Charging phenomena in dielectric/semiconductor heterostructures during x-ray photoelectron spectroscopy measurements

摘要

The determination of the valence band offset (VBO) by x-ray photoelectron spectroscopy (XPS) is commonly performed using the so-called Kraut's method that was developed for VBO determination in semiconductor/semiconductor heterojunctions. Although the physical model, which is the basis of the method, can be safely extended to dielectric/semiconductor (D/S) heterojunctions, in these systems a careful evaluation of the experimental results is necessary due to the differential charging phenomena originating at D/S interface during x-ray bombardment. As a consequence, precise determination of the VBO requires an accurate calibration of the energy scale in order to remove artifacts induced by the progressive charging of the oxide during the XPS measurement. In this work a detailed analysis of the band alignment between e-beam evaporated amorphous HfO_2 films and Si substrates is reported. The HfO_2/Si heterojunction was selected as a prototype for this study since HfO_2 based dielectrics have already been implemented as gate dielectrics in real devices and have been the subject of a wide number of publications providing controversial results in terms of VBO values. A clear dependence of the binding energy of the Hf 4f and O ls core lines on the thickness of the HfO_2 film is identified. The time evolution of these signals indicates that different steady states are reached after prolonged x-ray bombardment depending on the thickness of the HfO_2 films. On the basis of the original work of Iwata et al. [J. App. Phys. 79, 6653 (1996)], a rigorous method to remove these artifacts and empirically determine the real band offsets in D/S heterojunctions is proposed and validated by comparison with internal photoemission and photoconductivity data obtained on the same set of samples.