First-principles study of the electronic structure and optical properties of Ce and C Co-doped AlN

Aluminum nitride (AlN) is an ultrawide-bandgap semiconductor whose large intrinsic band gap limits low-energy interband optical absorption. This work compares Ce substitution at the Al site, C substitution at the N site, and Ce-C co-substitution in wurtzite AlN to clarify how Ce 4f/5d states and C 2p states modify the local structure, band-edge electronic states, and optical response. Among five neutral Ce_Al-C_N configurations considered in a 2 × 2 × 2 supercell, the nearest-neighbor Ce-C pair has the lowest total energy, with the other configurations lying 6.6-17.7 meV higher. The selected Ce-C pair also has a negative binding energy of -3.26 eV relative to the corresponding isolated single-substitution reference supercells. Structural relaxation shows lattice expansion after substitution, with the largest volume increase of 7.93% obtained for AlN:Ce-C. The calculated band gap of intrinsic AlN is 4.19 eV, whereas the effective electronic gaps of AlN:C, AlN:Ce, and AlN:Ce-C are 3.59, 2.18, and 2.04 eV, respectively. Since AlN:Ce-C is only 0.14 eV smaller in effective gap than AlN:Ce, the role of Ce-C co-substitution is interpreted mainly through defect-pair energetics, orbital redistribution, and local population changes rather than as a large additional band-gap narrowing. DOS/PDOS and population analyses show that C 2p states mainly modify the occupied valence-edge region, Ce 4f/5d states contribute near the band-edge and conduction-band regions, and Ce-C co-substitution induces finite spin population on C together with a nonzero Ce-C bond population. The scissors-corrected optical spectra show increased low-frequency dielectric and refractive responses, with ε₁(0)/n(0) changing from 3.805/1.951 for intrinsic AlN to 4.449/2.109 for AlN:Ce-C, while AlN:C gives the largest low-frequency values of 4.530/2.128. Additional low-energy interband absorption and attenuation features appear especially in AlN:C and AlN:Ce-C, associated with defect-related transitions involving C 2p and Ce-related states. These changes are interpreted as calculated interband optical-response redistribution within the present neutral-defect supercell framework, not as direct evidence of device-level optoelectronic performance.

First-principles calculations were performed using the CASTEP module in Materials Studio. The exchange-correlation interaction was described using the generalized gradient approximation with the Perdew-Burke-Ernzerhof functional, and on-the-fly-generated ultrasoft pseudopotentials were employed. Intrinsic AlN, AlN:Ce, AlN:C, and AlN:Ce-C were constructed from a fully relaxed 2 × 2 × 2 wurtzite AlN supercell containing 32 atoms. Single-doped models were constructed by replacing one Al or one N atom, corresponding to 6.25% substitution on the relevant sublattice. The co-doped model contains one Ce_Al-C_N pair, corresponding to xCe = yC = 0.0625 in Al_1-xCe_xN_1-yC_y, or a combined sublattice substitution level of 12.5%. A plane-wave cutoff energy of 700 eV and a 4 × 4 × 3 k-point mesh were used. Intrinsic AlN was calculated using non-spin-polarized GGA-PBE, C-doped AlN using spin-polarized GGA-PBE, and the Ce-containing systems using spin-polarized GGA-PBE + U. A Hubbard U correction of 5 eV was applied only to the Ce 4f orbitals. Band structures, TDOS/PDOS, Mulliken and Hirshfeld population analyses, representative bond populations, and optical properties were calculated. The dielectric function, complex refractive index, absorption coefficient, and reflectivity were obtained within the linear-response framework using a 2.01 eV scissors correction only for optical-property calculations.