Rapid quantitative analysis of ZnGa₂O₄（GZO） thin films using picosecond laser induced breakdown spectroscopy

  Objective   In recent years, with the rapid development of the research on nanomaterials, transparent conductive oxide nanofilms have been widely used in many fields such as flat display, liquid crystal display screen and thin film solar cell due to their good conductivity and high transmittance in visible light range. ZnGa₂O₄ (GZO) nanofilms are prepared by doping gallium elements in zinc oxide thin films, and its performance is close to that of traditional tin doped indium oxide (ITO) thin films. Radio frequency (RF) magnetron sputtering, as a mature preparation method for thin film materials, has been widely used in scientific research and industrial fields due to its advantages of stability and high film forming quality. However, in the preparation process of GZO thin film materials, changes in magnetron sputtering parameters often lead to differences in the composition ratio, resulting in different performance of the samples. Therefore, it is necessary to quickly analyze the composition ratio of the prepared GZO films, so as to analyze the performance of the sample and optimize the process parameters of magnetron sputtering. For this purpose, an available and effective analytical method was used to achieve the detection of the composition ratio of the prepared GZO films by radio frequency magnetron sputtering at different sputtering powers.

  Methods   During the deposition process of the GZO thin film, the sputtering powers affected the composition ratio of the samples, resulting in a difference in the performance of the GZO thin film, such as the transmittance (Fig.1) and optical band gap widths (Fig.2) of GZO films. In this work, the GZO thin films were analyzed by picosecond laser induced breakdown spectroscopy (PS-LIBS), and the critical element concentration ratios of GZO films were quantitatively analyzed.

  Results and Discussions   PS-LIBS experimental setup (Fig.3) and the corresponding LIBS spectroscopy of GZO thin film (Fig.4) were shown. Moreover, the plasma temperature and electron density produced by picosecond laser ablation of GZO film were calculated as 5 426.8 K and 4.2×1 016 cm⁻³, which satisfied the local thermodynamic equilibrium condition (Equ.4) so as to achieve the quantitative analysis. The results obtained by PS-LIBS showed that there is a certain relationship between the optical properties of the GZO thin films and the intensity ratios of the element spectral lines. With the increase of the sputtering power, the Zn/Ga spectral line intensity ratios and the concentration ratios show a consistent change (Fig.8). Taking the Zn/Ga ratio of the key component of the GZO thin films as the main analysis target, rapid quantitative analysis was carried out on the change of the ratios under different sputtering parameters. The calibration curves of GZO thin films were established with the Zn/Ga spectral line intensity ratios and its energy dispersive spectrometer (EDS) values (Fig.9), and the corresponding linear fitting coefficient was greater than 0.99 which showed good fitting results.

  Conclusions   In this study, PS-LIBS technology was used to analyze the Zn/Ga component ratios of GZO thin films deposited by RF magnetron sputtering under different sputtering powers. The linear fitting coefficient of calibration curve was up to be 0.998. The calculated plasma temperature (T=5 426.8 K) and electron density (Ne=4.2×1 016 cm⁻³) ensured the accuracy of quantitative analysis. The Zn/Ga intensity ratios detected by PS-LIBS under different sputtering powers were closely related to the optical properties of the GZO samples. Both the Zn/Ga intensity ratios and atomic content ratios decreased with the increase of sputtering power. Moreover, the corresponding optical band gap widths increased with the increase of gallium content in the GZO thin films, reaching the maximum value at the sputtering power of 95 W. It indicates that the PS-LIBS method has positive significance for the fast performance analysis of GZO thin films with its advantages of fast, real-time, in situ and micro-damage analysis, and it can also achieve real-time optimization of preparation parameters for GZO films deposited by radio frequency magnetron sputtering.