Multiple oscillator models for the optical constants of polycrystalline zinc oxide thin films over a wide wavelength range

Abstract

Zinc oxide (ZnO) films were prepared on Si(1 1 1) and quartz substrates using RF-magnetron sputtering in N2 plasma at room temperature. From the X-ray diffraction observations, it was found that all films are polycrystalline with a preferred orientation of (1 0 1). X ray photoelectron spectroscopy was used to analyze the chemical composition of the films by observing the behavior of the Zn2p3, O1s, N1s, and C1s lines. The thicknesses and optical constants of the ZnO thin films were determined using variable angle spectroscopic ellipsometry through the Genosc™ Herzinger–Johs parameterized semiconductor oscillator functions and multiple Gaussian oscillator models. Combining multiple oscillator types provided a very flexible approach to fitting optical constants over a wavelength range 190–1400 nm while simultaneously enforcing Kramers–Kronig consistency in the fitted ellipsometric parameters. Refractive indices of the films were determined to be in the range 1.68–1.93 and extinction coefficients in the range 4.56 × 10−6–0.23. A direct bandgap of 3.38 ± 0.03 eV was calculated from the extinction coefficient. Low temperature photoluminescence studies of the films exhibited one prominent peak at 3.41 eV. The equality of the ZnO thin films was obtained through the depolarization measurements.