Observation of the Direct Energy Band Gaps of Defect-Tolerant Cu₃N by Ultrafast Pump-Probe Spectroscopy

Abstract

Cu3N with a cubic crystal structure has been prepared from Cu on fused SiO2 under a flow of NH3:O2 between 400 and 600 °C. All Cu3N layers exhibited distinct maxima in differential transmission at ∼500, 550, and 630, 670 nm with the same spectral structure and shape on a ps timescale as shown by ultrafast pump-probe spectroscopy. We show that the maxima at 630 (≡1.97 eV) and 670 nm (≡1.85 eV) correspond to the M and R direct energy band gaps of Cu3N, in excellent agreement with density functional theory calculations of the electronic band structure. These findings are corroborated further by the fact that Cu3N as-deposited by reactive sputtering under 100% N2 at 25 °C and 10–2 mbar did not exhibit a fine spectral structure due to a smeared density of states, poor crystallinity, and a high density of defects, but annealing under NH3:H2 at 300 °C revealed a similar spectral structure to Cu3N obtained from Cu under NH3:O2. In contrast to the above, we suggest that the peaks at 500 (≡2.48 eV) and 550 nm (≡2.25 eV) might correspond to the M and R direct gaps of certain regions of Cu3N under strain that changes the lattice constant and band gap. We discuss the charge carrier generation and recombination mechanisms in terms of Cu interstitials and vacancies that are known to be energetically located near the band edges, thus allowing the observation of the direct energy band gaps in this defect tolerant semiconductor.