Efficiency droop of GaN lasers and LEDs

The density/current dependence of the internal efficiency (IQE) in GaN-based light emitting devices (LED) is commonly modelled using a cubic polynomial called the ABC law. Here, a linear density (N) dependence, AN, represents defect recombinations, a quadratic term, BN², is used for radiative losses and an Auger-like cubic term, CN³, is used to model the droop-causing losses. The model has been shown to be able to reproduce experimentally measured data quite successfully. However, when treating all three parameters, A, B, and C, as freely adjustable parameters fits of a single IQE curve usually leave a rather high degree of uncertainty. E.g., virtually identical results can be obtained varying the Auger coefficient by more than one order of magnitude if at the same time A and B are adjusted accordingly. This uncertainty not only obscures the accurate values for the strengths of the underlying mechanisms, but also prevents the model to be able to determine other dependencies like that on temperature which could yield more insight into which physical processes may be responsible for the droop.