The investigated Al0.07Ga0.93N/GaN heterostructure was grown on a thick GaN:Si layer by MOCVD. Inside the AlGaN barrier there was placed AlGaN quantum well. The structure was covered with a low temperature GaN cap layer. On the investigated structure there were performed photoreflectance (PR), electroreflectance (ER) and X-ray diffraction.
In the PR spectra there are present three features; one connected with transitions in GaN layer, second are Franz-Keldysh oscillations (FKOs) from AlGaN barrier, and the third line connected with transitions within AlGaN QW.
The ER measurement was performed for constant bias from 2 V to -7 V. In the spectrum there are also visible three features. For forward and low reverse bias there is visible line originating from AlGaN QW which due to the Stark effect is changing its position to lower energies. For low reverse bias there are visible two sets of FKOs from AlGaN barrier corresponding to two different values of electric field. This is due to the occupancy of the AlGaN QW and the presence of electric charge in AlGaN barrier. For higher reverse bias there are visible only one set of oscillations from AlGaN barrier, but in the spectrum appear FKOs from GaN layer. For this bias range the difference between calculated values of electric fields in GaN and AlGaN layers is connected with the concentration of polarization charge located in AlGaN/GaN interface which was found to be equal 2.5 × 1012 cm-2. This charge is induced by spontaneous and piezoelectric polarization present in those layers.
From the X-ray diffraction there were obtained lattice constants of AlGaN and GaN layers which allow to calculate the piezoelectric part of polarization. Combining this with ER results it was possible to obtain the difference of spontaneous polarization between GaN and AlGaN layer equal to 0.0030 C/m2. This value is in very good agreement with the theoretical predictions.