Burying non-radiative defects in InGaN underlayer to increase InGaN/GaN quantum well efficiency

The insertion of an InGaN underlayer (UL) is known to strongly improve the performance of InGaN/GaN quantum well (QW) based blue light emitting diodes (LEDs). However, the actual physical mechanism responsible for it is still unclear. We thus conduct a systematic study and investigate different hypotheses. To this aim, InGaN/GaN single (S) QWs are grown on sapphire and GaN free-standing substrates with or without InGaN UL. This allows us to conclude that (i) improvement of LED performance is due to a higher internal quantum efficiency of the InGaN/GaN SQW and (ii) reduction of structural defects is not at play. Furthermore, we show that neither the surface morphology nor the strain of the top GaN layer before the growth of the QW is affected by the InGaN UL. Finally, we find that the beneficial effect of the InGaN UL is still present after 100 nm of GaN. This result combined with band structure modelling rules out the hypothesis of higher QW oscillator strength induced by a reduction of the internal elect...