Crystallization Characteristics of Thermally-Induced Phase Transformations of Chalcogenide Alloys for Non-Volatile Memory Applications
Both Ag- and In-doped Sb2Te (AIST) and phase change materials located along the GeTe-Sb2Te3 pseudo-binary tie line have found widespread application as re-writable optical storage materials. Of this latter class of materials, the most widely used is Ge2Sb2Te5, often referred to simply as GST. The relatively low crystallization temperatures of AIST and Ge2Sb2Te5, however, are a drawback to repurposing these materials for possible new phase change random access memory (PCRAM) applications, as low crystallization temperatures can negatively impact archival stability. This is particularly true for automotive applications, where data retention for 10 years at 150°C is required. The work presented here employs temperature dependent resistivity measurements, static laser testing, and time-resolved x-ray diffraction to investigate the electrical contrast ratios, switching (i.e., crystallization) times, crystallization temperatures, and scaling behaviors of several potential new phase change memory alloys with the compositions GST-123, GST-254, GST-645, and Ge15Sb85.
In this study, static laser testing of amorphous and crystalline samples was used to determine crystallization times of all compounds/alloys researched here. The laser studies measured the change in reflectivity during (a) the crystallization of as-deposited, amorphous material and (b) the subsequent re-crystallization of amorphous material prepared from melt-quenched crystalline material. The alloys GST-123, GST-254, and Ge15Sb85 exhibited re-crystallization times comparable to that of the benchmark AIST and Ge2Sb2Te5, materials. In contrast, GST-645 exhibited extremely long switching times, suggesting it would be undesirable for PCRAM applications.
Time-resolved XRD was used to establish crystallization temperatures as a function of film thickness (2, 3, 5, 10, and 50 nm). This permitted determination of the lower limit of scalability for the phase change materials, i.e., the critical film thickness below which films would no longer crystallize. Both GST-123 and Ge15Sb85, as well as the benchmark material Ge2Sb2Te5, crystallized for all thin film thicknesses measured, while the GST-254 and GST-645 alloys crystallized at film thicknesses of 3 nm and greater. The second benchmark material, AIST, exhibited the worst scalability, crystallizing only for film thicknesses of 5 nm or greater. The crystallization temperatures of 50 nm thick films (measured at a heating rate of 1°C/s) were found to be 129°C, 236°C, 250°C, and 242°C for GST-123, GST-254, GST-645, and Ge15Sb85, respectively, compared to 156°C for Ge2Sb2Te5, and 170°C for AIST, the two benchmark materials. Thus all alloys except GST-123 showed higher crystallization temperatures than the benchmark materials, indicating better data retention properties. In addition, it was observed that the crystallization temperature increased with decreasing film thickness for all alloys studied. This inverse scaling behavior of the crystallization temperature with film thickness is beneficial as devices are miniaturized since it will lead to improved data retention properties.
In summary, the alloys GST-254 and Ge15Sb85 exhibited re-crystallization times comparable to the benchmark phase change materials coupled with substantially higher crystallization temperatures, making them promising candidates for future PCRAM applications. Of these two alloys, GST-254 displayed a much higher electrical contrast ratio and exhibited acceptable scaling characteristics, making it the leading candidate for future PCRAM applications.