Experimental studies of thorium ions implantation from pulse laser plasma into thin silicon oxide layers

摘要

We report the results of experimental studies related to implantation ofthorium ions into thin silicon dioxide by pulsed plasma fluxes expansion.Thorium ions were generated by laser ablation from a metal target, and theionic component of the laser plasma was accelerated in an electric fieldcreated by the potential difference (5, 10 and 15 kV) between the ablatedtarget and SiO2/Si(001) sample. Laser ablation system installed inside thevacuum chamber of the electron spectrometer was equipped with YAG:Nd3+ laserhaving the pulse energy of 100 mJ and time duration of 15 ns in the Q-switchedregime. Depth profile of thorium atoms implanted into the 10 nm thicksubsurface areas together with their chemical state as well as the band gap ofthe modified silicon oxide at different conditions of implantation processeswere studied by means of X-ray photoelectron spectroscopy (XPS) and ReflectedElectron Energy Loss Spectroscopy (REELS) methods. Analysis of chemicalcomposition showed that the modified silicon oxide film contains complexthorium silicates. Depending on local concentration of thorium atoms, theexperimentally established band gaps were located in the range of 6.0 - 9.0 eV.Theoretical studies of optical properties of the SiO2 and ThO2 crystallinesystems have been performed by ab initio calculations within hybrid functional.Optical properties of the SiO2/ThO2 composite were interpreted on the basis ofBruggeman effective medium approximation. A quantitative assessment of theyield of isomeric nuclei in "hot" laser plasma at the early stages of expansionhas been performed. The estimates made with experimental results demonstratedthat the laser implantation of thorium ions into the SiO2 matrix can be usefulfor further research of low-lying isomeric transitions in 229Th isotope withenergy of 7.8(0.5) eV.