Determination of the stoichiometry and trace impurities in thin barium strontium titanate perovskite layers by inductively coupled plasma-mass spectrometry

Abstract

The determination of the stoichiometry in semiconducting and non-conducting thin layers is of importance for the study of growth mechanisms and for the control of defects during development and production. An analytical procedure using inductively coupled plasma-mass spectrometry (ICP-MS) was developed, employing different ICP-MS instruments [one double-focusing sector field ICP-MS (DF-ICP-MS) and two quadrupole ICP-MS without and with a hexapole collision cell (ICP-QMS and HEX-ICP-QMS, respectively)] for the determination of the stoichiometry and trace impurities in thin BaxSryTiO3 perovskite films (BST) on silicon substrates after the dissolution of layers. The maximum sensitivity (138Ba+, 1720 MHz ppm−1; 88Sr+, 1330 MHz ppm−1; 48Ti+, 560 MHz ppm−1), lowest detection limit (Ba, 0.004 ng l−1; Sr, 0.007 ng l−1; Ti, 0.08 ng l−1) and best precision down to 0.11% relative standard deviation (RSD, at an analyte concentration of 1 µg l−1) were achieved in DF-ICP-MS. HEX-ICP-QMS yielded a better sensitivity (HEX-ICP-QMS, 100–240 MHz ppm−1; ICP-QMS, 23–50 MHz ppm−1) and lower detection limit (HEX-ICP-QMS, 0.09–3.4 ng l−1; ICP-QMS, 0.8–19 ng l−1) in comparison with conventional ICP-QMS. Precision values of 0.3 and 0.2% RSD were observed for Ba/Sr and (Ba + Sr)/Ti ratios, respectively, by ICP-QMS and HEX-ICP-QMS at an analyte concentration of 10 µg l−1. Besides ICP-MS, inductively coupled plasma-optical emission spectroscopy (ICP-OES) and X-ray fluorescence (XRF) analysis were used for the validation of the experimental results. The stoichiometric compositions measured by different methods are in good agreement taking into account the local inhomogeneity of element distribution in thin layers and the diffusion of elements in the substrate. Due to the high sensitivity of ICP-MS, the determination of 38 trace elements was performed in thin BaxSryTiO3 films (with a thickness less than 60 nm) with detection limits in the low ng g−1 range.