• The effects of X-ray-induced conductivity on the XPS charging shifts in WO 3 are studied. • No charging is observed for films thinner than the X-ray penetration depth. μ • Charging is evident for WO 3 layers significantly thicker than. μ • Sample tilting is a simple and effective way to alter the charging state. X-ray Photoelectron Spectroscopy (XPS) analyses of insulators are complicated by surface charging phenomena. In this paper the crucial effect of the X-ray-induced conductivity on the buildup of positive surface potential, up to now not explicitly considered in the literature, is demonstrated for the model case of WO 3 thin films grown on conducting substrates with the thickness d ox varying from 1 to 5000 nm. It is shown that high X-ray-induced conductivity prevents charging in layers significantly thinner than the X-ray penetration depth μ due to continuous creation of electron-hole pairs in the entire oxide volume, the electron injection from the bottom contact (due to photoemission), and the following secondary electron generation. Charging shifts are first observed once d ox > μ . Under such conditions, the conductivity in the part of the film closest to the substrate drops to the intrinsic level, which is too low to compensate for the photoemission. The close relationship between the core level charging shifts and the X-ray penetration depth is further verified by varying the sample tilt angle. Tilting the sample away from (towards) the X-ray gun increases (decreases) peak shifts. These results challenge the conventional understanding that charging is primarily determined by intrinsic electrical conductivity. It is demonstrated that for insulators with significant X-ray-induced conductivity, the conscious choice of key experimental variables such as film thickness, excitation energy and the X-ray incidence angle may limit, or even eliminate, the charging shifts.
Grzegorz Greczynski (Wed,) studied this question.