Optical Properties of Free Electrons in ZnO

The optical properties of indium-doped ZnO crystals, with free-carrier concentrations ranging from ${10}^{19}$ to 5×${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$, have been studied at room temperature. The condition of the crystal surfaces has been shown to influence radically the reflectivity in the near infrared. The optical effective mass for free electrons has been calculated from the dispersion in the near infrared to be $m_{\mathrm{opt}}=(0.265\mathrm{}\pm 0.015)m$, which is in close agreement with other determinations. Experiments with polarized radiation revealed no measurable anisotropy of effective mass. The "polaron" effective mass in ZnO is estimated and briefly discussed.