Study of Disorder-Induced Localised States in Semiconductor Nanostructures

Bhuyan, Sumi (2016) Study of Disorder-Induced Localised States in Semiconductor Nanostructures. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Semiconductor-based (group III-V) materials are of course among the most researched physical systems. The work presented in this thesis represents a very small increment toward the understanding of the physics of light emission in context of some such optoelectronic materials. Much of the research in semiconductor optoelectronics has focussed on band structure engineering through epitaxy and alloying. It is usually assumed that most of the physical properties can be modeled assuming virtual crystals (where the properties of the alloy smoothly interpolate between those of the parent compounds) and defect-free abrupt interfaces. Not very surprisingly, of course, there will also be defects and various other non-idealities. What has been interesting to us is the role these defects can play in enhancing the material performance in context of light emission. This thesis has been an effort to study physically interesting (in the sense of being universal) and positive attributes of defect states. We have focussed on three material systems, AlP/GaP quantum wells, ultra-dilute GaAsN alloy and high quality GaAs quantum wells. The experimental tools included steady-state and non-linear excitation-correlation photoluminescence spectroscopy, photoluminescence excitation spectroscopy. AlP/GaP quantum wells.{The thesis presents perhaps the most comprehensive spectroscopy study of the AlP/GaP heterostructure system. This material system has largely been ignored because both GaP and AlP have an indirect bandgap and the band alignement of AlP-GaP is also staggered (type-II). Therefore one would expect very poor light emission characteristics from this material. It was thus surprising that we observed rather bright light emission in PL studies from these samples. Through fairly comprehensive temperature and power dependent PL measurements on three sets of AlP/GaP quantum wells of width 2 nm, 3 nm and 4 nm respectively, we have found that the presence of defect states (acceptor hole states) results in a considerably higher emission as the spatial confinement considerably enhances the overlap between the electron and hole wave functions in both real and momentum space. Different experimental evidences are presented to substantiate this assertion. A novel back-of-the-envelope calculation is also presented to validate our point. Ultradilute GaAsN alloy.{While it has been known for more than 15 years that introducing a small fraction of substitutional nitrogen (which differs considerably from the arsenic ions in both electronegativity and size) in GaAs leads to dramatic decrease (`bowing') in its bandgap, it has been of interest to try to understand how the electronic wave functions in the system will actually evolve as one goes from the heavy doping to the dilute alloy limit. Focussing on an ultradilute alloy, we have observed the existence of two different set of states around the conduction band which are extended and localized respectively. From detailed temperature and power dependent PL measurements we have been able to infer the position of the mobility edge. The bound (localized) states strong phonon replicas. Mobilty edge in high quality GaAs quantum wells.{Finally we have revisited the question of understanding the inhomogeneously broadened emission line in in high quality GaAs quantum well with the emphasis on trying to determine the line demarcating the quantum well-like extended states from the quantum dot-like localized states, i.e., the mobility edge. Using picosecond-resolved excitation-correlation (EC) photoluminescence spectroscopy, we find that the EC signal shows an abrupt change in sign close to the centre of the emission line signifying a different nature (extended and localized respectively) of the states above and below and gap. These measurements supplement the previous (lower energy resolution) exciton diffusion measurements to determine the mobility edge.

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