Abstract
Time- and spectrally-resolved scanning near-field optical microscopy was applied to study spatial variations of photoluminescence (PL) spectra and carrier dynamics in polar InGaN/GaN single quantum wells (QWs) emitting from 410 nm to 570 nm. The main attention was devoted to variations of PL properties and carrier dynamics around V-defects. The PL intensity, peak wavelength, and linewidth, as well as the radiative and nonradiative recombination times, were found to be different in V-defect-rich and defect-free regions. The radiative lifetime close to the defects was longer up to several times, which is attributed to an increased electron and hole wave function separation in the QW plane. PL decay times, measured using excitation and collection through the near-field probe, were one to two orders of magnitude shorter than PL decay times measured in the far field. This shows that the near-field PL decay and the integrated PL intensity are primarily determined by the carrier out-diffusion from under the probe. Only in the immediate vicinity of the V-defects, the near-field PL decays due to the nonradiative recombination at dislocations. The area of such enhanced nonradiative recombination is limited to just a few percent of the total QW area. This shows that recombination via dislocations and V-defects does not play a decisive role in the overall nonradiative recombination and internal quantum efficiency of polar InGaN/GaN QWs.
Highlights
One of the most striking features of InGaN/GaN quantum well (QW) light emitting diodes is their high brightness
The radiative lifetime close to the defects was longer up to several times, which is attributed to an increased electron and hole wave function separation in the QW plane
In the immediate vicinity of the V-defects, the near-field PL decays due to the nonradiative recombination at dislocations. The area of such enhanced nonradiative recombination is limited to just a few percent of the total QW area. This shows that recombination via dislocations and V-defects does not play a decisive role in the overall nonradiative recombination and internal quantum efficiency of polar InGaN/GaN QWs
Summary
One of the most striking features of InGaN/GaN quantum well (QW) light emitting diodes is their high brightness. Despite the large (∼108 cm−2) density of dislocations in polar c-plane device structures heteroepitaxially grown on foreign substrates, the external quantum efficiency of state of the art violet and blue light emitting diodes (LEDs) at low currents is about 90%.1. The first one is carrier localization at band potential fluctuations that prevents the nonradiative recombination. The second mechanism is the formation of potential barriers around dislocations that prevent carrier capture into dislocation-related nonradiative recombination centers.. The second mechanism is the formation of potential barriers around dislocations that prevent carrier capture into dislocation-related nonradiative recombination centers.5 The origin of these barriers has been assigned to thin semipolar QWs present on {1011} facets of V-defects (sometimes referred to as V-pits).. The band potential fluctuations may be induced by InGaN alloy composition and QW width variations with the alloy composition probably playing the main role. The second mechanism is the formation of potential barriers around dislocations that prevent carrier capture into dislocation-related nonradiative recombination centers. The origin of these barriers has been assigned to thin semipolar QWs present on {1011} facets of V-defects (sometimes referred to as V-pits). The V-defects form at threading dislocations and/or In-rich clusters and have a shape of inverted hexagonal pyramids.
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