Temperature dependence of the A, B, and C excitons in ZnO over 5-400 K: A modulated reflectivity study.

Optical properties of ZnO, a wide gap semiconductor with wurtzite structure, have generated renewed interest in the material in the context of opto-electronic phenomena and applications. The A, B, and C excitons of ZnO, arising from the combined effects of crystal field and spin-orbit splittings of the valence band, are investigated in the temperature range 5- 400 K, exploiting electro-, photo-, and wavelength-modulated reflectivity. The specimens studied have natural isotopic composition. The temperature dependence of the A, B, and C excitonic band gaps, fitted with a two harmonic oscillator model\footnote{M. Cardona, Phys. Status. Solidi b \textbf{220}, 5 (2000); R. P\"{a}ssler, J. Appl. Phys. \textbf{89}, 6235 (2001)} following Manj\'{o}n \emph{et al.}\footnote{F. J. Manj\'{o}n \emph{et al.}, Solid State Commun. \textbf{128}, 35 (2003)}, yields the magnitudes of the zero-point renormalizations 262 meV (A), 227 meV (B), and 249 meV (C), respectively. Isotopically controlled ZnO is currently being investigated to determine the isotopic mass dependence of the zero-point renormalizations.