Spectroscopic Ellipsometry of InAs/(Al)GaAs Quantum Dots for Intermediate Band Solar Cells
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- Institutt for fysikk 
As a step towards reducing the CO2 emissions to the atmosphere, generating electricityfrom renewable energy resources instead of fossil fuels is a good contributor. Solar cellsconvert energy from sunlight directly into electricity, and they have long lifetimes andrequire little maintenance as they contain no mechanically moving parts. Even thoughefficiency has increased and prices have dropped significantly during the last decade, thereare still many steps that can be made towards making solar cells even more attractive.Intermediate band solar cells (IBSCs) have a theoretical potential of being 50 % moreefficient than the theoretically most efficient single band-gap solar cells, which is whatis used in today s commercial cells, without any major increase in cost. Many steps areyet to be taken to develop highly efficient IBSCs, and one way to increase the speed andreduce the costs of this development is to perform simulations of the cells, which can leadto a better understanding of how the cells work and which compositions provides the mostefficient cells.To simulate a solar cell, a good optical model is needed for that cell. Quantum dot (QD)IBSCs currently have no physically explainable, good optical models, and one way to createa model is by using spectroscopic ellipsometry (SE). In this thesis, InAs/(Al)GaAsQD-IBSCs are investigated using this technique. As a QD-IBSC has a relatively complexstructure, different parts of this structure are studied separately to reduce the amount ofunknowns in each measurement. This work looks into GaAs with varying doping, sampleswith two and ten layers of InAs QDs with GaAs and AlGaAs spacers, as well asfull solar cell structures. Most emphasis is put into understanding the two-layer QD samples,as these are the simplest QD structures and therefore contain the least number ofunknowns. Also, these samples have previously been measured using photoluminescencespectroscopy, atomic force microscopy, scanning electron microscopy and transmissionelectron microscopy, providing useful information for SE analysis.Different models from literature are applied in the modeling of QDs and wetting layer.The QD samples are first modeled only using bulk GaAs, oxide and surface roughness, beforea macroscopic model with bulk InAs values and an effective medium approximation(EMA) is applied, and at last an oscillator model of Lorentz oscillators is tried. This thesisfinds that the simple model outperforms the more detailed models, indicating that theassumptions made in the detailed models are not correct. Applying the oscillator modelgives some surprising results indicating a negative contribution to the pseudo-dielectricfunction (PDF) of the sample although the oscillators have positive amplitude. No explanationto this has been found, and it is not known if this is caused by a bug in thesoftware used for analysis or if it has a physical explanation. However, when combiningthe two detailed models by making an EMA of InAs and an oscillator model, the oscillatormodel contributes as expected to the PDF, outperforming the simple model for some ofthe measurements.Although the oscillator model showed some unexpected behaviour, it is still believedto be the model with most potential to make a good and physically explainable model forQDs.