Emanation thermal analysis in the characterization of zinc sulfide thin films prepared from different precursors
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M. Leskela,a) L. Niinisto,b) E. Nykanen, J. Rautanen, and P. Soininen Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02150 Espoo, Finland (Received 1 February 1993; accepted 27 August 1993)
Zinc sulfide thin films were prepared by the Atomic Layer Epitaxy (ALE) process from zinc acetate and zinc chloride and studied by emanation thermal analysis (ETA). The effects of different precursors and growth temperatures were evident in the ETA curves. In the films grown from zinc acetate, thermally induced changes were detected below 95 °C and above 400 °C which can plausibly be attributed to a higher amount of volatiles and to a polymorphic transition, respectively. The cubic to hexagonal transition was confirmed by DSC. Doping with terbium distorts the crystal structure and causes the peaks to become poorly discernible.
I. INTRODUCTION Thermoanalytical methods are not generally employed for the study of thin films because the common methods (TG, DTA, DSC) are not surface sensitive and, furthermore, require a relatively large sample size.1 Emanation thermal analysis (ETA) is based on the detection of emanation released from a sample after its radioactive labeling, and, thus, the method is inherently sensitive.2 As the penetration depth of labeling can be 50-100 nm, ETA is applicable for surface studies.3 The aim of the present investigation was to assess the value of ETA for the study of some technologically important thin films. The principle of the technique is described together with results obtained with zinc sulfide films. Thin films of zinc sulfide are currently being deposited on an industrial scale for use in electroluminescent flat-panel displays.4 A common dopant is divalent manganese producing bright yellow emission, but other activators, especially the rare earth ions, have been investigated as well in an attempt to produce a full-color display.5 Trivalent terbium gives rise to green emission,6 but due to the relatively high dopant concentration and the cation size mismatch, the activator does not form a simple solid solution. Codoping is also commonly applied to balance the oxidation state difference. Because normal soda glass is used as substrate, the films are processed and annealed at temperatures typically in the range 400-500 °C. Various physical methods have been used to characterize the quality and structure of the ZnS thin films. ^Permanent address: Department of Chemistry, University Helsinki, P.O. Box 6, FIN-00014 Helsinki, Finland. b 'Author to whom correspondence should be addressed.
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J. Mater. Res., Vol. 9, No. 1, Jan 1994
http://journals.cambridge.org
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These include XRD, ellipsometry, RBS and Auger spectroscopies, SEM, and TEM. 7 There is a need, however, to study also the dynamic processes taking place at elevated temperatures. Of special interest are the effects of the growth process and thermal treatment on the crystal structure and its possible polymorphic transitions as both cubic (zinc blende, sphalerite) and hexagonal
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