## 1 Introduction  
Recent progress in growth and fabrication technology has enabled the demonstration of InGaN-based light-emitting diodes (LEDs) with maximum external quantum efficiency (EQE) over \(-\)80% in the violet-blue spectral range [1, 2]. Currently, such LEDs still suffer from the efficiency droop, i.e., substantial non-thermal efficiency reduction at high operating currents mainly attributed to non-radiative Auger recombination in InGaN active regions. The history of studying the efficiency droop, identifying its dominant mechanisms, and searching for effective approaches for the droop reduction have been discussed in recent reviews [3, 4, 5, 6]. One more problem limiting the performance of InGaN-based LEDs is the so called \"green gap\", i.e., substantial EQE decline with the emission wavelength from blue (\(\sim\)430-440 nm) towards the green/yellow (\(\sim\)500-600 nm) spectral range. Indeed, the maximum EQE values of LEDs operating in the green, \(\sim\)40-50% [7, 8], and yellow, \(\sim\)20% [9], spectral ranges are remarkably lower than that in the violet/blue one, which cannot be attributed to insufficient light extraction from the LED dice [10]. In addition, a remarkable increase in the threshold current density and decrease in the wall-plug efficiency of InGaN-based laser diodes is also observed in the same spectral range [11, 12]. These \"green gap\" manifestations in both LEDs and laser diodes imply their common nature, pointing to some internal processes in the InGaN active regions. A natural explanation for the \"green gap\" attributes the decline of LED internal quantum efficiency (IQE) to crystal quality degradation in InGaN quantum wells (QWs) with high InN molar fractions (see, e.g., [13]). The degradation may occur by either extended or point defect generation. In particular, a higher InN molar fraction necessary for long-wavelength emission produces a larger lattice constant mismatch between InGaN and the rest of the LED structure. In turn, the mismatch may evoke stress relaxation in InGaN QWs accompanied by misfit dislocation formation, if the well thickness exceeds its critical value. Estimation of the (0001)InGaN/GaN critical thickness shows that it only becomes comparable with typical QW widths in the green/yellow spectral range [14]. Point defect generation and parasitic impurity incorporation are also expected to be enhanced at high InN fractions because of the lower growth temperatures normally used for increasing the indium content in InGaN QWs [15]. The impact of crystal quality on non-radiative carrier recombination in InGaN QWs can be assessed by analyzing the Shockley-Read-Hall (SRH) recombination coefficient \(A\) introduced by the empirical ABC-model. The model considers carrier recombination in InGaN QWs to occur via three main channels: SRH defect-mediated recombination with the recombination coefficient \(A\), radiative recombination with the coefficient \(B\), and Auger recombination with the coefficient \(C\). According to the original model of SRH recombination [16], its rate is proportional to the concentration of non-radiative centers. In addition, the value of \(A\) has also been found to correlate with the density of threading dislocations frequently controlling the rate of non-radiative recombination in nitride semiconductors [17]. Generally, the recombination coefficients (RCs) can be estimated from coupled measurements of LED efficiency and differential carrier lifetime [18; 19; 20; 21; 22; 23] as a function of operating current. Figure 1 summarizes the available data on the wavelength-dependent SRH recombination coefficient \(A\). Despite a big difference in the measurements, one can see a reproducible decrease of the \(A\)-coefficient at \(\lambda\) = 420-450 nm, which is beyond the \"green gap\". At longer wavelengths, 450-515 nm, where the \"green gap\" behavior of LED efficiency is already pronounced, the measured \(A\)-coefficients are practically independent of the wavelength.