Using ground-state and excited-state DFT to decipher 3d dopant defects in GaN

Predictive calculations of electronic excitations do not (not always) require theoretical frameworks that go beyond ground-state density functional theory (DFT). Using ground-state together with excited-state DFT in size-converged supercells, we decipher 3d defects in GaN from limited experimental data for defect levels and more common optical data probing excited states. Applying a local moment counter charge (LMCC) approach to avoid jellium-neutralization errors, defect levels in GaN:3d do not suffer a band gap problem and accurately predict observed levels. For self-consistent calculations of excited states, we implement an occupation-constrained DFT (occDFT) in a ground-state DFT code. The occDFT approach, used with a standard functional PBE, yields optical transitions quantitatively comparable (0.1-0.2 eV) to measurements. As a specific example, PBE-LMCC/occDFT predicts 1.28 eV (adiabatic) for the observed GaN:Mn(0) 1.42 eV photoabsorption. A partnered ground-state/excited-state analysis enables more confident defect identification—chemical fingerprinting. The results mandate extensive reinterpretation of experiments, e.g., the long-standing assignment of the 1.19 eV photoluminescence to Cr(1+) is incorrect. We predict design of alternate d² centers that are possible candidates for optically controlled quantum applications. A simple, fast occDFT approach is often effective and accurate for excited states in wide band gap systems.