Kanika Bansal

Temperature dependent reversal of voltage modulated light emission and negative capacitance in AlGaInP based multi quantum well light emitting devices

We report a reversal in negative capacitance (NC) and voltage modulated light emission from AlGaInP based multi-quantum well (QW) electroluminescent diodes under temperature variation. Unlike monotonically increasing continuous wave light emission with decreasing temperature, modulated electroluminescence and negative capacitance first increase to a maximum and then decrease while cooling down from room temperature. Interdependence of such electronic and optical properties is understood as a competition between defect participation in radiative recombination and field assisted carrier escape from the quantum well region during temperature variation. The temperature of maximum light emission must coincide with the operating temperature of a device for better efficiency.

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

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 compo...

Dynamics of electronic transitions and frequency dependence of negative capacitance in semiconductor diodes under high forward bias

We observed qualitatively dissimilar frequency dependence of negative capacitance under high charge injection in two sets of functionally different junction diodes: III-V based light emitting and Si-based non-light emitting diodes. Using an advanced approach based on bias activated differential capacitance, we developed a generalized understanding of negative capacitance phenomenon which can be extended to any diode based device structure. We explained the observations as the mutual competition of fast and slow electronic transition rates which are different in different devices. This study can be useful in understanding the interfacial effects in semiconductor heterostructures and may lead to superior device functionality.We observed qualitatively dissimilar frequency dependence of negative capacitive response under high charge injection in two sets of junction diodes which are functionally different from each other i.e. electroluminescent diodes and non-luminescent Si-based diodes. Using the technique of bias-activated differential capacitance response, we investigated the mutual dynamics of different rate processes in different diodes. We explain these observations as the mutual competition of fast and slow electronic transition rates albeit differently. This study provides a better understanding of the physics of junction diodes operating under high charge carrier injection and may lead to superior device functionalities.

Negative activation energy and dielectric signatures of excitons and excitonic Mott transitions in quantum confined laser structures

Mostly, optical spectroscopies are used to investigate the physics of excitons, whereas their electrical evidences are hardly explored. Here, we examined a forward bias activated differential capacitance response of GaInP/AlGaInP based multi-quantum well laser diodes to trace the presence of excitons using electrical measurements. Occurrence of “negative activation energy” after light emission is understood as thermodynamical signature of steady state excitonic population under intermediate range of carrier injections. Similar corroborative results are also observed in an InGaAs/GaAs quantum dot laser structure grown by molecular beam epitaxy. With increasing biases, the measured differential capacitance response slowly vanishes. This represents gradual Mott transition of an excitonic phase into an electron-hole plasma in a GaInP/AlGaInP laser diode. This is further substantiated by more and more exponentially looking shapes of high energy tails in electroluminescence spectra with increasing forward bias, which originates from a growing non-degenerate population of free electrons and holes. Such an experimental correlation between electrical and optical properties of excitons can be used to advance the next generation excitonic devices.

Voltage Modulated Electroluminescence and Forward Bias Impedance Characteristics of Light Emitting Semiconductor Devices

Voltage modulated electroluminescence and forward bias impedance characteristics of AlGaInP based multi quantum well light emitting devices are studied in frequency (up to 1 MHz) and temperature domain. We observed inductive like reactance for low frequencies and high biases. A frequency dependent modulated light output has also been observed which also increases when modulation frequency is lowered. We explained the occurrence of inductive response and correlated modulated light output at low frequencies by considering defect participation in fast radiative recombination process. Temperature variation of reactance and modulated light output supports our understanding based on defect contribution model. However, with temperature change, dynamics of charge carriers inside the quantum well starts to affect the optical as well as electrical response. This work can be useful in improving the efficiency of light emitting devices for their applications involving direct modulation. This can also help in understanding the physics of a semiconductor junction under high carrier injection.