Electroluminescence and inorganic phosphor science
Thesis or dissertation
- © 2007 Ioannis Metsios. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
The research is focussed on wide bandgap 11-VI semiconductors, and more specifically on ZnS and CdS, with applications as thin film electroluminescent displays in the expanding display device market. The science of electroluminescent semiconductors and inorganic salt precipitation is combined with a unique, thin film laser processing technique known as laser induced forward transfer or direct writing (the later terminology used mostly in the case of metal films).
Zinc sulfide and cadmium zinc sulfide films with a thickness ranging between 70 and 400 nin have been prepared in an aqueous chemical bath, on optically smooth, silica, or silica based, substrates. The chemical bath contained zinc and cadmium chlorides, thioacetamide, and urea, and the most successful combination of concentrations was found to be 2.6 mM, 56.36 mM and 167.71 mM, respectively. The solution was only slightly acidic, with a pH between 5 and 6.5, and a bath temperature of 90 to 92°C (as measured at I cm from the water surface of the bath) was found to be the most efficient. The films were doped with impurities, such as Cu, Ag and Mn in order to achieve specific luminescent characteristics.
A KrF excimer laser at 248 nin was used to transfer the films from their original substrate to a new one. The laser pulse was focussed on the chemically deposited films through the back of the transparent silica substrate. The detached film was transported across a gap of 15 µm and attached to the new substrate. A fluence between 0.5 and 0.7 jCM⁻² was found to give the best transfers, and also able to achieve multiple layer transfers over the same area of the target substrate. The transfers were performed in an argon atmosphere of 4x 10⁻² mbar pressure.
Ellipsometry and film reflectivity measurements were used to model and determine the film thickness of the chemically deposited films and the values obtained were confirmed by scanning electron microscopy. Ile latter, together with optical microscopy, atomic force microscopy and interferometry were exploited to investigate the structure of the chemically deposited and laser transferred films. It was found that a very thin ZnO film initially adhered to the substrate in the bath, on which the ZnS or CdZnS main film was attached as homogenously grown cluster beads or grown via ion by ion deposition. The homogeneously grown beads had a phase separation, containing the sulfide with the lowest Ksp in the centre, enclosed by highest Ksp sulfides, with the highest one as a shell. The phase separation between CuS and ZnS was also confirmed by extended X-ray absorption fine structure. The elemental composition of the chemically-prepared and laser-transferred films was investigated by energy dispersive X-ray analysis (EDX), inductively coupled plasma mass spectrometry and Raman microspectrometry, while the EDX and Raman methods also helped to confirm the phase separation between US and ZnS. Cathodoluminescence and photoluminescence measurements were employed to investigate the luminescence properties of the films, and the Mn doped films that were annealed at 700°C were found to be the most efficient cathode ray excited phosphors, while the Cu doped phosphors came next in efficiency, performing equally well under an electron or a UV laser beam (from a HeCd laser at 325 rim). Smaller luminescence peaks were also detected in Ag doped films. Transferred films showed similar luminescent properties to their original films, but with lower intensity. Thus the chemical bath deposition and laser transferring were successful, but the methods can be further improved.
- Department of Physical Sciences, The University of Hull
- Sands, David, 1960-; Young, Nigel A.
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