Two-and-a-half decades of progress in mid-infrared AlGaN/GaN inter-subband light emitters

Abstract

In this paper, the physics of AlGaN/GaN inter-subband (ISB) light emitters is presented. The important milestones of this research, including the relatively slow and winding early stages of device design, fabrication and characterization, will be highlighted. Special attention will be paid to the years around the turn of the millennium, during which we had already obtained most of the presented results. After several unsuccessful attempts to “push” these measurement results through a scientific journal review in 1999 already, they could be successfully published in “Applied Physics Letters” in 2014. In retrospect, five problematic areas are identified: First, a complete ignorance of the piezo- and pyro-electric polarization fields, although they were known since 1997 already. Second, a too low n-type doping, having resulted in an inefficient current transport and overly noisy signals at the interesting wavelength. Third, a coincidental match between GaN-based ISB transition and LO-phonon showing a narrow emission peak, which was consistently described many years later only. Fourth, the above-mentioned emission, which had revealed a pronounced spectral asymmetry. And fifth, the discovery of several experimental facts, which potentially allow a more careful analysis of this asymmetric emission. These five initially neglected issues, which were between 2014 and 2026, successively understood, will in this paper be concisely presented and carefully analyzed.