Re-evaluating photoluminescent defects in Cu$_2$O

TL;DR

This work re-evaluates the origins of below-band-gap photoluminescence lines in Cu2O using first-principles DFT (PBE and HSE06) across 2×2×2 and 3×3×3 supercells. By mapping defect-induced band-gap states, calculating formation enthalpies under realistic growth conditions, and applying a strict criterion to identify genuine in-gap states, the study finds no evidence that simple copper or oxygen vacancies generate mid-gap states, contradicting long-standing PL line assignments. Instead, oxygen interstitials consistently introduce localized in-gap states across functionals and cell sizes, with charge-dependent shifts suggesting multiple emission lines, while copper interstitials and split vacancies yield inconclusive results. The findings point to oxygen interstitials as plausible contributors to at least some PL lines and emphasize the need for excited-state calculations (GW/BSE) to quantitatively predict emission energies, informing Cu2O crystal-growth strategies for quantum-device applications.

Abstract

Cuprous oxide (Cu$_2$O) is of interest for several technologies, including solar cells, and more recently, quantum devices via Rydberg excitons. It's performance in these capacities is strongly affected by defects in the crystal. The current best diagnostic for the presence of defects in a sample is the photoluminescence (PL) spectrum, which shows a number of strong lines at energies below the band gap, with brightnesses dependent on the sample. However, the assignment of PL lines to particular defects has not been substantiated by modern theory. Using density functional theory (DFT), we investigate from first principles which native defects introduce electronic states within the Cu$_2$O band gap, and therefore would produce lines in the PL spectrum. We find that the accepted assignments of lines to simple oxygen and copper vacancies are unsupported, and propose a new assignment based on oxygen and copper interstitials, and (one of the possible) split copper vacancies, a significant step towards the use of PL as a diagnostic tool for Cu$_2$O crystal growth.

Figures (10)

• Figure 1: A sketch of an example PL spectrum showing the 4 main lines commonly observed, with the defect assignments made by Bloem bloem1958.
• Figure 2: Band structures for pure Cu$_2$O in supercells. From top to bottom: PBE band structure in a $2\times 2\times 2$ supercell; PBE band structure in a $3\times 3\times 3$ supercell; HSE06 band structure in a $2\times 2\times 2$ supercell. Occupied states are shown in green and unoccupied states are shown in orange. The dotted line denotes the Fermi level.
• Figure 3: Band structures for the simple copper vacancy, $V_\text{Cu}$. From top to bottom: PBE band structure in a $2\times 2\times 2$ supercell; PBE band structure in a $3\times 3\times 3$ supercell; HSE06 band structure in a $2\times 2\times 2$ supercell. Occupied bulk states are shown in green and unoccupied bulk states are shown in orange. The dotted line denotes the Fermi level. The simple copper vacancy does not exhibit defect states in the band gap.
• Figure 4: Band structures for the oxygen vacancy, $V_\text{O}$. From top to bottom: PBE band structure in a $2\times 2\times 2$ supercell; PBE band structure in a $3\times 3\times 3$ supercell; HSE06 band structure in a $2\times 2\times 2$ supercell. Occupied bulk states are shown in green and unoccupied bulk states are shown in orange. The dotted line denotes the Fermi level. The oxygen vacancy does not exhibit defect states in the band gap.
• Figure 5: Band structures for the octahedral form of the oxygen interstitial, O$_\text{i}^\text{oct}$. From top to bottom: PBE band structure in a $2\times 2\times 2$ supercell; PBE band structure in a $3\times 3\times 3$ supercell; HSE06 band structure in a $2\times 2\times 2$ supercell. Occupied bulk states are shown in green, unoccupied bulk states in orange, occupied defect states in blue and unoccupied defect states in red.