## 1 Introduction
This is evidence for the fact that crystal quality degradation is not the major reason for the efficiency decline in the \"green gap\". Only a dramatic rise in the \(A\)-coefficient observed in [20; 21] at \(\lambda\) > 515 nm may be attributed to intensive defect generation. On the other hand, the spectral range of the wavelength-independent \(A\)-coefficient is shown in [22] to be extended up to \(\lambda\) = 540 nm. [PERSON] et al. [24] argued that \(A\)-coefficient cannot be regarded as a measure of defect density in InGaN QWs because of its dependence on the overlap of the electron and hole wave functions. Using the experimental correlation of \(A\)-coefficient with the overlap integral from [24] and calculating the overlap integral for a typical QW width of 3 nm, the \(A\)-coefficient was predicted to decrease no greater than by a factor of 1.4 in the spectral range from 435 to 530 nm. Such a gradual variation is negligible compared to the changes in the \(A\)-coefficient actually observed at the wavelengths longer than 515 nm and shorter than 440 nm (Figure 1). So, the impact of the electron and hole wave function overlap can be disregarded while discussing the influence of defect density on the SRH recombination coefficient. Figure 1: Experimental Shockley-Read-Hall recombination coefficients as a function of emission wavelength of polar LEDs grown on different substrates. Another commonly accepted explanation associates the LED efficiency decline in the \"green gap\" with the spatial separation of electron and hole wave functions in (0001)InGaN QWs caused by polarization fields, increasing with the indium molar fraction in the alloys (see, e.g., [25]). The separation, also known as the quantum-confined Stark effect (QCSE), is expected to lower the radiative recombination coefficient (RRC) \(B\). In addition, a substantial rise in the Auger recombination rate with the polarization field is predicted theoretically in [26]. Both factors should lower the LED efficiency at longer wavelengths. In order to overcome the negative impact of polarization fields, the use of nonpolar and semipolar crystal orientations for LED fabrication has been suggested (see, e.g., [27]). Resulting, in particular, in the development of blue LEDs with extremely low efficiency droop [28], this idea did not come into action in full measure for the \"green gap\". Indeed, the comparison of numerous data on polar and nonpolar/semipolar LEDs, including those grown on native GaN substrates, demonstrated no remarkable difference between the slopes of the \"green gap\" efficiency decline in polar and nonpolar/semipolar LEDs in contrary to natural expectations [29]. Therefore, the built-in polarization field in the polar LED structures cannot be considered alone as the principal factor responsible for the \"green gap\", similar to the degradation of InGaN crystal quality. Recently, implication of carrier localization caused by composition fluctuations in InGaN alloys into the \"green gap\" problem has been demonstrated by atomistic simulations [30] and a simple semi-empirical model [22; 31]. The atomistic approach [30] considered the reduction in RRC caused by the spatially uncorrelated localization of electrons and holes inside a polar InGaN QW. Borrowing other RCs from experiment [20; 21], the authors of [30] attributed ~70% of the LED efficiency decline to QCSE and ~30% to the effect of uncorrelated electron and hole localization. The semi-empirical model developed for bulk InGaN alloys reproduced qualitatively well the experimental wavelength dependence of both RRC and the Auger recombination coefficient (ARC) from [20; 21], and qualitatively explained the anomalous rise in RRC with temperature reported in [22] but failed in the prediction of the absolute values of these RCs [31]. The latter fact was attributed to neglecting QCSE in polar QW-based LED structures. So, the relative roles of QCSE and carrier localization for InGaN-based LEDs still remain unclear despite the practical importance of this assessment.