Stoichiometry Driven Impurity Configurations in Compound Semiconductors*

Precise stoichiometry and departures therefrom in the composition of the tetrahedrally coordinated compound semiconductors allow impurity incorporation in more than one configuration. Ultra-high resolution infrared spectroscopy of CdTe:O at low temperatures reveals a unique pair of sharp lines, a non-degenerate $\nu _{1}$ = 1096.78 cm$^{-1}$ and a doubly degenerate $\nu _{2}$ = 1108.35 cm$^{-1}$ at 5 K, associated with the local vibrational modes of O$_{Te}$ in a (O$_{Te}$ -- V$_{Cd})$ complex in crystals grown with (CdTe + CdO + excess Te) or (CdTe + TeO$_{2})$ which enhances the occurrence of Cd vacancy (V$_{Cd})$; in contrast, a single, triply degenerate sharp line at $\nu _{0}$ = 349.79 cm$^{-1}$ observed at 5 K occurs in CdTe grown with (CdTe + CdO + excess Cd) in which the appearance of V$_{Cd}$ is inhibited. In the former, oxygen, O$_{Te}$, is bonded to three nearest neighbor Cd's with a nearby V$_{Cd}$. The latter corresponds to O$_{Te}$ attached to all the four nearest neighbor Cd cations. With increasing temperature, $\nu _{1}$ and $\nu _{2}$ approach each other and behave as a single triply degenerate line at $\nu _{0}^{\ast }$ for temperature T $\ge $ T$^{\ast } \quad \sim $ 300 K; the uniaxial (C$_{3v})$ symmetry of (O$_{Te}$ -- V$_{Cd})$ transforms to T$_{d}$ symmetry at T$^{\ast }$, acquired due to an increasing rate of bond switching among the four possible O$_{Te}$ -- V$_{Cd}$ directions as T approaches T$^{\ast }$.