A transmission electron microscopy study on the microstructural properties of Te-based chalcogenide thin films

Abstract

Recently, phase change non-volatile memory is rising as one of the candidates for the next generation solid state memory. The phase change nonvolatile memory, so-called phase change random access memory (PRAM) uses the electrical resistance difference between an amorphous and a crystalline state while rewritable compact disk (CD)/digital versatile disk (DVD) employs the optical reflectivity difference for operating. Moreover, PRAM uses current to heat up the phase change material instead of a laser beam to detect the reflectivity difference in optical storage media. In the PRAM, different electrical currents are applied for reversible phase transition in phase change material. Higher current and fast quenching lead the material to an amorphous state with a high resistance (reset state). Medium current for longer pulse time is used to re-crystallize the region to a crystalline state, which has a low resistance (set state). A much lower current with essentially no joule heating is used to distinguish between high and low resistance states. Among many other chalcogenide amorphous semiconductors, the Ge-Sb-Te (GST) system has been studied intensively for phase change memory, especially $Ge_2Sb_Te_5$, because of fast crystallization and better data storage lifetime characteristics. However, for achievement of high density commercial memory devices, some problems have to be solved which are listed as follows; stability at room temperature, high crystallization speed, and reduction of writing currents. Up to date, many researchers have tried to work out the above problems in various ways by typical elements doping into GST, especially into the $Ge_2Sb_Te_5$ alloy or decreasing the power consumption by reducing the area of the bottom-electrode contact (BEC). However, there have not been much promising developments. Therefore, fundamental research for an alternative GST phase change material is crucial. In this thesis, a transmission electron microscopy study regardin...