Electronic and optical properties of La,Cr co-doped SrTiO$_{3}$ epitaxial thin films

SrTiO$_{3}$ (STO) is a wide-gap semiconductor well suited for photocatalytic H$_{2}$ production due to the alignment of its band edges with the half-cell energies of the H$_{2}$O redox reactions. However, the wide optical gap of STO (3.3 eV) makes the material an inefficient light absorber in the visible spectrum, preventing formation of electron-hole pairs needed for photocatalysis. Cr dopants on the Ti site have been shown to reduce the optical bandgap if the Cr ion is in the 3$+$ state. However, charge conservation in Cr-doped STO dictates that a Cr ion must either be in the 4$+$ oxidation state or compensate the 3$+$ state via an oxygen vacancy. In each case, defect electronic states occur, reducing the efficiency of electron-hole pair formation. To compensate this, others have shown that doping STO with La and Cr ions in equal quantities can promote the formation of the Cr$^{3+}$oxidation$^{\, }$state. In this work, we examine the electronic and optical properties of La,Cr-doped STO films grown using oxide molecular beam epitaxy. Films were characterized via \textit{in situ} x-ray photoelectron spectroscopy to measure valence band and core level energies, confirming that most Cr ions are in the 3$+$ state. Optical absorption measurements show that the optical bandgap is reduced by 0.8 eV from that of undoped STO.