A contrast in the electronic structures of B ion implanted ZnO thin films grown on glass and silicon substrates by using x-ray absorption spectroscopy

Abstract

Fabrication of suitable c-axis oriented boron doped zinc oxide (BZO) thin films is vital for many applications. The properties of films critically depend on film preparation methods and the type of substrate. Furthermore, ion implantation at a specific position or depth of a sample substantially affects the properties of films. In this study, a contrast between the electronic structure of ZnO thin films grown on glass and silicon (Si) substrates and then implanted with B ions at 40 keV with a concentration of 5 × 1013 ions/cm2 was investigated. The B ions were implanted using a 3 MV tandem accelerator. Secondary ion mass spectrometry, atomic force microscopy, and x-ray diffraction were used to study the distribution and depth of B ions, surface morphology, and the crystallinity of ZnO and BZO thin films. The preferential orientation, texture, surface roughness, and grain size of the ZnO thin films were affected by B ion implantation. O K-, Zn L3-, and K-edge x-ray absorption near-edge structure (XANES), and Zn K-edge extended x-ray absorption fine structure techniques were used to investigate the electronic structure of the BZO thin films and to determine the substrate influence. O K- and Zn L3-edge XANES spectra of the BZO/glass thin films revealed an enhanced electron transfer from Zn to O as compared to the BZO/Si thin film, implying that B implantation increased the negative effective charge on the O ions. Interestingly, in contrast to the definition of electronegativity, an increase (decrease) in the number of O 2p (Zn 3d/4sp) unoccupied states due to an enhanced O 2p–Zn 3d/4sp hybridization is observed in the BZO/Si thin film as compared to the BZO/glass thin film, suggesting an increase in the number of O 2p-dangling bonds. This would affect the conductivity and luminescent behavior of the ZnO/Si thin films after B doping, which will be useful in optoelectronic applications.