Fractal quantum well heterostructures for broadband light emitters

We examine carrier relaxation and radiative recombination in AlGaAs-based near IR and AlGaInP-based visible fractal quantum well heterostructures. Through temperature dependent photoluminescence, we demonstrate that enhanced population of higher lying energy levels can be achieved by varying the thickness of the layers in the fractal heterostructure. This distribution of carriers results in room temperature emission over a relatively broad range of wavelengths: approximately 700-855 nm for AlGaAs structures and 575-650 nm for AlGaInP structures. Spectra are compared to theoretical calculations to evaluate the nonequilibrium nature of the carrier distributions. Time resolved photoluminescence measurements demonstrate an approximately linear relationship between the radiative decay time and the layer thickness of the structure. Correspondingly, integrated luminescence measurements at room temperature reveal a factor of four increase in the light output efficiency of the structure as the fractal layer thickness is increased from 50 angstrom to 400 angstrom. The applicability of these heterostructures to broadband LEDs is discussed.