Structure And Electrical Properties Of GeSbTe Thin Films For PCRAM Application

Phase-change random access memory (PCRAM) is considered as one of the most promising candidates for the next generation memory due to its advantages of fast write/erase speed, low power consumption, nonvolatility, long data retention time and compatibility with CMOS process. PCRAM has attracted more and more attention and has been studied intensively in recent twenty years because of the prospect of replacement of current memories, such as SRAM, DRAM and FLASH. It may become the main memory in the future. GeSbTe chalcogenide phase-change materials have been successfully used in commercialized optical storage memories and also are one of the most important materials for PCRAM application. In this thesis, we mainly focused on the study of structures, surface morphologies and electrical properties of GeSbTe films.We deposited GeSbTe thin films by electron beam evaporation system. XPS measurement was used to determine the atomic ratio of the compound. Results of XRD measurement indicates that the microstructure of GeSbTe changed from amorphous to face-centered cubic structure initially and finally to stable hexagonal structure with annealing temperature increased. Optical absorption coefficient and optical band gap of annealed GeSbTe films were obtained by transmission measurement using spectrophotometer with wavelength ranged from 600nm to 2500nm. We calculated the optical band gap of amorphous and crystalline GeSbTe.AFM was used to characterize the surface morphologies of Ge1Sb2Te4 and Ge2Sb2Te5 thin films annealed at the same temperature but for different time. We obtained the distribution of the sizes of the crystallization grains which shows that the sizes of crystalline grains increased with the annealing time increased. The results of the AFM measurement show that at the beginning of the crystallization, crystallization speed is determined by nucleation speed and growth speed. But with the annealing time increased, crystallization speed is mainly affected by growth speed. We also got that Ge2Sb2Te5 crystallizes faster than Ge1Sb2Te4 under the same condition.The thermal-induced crystallization of GeSbTe films deposited by electron beam evaporation was studied. The films were heated in-situ at different heating rates or isothermally at different temperatures with a high temperature probe station. The resistance of the films shows two pronounced drops with temperature increased which correspond to two structural changes and we calculated the activation energy of the both phase transition. We fabricated GeSbTe/n-Si heterojuctions and current-voltage patterns of the heterojuctions were measured. The results show that both amorphous-GST/n-Si and fcc-GST/n-Si heterojuctions have typical p-n junction voltage-current characteristics. We also illustrated the band diagrams of GeSbTe/n-Si heterojuctions.