# Performance of Nanostructures within InGaN-Based Multiquantum-Well Light-Emitting Devices
## 3 Results and Discussion
The temperature dependence of the peak energy does not follow [PERSON]'s law. The \"S-shaped\" temperature dependence of the EL peak energy is the fingerprint of the existence of localized band-tail states [24; 25]. At low temperatures, carriers are randomly distributed among the potential minima caused by the inhomogeneous potential fluctuations. As the temperature slightly increases, the carriers gain sufficient thermal energy to overcome small potential barriers and relax into lower energy tail states where the radiative recombination finally takes place. In the mid-temperature range, the blueshift of the peak energy is a result of the thermal population of higher energy states of the density of states [26; 27]. Here a transition from the localized states in the quantum well takes place. As the temperature is even higher, the blueshift behavior is smaller than the temperature-induced band-gap shrinkage, thus the peak position exhibits a redshift behavior. A similar thermal action is conducted for the sample with MQBs, as shown in Figure 3b. The amounts of blueshift of peak energy are 40.4 and 46.7 meV for the samples with the GaN barrier and with MQBs, respectively, implying a wider energy scale distribution of the band potential profile fluctuations and a stronger localization effect for the MQB sample. In InGaN/GaN MQWs, the indium compositional inhomogeneity causes potential fluctuations, which has been proposed as the cause of the localization effect [28; 29]. Analysis of the contribution of structure-related heteropotential fluctuations on the recombination mechanisms show that a compositionally inhomogeneous InGaN/GaN heterosystem can be regarded as a partially disordered structure. The nanostructures of InGaN/GaN MQWs can be viewed as disordered collections of InGaN nanocrystallites. To determine the size distribution of the nanocrystallites, we investigated the HRXRD line profile of the samples. Figure 4 shows the measured XRD patterns of the samples. The major peaks and the shoulders on the left correspond to the features of GaN and InGaN, respectively. Superlattice satellite peaks emerging from the InGaN thin layer are indicative of indium incorporation in the barriers. For the sample with multiquantum barriers, the XRD linewidths were observed to broaden, indicating crystalline randomization and interface roughness of the sample. To determine the size distribution of nanocrystallites in our samples, the XRD spectra were analyzed using the Warren-Averbach method in Fourier space [30; 31], and the calculation results are shown in Figure 5. We can see that the sample with multiquantum barriers exhibits a wider breadth of nanocrystallite size distribution. The broader nanocrystallite size distribution implies that fluctuation in the overall composition is enhanced for the sample with MQBs, coinciding with the inferences derived from the EL measurements. The effect of MQB nanostructures on the operation of the light-emitting devices is further investigated by discussing the EL efficiency of the samples. To show the variations in EL efficiency  
Figure 3: Electroluminescence spectra of InGaN/GaN multiquantum well (MQW) light-emitting devices (**a**) with conventional GaN barriers and (**b**) with multiquantum barriers in the temperature range 20–380 K. The driving current is 20 mA. at increasing temperatures, the integrated EL intensity is divided by the injection current, which is proportional to the EL external quantum efficiency. The calculated result of the samples is shown in Figure 6. With the injection current of 20 mA, it is clearly seen that the EL efficiency of the sample with MQBs is significantly higher than that of the sample with GaN barriers. The improved performance of the light-emitting device can be attributed to the enhanced fluctuation in the overall indium composition of the sample with MQBs, which causes a stronger localization effect in the active layer and a better carrier spreading of the device [32].