Phase-change Materials And Technique Of Cell Element For Chalcogenide Random Access Memory

This dissertation makes up of fabrication and properties of Ge₂Sb₂Te₅ phase-change material, the effect of nitrogen, oxygen and boron doping on the structure and electrical properties of Ge₂Sb₂Te₅ film, and the fabrication technique and electrical properties of the cell element of chalcogenide random access memory (C-RAM).At first, the progresses of chalcogenide semiconductor memory are reviewed systematically. The memory principle, development history, and application potential of C-RAM are briefly introduced. The major advantages, phase-change materials, and development trend of C-RAM are discussed particularly. A detailed and comprehensive overview on the device design, realization of nano-structure for cell element, and some key issues of the fabrication techniquere are given.The Ge₂Sb₂Te₅ phase-change film with high quality is successfully fabricated, which is very suitable for C-RAM device. The structure, atomic bonding state and electrical properties of Ge₂Sb₂Te₅ are studied systematically. The crystallization of Ge₂Sb₂Te₅ thin films was investigated by using differential scanning calorimeter, X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy (XPS). It is indicated that phase transitions from amorphous state to the meta-stable face centered cubic (FCC) structure with crystallization temperature of 175℃ .and from FCC to hexagonal structure with crystallization temperature of 365 °C take place. The sheet resistance difference between amorphous and crystalline Ge₂Sb₂Te₅ film is very large and can reach six orders, which is enough for application in C-RAM.Doping nitrogen, oxygen and boron into Ge₂Sb₂Te₅ film has great effect on its structure and electrical properties. The unit cell of Ge₂Sb₂Te₅ film is distorted and phase separation even takes place due to nitrogen/oxygen/boron ion implantation. However, phase separation no longer takes place and phase transition of Ge₂Sb₂Te₅ film from FCC to hexagonal structure was suppressed by nitrogen/oxygen/boron implantation when the dose is large and the annealing temperature is high. From the results of XRD, Raman spectra, and XPS measurements, N atom bonds to Ge atom forming Ge₃N₄ and O atom bonds to Ge atom forming GeO₂. Two obvious steps were observed in the sheet resistance-temperature curve of Ge₂Sb₂Te₅ film with a minor nitrogen or boron implant dose, which is very...