Laser processing of solution based antimony doped tin oxide thin films

• Main Author: Abeywickrama, N. U.
• Published: Nottingham Trent University 2013
• Subjects: 621.36
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.629326

Antimony doped Tin oxide (SnO2:Sb, or ATO) is of interest as an alternative to Indium Tin Oxide (ITO) for large area optoelectronic applications. There is a particular interest in the potential for solution based coatings based on nanoparticulate suspensions of SnO2:Sb. However, solution processed films typically require a high temperature (~700⁰C) annealing step to achieve the desired electrical and optical properties. This is disadvantageous for applications that would benefit from low cost, low temperature/flexible substrates. As an alternative to conventional high temperature annealing, excimer laser processing can provide highly localized energy dissipation, and is an attractive technique to functionalise coated materials. Therefore the work presented in this thesis investigates the use of excimer laser processing to optimise the electrical and optical properties of solution deposited SnO2:Sb thin films for use in electroluminescent display devices. Thin films of SnO2:Sb were deposited using dip coating, inkjet printing and the spin coating technique. By varying the numbers of spin coatings deposited, a series of samples were prepared on Eagle XG glass substrates with different thicknesses of SnO2:Sb (ranging from 0.2 μm to 1.4 μm). The initial sheet resistance, optical transmission and crystal structure of the deposited films was studied. The films were subsequently post processed using three different annealing techniques: (i) Laser Processing: samples were laser processed in air to optimise the sheet resistance and optical transmission. Excimer (KrF, 248 nm, 20 ns pulse) irradiation of between 1 and 1000 pulses was applied at fluences in the range 20-70 mJ/cm2. (ii) Thermal Annealing: samples were thermally annealed at temperatures in the range 100⁰C - 700⁰C in air. (iii) Combined Processing: samples were initially annealed in air at temperatures in the range 100⁰C - 400⁰C and then KrF laser processed with the optimum laser fluence (60 mJ/cm2) and number of laser pulses (1000). It has been found that grain boundary scattering limits the electron conduction when films were laser processed or low temperature thermal annealed and with higher temperature thermal annealing ionized impurity scattering dominates. The sheet resistance of the as-deposited films were in the MΩ range with the optical transmission was >85% (at 550 nm). Further to KrF laser processing, the sheet resistance of the SnO2:Sb films was reduced to around 20 kΩ/sq and the optical transmission remained >75%. The films that were thermally annealed at 700⁰C showed a resultant sheet resistance of around 120 Ω/sq, and the combined processed films showed a resultant sheet resistance of 700 Ω/sq, which facilitated use as transparent electrodes in electroluminescent device fabrication. TEM studies indicated grain growth associated with the post processing for all three techniques, and Hall effect measurement confirms improvement in carrier concentration (i.e. 2 x 1019 cm3 for KrF laser processed films and 2.8 x 1020 cm3 for thermally annealed films as compared to 1.8 x 1017 cm3 for as-deposited). Luminescent studies of Alternating Current Electroluminescent (ACEL) display devices fabricated with spin coated SnO2:Sb transparent electrodes showed that the combined processing of low temperature thermal annealing (400C) followed by laser treatment gives the same order of luminescence as high temperature (700C) processed devices. The results presented demonstrated that spin coating deposition followed by combined low temperature/photonic post processing offers the potential to use solution processed SnO2:Sb layers for low temperature, low cost transparent electrode applications.