The Selective Synthesis of Molybdenum Silicides from Modulated Elemental Reactants
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layers thin enough that the multilayers would interdiffuse before nucleation occurred. If a relatively homogeneous amorphous mixture of the elements could be obtained, we could then vary the composition of the amorphous phase, by changing the thickness ratios of the initial multilayer, to determine the dependence of phase formation on composition. To fuirther test this composition dependence, we use Kissinger analysis" of non-isothermal DSC data to estimate the activation energy for nucleation and growth as a function of composition. EXPERIMENT Multilayer samples were prepared in a high vacuum deposition chamber using electron beam evaporation sources, with a background pressure of 5X10" 7 torr. The elements were deposited at a rate of 0.5 A/sec. under the control of quartz crystal thickness monitors. The multilayers were simultaneously deposited on silicon substrates and silicon wafers coated with polymethyl methacrylate (PMMA). The samples deposited on silicon substrates were used to determine the layering by low angle x-ray diffraction. Multilayers deposited on PMMA coated wafers were soaked in acetone to dissolve the polymer, and the "floated" multilayer was collected on PTFE filters. The multilayers removed from the substrate were subjected to differential scanning calorimetry (DSC) experiments, and the data used for subsequent Kissinger analysis. The evolution of phase formation was followed by high angle x-ray diffraction and electron diffraction. Compositions were determined by electron microprobe analysis. RESULTS and DISCUSSION In order to assure that a sufficiently mixed amorphous intermediate is obtained before nucleation occurs, the multilayers used in this study contain repeat layer thicknesses of 30A or less. In addition, to obtain sufficient mass for Kissinger analysis several hundred repeat layers were deposited, giving a total thickness of from 3000A-5000A. Because of the thin layers and large total thicknesses, no Bragg maxima or front back interference patterns were observed by low angle diffraction in the majority of samples. However, if the total number of repeat layers was smaller or the repeat layer thickness was larger, the expected Bragg maxima were observed. Figure I shows the low angle x-ray diffraction data collected on a 7 layer sample with an intended repeat layer thickness of 75A and composition near 3:1 Mo:Si. In the unannealed multilayer, shown in the upper curve, a Bragg maxima resulting from the modulation of electron density in the multilayer sample is clearly observable. When annealed for twenty minutes at 600'C this Bragg maxima decreases to near the intensity of the subsidiary maxima which arise from front back interference. As will be seen from subsequent data, multilayers with composition near 3:1 Mo:Si formed a crystalline compound at approximately 720'C. This result indicates that significant interdiffusion occurs below the formation temperature of compounds even in samples with thicknesses larger than those used for DSC analysis. Differential scanning calorimetry data col
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