Deposition of Thin Film SrFeO 2.5+X by Pulsed Laser Ablation
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DEPOSITION OF THIN FILM SrFeO2.5x BY PULSED LASER ABLATION BRIAN W. SANDERS AND MICHAEL L. POST National Research Council of Canada, Institute for Environmental Chemistry, Ottawa, Canada, KIA 0R6. ABSTRACT Pulsed laser ablation has been used to deposit thin perovskite films based on the formula: SrFeO 2 .5+x (x = 0 to =0.5). Good quality films have been deposited on (1102) and AT cut quartz substrates at temperatures ranging from 300 to 1130 K. Films grown below 770 K showed little or no preferential crystallinity. Temperatures around 913 K produced films predominantly oriented (200), while temperatures greater than 1000 K produced films with (110) orientation. Films were grown from pellets of two compositions (SrFeO 2 .5 , and SrFeO,, 3 ). The former had a higher ablation threshold than the latter. The atmosphere during cooling had a greater effect on the film's oxygen content than the growth atmosphere. Films cooled in vacuum had the brownmillerite structure (x--0), whereas films cooled in 53.3 kPa oxygen had the cubic perovskite structure (x=0.5). INTRODUCTION The current world focus on environmentally "green" procedures produces the requirement for accurate sensing of chemical processes. Oxygen concentration, in particular, can be used for monitoring many reactions. More specifically oxygen sensing plays an important role in improving automobile combustion and emission control. In addition to the automotive market, there is also concern over ground level ozone concentrations. Any material capable of measuring ozone would benefit society. Compounds with varying oxygen stoichiometry might be suitable for both of these sensor applications. The perovskite, SrFeO 2 .5 + x (x=0 to - 0.5) has a variable oxygen content. It has been shown that, at temperatures of 573 K, the oxygen content of this compound quickly changes in response to changing oxygen partial pressures 1 ' 2 . The change in oxygen content might be monitored through a change in the optical properties of the material or if the material was deposited on AT cut quartz oxygen gain/loss could be followed by changes in the resonance frequency of the quartz crystal . In order to exploit these sensing techniques fully, it would be useful to have the material in thin film form. There are many methods of depositing thin films but one of the best for refractory metal oxides is pulsed laser ablation. This technique allows stoichiometric (in metal content) deposition of material in various atmospheres ranging from vacuum to a few tens of kPa of oxygen3 . This allows for varying oxygen stoichiometry to occur during growth. EXPERIMENTAL Figure 1 shows a schematic of the deposition system. A Lumonics KrF excimer laser (X = 248 nm) was used for the ablation. The laser pulse rate was 10 Hz. The power density on the target varied from 0.69 to 3.85 J/cm2. Targets were mounted in a copper plate. The targets used consisted of 1.25 cm diameter pressed (10 tonne) powder pellets of SrFeO 2 .5 + x (x--O or x=0.5). To create an oxygen deplete target (x--0) a pellet was annealed in
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