Effect of thickness and carrier density on the optical polarization of Al_0.44Ga_0.56N/Al_0.55Ga_0.45N quantum well layers

The thickness and carrier density of AlGaN quantum well (QW) layers have a strong influence on the valence subband structure, and the resulting optical polarization and light extraction of ultraviolet light-emitting diodes. An ultraviolet-emitting (270-280nm) multiple quantum well heterostructure consisting of 3 periods of Al_0.44Ga_0.56N/Al _0.55Ga_0.45N with individual layer thicknesses between 2-3.2nm is studied both experimentally and theoretically. The optical polarization changes to preferentially polarized perpendicular to the QW plane as the QW thickness increases or the carrier density increases. Calculations show these trends are due to (a) a larger decrease in overlap of conduction band to light and heavy hole envelope functions compared to crystal-field split-off envelope functions, and (b) coupling between the valence subbands where higher heavy hole subbands couple to lower light hole and crystal-field split-off subbands. These changes in the valence band have a profound effect on the optical polarization, emission patterns, and eventual light extraction for ultraviolet emitters at these compositions and thicknesses, and need to be controlled to ensure high device efficiency.