Electrical and Optical Investigations of the Condensed Matter Physics of Junction Diodes under Charge Carrier Injection

Abstract

In this thesis we present our studies of the dielectric properties of junction based diode structures under charge carrier injection. This regime remains less explored even after decades of research in the field of semiconductors. However, it is a necessary working condition for a set of diodes like electroluminescent diodes, producing light emission and hence important from the fundamental and application points of view. Nevertheless, the presence of a large number of free charge carriers in this regime, giving rise to different time scale processes and possibility of carrier interactions imposes experimental challenges and complexities. In such a scenario, conventional characterization techniques and understanding of junction diodes based on depletion approximation and electrostatics break down. Hence, we modified the existing techniques and developed new techniques and analyses to probe electroluminescent diode structures under charge injection. We observed counter intuitive response beyond the available understanding of these structures. We probed the connection between electrical and optical properties and its manifestations to understand carrier transport in these active devices. We used impedance spectroscopy and developed voltage modulated electroluminescence spectroscopy to study mainly AlGaInP based multi quantum well electroluminescent diodes (ELDs) for relatively low frequencies. We observed that under high injection, reactance of the diodes acquires inductive like behavior, which is perceived as negative capacitance (NC). Occurrence of NC was found to be accompanied by the onset of modulated light emission. Magnitudes of both increase in a correlated fashion with decreasing modulation frequencies. These observations are technologically important but are beyond the conventional diffusion capacitance model of the diode under forward bias. We explained that this interdependence of electrical and optical properties in case of ELDs is due to the participation of slow non-radiative defect channels in the fast radiative recombination dynamics. We theoretically demonstrated how NC can appear in the admittance response of a diode in general. We identified that the positive valued and increasing current transient xvi after the application of a forward bias step is the signature of NC. During the temperature scan of these devices, we observed that the quantum confinement affects the response under modulation in a way that is not apparent from room temperature studies. For low temperature regime, low frequency defect response causes a counterintuitive increase in modulated light emission with increasing temperature. At higher temperatures, thermally activated escape of charge carriers from the quantum well starts to affect the device response. As a result, modulated light output maximizes in a certain temperature range below room temperature. We suggest that for better efficiency of devices in high frequency (~GHz) direct modulation applications, it is desirable to reduce low frequency response which prevents charge carriers from following the fast modulation. Nevertheless, quantum well parameters should be tuned to get maximum modulated light output around working temperature of the device. We further observed that the frequency domain steady state electrical impedance response of electronic rate processes activated with dc bias reveals an intricate picture of the carrier dynamics. We demonstrated that the sudden change in frequency dependence of this bias activated response, after the onset of light emission, points towards the presence of an excitonic state. This response vanishes with increasing bias indicating Mott transition. Hence for the first time, we electrically identified the possible signatures of the formation of excitonic states and their subsequent Mott transition to electron hole plasma. We further applied our techniques and analyses to different ELDs and functionally different Silicon-based diodes. We demonstrated that our understanding can be generalized to explain the dissimilar frequency dependent reactance in these functionally different junction diodes under charge carrier injection.