Piezoreflectance of Diluteα-Phase Ag-In Alloys

The fractional change in reflectivity due to strain $\frac{\Delta R}{R}$ of evaporated film samples of four silver-indium alloys was measured in the energy range 2.5-4.5 eV. The films had indium concentrations of 0.4, 1.6, 3.3, and 5.0 at.%. A Kramers-Kronig inversion of $\frac{\Delta R}{R}$ yielded the change, due to strain, of the phase shift of the reflected electric field. The strain-induced changes of the real and imaginary parts of the complex dielectric constant, $\Delta {\epsilon }_1$ and $\Delta {\epsilon }_2$, were calculated. Interpretation of the structure in $\Delta {\epsilon }_2$ of silver and the four alloys yielded the following conclusions. (a) The structure in $\Delta {\epsilon }_2$ at 4.06 eV in silver is due to the ${L_2}^{\prime }\to L_1$ (Fermi surface to conduction band) transition and not to the $L_3\to {L_2}^{\prime }$ ($d$ band to Fermi surface) transition as previously suggested. (b) The ${L_2}^{\prime }\to L_1$ band gap in silver is 4.15 eV and decreases to about 3.7-3.8 eV in the 5.0% indium alloy. (c) Indirect transitions contribute to ${\epsilon }_2$ and $\Delta {\epsilon }_2$ below the direct ${L_2}^{\prime }\to L_1$ transition energy in the silver-indium alloys.