Visible and near-infrared emission images of (In,Ga,Al)N-based 450nm emitting-diode lasers

Except for their primary emission, diode lasers frequently show emissions at lower photon energies. We present a study in which we record and analyze emission images of (In,Ga,Al)N-based 450 nm emitting diode lasers. Imaging is realized in the spectral ranges of two broad secondary emission bands, which are peaking in the yellow region at 580 nm (VIS) and in the infrared at 875 nm (IR). Both bands have their principal origin in the active region of the device. The VIS emission spectrum looks like the well-known yellow GaN-emission, but comes exclusively from the active region. It is very likely an electroluminescence that involves trapping of non-equilibrium carriers into defects located in the active region, followed by radiative recombination under emission of VIS photons. The IR emission involves also emission from the active region, but significant contributions are also observed in the substrate. The latter contribution could be generated by absorption of spontaneous primary emissions there. Moreover, we modelled emission images by raytracing. This allows the determination of absorption coefficients and refractive indexes of the active region, the unpumped epitaxial layer, and the substrate. The VIS signal from the active region proved to be proportional to the nonequilibrium carrier concentration. This makes it potentially interesting for analytical purposes, e.g., the imaging of carrier concentration profiles.