Non-radiative recombination at point defects in GaN
Non-radiative recombination limits the efficiency of GaN based light-emitting diodes. Our project aims to evaluate the non-radiative recombination mechanism of multiphonon emission centers in GaN by using first-principles calculation.
The invention of efficient blue light-emitting diodes (LEDs) by Akasaki, Amano and Nakamura has a deep and fast impact on the industrialization of LEDs for general lighting, as recognized by 2014's Nobel prize in physics. Despite of the successful story of commercialization of LEDs, there are some challenges remain to be tackled. One of them is the efficiency restriction caused by non-radiative recombination.
Capture cross sections are key parameters to evaluate the non-radiative recombination efficiency. In this project, we develop a novel approach to locate the transition state and further obtain the capture cross sections by analytical method.
We first benchmark our theory against experiment based on prototype GaP with substitutional oxygen, whose accurate DLTS data are available. The calculations are based on density functional theory with hybrid functional of Heyd-Scuseria-Ernzerhof (HSE) and the projector-augmented wave method. We here propose a novel method to generate reaction coordinates for capture processes by employing defect level occuptation as a natural reaction coordinate.
We obtained very satisfactory results for the band gap and direct optical transitions compared with experimental data, which shows the HSE functional well solves the band gap underestimation problem and places the defect states correctly within the gap. Substitutional oxygen is a deep defect center, which come along with large lattice relaxations. By following our methodology, we construct configuration coordinate diagrams, from which we could obtain the hole capture barrier. Then, we get the hole capture cross section by analytical model. Our calculated capture cross sectios agree to within one order of magnitude with experimental value.
We further apply our scheme to group III nitrides (AlN, GaN, InN) and prove that nitrogen vacancies act as efficient non-radiative recombination centers.