# III-Nitride Multi-Quantum-Well Light Emitting Structures with Selective Carrier Injection
## 3 Results  
The active region of monochromatic blue and green-emitting LEDs studied in this work includes three identical 4 nm-wide GaInN QWs separated by 20 nm wide regions designed with or without the IBLs. In plain-barrier design, the regions separating the active QWs consist of GaN plain barriers only, while in IBL-designed structures, these regions comprise a 12 nm wide AlGaN IBL interposed between two adjacent QW barrier layers with 4 nm wide GaN buffers on each side. Figures 1 and 2 illustrate the active region layouts. In this example study, only Al compositions and doping levels of the IBLs have been varied, while the inter-QW separation was kept the same in all simulated structures to facilitate comparison between IBL and plain-barrier designs. LED layout also includes an external p\({}^{+}\)-doped 20 nm Al\({}_{0.15}\)Ga\({}_{0.85}\)N electron blocking layer (EBL) located on the p-side of the active region. Modeling shows, however, that the external EBL, while suppressing the electron leakage, can hardly affect the uniformity of MQW-carrier distribution across the active region at LED operational currents, and therefore, provides little impact on relative QW injection [3]. QW radiative characteristics used in our example simulations are listed in Table 1. QW parameters have been calculated by independent QW simulation [5] assuming 40% values of interface polarization charges and using band structure characteristics taken from a single source [15]. We also adopted a conduction band-valence band offset ratio of 0.6/0.4 established by the analysis of carrier leakage [16]. It should be noted that in a III-nitride material system, the characteristic parameter values have large variances, especially in the quantum wells. For our illustrative study, we tried to avoid the extremities of specific models and used parameter values which represent only the main features of the material system, which are the steady decrease of radiative and Shockley-Reed-Hall (SRH) recombination rate coefficients with increasing QW emission wavelength [17]. Parameter values in Table 1 represent this trend. The increase of SRH lifetimes in QWs with higher indium compositions corresponds to the higher level of compositional fluctuations which prevent the carriers from diffusing to SRH recombination centers [18, 19]. Moderate variation of radiative rec  
Figure 2: Recombination rate profiles across the active region of (**a**) blue-emitting and (**b**) green-emitting LEDs at a nominal injection current of 50 A/cm\({}^{2}\). Dashed lines indicate the radiative recombination rates. due to efficient intra-QW polarization charge screening at high injection levels [20] paired with the possible balancing effect of compositional fluctuations in long-wavelength emitting QWs [21]. Auger coefficients for different QWs were assumed to be similar according to reference [17]. The relatively high values of QW Auger coefficients account for the enhancement of Auger processes in narrow QWs [22] which complies with the currently dominant view on the efficiency droop in III-nitrides as Auger-related phenomena [23; 24]. The example values of the layer (2D) and bulk (3D) Auger coefficients are within the range of reported experimental findings [25; 26; 27]. The increase of capture times qualitatively reflects stronger carrier overshoot of QWs with higher polarization barriers [28]. Calculated band profiles of plain-barrier and IBL-designed LEDs simulated at the same with injection levels of 50 A/cm\({}^{2}\) are compared in Figure 1. The figure subplots also show the quasi-Fermi levels for mobile carriers controlled by drift-diffusion transport and the internal potential distribution inside the active region. Apparently, the IBL active region design, at the expense of slightly higher LED operational voltage of about 0.1 V per IBL, provides a more favorable potential profile around marginal n-side QWs.