Study of opto-electrical properties of excitonic heterostructures and measurement of spatio-temporal coherence using a single interferometer

Abstract

This thesis presents opto-electrical studies of III-V based quantum heterostructures to understand the underlying many-body physics of excitons or electron-hole pairs. With the progress of advanced fabrication techniques over the years, high quality samples with cleaner interfaces are realized. That provides greater control over the dynamics of these excitons and offers an opportunity to explore the fundamental physics and utilize these for various applications. Experimental investigations of quantum correlations of optical and electrical properties of these excitons can also be intriguing. In this thesis, we also developed a modified optical interferometer that can measure such quantum correlations of excitons through optical emissions.First, we probed an III-V quantum dot (QD)-quantum well (QW) heterostructure based light emitting p-i-n diode using electroluminescence measurements under a wide range of carrier injections and at different temperatures. The initial lower current bias at ~8 Kelvin shows luminescence originates only from the QW even in the presence of QDs having lower energy levels. However, above some threshold levels of carrier injections, light emissions from QDs start. Further increase in current contributes to the exponential increase in QD emission while QW emission remains saturated. This behaviour points toward a potential barrier between QW and QD, which, we argue, can form by diffusion of the electrons from the conduction band of QW to the conduction band of QDs due to their energy difference. With the help of the study of electroluminescence vs bias current and voltage, we approximated an empirical formula that quantified the role of this barrier in QD emission. Furthermore, we separately analysed the emission from QD and QW for temporal coherence with respect to bias current and temperature. The observed decline in optical coherence of QDs with increasing bias is attributed to the non-radiative Auger recombination process, while the same reason explains the unanticipated increase of optical coherence with increasing temperatures. This study shows the importance of optical coherence measurements not only from the point of view of basic physics but also from the perspective of how these measurements can help one to achieve the desired level in applications involving light emitting devices. Realising the usefulness of optical coherence measurement in condensed matter physics of light emission, we developed a modified Michelson interferometer to measure first order spatial and temporal coherence in a single setup as our second project. We modified the standard Michelson interferometer to design a compact setup on a single platform. In this project, we tackled some of the fundamental issues of optical coherence measurement using a single optical setup which can have multiple usages. Time delays in optical coherence measurement are often provided by two independent instruments making fine and coarse delays. With our ‘curve overlap’ technique, we are able to combine the results of two instruments while keeping the fine precision over to the range coarse measurements. The instrumentational issues of mixing of optical coherence, which was usually overlooked in dual coherence measurements, are worked out with our 'temporal filtering' method, where the temporal delay is filtered out during spatial coherence results. Moreover, this setup can be placed outside at room temperature to measure spatio-temporal optical coherence of samples placed inside a cryostat. Third, we proceeded with the study of a 0D-2D system where InAs QDs are physically separated by AlAs layer from GaAs triangular quantum well (TQW). QDs are fairly uniformly spread over the investigated area of ~200 μm2. Under the reverse bias, holes present within these QDs form spatially indirect excitons with electrons of the TQW when photoexcited with light having photon energies > GaAs bandgap. This system of millions of dipolar indirect excitons responded coherently to applied bias and showed collective oscillations of average electrical polarization of these excitonic dipoles measured using photocapacitance. The oscillations are related with coherent resonant tunneling (CRT) of electron between TQW to QD. Moreover, the energy matching condition required for resonant CRT restricts the momentum distribution of electrons for direct photoexcitation of indirect excitons. This results in narrow range of momentum of electrons participating in resonant CRT. As a result, these momentum selective electrons of TQW, in parallel with resonant CRT drive these excitons to spontaneously orientate their dipoles along the direction of applied bias. This coherent collective behaviour of millions of excitons is possible if such a phase coherent macroscopic quantum states of excitons are already present due to excitonic Bose-Einstein Condensation (BEC). In this thesis, we further investigated the behaviour of excitonic condensation under varying photoexcitation intensity and we notice collective oscillations of excitonic polarization like Rabi oscillation of a two level quantum system. We eliminated the possibility of laser coherence influencing the coherent behaviour of excitons by repeating the same experiments with two independent lasers and also with a completely incoherent white light. These experiments showed polarization interference which suggest the presence of spatially correlated coherent state of excitons. We investigated the sample at different temperatures and observed such quantum coherent behaviour below 100 Kelvin. All these experimental evidences repeatedly indicate the presence of Bose-Einstein condensate formed by these millions of 0D-2D indirect excitons. In conclusion, we studied fundamental properties of direct bandgap, quantum heterostructures with opto-electronic techniques to understand the many-body physics of excitons and further improve the targeted applications using optical coherence measurements. We surveyed the basics of optical coherence measurements and offered a new instrumentation as well.