Localized Vibrational Modes of $\mathrm{O_{Te}}$ and ($\mathrm{O_{Te}-V_{Cd}}$) Centers in CdTe: Fundamentals and Second Harmonics$^{*}$

In CdTe, grown with excess Cd vacancies ($\mathrm{V_{Cd}}$), oxygen replacing Te ($\mathrm{O_{Te}}$) displays a pair of fundamental localized vibrational modes (LVMs), $\nu_{1} = 1096.78$ cm$^{-1}$ and $\nu_{2} = 1108.35$ cm$^{-1}$. They are ascribed to the non-degenerate $\Gamma_{1}$ ($\nu_{1}$) and the doubly degenerate $\Gamma_{3}$ ($\nu_{2}$) LVMs of ($\mathrm{O_{Te}-V_{Cd}}$) centers with nearest neighbor Cd missing, having $C_{3v}$ symmetry and $\hat{\bf{c}}$ axis along $\langle 111\rangle$. In CdTe grown with conditions suppressing $\mathrm{V_{Cd}}$, $\mathrm{O_{Te}}$ occurs with all the four Cd nearest neighbors, and exhibits a triply degenerate $\Gamma_{5}$ LVM at $\nu_{0} = 349.79$ cm$^{-1}$ of $T_{d}$ symmetry.[1] The harmonics of ($\mathrm{O_{Te}-V_{Cd}}$), i.e., of $\nu_{1}$ and $\nu_{2}$ occur at $\nu_{4} = 2198.66$ cm$^{-1}$ and $\nu_{5} = 2210.5$ cm$^{-1}$. The temperature dependence of both ($\nu_{1}$, $\nu_{2}$) and ($\nu_{4}$, $\nu_{5}$) pairs display a remarkable behavior: $\nu_{1}$ and $\nu_{2}$ approach each other and coalesce at $T^{*} \sim 300$ K, as do $\nu_{4}$ and $\nu_{5}$; beyond $T^{*}$ they behave as a triply degenerate $\nu_{0}^{*}$ and $\nu_{s}^{*}$, respectively. The relative intensity of $\nu_{2}$ : $\nu_{1}$ approaches $2$ as $T \rightarrow T^{*}$ while that of $\nu_{5}$ : $\nu_{4}$ approaches $1/2$. These features find a convincing explanation on the basis of the dynamic switching of the ($\mathrm{O_{Te}-V_{Cd}}$) dangling bond among the four $\langle 111\rangle$ axes and, for $T \geq T^{*}$, these centers \textquotedblleft acquire\textquotedblright ~$T_{d}$ symmetry. With its $T_{d}$ symmetry, $\mathrm{O_{Te}}$ displays a single second harmonic $\nu_{s}$ at $695.72$ cm$^{-1}$. [1] Chen \textit{et al.}, Phys. Rev. Lett., \textbf{96}, 035508 (2006). $^{*}$Work supported by NSF (DMR 0405082)