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

TL;DR

This study investigates how sub-bandgap defect states influence electroluminescence and negative capacitance in light-emitting diodes, revealing their impact on device efficiency and carrier dynamics at high frequencies.

Contribution

It provides experimental evidence linking sub-bandgap states to negative capacitance and light emission behavior, highlighting their role in device performance.

Findings

Negative capacitance correlates with the onset of voltage modulated electroluminescence.

Sub-bandgap defect states participate actively in minority carrier recombination.

High-frequency operation can reduce external quantum efficiency due to these states.

Abstract

Voltage modulated electroluminescence spectra and low frequency ({\leq} 100 kHz) impedance characteristics of electroluminescent diodes are studied. Voltage modulated light emission tracks the onset of observed negative capacitance at a forward bias level for each modulation frequency. Active participation of sub-bandgap defect states in minority carrier recombination dynamics is sought to explain the results. Negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the 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 modulation frequencies support the dynamic influence of these 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. We also discuss how these states can actually affect the net density of minority carriers available for radiative recombination. Results indicate that these sub-bandgap states can suppress external quantum efficiency of such devices under high frequency operation commonly used in optical communication.