## 4 Conclusions  
Studies of the electro-optical response of PIN-diodes based on In\({}_{0.055}\)Ga\({}_{0.945}\)N/GaN and In\({}_{0.15}\)Al\({}_{0.85}\)N/GaN, In\({}_{0.18}\)Al\({}_{0.82}\)N/GaN, In\({}_{0.21}\)Al\({}_{0.79}\)N/GaN quantum well heterostructures revealed selective excitation of the three valence-split bands of the GaN barrier structure, \(E_{a}\), \(E_{b}\), and \(E_{c}\), in deep well/shallow barrier InAlN/GaN DHS, which opens new possibilities for the fine adjustment of UV emission components. The strength of the polarization field extracted from the ER spectra of the  
Figure 11: EL emission spectra of PIN-diodes based on In\({}_{\text{v}}\)Al\({}_{1\text{-v}}\)N/GaN DHS in dependence of In-fraction x = 15, 18, and 21% and AC bias-voltage of (**a**) 2 V\({}_{\text{P-P}}\) and (**b**) 8 V\({}_{\text{P-P}}\).  
In\({}_{0.21}\)Al\({}_{0.79}\)N/GaN DHS \(E_{pol}\) = 5.4 \(\pm\) 1.6 MV/cm is in excellent agreement with the field value measured by CVM \(E_{pol}\) = 5.1 \(\pm\) 0.8 MV/cm [45]. The strength and direction of the internal polarization field \(E_{pol}\) = \(-\)2.3 \(\pm\) 0.3 MV/cm determined from Franz-Keldysh oscillations, in the electro-modulated spectra of the In\({}_{0.055}\)Ga\({}_{0.945}\)N/GaN DHS under flat-barrier conditions, are in agreement, within experimental and calculation errors, with the capacitance-voltage measurement results \(E_{pol}\) = \(-\)1.2 \(\pm\) 0.4 MV/cm, which justifies the application of both optical and electrical characterization techniques for comparative studies of polarization fields in group-III nitrides. In addition, the (total) polarization field strength of In\({}_{\rm x}\)Ga\({}_{1\rm x}\)N (x = 0.055) obtained by ER and CVM and the strength \(E_{pol}\) = \(-\)0.50 \(\pm\) 0.07 MV/cm obtained by CVM on In\({}_{\rm x}\)Ga\({}_{1\rm x}\)N (x = 0.08), in a previously published work [46], are in agreement, within statistical fluctuations, with the theoretically predicted values [19; 26; 38]. As expected, the (absolute) polarization field value extracted from the FKOs of (0001) oriented In\({}_{0.055}\)Ga\({}_{0.945}\)N/GaN, in this work, is significantly higher than the value \(E_{pol}\) = \(-\)0.575 \(\pm\) 0.150 MV/cm measured by FKOs on a semipolar (1122) oriented In\({}_{0.12}\)Ga\({}_{0.88}\)N quantum well [99].  
The scientific research was funded by the Staff-Mobility Programmes of the European Union.  
This work was elaborated by author [PERSON]. [PERSON], staff member of the National Technical University of Athens (NTUA), in the framework of the Staff Mobility Agreement between the NTUA and the Technical University of Berlin (TU-BERLIN) supported by the ERASMUS Programme of the European Union. The group-III nitride based PIN-diodes were provided by TU-BERLIN and its Cooperative Institutions. NTUA staff members [PERSON] (Chemical Engineering) and [PERSON] (Electrical Engineering) are acknowledged for commenting on the PIN-diodes equivalent circuits.  
No conflict of interest in relation to the \"Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques\".