Evidence of strong electron-phonon interaction in a GaN-based quantum cascade emitter

We present a GaN-based quantum-cascade device whose inter-subband emission shows strong electron-phonon interaction. To generate the luminescence, an external electrical feld - which partially screened the internal polarization - had to be applied. In low intensity spectra, a pattern of secondary peaks occurs. Each side-peak is separated from its fundamental inter-subband transition by a characteristic phonon energy, which shifts with applied feld at the same rate as the main transition. At high intensity, there exists resonance between the 92 meV LO-phonon and the vertical inter-subband transition. A strong electrical feld of >1 MV cm!1 reduced via QCSE the transition energy from 230 meV to 80 meV. Additionally, the low active region doping necessitated large operating voltages. Besides the emission of mid-infrared radiation, the elevated voltage generated lots of phonons. At an electrical feld of 1.02 MV cm!1, the frequency-shifted inter-subband luminescence became resonant with the LO-phonon. The effects of this resonance will be discussed.