The impact of trench defects in InGaN/GaN light emitting diodes and implications for the “green gap” problem

Abstract

The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a model that the “green gap” problem might relate to localized strain relaxation occurring through defects.

Radiation-induced alloy rearrangement in InxGa1−xN

“…So far, inhomogeneous indium distribution, dislocations, and surfacerelated defects have been reported. [11][12][13][14] Cation vacancies have been predicted to play a crucial role in Shockley-ReadHall recombination in wide-band-gap semiconductors. 15 Experimental studies of vacancy-type defects in thin-film III-nitrides have been extensively performed by positron annihilation spectroscopy (see Refs.…”

Effects of quantum well growth temperature on the recombination efficiency of InGaN/GaN multiple quantum wells that emit in the green and blue spectral regions

“…It has been suggested that defect formation may occur as a consequence of either the higher In content or lower QW growth temperature, which may act as a contributory factor to the "green gap." 10 In particular, it has been shown that the use of low growth temperatures can lead to an increased density of structural defects 11 and impurity incorporation in GaN 12 and InGaN. 13 There is also a growing body of evidence that point defects are introduced into InGaN layers as the indium composition is increased.…”

The recombination mechanisms leading to amplified spontaneous emission at the true-green wavelength in CH3NH3PbBr3 perovskites

“…However, beyond 520 nm, toward a true-green wavelength of 555 nm, high lattice-and thermal-mismatch cause the nucleation of threading dislocations in materials with a drastic reduction in internal quantum efficiency, which constitutes the efficiency roll-over and "green gap" in solid-state lighting. 1 An investigation into alternative materials technology reveals that high photoluminescence quantum efficiency (PLQE) of approximately 70% has been demonstrated in solutionprocessed organic-inorganic hybrid perovskite semiconductors. 2 Previous studies have successfully identified materials for light emitters in the form of vertical microcavities, 2,3 whispering gallery cavities, 4 spherical resonators, 5 and free cavity.…”