## 3 Results and Discussion  
Previously, we reported that less strain in the \(u\)-GaN template layer can be achieved by replacing a conventional low temperature GaN buffer layer with a sputtered-AlN buffer layer on a \(c\)-plane sapphire substrate, where the crystalline quality of both samples was comparable [20]. Since the strain in the epitaxial layer after cooling down is greatly impacted by the strain of an adjacent layer during growth, one can thus infer that the strain and piezoelectric field in the MQWs active region is significantly influenced by the strain of the underlying layer [21; 22; 23]. In our structure, the epitaxial structure of the green LEDs is identical except for the buffer layer, thus, the strain in the \(u\)-GaN template layer can be considered a factor that limits the QCSE in the MQWs. To investigate the effect of the buffer layer on the piezoelectric field in the MQWs induced by strain, we experimentally measured the internal electric field of GaInN/GaN MQWs by means of reverse-biased ER spectroscopy. The detailed experimental set-up is described elsewhere [24]. The direction of the internal electric field induced by polarization in the GaInN/GaN MQWs grown on a \(c\)-plain sapphire substrate is opposite to that of the built-in electric field formed by a \(p\)-\(n\) junction [25]. Hence, the total internal electric field gradually decreases as the applied reverse bias increases. When the applied reverse bias reaches a compensation voltage (\(V_{C}\)), a flat band condition is achieved. That is, one can then estimate the piezoelectric field by measuring \(V_{C}\). When the applied electric field further increases, i.e., \(V>V_{C}\), a phase inversion of the ER peak signal is expected [24; 26]. In other words, investigating the peak intensity of the ER spectrum as a function of applied reverse bias, i.e., a phase inversion of the ER peak signal, is a useful way to measure \(V_{C}\)[27]. Figure 2a shows the peak intensity in the ER spectra plotted against the applied reverse bias for LED A and LED B. A fifth-order polynomial function has been used to fit the experimental data and to estimate \(V_{C}\). The \(V_{C}\) is determined by estimating a reverse-bias voltage at which point the fitting curve crosses zero [28]. Beyond this point, the peak intensity of the ER spectra turns negative, indicating a phase inversion of the ER signal. From the results in Figure 2a, the \(V_{C}\) was estimated to be \(\sim\)\(-\)38 V for LED A and \(\sim\)\(-\)46 V for LED B. Once \(V_{C}\) is determined, the piezoelectric field (\(F_{PZ}\)) can be obtained from the following equation [28]:  
\[F_{PZ}\cong\frac{2(V_{bi}-V_{C})}{w(V_{C})}, \tag{1}\]  
where \(V_{bi}\) is the built-in voltage and \(w(V_{C})\) is the depletion width at \(V_{C}\). Here, we selected 2.7 V as the \(V_{bi}\) and \(w(V_{C})\) was determined from the C-V curve. Note that \(C=\varepsilon A/w\), where \(C\) is the capacitance, \(\varepsilon\) is the dielectric constant, and \(A\) is the junction area [29]. From the C-V curve, \(w(V_{C})\) was estimated to be \(\sim\)233 nm at \(-\)38 V for LED A and \(\sim\)261 nm at \(-\)46 V for LED B. By combining the results for \(V_{C}\), \(V_{bi}\), and \(w(V_{C})\), the \(F_{PZ}\) was calculated to be \(\sim\)3.48 MV/cm for LED A and \(\sim\)3.72 MV/cm for LED B. The MQWs active layer are suffered from the compressive strain due to the difference in lattice mismatch between GaInN and GaN. Thus, measured piezo electric field in Figure 2b is caused by a compressive strain. The obtained \(F_{PZ}\) values of the samples fall between the theoretical expectationsreported by [PERSON] et al. [30] and [PERSON] et al. [31], as depicted in Figure 2b. That is, the obtained values of \(F_{PZ}\) are reasonable considering the theoretical expected values (\(\sim\)3.72 MV/cm by [PERSON] et al. and \(\sim\)3.41 MV/cm by [PERSON] et al. for 23% of GaN/GaInN strained layer).