Abstract
Although novel resistive random access memory (RRAM) with 3D integration shows high potential for down scaling beyond charge-trapping flash (CTF) nonvolatile memory (NVM) at sub-25 nm nodes, the large forming voltage, high set/reset currents, poor switching uniformity and low cycling endurance are other challenges. In this dissertation, it is demonstrated ultra-low power nonvolatile RRAM devices with superior memory characteristics can be achieved by using stacked metal-insulator-metal (MIM) structures with covalent-bond germanium oxide (GeOx) and oxygen-deficient metal oxides (e.g., HfON, TaON, SrTiO3, TiO2). These low-power RRAM devices show excellent resistance switching characteristics such as large high- to low-resistance state (HRS/LRS) ratio of >100X, good data retention, fast speed of <100 ns and cycling endurance of >10^6 cycles. Our studies reveal that hopping conduction mechanism in LRS provides a large internal resistance to reach low self-compliance switching set/reset currents. Using novel stacked GeOx on metal-oxide SrTiO3 to form the cost-effective Ni/GeO/SrTiO/TaN resistive switching memory, low set power of small 4 uW, reset power of 16 pW, good data retention at 85C, fast 50 ns switching time and good 106 cycling endurance are realized. Another technique used for further saving power is to employ HfON to replace of narrow-bandgap SrTiO3, which can lower set power to sub-uW and reach ultra-low 8 fJ switching energy. The improved 125C retention than previous GeOx/SrTiO3 RRAM can be ascribed to higher activation energy to maintain stable resistance state under high-temperature retention test.Furthermore, the high performance GeOx/HfON RRAM has been demonstrated on low-cost Polyimide substrate. Only very low set poer of 4.8 uW and reset power of 1 nW are needed to reach bi-stable resistance state, which lead to a large memory window with HRS/LRS ratio of 9x10^2. Also, good retention of 85C for 10^4 sec and excellent endurance of 10^5 cycles at a fast 50 ns are obtained simutaneously. To further imporve switching stability and cycling endurance, we propose a tri-layer RRAM using nano-crystal TiO2 and TaON buffer layer, the Ni/GeOx/nc-TiO2/TaON/TaN RRAM shows the self-compliance set/reset currents, low 0.7-pJ switching energy, narrow current distribution and long 10^10 cycling endurance. Such long endurance is 5 orders of magnitude higher than the existing Flash memory at the close sub-pJ switching energy.