## 3 Results and Discussion
Therefore, we experimentally confirmed that the sputtered-AlN buffer layer of LED A leads to less strain and a lower \(F_{PZ}\) in MQWs compared to that of a low temperature GaN buffer layer in LED B. Next, to investigate the effect of the buffer layer on the electrical properties of the LEDs, we studied the I-V curves in linear and semi-log scales, see Figure 3a,b for LED A and LED B, respectively. No remarkable difference is observed in the linear scale graph (Figure 3a), while one can clearly observe in the semi-log scale graph (Figure 3b) that there is a different current component under low forward bias (from 0 V to 2 V). There is no remarkable difference in the leakage current under reverse bias (results not shown in this paper). It is well-known that threading dislocations (TDs) and V-defects induce an additional current path that encourages leakage current by tunneling processes under low forward bias [32; 33; 34]. Thus, the larger leakage current under low forward bias is possibly due to higher TD and V-defect densities in MQWs of LED B. In general, the main contribution to the generation of dislocations and defects in a GaInN/GaN layer is the strain energy, namely, the strain energy is dissipated through the  
Figure 3: Current-voltage characteristics (I–V curve) for LED A and LED B (**a**) linear scale and (**b**) in semi-log scale. Figure 2: (**a**) The peak intensity of the ER spectra and the fitting curves plotted against the applied reverse bias for LED A and LED B. (**b**) Measured piezoelectric field values (dots) and the theoretically calculated values (solid lines). formation of a dislocation [35; 36]. In this case, we infer that the larger strain energy of LED B introduces a higher density of TDs and V-defects, which eventually acts as a leakage current path and leads to tunneling leakage dominating the current flow under low forward bias, as shown in Figure 3b. Since the forward leakage current typically has a great impact on device reliability, electrostatic discharge characteristics, and ultimately device failure, one can say that a reliable LED device with a long lifetime could be expected by replacing a buffer layer with a sputtered-AlN layer [37]. Next, we investigated the optical properties by considering the EL spectrum dependence on the driving current for LED A and LED B (see Figure 4a,b). The wavelength at peak EL intensity (\(\lambda_{\text{peak}}\)) was measured as \(\sim\)536 nm for LED A and \(\sim\)543 nm for LED B at 0.5 mA, and \(\sim\)524 nm for LED A and \(\sim\)526 nm for LED B at 50 mA. The blue shifts for LED A and LED B were \(\sim\)52 meV and \(\sim\)74 meV, respectively, namely, a larger blue shift of \(\lambda_{\text{peak}}\) is observed in LED B. A blue shift that depends on driving current is typically caused by two mechanisms; (i) the QCSE and screening of the piezoelectric field owing to injected carriers [15], and (ii) the filling of the band-tail states by the injected carriers [29]. Here, we think that the QCSE is the dominant factor in this experimental result. The small blue shift in \(\lambda_{\text{peak}}\) caused by less strain indicates some mitigation of the QCSE and a reduced piezoelectric field in LED A. Thus, we believe that this is further evidence of strain relaxation in MQWs by replacing a buffer layer with a sputtered-AlN layer. Lastly, to investigate the emission performance of the LEDs, we measured the L-I and IQE-I curves of LED A and LED B, which are depicted in Figure 5a,b, respectively. Note that the measured light output power is a relative value. All light output powers of the LEDs in Figure 5a increase with increasing driving current. The light output powers of LED A and LED B are \(\sim\)2.62 and \(\sim\)1.91 at 50 mA, which indicates that the light output power of LED A is \(\sim\)1.37 times greater than that of LED B. Note that the EQEs at 50 mA are estimated as \(\sim\)15.8% and \(\sim\)11.5% for LED A and LED B, respectively. Recently, we proposed a reliable and convenient method to determine the IQE of GalnN-based LEDs [38].