GeTe And SbTe Based Phase-change Materials With High Speed And Good Data Retention

In the past few decades, with the rapid development of integrated circuits process technology, the size of the semiconductor devices became smaller and smaller. The conventional semiconductor memory technologies such as SRAM, DRAM and Flash started to expose their physical limits and it was difficult for them to achieve higher density. For example, the large cell size of SRAM makes it difficult to constitute cache memory in higher density, and NOR Flash also can no longer to be scaled down below40nm. Meanwhile, DRAM and NAND Flash will encounter similar blocks as their technologies scale down below20nm.However, the market is always demanding for higher density memories, which thus encourage the development of novel memory technologies such as FRAM, MR AM, RRAM, and PCRAM. In particular, PCRAM has drawn much attention as the most promising candidates for the next-generation rewritable nonvolatile memories. In order to improve the performances of PCRAM, the present paper proposes a series of new phase-change materials and investigates the phase-change behaviors of these materials systematically using different methods. In the present paper, several innovative results have been obtained:1. Influences of N doping on the phase-change behaviors of GeTe and Ge3Te2have been evaluated. The doped materials with good data retention, high reliability, and low power consumption have been obtained.N-doped GeTe with N concentration of9.81%has been investigated. The crystallization temperature of GeTe increases from187to372℃and the10-year data retention reaches241℃after N doping. The PCRAM cell based on N-doped GeTe film shows much lower power consumption than the one based on Ge2Sb2Te5. Cycle number of～106has also been achieved for the N-doped GeTe based cell. In addition, in order to avoid the separation of Te caused by N doping, stoichiometric GeTe has been replaced by the nonstoichiometric Ge3Te2. The N-doped Ge3Te2material with different N contents has been systematically studied for thermal, optical, structural and electrical properties. It is found that N atoms bonding to Ge atoms form GeNx that piles up at the grain-boundaries. Thus the active region of the phase-change material has been restricted by GeNx and lower power consumption of the device is achieved.2. Novel Al-Sb-Te phase-change materials with high speed, good data retention and low melting point have been developed in this paper. Particularly, the device performances of the AI-Sb-Te material system have been demonstrated.Al-Sb-Te materials are obtained by introducing Al element to SbTe that shows a fast crystallization speed. Al atoms prefer to bond to Sb and Te atoms, which further increase the thermal stability of the amorphous material and lead to better data retention. Some better material compositions such as Al0.8Sb3Te and Al1.3Sb3Te are intensively studied. The best10-year data retention for the Al-Sb-Te materials is as high as131℃. The SET time of the PCRAM cell based on Al-Sb-Te material is only7ns. The Al-Sb-Te based cell also shows much lower operation voltages than the one based on Ge2Sb2Tes. Al-Sb-Te material also has good endurance (-2.5×104cycles) and an enough resistance ratio of100.3. Wx(Sb2Te)1-x phase-change materials have been proposed and demonstrated a good candidate for PCRAM applications. The outstanding performance of the material system lies in the good trade-off between programming speed and data retention.Crystallization temperature, crystalline resistance, and10-year data retention of Wx(Sb2Te)1-x increase markedly with increasing W concentration. A data retention temperature of173℃is achieved for the Wx(Sb2Te)1-x film with12%W content. The Wx(Sb2Te)1-x films crystallize quickly into a stable hexagonal phase with W uniformly distributing in the crystal lattice, which ensures faster SET speed and better operation stability for the application in practical device. PCM device based on W0.07(Sb2Te)0.93shows ultrafast SET operation (6ns) and good endurance (1.8×105cycles). W-Sb-Te material is a promising candidate for the trade-off between programming speed and data retention. 4. W-Ge-Te phase-change materials have been developed for PCRAM applications in the present paper. W-Ge-Te material shows remarkable overall performance because of its fast crystallization speed, outstanding thermal stability and reduced element segregation.In the W-Ge-Te material, W atoms bonding to Ge and Te atoms serve as substitutional impurities. During the crystallization process, diffusion of Ge and Te atoms is restricted by W atoms that have much larger atomic mass, which further leads to more uniform crystallization of the material. W atoms serve as nucleation centers and attract the surrounding Ge and Te atoms to build crystal grains quickly. W0.1(GeTe)o.9film has a10-year date retention temperature of225℃and an ultrafast crystallization time of3ns. As short as10ns width, voltage pulse can realize reversible operations for W0.1(GeTe)o.9based PCRAM cell. Good endurance (～106cycles) has also been obtained for the cell.