Photocurrent measurements on thin passive films at high local resolution

Abstract

Photocurrent spectra i ph( λ) and UV laser scanning measurements at high lateral resolution for the investigation of local properties of the Ti TiO 2 system are presented. The measurements are carried out on Ti single crystals of known orientation. It is emphasized and proved that the interpretation of i ph values requires consideration of multiple internal reflections within the ultrathin TiO 2 layers. An extended Butler-Gärtner model is applied which explains the i ph( d)-curves. The model enables determination of both electronic and optical layer properties such as optical constants, extension of the space charge layer, and defect state concentration. These layer quantities are correlated with the crystal orientation of the Ti substrate. Thickness dependent photocurrent measurements on oxide thickness gradients generated by UV laser illumination showed an excellent qualitative agreement with simulations according to the model in a wide thickness range. The largest growth factor for the laser induced oxides was found for the (0001) orientation, which is the densest packed surface. The oxide gradient generated on this surface covered the range from 30 to 80 nm. In this range, i ph runs through one maximum and a successive minimum as predicted by the theory. However, the comparison of electronic parameters ( ( ϵ(0) N D )- values ) determined by a fit according to this model and impedance measurements still shows significant deviations demanding further improvement of the model.