Interstitial silicon ions in rutile $\mathrm{Ti}{\mathrm{O}}_2$ crystals

Electron paramagnetic resonance (EPR) is used to identify a new and unique photoactive silicon-related point defect in single crystals of rutile $\mathrm{Ti}{\mathrm{O}}_2$. The importance of this defect lies in its assignment to interstitial silicon ions and the unexpected establishment of silicon impurities as a major hole trap in $\mathrm{Ti}{\mathrm{O}}_2$. Principal $g$ values of this new $S=1/2$ center are 1.9159, 1.9377, and 1.9668 with principal axes along the $\left[\overline{1}10\right],\left[001\right]$, and $\left[110\right]$ directions, respectively. Hyperfine structure in the EPR spectrum shows the unpaired spin interacting equally with two Ti nuclei and unequally with two Si nuclei. These silicon ions are present in the $\mathrm{Ti}{\mathrm{O}}_2$ crystals as unintentional impurities. Principal values for the larger of the two Si hyperfine interactions are 91.4, 95.4, and 316.4 MHz with principal axes also along the $\left[\overline{1}10\right],\left[001\right]$, and $\left[110\right]$ directions. The model for the defect consists of two adjacent Si ions, one at a tetrahedral interstitial site and the other occupying a Ti site. Together, they form a neutral nonparamagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^0$ complex. When a crystal is illuminated below 40 K with 442-nm laser light, holes are trapped by these silicon complexes and form paramagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defects, while electrons are trapped at oxygen vacancies. Thermal anneal results show that the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ EPR signal disappears in two steps, coinciding with the release of electrons from neutral oxygen vacancies and singly ionized oxygen vacancies. These released electrons recombine with the holes trapped at the silicon complexes.

Figure 1

Schematic of the rutile $\mathrm{Ti}{\mathrm{O}}_2$ crystal structure, showing one of the two equivalent $\mathrm{Ti}{\mathrm{O}}_6$ octahedra and its eight neighboring Ti neighbors.

Figure 2

EPR spectrum of the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defect in an as-grown oxidized rutile $\mathrm{Ti}{\mathrm{O}}_2$ crystal. The sample temperature was 40 K, the magnetic field was along the $\left[001\right]$ direction, and the microwave frequency was 9.520 GHz. The sample was exposed to 442-nm laser light while acquiring the spectrum.

Figure 3

EPR spectrum of the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defect in a rutile $\mathrm{Ti}{\mathrm{O}}_2$ crystal diffused with lithium. The sample temperature was 40 K, the magnetic field was along the $\left[001\right]$ direction, and the microwave frequency was 9.520 GHz. There was no laser light on the sample.

Figure 4

EPR spectrum of the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defect in a rutile $\mathrm{Ti}{\mathrm{O}}_2$ crystal diffused with lithium. The temperature was 40 K, the field was along the $\left[100\right]$ direction, and the microwave frequency was 9.520 GHz. There was no laser light on the sample.

Figure 5

EPR angular dependence (in three high-symmetry planes) of the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defect. Red lines represent defects with no ${}^{29}\mathrm{Si}$ nuclei, whereas the green and blue lines represent the two separate ${}^{29}\mathrm{Si}$ nuclei. Discrete points are experimental results. Solid curves were calculated using the parameters in Table 1 and a microwave frequency of 9.520 GHz.

Figure 6

EPR isochronal pulsed anneal results. The crystal was initially illuminated at 20 K and then kept in the dark for the remainder of the experiment. The green curve is the neutral oxygen vacancy, the red curve is the singly ionized oxygen vacancy, and the blue curve is the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defect.

Figure 7

(a) A projection on the $\left[1\overline{1}0\right]$ plane of the regular $\mathrm{Ti}{\mathrm{O}}_2$ crystal. Oxygen ions are red, and titanium ions are blue. The oxygen ion labeled ${\mathrm{O}}_3$ is in front of the plane, and the oxygen ion labeled ${\mathrm{O}}_4$ is behind the plane. A bold × marks the tetrahedral interstitial position along the $\left[110\right]$ direction. (b) Model of the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defect.