Chemical nature of N-ions incorporated into epitaxial ZnO films†

Efficient p-type doping of zinc oxide (ZnO) is hindered on the one hand by the strong native n-type doping of the ZnO, on the other hand, compensation effects (defect generation due to p-doping) tend to preserve the n-type doping. Incorporation of nitrogen is proposed as a promising method to achieve p-type ZnO, because the ionic radii of nitrogen and oxygen are comparable. Therefore, nitrogen atoms can replace oxygen. Nevertheless, a reliable and stable p-doping is still an unmatched challenge. This work will focus on the chemical nature of nitrogen implanted by ion irradiation into metal-organic MBE grown ZnO layers on sapphire substrate. The incorporated nitrogen was investigated by photoelectron spectroscopy using synchrotron radiation (PES) and monochromatised Al Kα (mXPS), and near edge X-ray absorption spectroscopy (NEXAFS). The preparation conditions were varied for preferential incorporation of the different nitrogen species. The three main N1s-PES components were assigned to different nitrogen compounds (molecular N2, NO bonds and NZn bonds) with the help of NEXAFS data. In addition, the thermal stability of the nitrogen compounds were investigated. These results may lead to an optimisation of the nitrogen implantation process for a better doping efficiency.