Comparison of the Crystallization Behaviors in As-Deposited and Melt-Quenched N-Doped Ge2Sb2Te5 Thin Films

Abstract

The crystallization behavior of nitrogen-doped Ge 2 Sb 2 Te 5 (NGST) thin films was examined using a static laser heating method and transmission electron microscopy. Reflectance changes were examined, and the fraction of crystalline phase present was measured on a nanosecond time scale for various laser powers and duration times. A modified laser sequence was employed to determine the crystallization of the melt-quenched amorphous phase. Compared to an as-deposited NGST, the melt-quenched NGST showed a significantly faster crystallization with a shorter incubation time. The microstructures of the melt-quenched NGST systems demonstrated the presence of a number of fine crystalline grains upon crystallization, which is far different from the case of an as-deposited NGST. The findings indicate that the grain size is smaller in the crystallized NGST with a higher nitrogen content in the melt-quenched amorphous films.Phase change behaviors in Ge 2 Sb 2 Te 5 (GST)-based materials have been a subject of great interests for their applications in phase change random access memory (PRAM) devices as well as digital versatile disc (DVD). Because GST was employed to optical applications, many research groups reported the phase change behaviors of phase change materials used for DVD or optical devices. 1-4 In electronic devices, too, recently the reversible phase transformation between crystalline and amorphous phases has been intensively investigated for PRAM device applications. 5-7 Among the findings reported are that such material properties are significantly changed by doping processes. In particularly, the addition of nitrogen is known to be effective in modifying phase change behaviors by suppressing crystallization from an amorphous state. [8][9][10][11][12][13][14] The use of such materials with appropriate properties is critically important in PRAM device operations because the nature of the crystalline phase is directly related to device parameters and consequent electrical properties.Despite the previous reports on the advantages of nitrogen doping, most of the conclusions are based upon data obtained under experimental conditions that are different from those in actual PRAM device operations. The most apparent difference is the use of the nitrogen-doped Ge 2 Sb 2 Te 5 (NGST) in an as-deposited amorphous state for the study of the mechanism of crystallization, while the melt-quenched amorphous state is the case in the actual PRAM device operations. In fact, several research groups, including us, have reported that the crystallization kinetics in the undoped GST films is significantly different between the as-deposited amorphous and melt-quenched amorphous states. 14-18 Such discrepancies can pose critical issues in understanding the crystallization kinetics and simulating device parameters in actual PRAM device operations. Therefore, for applications to PRAM devices, the phase change kinetics of NGST materials should be examined under experimental conditions that approximate actual device operations, such as the u...