S-shaped negative differential resistance in III-Nitride blue quantum-well laser diodes grown by plasma-assisted MBE
Group III-Nitride semiconductor quantum heterostructures have revolutionized efficient visible light emitters . Even though efficient nitride light emitting diodes (LEDs) and laser diodes (LDs) are now commercially mature, there as several physical effects in them that are poorly understood. Furthermore, light emitters in the UV  and green and longer wavelengths [3, 4] remain challenging. The presence of built-in electric fields due to spontaneous and piezoelectric polarization in these quantum heterostructures leads to rather remarkable effects in the N-shaped negative differential resistance (NDR) in resonant tunnel diodes (RTDs) . Much rarer are S-shape NDR, which was reported recently in GaN tunnel switch diodes . To our surprise, we have observed strong, persistent, S-shaped NDR in GaN quantum well laser diodes at room temperature. The S-NDR in the laser diode is strong enough to drive an external circuit into sustained oscillations, and is likely caused by the strong internal polarization fields.