XPS Ti 2p spectra of LaAlO3/SrTiO3, LaGaO3/SrTiO3 and NdGaO3/SrTiO3 for different layer numbers.
(a) Ti 2p spectra of LaAlO3/SrTiO3 and SrTiO3 for comparison. The spectra are subtracted by a Shirley background and normalized to the area of the Ti4+ 2p3/2 peak. The energy scale is referenced to the Ti 2p3/2 peak maximum. The Ti3+ states appear as a low energy shoulder of the main Ti4+ peak. (b–g) Ti3+ region as a function of layer number and emission angle for the three different materials. For comparison, the gray baseline in all graphs represents the spectrum of a SrTiO3 substrate.