Nondestructive spectroscopic characterisation of visible resonant cavity light emitting diode structures

Photomodulated reflectance (PR), conventional reflectance (R), and edge emission electroluminescence (EL) studies are performed at room temperature on bare as-grown wafers of CaInP/AlCaInP/AlGaAs resonant cavity light emitting diode (RCLED) structures, designed for red emission (650-670 nm). The interaction between the cavity mode (CM) and the quantum well (QW) excitonic transitions is altered by varying the angle of incidence of the probe beam, which shifts the CM wavelength, but not the QW PR features. Both features are clearly observed in the nondestructive, noncontact, PR measurements, at all angles of incidence, although only the CM feature is visible in the R spectra. A higher order QW transition is also observed in the PR. Close to the resonance between the CM and QW transitions, an enhancement of the PR signal is clearly observed. This could provide a sensitive way of qualifying RCLED wafers prior to fabrication, and estimating device yields. To refine the analysis, the PR spectra are also fitted with simplified version of a line-shape model developed in our previous studies on vertical-cavity surface-emitting laser structures. Finally, an edge emission EL method which does not need direct electrical contacts, is used as an easy way of confirming the QW ground state transition. The application of these techniques to RCLED characterisation and fabrication is discussed.