Thermal effects on spectral modulation properties of high-power light-emitting diodes

Abstract

We report on the effect of transient selfheating on the spectral modulation of electroluminescence (EL) in high-power light-emitting diodes (LEDs). In AlGaInP LEDs, which emit due to the band-to-band recombination of free carriers, the oscillation of junction temperature was found to result in that the modulation depth has a drop around the peak photon energy, an increased magnitude at lower energies, and a linear increase with photon energy at higher energies. These properties of the EL modulation spectrum can be explained by a model that takes into account the thermal modulation of band gap energy and carrier distribution function. In InGaN LEDs, almost no thermal effect on EL modulation was found around the peak photon energy and at lower energies, whereas at higher energies, the modulation depth also increases with photon energy. Such a spectrum of EL modulation depth can be understood in terms of localized carrier effect at peak photon energy and lower energies and of free carrier heating at higher energies. The frequency dependence of modulation depth at particular photon energies was shown to sensitively replicate the thermal response function of the LEDs.