Voltage modulated electro-luminescence spectroscopy to understand negative capacitance and the role of sub-bandgap states in light emitting devices

Abstract

Voltage modulated electroluminescence spectra and low frequency (≤100 kHz) impedance characteristics of red electroluminescent diodes under forward bias are investigated. Light emission under periodic voltage modulation tracks the onset of observed negative capacitance for each modulation frequency. Active participation of sub-bandgap defects including the shallower states in minority carrier recombination dynamics is sought to explain the results. The phenomenon of negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the injected minority carrier reservoir due to radiative recombinations mediated by slowly responding sub-bandgap defects. Experimentally measured variations of the in-phase component of modulated electroluminescence spectra with forward bias levels and with modulation frequencies support the dynamic influence of these sub-bandgap states in the radiative recombination process. Predominant negative sign of the in-phase component of voltage modulated electroluminescence signal further confirms the bi-molecular nature of light emission. Effect of these states on the net density of minority carriers available for radiative recombination is discussed. These sub-bandgap states can even supress the external quantum efficiency of such devices under high frequency operation commonly used in optical communication.