It is demonstrated that the strain-induced piezoelectric field in GaInN/GaN and GaN/GaInN single quantum wells leads to a Stark-shift of the optical transitions and a reduction of oscillator strength due to the spatial separation of the electron and hole wave functions. The redshift of time-resolved photoluminescence spectra and the Stokes-shift between absorption and emission can be well explained by the piezoelectric field. Secondly, this work studies asymmetric group-III-nitride quantum well

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... structures consisting of either a GaInN quantum well sandwiched by AlGaN and GaN barriers or asymmetrically doped GaN barriers. The study on the asymmetric GaInN/AlGaN/GaN quantum wells reveals that the oscillator strength critically depends on the position of the AlGaN barrier layer. The emission energy of GaInN/GaN quantum wells with doped GaN barriers shifts towards higher energy compared to that of undoped samples due to screening of the piezoelectric field, but only when the GaN cladding layer below the quantum well is doped. These results indicate a lack of inversion symmetry in GaInN/GaN single quantum wells and provide clear evidence for the existence of the piezoelectric field. Furthermore, both asymmetric structures allow us to determine consistently the direction of the piezoelectric field, which points towards the substrate. Thirdly, time-resolved study reveals multiple emission lines in multiple quantum wells. They are attributed to intra- and interwell transitions which are split by the piezoelectric field.