Spectroscopy and microscopy of localised and delocalised excitons in InGaN-based light emitting diodes and epilayers

A comparison of the electroluminescence (EL) spectra and photocurrent (PC) spectra of commercial Nichia ‘chip-type’ diodes was undertaken in order to clarify the energy relationship between localised and delocalised states. In addition, InGaN epilayers were studied using comparative scanning electron microscopy (SEM), cathodoluminescence (CL) and energy dispersive X-ray (EDX) imaging and optical absorption. The existence of localised states is demonstrated by the so-called ‘Stokes’ shift’ between emission and absorption peaks. After clarifying the definition of this quantity we are able to establish that it scales linearly with the emission energy. An explanation of the optical results is found in a detailed picture of the microscopic nature of the InGaN alloys, with regions of relatively high In content found within the matrix, on a size scale much larger than that of a quantum dot.