Piezoresistivity in the Oxide Semiconductor Rutile (TiO2)

The piezoresistive effect in single crystals of the oxide semiconductor rutile (Ti${\mathrm{O}}_2$) has been studied. The fourth-rank piezoresistive tensor for the $D_{4h}$ tetragonal symmetry of rutile may be expressed in terms of seven piezoresistive coefficients. Room-temperature values for these coefficients are presented for different resistivity material in the range from approximately 1000 to 0.1 ohm cm, resulting from various concentrations of oxygen deficiency in the nonstoichiometric Ti${\mathrm{O}}_2$ lattice. Some of these coefficients were also measured at 77° and 380°K. The measured piezoresistive coefficients are of the order of +${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^2$/dyne. The longitudinal piezoresistive coefficient measured along the "$c$" axis, ${\pi }_{33}$, diminishes with increasing oxygen deficiency, while that along the "$a$" axis, ${\pi }_{11}$, increases with oxygen deficiency. This trend is interpreted in terms of the impurity band conduction model proposed for rutile.