Role of localized quantum well excitons in InGaN quantum well structure correlated with microstructural analysis

In_xGa_1-xN multiple-quantum-well laser diode structure, which lased at 405 nm, was shown to have atomically-flat interfaces between each layer. Nanometer-probe compositional analysis showed that InN mole fraction, x, in the wells and barriers are approximately 6 % and 2 %, respectively, which agreed with the result obtained from high-resolution x-ray diffraction measurement. The Stokes-like shift (SS) at 300 K was 49 meV, being approximately 65 % of the luminescence linewidth. The localization depth, E₀, of quantum-well (QW) excitons as estimated to be 35 meV at 300 K though the compositional fluctuation in the well was as small as 1 % or less (detection limit) within adjacent 20-30 nm lateral length scale. Since the well thickness fluctuation is insufficient to reproduce SS or E₀, effective bandgap inhomogeneity is attributed to be due to large bandgap bowing in InGaN. The spontaneous emission was thus assigned as being due to the recombination of QW excitons weakly localized in exponential tail-type potential minima in the QW. The size of localization is smaller than the quantum-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)]-size. Such small bandgap inhomogeneity can be leveled by injecting high density carriers under lasing conditions, which can explain the general experimental finding that the quantum efficiency decreases with increasing carrier density in InGaN QW devices due to free carrier trapping into threading dislocations.