## 2 Materials and Methods  
InGaN/GaN and InAlN/GaN PIN-diodes with In\({}_{0.055}\)Ga\({}_{0.945}\)N and three different In\({}_{\rm x}\)Al\({}_{1\rm x}\)-N compositions (In\({}_{0.15}\)Al\({}_{0.85}\)N, In\({}_{0.18}\)Al\({}_{0.82}\)N, and In\({}_{0.21}\)Al\({}_{0.79}\)N), in a double heterostructure (DHS) grown on (0001) sapphire substrate by metalorganic vapor-phase epitaxy (MOVPE), were fabricated and structurally characterized as described elsewhere [44; 45; 46]; the thickness of the central ternary nitride layer was determined there by high-resolution transmission electron microscopy (HRTEM). A PIN-diode with a GaN layer instead of InGaN or InAlN served as a reference sample. Schematic side-view representations of the reference and the DHS samples are illustrated in Figure 1a,b, respectively. In addition, distant and close top-view photos of the PIN-diodes are shown in Figure 1c,d, respectively. A brief description of the DHS samples is given in the following:  
The quantum well layer of nominal thickness 9.5 nm InGaN in InGaN/GaN and 4 nm InAlN in InAlN/GaN is sandwiched between two intrinsic GaN layers (i-GaN); the total thickness of the intrinsic (unintentionally doped) region is 23 nm in both the DHS and the GaN reference. The composition of the InGaN and InAlN layers was obtained from high-resolution x-ray diffraction scans (HRXRD) of the (0002) reflection [44; 45; 46]. The adjacent n- and p-GaN layer sequences contain, at the n-side, on top of the (0001) sapphire substrate: n-doped GaN:Si layer (200 nm, \(3\times 10^{18}\) cm\({}^{-3}\)), graded n-doped GaN:Si layer (100 nm, \(10^{18}\)-\(10^{17}\) cm\({}^{-3}\)), low n-doped GaN:Si layer (400 nm, \(1\times 10^{17}\) cm\({}^{-3}\)); at the p-side: p-doped GaN:Mg (150 nm, \(3\times 10^{18}\) cm\({}^{-3}\)) capped with highly p-doped GaN:Mg contact layer (10 nm, \(1\times 10^{20}\) cm\({}^{-3}\)). Following the epitaxial heterostructure growth, PIN-diodes were fabricated by standard lithography and metallization techniques using Ni/Au as p-contact and Ti/Al/Ti/Au as n-contact [47].  
The ER and EL measurements were performed using a self-developed modulation spectrometer equipped with a Xe arc spectral lamp (Oriel 75 W), two single diffraction grating monochromators, installed the first one (CVI DK240 1/4 m) in the path of the incident and the second one (SPEX 1704 1 m) in the path of the detected light beam, and a Si-diode detector. In the ER experiments, a function generator (Thurlby-Thandar TG215 2 MHz) was used to modulate the InGaN/GaN and InAlN/GaN reflectance at AC voltages of 2, 4, and 6 V\({}_{\text{P-p}}\) and frequencies of 10, 30, and 60 Hz. In the EL experiments, the function generator supplied the DC voltage of the PIN-diodes ranging from the diode threshold-voltage 1.6-2.5 V up to 4 V. The PIN-diodes were contacted on a large planar n-contact and 200 \(\times\) 200 \(\upmu\)m\({}^{2}\) semi-transparent p-contacts. The detected reflectance changes (\(\Delta\)R), in the ER experiments, and the electroluminescence emission, in the EL experiments, were amplified by lock-in amplifier (Stanford Research Systems SR530) techniques.  
Figure 1: Schematic side-view representation of PIN-diodes based on (**a**) the GaN reference and (**b**) the InGaN/GaN and InAlN/GaN DHS; (**c**) distant and (**d**) close top-view photo of the PIN-diodes.