Material Gain in Polar GaInN and AlGaN Quantum Wells: How to Overcome the ‘Dead’ Width for Light Emitters in These QW Systems?

Polar GaInN and AlGaN quantum wells (QWs) are widely used in light emitting diodes and laser diodes (LDs). However, the widths of such QWs are usually limited to a few nanometers in order to ensure a sufficiently large overlap between the wave functions of the ground electron and the ground hole state. By increasing the QW width we enter the area of ‘dead’ width where, the overlap of the electron and hole wave functions decreases almost to zero and the luminescence efficiency drastically deteriorates. Therefore, it is assumed that wide QWs are not suitable for light emitters and very wide (8-15 nm) QWs are not considered as a promising gain medium for LDs. Hence such QWs are very rarely studied both experimentally and theoretically. In this work the material gain is calculated for Ga_0.8In_0.2N/GaN and Al_0.8Ga_0.2N/AlN QWs with a width varying in the range of 2-15 nm. We observed that the material gain at fixed carrier concentration for these QWs drops to zero with the increase in the QW width and reaches negative values in the width range of ∼4-8 nm even for high carrier concentrations, but after exceeding a certain width to ∼8-12 nm it begins to increase rapidly and reaches the values greater than those observed for narrow QWs. This phenomenon is related to the screening of the built-in electric field by carriers, which is easier for wide QWs, and the reduction of the distance between the energy levels for electrons and holes. For the latter reason, optical transitions between higher energy states make a very significant contribution to the positive material gain.