Observation of hot luminescence and slow intersubband relaxation in GaN/AlGaN multi-quantum-well structures

In this work we discuss the optical properties of GaN/AlxGa1-xN (x=0.1,0.25) multiple-quantum-well structures grown by plasma-assisted molecular-beam epitaxy. Photoluminescence lines related to the fundamental and excited electronic levels are identified, in good agreement with simulations of the electronic structure. Temperature dependent studies reveal an anomalous behavior of the photoluminescence intensity, which is the quenching of the e1-hh1 line, while the e2-hh1 and e3-hh1 lines become dominant at room temperature in samples with 11% and 25% Al in the barriers, respectively. This behavior is explained by carrier filling of e2 and e3 and by the influence of the internal electric field on the oscillator strength of the different transitions. Long intersubband relaxation times ( > 40 ps) have been measured as a consequence of the quenching of the longitudinal optical (LO)-phonon emission when the intersubband transition energy is below the LO-phonon energy.