Abstract
Phase-change random access memory (PCRAM) is one of the most important emerging non-volatile memories. The purpose of this dissertation is to study the effect of modification on new phase-change materials for applications in PCRAM. We worked out Ga-Te-Sb and Si-doped Ga-Te-Sb alloys for highly thermal stable PCRAM and Ga-Sb for multi-level storage applications. Meanwhile, Si-doped Ga-Sb alloys are suitable for high-speed operation while retain excellent thermal stability. Eutectic Ge-Al alloy for PCRAM was also evaluated. Finally, solid-state phase transformation based on Fe-C alloy is proposed. The Ga2TeSb7 films exhibit a high crystallization temperature (Tx: 253 °C), high temperature corresponding to 10-year data retention (T10y: 201 °C) and high activation energy of crystallization (Ea: 5.76 eV) hence providing outstanding thermal stability. X-ray diffraction (XRD) analysis showed that amorphous Ga2TeSb7 crystallizes into single R3m Sb-structure. The density change of Ga2TeSb7 film is 8.9 %, which is smaller than GST (9.5 %). Ga2TeSb7 memory cells demonstrate fast SET/RESET operation within 10 ns and excellent cycling endurance up to 109 cycles. Si-doped Ga2TeSb7, Si29(Ga2TeSb7)71, showed further improved thermal stability (Tx: 340 °C, T10y: 254 °C, Ea: 5.2 eV). Data retention of Si29(Ga2TeSb7)71 memory cells is beyond 1500 s at 250 °C. The memory cell can be operated using set and reset pulses at 20 μs and 10 ns, respectively, over 106 cycles. X-ray photoelectron spectroscopy (XPS) results showed the formation of Si-Te bonds as Si content is greater than 10 at.%. Based on bonding status revealed by XPS, prediction of glass-transition temperature (Tg) and Tx can thus be more dependable. Tx, T10y and Ea of Ga19Sb81 thin films are 228 °C, 156 °C and 4.7 eV, respectively. Ga19Sb81 memory cell can be operated within 10-100 ns. Most importantly, it shows multi-level capability (3 bits/cell). Tx, T10y and Ea of Si-doped Ga19Sb81 films (Si at.% : 1~14) are 236-284 °C, 171-217 °C and 5.3-9.1 eV, respectively. The memory cells of Si4(Ga19Sb81)96 film show reversible switching within 40-200 ns and that of Si9(Ga19Sb81)91 film is 100 ns. The endurance for Si4(Ga19Sb81)96 and Si9(Ga19Sb81)91 memory cells are ~103 and 104, respectively. All of our developed materials showed remarkable thermal stability than the benchmark material Ge2Sb2Te5 (Tx: 176 °C, T10y: 93 °C, Ea: 3.1 eV) while having satisfactory operation speed. They have the chance to replace NOR Flash even the DRAM technology. Ge-Al film with eutectic composition can be set and reset in 10 ns and 80 ns, respectively. Cycling times can be up to 104. Fe-Ni-C film can be set and rest within 10 ns while showing reversible switching over 105. Others such as Si-based and Sb-based films also show potential applications in PCRAM.