(551d) Investigation of Metal Oxide Surface Fermi Level

Buried solid-solid interfaces of metal oxides in thin film structures are arising with increasing frequency in electronics applications such as memory resistors. Yet certain electrical properties of these interfaces, such as the degree of fixed charge buildup, can be difficult to characterize. This charge buildup can be very sample-dependent for some oxides, and depend only on a small fraction of the total surface sites (< 1 x 10^-3). Photoreflectance (PR), a type of optical modulation spectroscopy, can be utilized to detect and sometimes quantify the presence of electric fields near solid-solid interfaces as long as the overlayer is transparent to the probe light. The present work describes the applicability of PR to characterizing metal oxide interfaces using thin film TiO₂ as an example metal oxide. The approach involves the synthesis of thin film polycrystalline anatase TiO₂ on Si(100) and quartz by atomic layer deposition using Ti(OCH(CH₃)₂)₄ and H₂O as precursors. Film composition and microstructure are characterized using x-ray spectroscopy, x-ray diffraction, and x-ray reflectivity. PR is then utilized to examine the effect of film thickness, uniformity, and crystallinity on the position of the surface Fermi level. Spectra reveal the existence of distinct transient effects evolving on two different time scales. A several minute effect manifests as a sloping PR signal baseline and spectral phase change and is attributed to charge exchange between surface/interface/grain boundary states and the bulk bands. A several week effect shows up as a decrease in overall PR signal amplitude and could arise from chemical reactions at the solid interface.