Effect of plasmon–phonon interaction on the infrared reflection spectra of Mg_xZn_1-xO/Al₂O₃ structures

The effect of plasmon–phonon interaction on the vibrational properties of the Mg_xZn_1-xO films on anisotropic substrate is studied versus Mg content (x), film thickness (0.5–20 μm), free carrier concentration (1×10¹⁶–5×10¹⁸ cm⁻³) and damping coefficients using infrared reflection spectroscopy. The mathematical model with additive and phenomenological contribution of several oscillators to dielectric permittivity of Mg_xZn_1-xO material was developed. The infrared reflection spectra were simulated in the range of “residual rays” of the film and the substrate for Mg_xZn_1-xO/Al₂O₃ structures using self-consisted parameters of bulk ZnO, MgO and Al₂O₃ materials. Based on the Kramers–Kronig relationship, the frequency range where film reflectivity is sensitive to the variation of film doping and thickness was determined. The frequencies and damping coefficients of TO and LO modes of the oscillators, static and high-frequency dielectric permittivity for orientation E⊥c were obtained with high accuracy. Main attention was paid to the compositions which were in hexagonal structure. Experimental infrared reflection spectra were recorded for the films with x = 0.25 additionally doped with manganese and their simulation was performed based on the model developed. The free carrier concentration and mobility as well as film conductivity were determined. The results obtained showed the utility of infrared reflection spectroscopy for the investigation of textured alloy films. This non-destructive and contactless method can be implemented for the determination of optical properties of other semiconductor films.