Effects of Indium Segregation and Well-Width Fluctuations on Optical Properties of InGaN/GaN Quantum Wells

Abstract

We report the results of calculations for the energies of confined electrons and holes and their wavefunction overlap in In x Ga 1--x N/GaN quantum wells (QWs) with an indium concentration of x = 15% in the well material. It is known that the observed increase in the photoluminescence lifetime with increasing well width can be explained qualitatively by the reduction in overlap of the electron and hole wave functions, which is caused by the piezoelectric field in the strained QW material. We show that the energy dependence of the lifetime measured across the emission line can be explained in a similar way, as the result of AE1-monolayer variations in the QW width. We also calculate the energies and electron-hole wave-function overlap for carriers trapped within indium-rich regions of the QW, taking into account the relaxation of the strain field in and around the indium fluctuation. Our results indicate that well-width fluctuations lead to a stronger energy dependence of the lifetime: the magnitude of the effect is the same order as in experiment, and shows a similar increase with increasing well width. IntroductionThe measured photon energy of the photoluminescence (PL) from InGaN quantum wells (QWs) shows a consistent red-shift, compared with the band gap of bulk InGaN alloy material of the same composition. In addition, the PL decay time depends on the photon energy, increasing as the energy of the emission is scanned from the high to the low energy side of the emission line.Various models have been put forward in partial explanation of these results. Firstly, it has been postulated [1] that there exist localized states whose energies extend into the band gap of the InGaN QW with a density that decreases exponentially away from the band edges. In this band tail model it can be shown that the decay time increases towards lower transition energies, but the model offers no explanation for the strong dependence on well width of the decay time. Moreover, if the timescale of the PL decay is to be explained by the relatively slow relaxation of carriers through these localized states, then, by a similar argument, the onset of the PL should be expected to be delayed on a similar timescale. No such effect on the rise time is observed.A second model proposed does not require the existence of localized states, but instead relies on the quantum confined Stark effect (QCSE) in the InGaN wells [2]. Strong spontaneous and piezoelectric polarization effects [3] are expected, the latter arising from the large difference between the in-plane lattice parameters of InN and GaN. The electron and hole wave functions are confined near the edges of the QW and this accounts for both the large downward shifts in emission energy (compared with the

Improving radiative recombination efficiency of green light-emitting diodes

“…It has been proved via both transmission electron microscope and atom probe tomography that there are one-/two-monolayer high islands on the top of the InGaN QW interface while the bottom interface of the QW being smooth [40][41][42]. Calculations have shown that, due to the in-built electric field along the growth direction, these monolayer well-width fluctuations have a strong effect that decreases the transition energy by about 60 meV in typical green or blue QWs [41,43], which may again localise carriers.…”