First-principles investigation of a bistable boron-oxygen interstitial pair in Si

Local density functional calculations are used to predict and compare the properties of the two distinct interstitial boron-interstitial oxygen $\left({\mathrm{B}}_i{\mathrm{O}}_i\right)$ complexes recently reported in the literature. The electronic and free energies, as well as the small transformation barrier, suggest that both forms of the defect are present at the temperature at which the defect forms. The vibrational spectra of the defects are predicted. The electrical levels of the defect are calculated and compared to experimental data. The existence of two forms of the ${\mathrm{B}}_i{\mathrm{O}}_i$ defect may have implications for the lifetime degradation of space-based Czochralski-silicon solar cells.

Figure 1

Equilibrium structures of the $\{{\mathrm{B}}_i,{\mathrm{O}}_i\}$ defect: (a) ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_a$ and (b) ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_b$ and (c) conventional cell of Si to help the reader. Boron, oxygen, and silicon atoms are represented in black, white, and grey, respectively. Bond lengths are indicated in Å.

Figure 2

(Color online) Temperature dependence of the electronic plus vibrational free energy difference between the ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_a$ and ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_b$ defects in the $+1$ (solid red line) and 0 (dashed black line) charge states. The solid ($+1$ charge state) and open (0 charge state) squares on the $T=0\mathrm{K}$ axis show the total eletronic energy differences without zero-point energies.

Figure 3

(Color online) Minimum energy path for the transformation of ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_a$ into ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_b$. The zero-point energies are ignored in this calculation. The data for the positive and neutral charge states are represented by filled squares and empty squares, respectively. Energies are given with respect to ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_a$. Interpolation lines are added to help the visualization. Projections in the (110) plane of the equilibrium (a and b) and saddle point (S) structures are also shown. Silicon, boron, and oxygen atoms are represented in grey, light grey, and white, respectively.

Figure 4

(Color online) Relative amplitude of the B (solid green lines) and O (dashed red lines) oscillations versus normal mode frequency for the neutral ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_a$ (top) and ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_b$ (bottom) defects. The frequencies of the more strongly localized modes are given.

Figure 5

(Color online) Relative amplitude of the B (solid green lines) and O oscillations (dashed red lines) versus normal mode frequency for the ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_a$ (top) and ${\{{\mathrm{B}}_i,{\mathrm{O}}_i\}}_b$ (bottom) defects in the $+1$ charge state. The frequencies of the more strongly localized modes are given.