Microscopic Dynamics of Silicon Oxidation

We study the silicon oxidation process and the dynamic structure of the $\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}$-Si (001) interface using a grand canonical Monte Carlo approach. We find that Si-O-Si bridge bonds are the main building blocks of the advancing interface, and we identify a kinetic pathway that continually creates new bridge bonds. Oxidation proceeds by local events, with little evidence of “step flow” in the simulation. Yet the interface remains remarkably smooth and abrupt as it advances.

Figure 1

Structure of the Si-$\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}$ system at successive stages of oxidation. Silicon atoms are shown as gray (yellow in color), oxygen as black (red in color). Arrows indicate bridge bonds. Horizontal and vertical directions are (001) and (110), respectively.

Figure 2

Amount of oxide grown vs MC “time,” for different values of oxygen chemical potential $\mu$. Here, one time unit equals 100 000 attempted MC moves, and one monolayer (ML) of oxide refers to incorporation of enough oxygen to fully oxidize one (001) atomic layer of Si, i.e., $1.4\times {10}^{15}{\mathrm{c}\mathrm{m}}^{-2}$. Two independent simulations are shown for each value of $\mu$. The inset shows average reaction rate $R$ (slope of growth curves) vs chemical potential $\mu$; line is a smooth fit to guide the eye.

Figure 3

Schematic illustration of the reaction pathway seen in the simulations, which preserves interfacial bridge bonding. (a) Idealized initial interface, with fully bridge-bonded $2\times 1$ structure. Arrow indicates bridge bonds. (b) Same structure rotated $90°$ about interface normal; bridging oxygen atoms are partially hidden behind Si atoms labeled A and C. (c) A bond-switching move replaces bonds A-B and C-D with bonds parallel to the interface (A-C and B-D). (d) Additional oxygen is incorporated in bond B-C and elsewhere near the bond-switching event. (e) Repeating the process returns the system to the initial bridge-bonded structure, but with one atomic layer of Si converted to $\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}$, and with the interface structure rotated by $90°$. (f) The energetics of the process is summarized schematically.

Figure 4

(a) The interface width $\sigma$ and the amount of each suboxide component during growth, versus the thickness of $\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}$ grown. The system has lateral size $6\times 6$, and the simulation starts from an ideally sharp interface. (b) The spatial distribution of the suboxide components with respect to the interface.

Figure 5

Interface motion at a cross section through the system. Top curve is initial flat interface. Other curves correspond to the interface position after increments of ten oxygen atoms in the $6\times 6$ cell, i.e., 0.14 ML. For visual clarity, successive curves are displaced downward by $0.5\AA$ per line, in addition to the actual interface motion.