## 4 Summary  
In summary, we have demonstrated the electrical and optical performances of GaInN-based green LED devices with a sputtered-AlN buffer (LED A) and a conventional low temperature GaN buffer layer (LED B). We experimentally confirmed by means of reverse bias ER spectroscopy and the smaller blue shift in the EL spectrum that there is a larger reduction in the piezoelectric field in LED A compared to that of LED B (approximately 9%). The I-V curve shows different conduction process in the low forward bias region, i.e., tunneling leakage dominates the current in LED B, which is due to the generation of TDs and V-defects that relax the strain energy. These defects are fatal to device reliability and lifetime. We obtained \(\sim\)56% and \(\sim\)51% of the peak IQE, and \(\sim\)52% and \(\sim\)38% of IQE at 50 mA, and \(\sim\)4% and \(\sim\)13% efficiency droops for LED A and LED B, respectively. We believe that the improvement in IQE and light output power is due to a reduction in carrier overflow which is limited by the strain and the piezoelectric field in MQWs. We show that the crystalline quality and strain in the active region can be improved by just replacing the buffer layer, which eventually contributes to high performance and reliability of the LED device. Therefore, we can conclude that replacing a conventional low temperature GaN buffer layer with a sputtered-AlN buffer layer is a very effective way for improving device performance, such light output power, and the IQE of green GaInN-based LEDs. Additionally, it can be expected that the reliability and lifetime could also be  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Piezoelectric Field** & **Blue Shift** & **IQE at 50 mA** & **Efficiency Droop** \\  & **[MV/cm]** & **[meV]** & **[\%]** & **[\%]** \\ \hline LED A & \(\sim\)3.48 & 52 & 52 & 4 \\ LED B & \(\sim\)3.72 & 74 & 38 & 13 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Measured data of LED A and LED B.  
Figure 5: Results of (**a**) light output power-current characteristics (L–I curve) and (**b**) internal quantum efficiency-current characteristics (IQE–I curve) for LED A and LED B on a linear scale.  
improved. We believe that further improvements can be achieved by the optimization of a sp-AIN buffer layer such as thickness and sputtering power.  
[PERSON], [PERSON]-P.H., [PERSON] and [PERSON] conducted the experiments and data analysis under the advising of S.K., [PERSON], [PERSON] and [PERSON]; [PERSON]-[PERSON] and [PERSON] mainly wrote the manuscript; S.K. read, edited and commented on the manuscript.  
This research is supported by MEXT Private University Research Branding Project and JSPS KAKENHI for Innovative Areas [No.16H06416].  
Authors would like to thank [PERSON] and [PERSON] at Hanyang University, Korea for supporting of ER measurement. This work was supported by MEXT Private University Research Branding Project, MEXT Program for research and development of next-generation semiconductor to realize energy-saving society, JSPS KAKENHI for Scientific Research A [No.15H02019], JSPS KAKENHI for Scientific Research A [No.17H01055], JSPS KAKENHI for Innovative Areas [No.16H06416], and Japan Science and Technology CREST [No.16815710].  
The authors declare no conflict of interest.