Observation of Intersubband Transition from the First to the Third Subband (e1–e3) in GaN/AlGaN Quantum Wells

We present the first observation of intersubband transition (ISBT) from the ground state (e1) to the third subband (e3) in GaN/AlGaN quantum wells (QWs). A clear peak of the ISBT e1–e3 was observed in GaN/Al<INF>0.56</INF>Ga<INF>0.44</INF>N QWs. This demonstrates that a strong built-in field due to the piezoelectric and spontaneous polarization effects exists in the structure. The polarization field results in a symmetry breaking of a QW, and thus ISBT e1–e3, which in a symmetrical square well is normally forbidden by parity. The band-to-band photoluminescence emission at energies below the GaN band gap also shows the existence of a polarization field.