Hydrogen donors in ${\text{SnO}}_2$ studied by infrared spectroscopy and first-principles calculations

Hydrogen is a potentially important source of $n$-type conductivity in oxide materials. We have investigated hydrogen in tin oxide $\left({\text{SnO}}_2\right)$, a wide-band-gap semiconductor with applications as a transparent conductor and in gas sensors. Infrared (IR) spectroscopy and electrical measurements indicate that hydrogen binds to a host oxygen atom and increases the conductivity. First-principles calculations confirm that interstitial hydrogen acts as a shallow donor $\left({\text{H}}_i^{+}\right)$. Our calculations also indicate that ${\text{H}}_i^{+}$ diffuses easily and combines with Sn vacancies into stable ${\left(V_{\text{Sn}}\text{-H}\right)}^{-3}$ complexes, with the calculated O-H frequencies in agreement with the experimental values. These results suggest that interstitial hydrogen acts as a shallow, mobile donor in a range of oxide materials.

Figure 1

IR spectra of OH and OD vibrational modes in samples annealed in hydrogen (bottom) and deuterium (top). Spectra are baseline corrected and offset for clarity. Inset: polarized IR spectra.

Figure 2

IR spectra of hydrogen-annealed sample just after annealing $\left(t=0\right)$ and after 11 days at room temperature. Assignments for the interstitial H donor and defect-H complexes are shown. Spectra are offset for clarity.

Figure 3

IR spectra of deuterium-annealed sample just after annealing $\left(t=0\right)$, after 1 yr at room temperature, and after annealing in air. Assignments for the interstitial D donor and defect-D complexes are shown. Spectra are baseline corrected and offset for clarity.

Figure 4

(Color online) Formation energies of vacancies and H-related defects in ${\text{SnO}}_2$. O-rich and O-poor conditions are shown in the absence of temperature- and pressure-dependent contributions.

Figure 5

(Color online) Structures of several energetically favorable H-related defects ${\text{SnO}}_2$. (a) Isolated interstitial, ${\text{H}}_i^{+}$. (b) Substitutional ${\text{H}}_{\text{O}}^{+}$. The ${\left(V_{\text{Sn}}\text{-H}\right)}^{-3}$ complex is shown for the two possible configurations, (c) $ax$ and (d) $eq$. The $V_{\text{Sn}}$ site is shown as a transparent atom. The $eq$ $\left(ax\right)$ bonds are shown as black (gray) dashed lines.