Beyond Standard DFT: Accurate Electronic and NLO Characterization of ZnSe and GaN for QPM Device Design using HSE06 Hybrid Functional

Designing efficient optoelectronic devices requires a precise understanding of the bandgap, a parameter often poorly predicted by standard DFT. In this study, we tackle this "bandgap problem" for cubic and (space group ) by employing the hybrid HSE06 functional. Our calculations, performed with high energy cut-offs of and , yielded accurate bandgap values of for and for . These hybrid results represent a significant correction over traditional / underestimations. To ensure a consistent description of response properties, the BLYP functional was utilized to calculate the second-order nonlinear coefficients and the elastic constants ). The obtained values ( for and for ), combined with our refined HSE06 electronic structures, allowed for a precise theoretical determination of coherence length ( ) and the calculated Quasi-Phase Matching (QPM) periods for harmonic generation. Our findings demonstrate that using hybrid functionals provides a robust and physically sound basis for evaluating the potential performance of frequency-conversion devices based on wide-bandgap semiconductors, offering high-precision parameters for future experimental development.