Evidence for a New Class of Defects in Highly $n$-Doped Si: Donor-Pair-Vacancy-Interstitial Complexes

Electron channeling experiments performed on individually scanned, single columns of atoms show that in highly $n$-type Si grown at low temperatures the primary electrically deactivating defect cannot belong to either the widely accepted class of donor-vacancy clusters or a recently proposed class of donor pairs. First-principles calculations suggest a new class of defects consisting of two dopant donor atoms near a displaced Si atom, which forms a vacancy-interstitial pair. These complexes are consistent with the present experimental results, the measured open volume of the defects, the observed electrical activity as a function of dopant concentration, and the enhanced diffusion of impurities in the presence of deactivated dopants.

Figure 1

Schematic diagram for forming electrically deactivating defects from (a) two active Sb atoms in Si. (b) An ${\mathrm{S}\mathrm{b}}_{2}V$ is formed by breaking bonds 2 and 3 and creating a Si vacancy. (c) A DP(2) is formed by breaking bonds 1 and 3 and forming a new Si-Si bond, shown bold. The resulting displacements of the Sb and Si atoms are indicated.

Figure 2

(top left) STEM detector configuration: The dashed line is high-angle scattering caught on the upper detector, the light line is low-angle scattering caught on the lower detector, and the heavy line is unscattered electrons, which are not detected. (top right) Simulated visibility of a single Sb atom in $40\AA$ of Si as a function of $\Delta r_{\mathrm{S}\mathrm{b}}$, the projected off-column displacement of the Sb atom. The Sb atom is not visible in the low-angle image for $\Delta r_{\mathrm{S}\mathrm{b}}>0.3\AA$. (bottom left) Simulated images of a DP(2) defect. The off-column Sb atom is visible in the high-angle image, but not in the low-angle. (bottom right) Experimental images. The brightest spots are atomic columns containing at least one Sb atom. These features appear on the same columns in both high- and low-angle images, indicating experimentally that for these Sb atoms $\Delta r_{\mathrm{S}\mathrm{b}}<0.3\AA$. Images have been smoothed and had pixels added by interpolation.

Figure 3

Schematic of the $\mathrm{D}\mathrm{P}(2)V\mathrm{\text{−}}I$ deactivating defect. Referring to Fig. 1, $\mathrm{D}\mathrm{P}(2)V\mathrm{\text{−}}I$ is formed by breaking bond 2 and forming a Si vacancy-interstitial pair. The resulting Sb-atom displacements are smaller than in either ${\mathrm{S}\mathrm{b}}_{2}V$ or DP(2).