Effect of free carriers on optical functions of n-type ZnO:Ga films

Thin films of Ga (0.9- 9.3 at.%) doped ZnO were prepared by e-beam evaporation in vacuum and their electronic and optical properties characterized using far-infrared reflectivity spectroscopy. The optical conductivity σ, dielectric e and electron loss energy -Im(1/e) functions of Ga doped ZnO films have been calculated via Kramers-Kronig transformation of the reflectivity spectra and analyzed by the generalized Drude (one-component) and Drude-Lorentz (two-component) models. Using Lorentzian oscillator to simulate the spectroscopic data it is revealed the presence of bound optically active electrons in mid-infrared 3500-4000 cm-1 range, whereas one-component Drude model shows frequency dependent scattering rate and enhanced low-frequency effective mass. Comparison carrier transport properties (charge concentration N, optical electron mobility μopt, and resistivity ρDr) derived from Drude analysis with those obtained by Hall measurements shows that the electron scattering from the grain boundaries makes significantly contribution to the electron mobility of ZnO films studied.