Impact of Oxygen on the Properties of Cu₃N and Cu_3–<i>x</i>N_1–<i>x</i>O<i>_x</i>

Abstract

Cu₃N layers with a thickness of 40 nm were deposited\nby reactive sputtering using Ar: 10% N₂ and 100% N₂, after which they were annealed under NH₃/H₂ between 300 and 500 °C. These exhibited distinct maxima\nin differential transmission at ∼2.46, 2.30, 2.05, and 1.9\neV on a picosecond time scale, as shown by ultrafast pump-probe spectroscopy.\nWe show that the maxima at 1.9 and 2.05 eV correspond to the M and\nR direct energy band gaps of bulk Cu₃N. In contrast, the\nhigher energy maxima at 2.46 and 2.30 eV are related to the occurrence\nof strained Cu₃N in the vicinity of the surface due to\nsurface oxidation upon exposure to the ambient. This is corroborated\nby the fact that we observed a suppression of the high energy maxima\nat 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.\nIt is also consistent with the fact that we observed a suppression\nof the low-energy peaks at 1.9 and 2.05 eV upon the intentional incorporation\nof oxygen during the deposition of Cu_3–<i>x</i>N_1–<i>x</i>O<i>_x</i>. We describe these findings in conjunction with density\nfunctional theory calculations of the electronic band structure of\nCu₃N and Cu_3–<i>x</i>N_1–<i>x</i>O<i>_x</i>,\nfrom which we find that oxygen is preferably incorporated as a shallow\ndonor without giving rise to midgap states and may be used to tailor\nthe direct energy band gaps of this defect-tolerant semiconductor,\nwhich in turn is important in the context of solar cells.