The influence of Bi doping upon the phase change characteristics of Ge2Sb2Te5 alloys has been investigated using four-point-probe electrical resistance measurements, grazing incidence x-ray diffraction (XRD), x-ray reflectometry (XRR) and variable incident angle spectroscopic ellipsometry, a static tester and atomic force microscopy. For a Ge2Sb2Te5 alloy doped with 3% Bi, two transition temperatures are observed in the temperature dependent sheet resistance measurements at 136°C and 236°C, respectively. The evolution of structures upon annealing, investigated by XRD, reveals that the first transition is caused by the crystallization of the amorphous film to a NaCl-type structure, while the second transition is related to the transition to a hexagonal structure. The density values of 5.87±0.05gcm−3, 6.33±0.05gcm−3, and 6.41±0.05gcm−3 are measured by XRR for the film in the amorphous, NaCl-type, and hexagonal structure, respectively. Ultrafast crystallization, which is correlated with a single NaCl-structure phase and the reduced activation barrier, is demonstrated. Sufficient optical contrast is exhibited and can be correlated with the density change upon crystallization.
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Wang et al. (2004) studied this question.
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