Cl surface with a Cl vacancy. Si atoms are shown in gray and Cl in green. The silicon atom holding the DB is denoted as Si$_{DB}$. (b) Schematic representation of spin-resolved DOS of the Si atom holding the DB$^+$, DB$^0$, or DB$^-$. Filled levels of the DB are shaded. Experimental (c--e) and simulated (f--h) empty state STM images of the DB$^+$, DB$^0$, and DB$^-$ on the Si(100)-2$\times$1-Cl surface. Experimental images were obtained at 3.9 V (c), 3.6 V (d), and 1.8 V (e). Theoretical images were calculated at 2.5 V (f), 2.3 V (g), and 2.0 V (h). Silicon dimers are marked by dumbbells in (c).
• Figure 2: (a) Schematic representation of the formation energy as a function of the Fermi energy for different charge states of the DB on the halogenated Si(100)-2$\times$1 surface. The DB$^+$, DB$^0$, and DB$^-$ are in equilibrium with the sample if the Fermi level of the sample (E$_F$) is located in ranges I, II, or III, respectively. (b) Energy band diagram for empty-state STM imaging of the DB on the halogenated Si(100)-2$\times$1 surface. In three different charge states, the DB has different levels. A positive voltage applied to the sample induces the upward band bending, which can be used to change the position of the DB levels relative to the E$_F$. E$^T_F$ is the Fermi level of the tip.
• Figure 3: Changing of the DB charge state from positive to negative by decreasing the positive bias voltage. (a--c) Empty state STM images of the Si(100)-2$\times$1-Cl surface with the DB (P-doped Si). STM images are independent of whether the voltage goes from high to low or vice versa. Note that doubling the current (from 1 to 2 nA) when recording an STM image at 2.7 V (b) did not result in a change in the DB visualization.
• Figure 4: Changing of the DBs charge state by varying the sample voltage. Empty state STM images (10.0$\times$5.6 nm$^2$, I$_t$ = 2 nA, B-doped Si) of the Si(100)-2$\times$1-Cl surface with several Cl vacancies. Initially, all DBs were neutral (a). When DB2 and DB4 were charged (b), their charge states changed with voltage variation, and the unusual visualization was observed (b--j). The voltage variations have no effect on the neutral vacancies with DB1 and DB3 (a--e). When an electron was removed from DB1 (f), DB1 began to change its state when the voltage was changed (g--j). The slow scan direction proceeded from bottom to top.
• Figure 5: Spontaneous transition from the DB$^0$ to DB$^+$ on the Si(100)-2$\times$1-Cl surface. Empty state STM images (7.4$\times$6.8 nm$^2$, $U_s =+3.8$ V, I$_t$ = 1.0 nA, B-doped Si) were sequentially recorded. The DB$^0$ in the upper right corner looks dark (a), but after an electron removal (b), it becomes positively charged and looks bright (c) as the DB$^+$. In (b), the slow scan direction proceeded from top to bottom.