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dc.contributor.advisorReenaas, Turid Worren
dc.contributor.advisorKildemo, Morten
dc.contributor.advisorBrakstad, Thomas
dc.contributor.authorWaalekalv, Magnus Langøien
dc.date.created2016-06-15
dc.date.issued2016
dc.identifierntnudaim:14741
dc.identifier.urihttp://hdl.handle.net/11250/2416771
dc.description.abstractIntermediate band solar cells (IBSC) has a great potential to increase the efficiency of solar cells and reduce the costs of the generated electricity. To realize IBSC that contain one or more sub-band gap in the main band gap of the semiconductor needs to be developed. In this master thesis, non-destructive spectroscopic ellipsometry (SE) measurements are used to determine the dielectric function (DF) of the samples. The measurements are performed on different series of reduced molybdenum oxides, MoO3-x, thin films. The samples had been fabricated by either pulsed laser deposition (PLD) or spin coating (SC) before the thesis work started. To determine the DF, an optical model for the samples had to be developed. A model developed previously for another potential IB material (chromium doped ZnS) was used as a staring point, but turned out not to be suitable for the MoO3-x samples. Initially, information about the thickness and roughness was used to develop the model, and later also information about composition and resistivity, available for some of the samples, was used. Information from cross-sectional scanning electron microscopy and atomic force microscopy images were used to develop the layer structure of the model. For the PLD samples, the final model consisted of three layers and it was assumed to consist of the same material, but with varying amount of void, on top of a substrate. The bottom layer had no void, the uppermost layer (surface roughness) had 50 % void, and in the middle layer the amount of void was used as a fitting parameter between 0 and 50 %. The SC samples were more dense than the PLD samples, and thus the middle layer was not included. A single oscillator layer was developed for the material in both the PLD and SC samples. This oscillator layer accounts for contributions from band to band transitions across the band gap, as well as sub-band gap transitions caused by intermediate, or sub-stoichiometric, phases of MoO3-x and contributions from free electron excitations. The final model was used to obtain the DF for most of the samples, but not for the PLD samples grown at the highest temperatures: for these samples it is likely that the three layers consist of different material, and thus the developed model, assuming the same material in each layer, failed. For the low growth temperature series from PLD and the SC samples, reasonable dielectric functions were obtained, that matched the information about the samples obtained from other characterization techniques, and also matched DFs reported for MoO3-x found in literature. Using the finale model, fitted values for the band gap and other material properties could be obtained. Based on this, we can conclude that the low growth temperature series from PLD consist of mainly MoO3 for the the films made with high oxygen partial pressure in the vacuum chamber during deposition, with very little sub-band gap absorption, meaning that intermediate band states are most likely not present in these samples. In contrast, the low growth temperature series deposited with a low oxygen partial pressure, consist of MoO2 (i.e. highly reduced MoO3, but also of intermediate phases such as Mo4O11, leading to significant sub-band gap absorption. The samples made by SC had been annealed in a hydrogen atmosphere to form oxygen vacancies, because it was known from literature that this could result in an intermediate band. From the fitted values of the band gap and the other parameters we could confirm earlier findings that for the thickest films, only the highest annealing temperatures lead to formation of significant amounts of sub-band gap states. In contrast, for the thinnest films, that also were found to consist of MoO3, an increasing amount of sub-band gap absorption was seen with increasing annealing temperature. The sub-band gap absorption was related to intermediate phases (for instance Mo4O11) and MoO2 for the highest annealing temperature. Suggestions for further development of the model are also included in this thesis work, as well as a discussion on what deposition technique seems to be more suitable to make an IB material based on MoO3-x
dc.languageeng
dc.publisherNTNU
dc.subjectFysikk og matematikk, Teknisk fysikk
dc.titleSpectroscopic Ellipsometry of MoO3-x
dc.typeMaster thesis


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