For Phase-change Memory Chalcogenide Compounds And Device Research

The recent development of PCRAM technology is fast. It only takes four years from the Intel company published the first paper about PCRAM to the Samsung company published the paper in the 2005 VLSI symposium about the 256M PCRAM test chip. In the 21^st nonvolatile semiconductor memory symposium, the Intel Company declaimed:" the 32nm nod beyond belongs to PCRAM." As PCRAM has very simple process, fast operation speed, good retention time, and good CMOS compatibility, it is potential of next generation nonvolatile memory. It makes the PCRAM an international research focus. However the PCRAM has several serious problems: high RESET current, distribution of resistivity, thermal stability and so on. With the contents divided into five sections, this thesis focuses on two main topics, i.e., reducing high RESET current and improvement of the thermal stability.Section 1 focuses on the annealing temperature and thickness effects on the structure and electric property. With an increase of annealing temperature, the first steep decrease of the resistivity is due to the crystallization of the amorphous phase into fcc phase, while the second steep decrease is primarily caused by structural transformation from "fcc" to "hex" and growth of the crystal grains. it is confirmed that the amorphous Ge₂Sb₂Te₅ film crystallizes in fcc crystal structure around 180℃, and the fcc changes to hex structure accompanying a small exothermal peak around 250℃. Meanwhile, the grain size increases markedly with the annealing temperature. The Raman spectra show the peaks of (Te₂) Sb—Sb (Te₂) and GeTe₄ both appear in amorphous GST, while only the peaks of GeTe₄ appear in crystalline state. The Raman spectra are similar in the laser annealing samples and temperature annealing samples. The resistance increase and Hall mobility increase with the film thickness decrease. The grain sizes of films have a decrease as the film thickness decreases. The AFM measurement shows there are holes in the film when film thickness decreases to 10 nm and film is porous.In section 2, N doping effects on structure and electric property are studied firstly. Studies show nitrogen doping increases crystallization temperature. Nitrogen doping extends the temperature range for fcc structure existence by increasing phase change temperature, so three relatively stable resistivitystates can be observed in resistivity plot of nitrogen doped Ge₂Sb₂Te₅, which makes Ge₂Sb₂Te₅ multi-level storage possible. The XRD show the grain is fined after N doped. In this study, the lattice parameter slightly increases with the increment of nitrogen dose. Second, the effect of Si doping on the GST is studied. The crystallization temperature increases with increasing Si dopant concentration, which indicates Si doping can improve the thermal stability of Ge₂Sb₂Te₅ amorphous state. Phase transition from fcc to hex was suppressed by more Si doping. The Si doping increase the resistivity of GST, while the ration of amorphous and crystalline resistivity does not decrease. Then the thermal stability of N and Si doped GST were studied by in situ resistance measurement. The results show the N and Si doping improved the thermal stability of GST. We have studied the electrical conduction properties of Si- and N-doped GST. An increase of the band gap has been observed with increasing dopant concentration both in Si-doped and N-doped samples. It is also found that the phase change activation energy reaches the maximum value with 4at% of Si, and then decreases as the dopant concentration increases in Si-doped samples. The phase change activation energy increases monotonically with increasing dopant concentration in N-doped samples.In section 3, the effect of film thickness on the GST thermal stability is investigated by in situ resistance measurement. The results show The crystallization temperature increases and incubation time longer from 135℃ to 148℃ with the film thickness decrease from 110nm to 11nm.The result is explained by the thermal dynamic theory.In section 4, a one-dimensional heat conduction model is developed for a phase change random access memory device. The required current level for a reset operation, which corresponds to the phase switching from a crystalline to an amorphous phase of Ge₂Sb₂Te₅, was investigated by calculating one-dimensional temperature profiles for the memory cell structure. The simulation results show current level for a reset operation can lower by increase the thickness of GST, increase resistivity of GST, and smaller bottom electrode area. The simulation results it is a method to lower the RESET current by adding a heating layer. At last, an optimized example is given.In section 5, an novel edge contact structure PCRAM cell is designed and manufactured. Two key processes of adhering of GST and the etching ofbottom electrode are investigated. The test of sample shows that N doping of GST lower the V_th of PCRAM and the smaller electrode area lower the RESET current.