Photoluminescence Spectroscopy of Cuprous Oxide: Bulk Crystal versus Crystalline Films

Cuprous oxide (Cu2O) crystals and films of 10-65 nm thickness are investigated via electron diffraction, scanning tunneling microscopy (STM), and photoluminescence (PL) spectroscopy at temperature between 100 to 300 K. While the chemical composition and surface morphology of both systems are identical, large differences are found in the optical response. Bulk Cu2O shows pronounced PL peaks at 620, 730, and 920 nm, compatible with the radiative decay of free and bound excitons, whereas broad and asymmetric peaks at 775 and 850 nm are found for Cu2O films grown on Au(111) and Pt(111) supports. The latter represent the PL signature of VO2+ and VO+ defects, being inserted with substrate-dependent concentrations due to the oxygen-poor preparation of the Cu2O films. Despite the strong V-O signature in PL, all Cu2O samples show p-type conductance behavior in STM spectroscopy, indicating an abundance of Cu defects in the lattice. The fact that O vacancies still govern the thin-film PL is explained by a more efficient recombination via V-O- than V-cu-emission channels, as the latter requires exciton formation first. Herein, the high sensitivity of low-temperature PL to probe the defect landscape of dielectrics, being neither reached by photoelectron spectroscopy nor scanning probe techniques is demonstrated.