Material exlorations for applications in phase-change memory

• Main Authors: Kao, Chin-Fu, 高金福
• Other Authors: Chin, Tsung-Shune
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/93023459241859760995

博士 === 國立清華大學 === 材料科學工程學系 === 98 === Disclosed in this dissertation are the exploration of new materials and test-cells prepared thereof for applications in phase-change memory (PCM). We worked out novel Ga-Te-Sb alloys for highly thermal stable PCM, and Ge-based materials for a brand-new ‘eutectic memory’. Single element PCM, being the ultimate pursuit, is also exemplified in Ge-core of core-shelled Ge-AlOx nanowires. The memory devices made of new Ga-Te-Sb compositions, Ga2Te3Sb5, Ga3Te2Sb12 and Ga3TeSb8, exhibit outstanding performances, such as lower power consumption, high speed operation (20 ns) and extreme thermal stability, as compared to our reference cells made of Ge2Sb2Te5 (GST225). The Ga-Te-Sb thin films with a high crystallization temperatures (Tx, 228-276 oC) and enhanced activation energy of crystallization (Ec, 4.3-5.76 eV evaluated by isothermal method), present outstanding thermal stability. The data-retention temperature for ten-year archival lifetime of Ga3TeSb8 films (Tx= 276 oC, Ec= 5.76 eV) reaches record high 210 oC. Ga3TeSb8 devices exhibit endurance more than 5x106 cycles at room temperature (RT), and can be normal-operative at 100 oC for more than 3x105 cycles. A lowest programming current is achieved in Ga3Te2Sb12 devices, showing 34% lower than those of GST225. Besides, the Ga2Te3Sb5 test-cells also demonstrate a capability of three-level resistance changes in a single memory unit. This makes possible a three-level per cell memory to facilitate higher memory capacity. Ge-based materials was proposed and exemplified by test-cells made of Ge-Cu and Ge-Al alloys which are fully compatible with IC processing. Crystallization mechanism of Ge-Al and Ge-Cu films belongs to highly growth-dominated (GD) and nucleation-dominated (ND) regime, respectively. The ND/GD ratio of Ge-Cu alloys is linear versus Cu content. Devices made of a Ge-Al-Cu alloy can be SET/RESET within a short pulse-width 5 ns. Although the set-reset currents of Ge-based testing-cells are 50 % higher than that of Ge2Sb2Te5 cells, the thermal stability and endurance results are much better. The endurance result of Ge-Al devices is greater than 107 cycles at RT, even though intrinsic phase separation Ge-Cu and Ge-Al alloys are observed. Due to their high Tx (~300 oC) and high Ec, Ge-Al devices display an endurance of 3x106 cycles operative at 160 oC. We synthesized core-shelled Ge-AlOx nanowires (NWs) through vapor-solid process without any catalysts. Ge-AlOx NWs comprise a core of Ge-phase with a diameter 20-40 nm. We verified, using Ge-AlOx devices prepared by focus-ion-beam, reproducible electrical switching characteristic of phase-change mechanism. Repeated operations are possible with programming DC current less than 0.1 mA, at applied voltages 4.5 V for SET and 14 V for RESET and at least 30 cycles when tests stopped.