Impact Of Oxygen On The Properties Of Cu3n And Cu3-Xn1-Xox

Cu3N layers with a thickness of 40 nm were deposited by reactive sputtering using Ar: 10% N-2 and 100% N-2, after which they were annealed under NH3/H-2 between 300 and 500 degrees C. These exhibited distinct maxima in differential transmission at similar to 2.46, 2.30, 2.05, and 1.9 eV on a picosecond time scale, as shown by ultrafast pump-probe spectroscopy. We show that the maxima at 1.9 and 2.05 eV correspond to the M and R direct energy band gaps of bulk Cu3N. In contrast, the higher energy maxima at 2.46 and 2.30 eV are related to the occurrence of strained Cu3N in the vicinity of the surface due to surface oxidation upon exposure to the ambient. This is corroborated by the fact that we observed a suppression of the high energy maxima at 2.46 and 2.30 eV by increasing the thickness of the Cu3N layers from 40 to 80 nm, which also rules out intervalley transfer. It is also consistent with the fact that we observed a suppression of the low-energy peaks at 1.9 and 2.05 eV upon the intentional incorporation of oxygen during the deposition of Cu3-xN1-xOx. We describe these findings in conjunction with density functional theory calculations of the electronic band structure of Cu3N and Cu3-xN1-xOx, from which we find that oxygen is preferably incorporated as a shallow donor without giving rise to midgap states and may be used to tailor the direct energy band gaps of this defect-tolerant semiconductor, which in turn is important in the context of solar cells.