Thermovoltaic response of thin-film bilayered structures based on zinc oxide

A method for measuring the thermovoltaic effect in heterogeneous media with a gradient distribution of the dopant concentration, leading to a gradient distribution of the charge carrier concentration, is proposed. Samples of zinc oxide doped with iron were obtained by ion beam sputtering on a thin foil substrate of tantalum (to measure the thermovoltaic effect), citall (to measure the Hall effect) and silicon to study the structure. The content of the dopant in the samples varied from x_Fe = 0.34 to 4.18 at.%. X-ray diffraction phase analysis studies have shown that the crystal structure of hexagonal zinc oxide is characteristic for all samples. The films are predominantly oriented in the direction [002]. The concentration of charge carriers in the layers of experimental samples, determined using the Hall effect on the ECOPIA 5500 installation in a constant magnetic field with a strength of 0.5 T, varies between 10¹⁶–10²⁰ cm^-3. The samples had an electronic type of conductivity.

To study the thermovoltaic effect, two-layer zinc oxide samples doped with iron and having different concentrations of charge carriers were synthesized. The thermovoltaic effect was studied using the example of two-layer thin-film samples based on zinc oxide with different content of alloying iron impurities using the proposed technique. It was found that the highest value of the thermovoltaic response (U ~ 80 µV) is observed in a two-layer thin-film sample with a greater difference in the concentration of charge carriers between the layers (Δn ≈ 2·10³ cm^-3). The observed saturation of the thermovoltaic response is associated with the onset of dynamic equilibrium between the processes of diffusion of charge carriers from a layer with a high concentration of carriers to a layer with a low concentration and the process of carrier drift due to an internal electric field.

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