Studying the Physics of Excitons in Semiconductor Quantum Heterostructures

Abstract

Semiconductor science and technology has experienced a giant leap in the last few decades due to the invention of sophisticated instruments for efficient growth of semiconductor materials. These eventually facilitated the growth of few nanometers thin, uniform epitaxial layers and heterostructures with precise control of charge carriers even at such small length scales. All these had allowed physicists to explore and manipulate quantum properties of semiconductor structures for both basic physics studies and to realize novel technological applications. Excitons are one of the most commonplace and elementary excitations happen in semiconductor systems. Contributions of these excitons become even more interesting in quantum heterostructures due to experimental detection of excitonic electro-optical effects and large non-linear optical effects even at room temperatures. Nevertheless, past studies of excitons mostly focussed on studying its optical properties. However, excitons as a composite quasi particle of negatively charged electron and positively charged hole always possess electric dipole moments. Thus probing the dielectric polarization of excitons using various opto-electrical techniques will be helpful to explore the many-body physics of semiconductor heterostructure devices. Here, in this thesis, we mainly aim to probe this inherent dipolar nature of these excitons using voltage and frequency-dependent capacitance and photocapacitance measurements in quantum heterostructure devices. We would like to mention here that these techniques of sensing dielectric polarization of excitons were not commonly used in the past. We also reiterate and explain why some population of excitons in quantum-confined heterostructure of III-V semiconductors can survive at high enough temperatures such that thermal energy kBT is more than the binding energy of excitons. This is in contrast to widespread misunderstandings of treating excitons as mostly low-temperature objects. We also demonstrate that voltage-activated rate processes can actually mimic the standard temperature-dependent activation of defect related transitions in case of excitons. Experimental observation of voltage modulated ‘negative activation energy’ is connected with the presence of excitonic bound states in quantum confined laser diodes, which is a novel approach to probe such excitonic bound states. Estimated binding energy of excitons using such methods also corroborates previously reported values for these III-V materials. Thereafter, we extend this line of investigation to probe electrical signature of excitonic Mott transition in these light emitting quantum heterostructures. Moreover, photocapacitance spectroscopy was hardly used to probe and understand the applied bias induced changes in dielectric polarization of spatially indirect excitons. We demonstrated for the first time that one can detect distinct indirect excitonic signature in the bias induced photocapacitance spectra in single barrier GaAs/AlAs/GaAs heterostructure system at room temperature. Variation of excitonic peak energy with applied bias allows us to estimate the dipole moments and polarizability of these small populations of spatially indirect excitons. Excitonic dipole moment and applied electric field were found to be oppositely oriented in these single barrier structures. Furthermore, excitonic many-body states like indirect trions, Fermi-edge singularities were also experimentally detected by us using photocapacitance spectroscopy at a relatively higher temperature of 100 K which was not possible using photoluminescence (PL) method. This certainly indicates better sensitivity of the photocapacitance based dielectric polarization measurements to sense these excitonic complexes in comparison to PL. We further study interactions between zero-dimensional (0D) and two dimensional (2D) quantum structures wherein InAs quantum dots were embedded inside AlAs barrier within a GaAs/AlAs/GaAs heterostructure. Distinct, periodic quantum oscillations with peaks and valleys in the bias-driven photocapacitance and photocurrent measurement were observed at 10 K. Correspondingly, we noticed periodic presence and absence of sharp, prominent excitonic signatures in the photocapacitance spectra in the peak and valley voltages, respectively. This observation indicates critical involvement of excitons in generating such interesting coherent oscillations. We explain these oscillations due to the involvement of coherent resonant tunnelling of mostly electrons between quantized energy levels of InAs quantum dots and the GaAs triangular quantum wells in this coupled 0D-2D quantum structure. Moreover, quantum nature of photocapacitance and its correspondence to excitonic Bohr radius is also experimentally demonstrated in the same heterostructure system. Finally, we want to emphasize that further detailed understanding of such microscopic dynamics of excitons and excitonic complexes will certainly be possible using these sensitive capacitive measurements. Therefore, we propose that capacitance and photocapacitance spectroscopy may turn out to be quite useful experimental techniques to explore the many-body physics of dipolar excitons in quantum heterostructures. We sincerely hope that research methods and observations included in this thesis will thereby be able to fulfil some of these long-cherished goals of excitonic condensed matter system by revealing how such dipolar excitonic complexes can be sensed, probed and experimentally controlled using these tools, to begin with.