In-situ TEM Study of the Crystallization of Thin Films composed of Sb 3.6 Te with Ge
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In-situ TEM Study of the Crystallization of Thin Films composed of Sb3.6Te with Ge Bart J. Kooi, Willemijn M.G. Groot, Jeff Th.M. De Hosson Dept. of Applied Physics, Materials Science Center and Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ABSTRACT This paper addresses the crystallization of amorphous Sb3.6Te films (40 nm thick) and 5 at.% Ge containing Sb3.6Te films (10, 20 and 40 nm thick) as studied using in-situ annealing in a Transmission Electron Microscope (TEM). These materials show growth-dominated crystallization, in contrast to Ge2Sb2Te5 that shows nucleation-dominated crystallization. Particularly the crystal-growth velocity in these systems is measured as a function of temperature from which the activation energy for growth can be derived. The strong effect of the 5 at.% Ge addition on the total crystallization behavior is revealed: Ge increases the crystallization temperature (from 95 to 150 0C), increases the activation energy for growth (from 1.58 to 2.37 eV), increases the nucleation rate and decreases the growth anisotropy. EXPERIMENTAL Homogeneous master alloys of Sb3.6Te and of Sb3.6Te containing 5 at.% Ge were produced by mixing the pure components (Ge:6N, Sb and Te both 5N) in evacuated quartz tubes at 750 0C. Pieces of the ingot were positioned in pockets for electron beam evaporation. As substrates 10 nm thick Si-nitride membranes were used. A Varian electron beam evaporator with thickness monitor was used for the deposition of 10, 20 and 40 nm thick amorphous films. Specimens were stored in vacuum to prevent oxidation of the films. For TEM a JEOL 2010F operating at 200 kV was used. A Gatan double tilt heating holder (model 652 with a model 901 SmartSet Hot Stage Controller) was used that employs a PID controller for accurately controlling of the temperature (within ±1 0C) and for a fast ramp rate to attain the desired final temperature without overshoot. RESULTS AND DISCUSSION Sb3.6Te The isothermal growth of Sb3.6Te crystals in a 40 nm thick amorphous films was monitored by in-situ TEM in the temperature range 85-115 0C. Two problems had to be solved before growth rates of crystallization could be measured properly. We noted that the electron beam of the TEM, that irradiates a part of the sample during heating, clearly influenced the crystallization rate. Careful analysis showed that the electron beam only increases the nucleation rate (most prominent at low temperatures just above the crystallization temperature Tc), but does not noticeably affect the growth rate during crystallization. The second problem is connected to anisotropy in growth rates. This anisotropy is a logical consequence of the growth of single crystals where different crystallographic directions exhibit different growth rates. Fortunately, most crystals showed a strong preference to nucleate with the [0001] axis of the R 3 m structure perpendicular to the film surface. These crystals have
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a triangular shape (when observed plan-vie
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