Preparation and characterization of laser-ablated multicomponent chalcogenide thin films
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I. INTRODUCTION
A problem in making multicomponent films from bulk starting materials is to produce the desired stoichiometry in the films. The departure from stoichiometry differs for various methods of preparation. In vacuum thermal evaporation from a single source, the rates of deposition of the various components depend on their rates of evaporation from the starting material and on their sticking coefficients at the films. Because the components may have significantly different vapor pressures, and these vapor pressures depend sensitively on temperature and on the environment of the components, one must design targets having appropriate starting compositions to end up with the desired film stoichiometry. Another approach is to use multiple vapor sources, one for each component, but this adds considerable complexity. In flash evaporation, the problem of departure from stoichiometry is often significantly reduced but other problems such as "chunk" deposition occur which, for our planned optical transmission studies, were deemed undesirable. In sputtering one has a problem analogous to evaporation in that the various components have different sputtering rates and that these rates are environment-dependent. Laser ablation1 appears to offer an attractive alternative to the above methods in solving the problem of departure from stoichiometry. It is relatively simple to set up (once you have the laser of requisite power). Moreover, the ablation process, because of the deposition of a large amount of energy over a short period at the source surface, is expected to "ablate" or remove a few monolayers from the target irrespective of the target composition. In this work we describe the preparation of thin film samples by laser ablation and compare their stoichiometries to those of the starting materials by protoninduced x-ray emission (PIXE) measurements. We also compare their properties to those of thermallyJ. Mater. Res., Vol. 5, No. 3, Mar 1990
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evaporated films studied both by PIXE and by energy dispersive x-ray emission (EDX) using an electron beam in a scanning electron microscope. II. SAMPLE PREPARATION
In our particular case, we were interested in preparing thin films of glassy Gei_xSn^Se2 for optical transmission measurements.2 For this purpose, sample homogeneity and uniformity of sample thickness are important as well as stoichiometry. The samples were ablated onto cleaned pyrex microscope slides in the 10~5 Torr vacuum chamber shown in Fig. 1. Since the ablated plume tends to be oriented perpendicularly to the target surface3 and our target surfaces were irregular, we rotated the target at 0.1 Hz to produce an average in the plume direction. Moreover, the multiple substrate
Rotating slide holder cup Motor #2
Rotating target table
Motor #1
FIG. 1. Schematic drawing of laser ablation vacuum chamber showing some details. © 1990 Materials Research Society
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D. Islam, C. E. Brient, and R. L. Cappelletti: Laser-ablated mu
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