Yimin Chen

Enhanced thermal stability and electrical behavior of Zn-doped Sb2Te films for phase change memory application

Zn-doped Sb2Te films are proposed to present the feasibility for phase-change memory application. Zn atoms are found to significantly increase crystallization temperature of Znx(Sb2Te)1−x films and be almost linearly with the wide range of Zn-doping concentration from x = 0 to 29.67 at.%. Crystalline resistances are enhanced by Zn-doping, while keeping the large amorphous/crystalline resistance ratio almost constant at ∼105. Especially, the Zn26.07(Sb2Te)73.93 and Zn29.67(Sb2Te)70.33 films exhibit a larger resistance change, faster crystallization speed, and better thermal stability due to the formation of amorphous Zn-Sb and Zn-Te phases as well as uniform distribution of Sb2Te crystalline grains.

Crystallization behaviors of ZnxSb100−x thin films for ultralong data retention phase change memory applications

ZnxSb100−x films with low Zn content are crystallized in a single-step process with Sb, while the film (Zn/Sb ratio is about 1:1) exhibits a two-step crystallization process with ZnSb metastable and stable phases. Importantly, ZnSb films have higher crystallization temperature (~257 °C), larger crystalline activation energy (~5.63 eV), better 10 year-data-retention (~201 °C) and lower melting temperature (~500 °C).

Improved phase-change characteristics of Zn-doped amorphous Sb7Te3 films for high- speed and low-power phase change memory

This work was financially supported by the International Science & Technology Cooperation Program of China (Grant No. 2011DFA12040), the National Program on Key Basic Research Project (973 Program) (Grant No. 2012CB722703), the Natural Science Foundation of China (Grant Nos. 61008041 and 60978058), the CAS Special Grant for Postgraduate Research, Innovation and Practice, the Program for Innovative Research Team of Ningbo city (Grant No. 2009B21007), and sponsored by K. C. Wong Magna Fund in Ningbo University.

Reversibility and stability of ZnO-Sb2Te3 nanocomposite films for phase change memory applications

(ZnO)x(Sb2Te3)1-x materials with different ZnO contents have been systemically studied with an aim of finding the most suitable composition for phase change memory applications. It was found that ZnO-doping could improve thermal stability and electrical behavior of Sb2Te3 film. Sb2Te3-rich nanocrystals, surrounded by ZnO-rich amorphous phases, were observed in annealed ZnO-doped Sb2Te3 composite films, and the segregated domains exhibited a relatively uniform distribution. The ZnO-doped Sb2Te3 composite films, especially with 5.2 at% ZnO concentration were found to have higher crystallization temperature, higher crystalline resistance, and faster crystallization speed in comparison with Ge2Sb2Te5. A reversible repetitive optical switching behavior can be observed in (ZnO)5.2(Sb2Te3)94.8, confirming that the ZnO doping is responsible for a fast switching and the compound is stable with cycling. Therefore, it is promising for the applications in phase change memory devices.

Sb-rich Zn–Sb–Te phase-change materials: A candidate for the trade-off between crystallization speed and data retention

We explored the structural and physical properties of Sb-rich Zn–Sb–Te films in order to combine the good thermal stability of ZnSb with the high crystallization speed of Sb2Te. The films generally exhibit two different crystallization characteristics described as follows: amorphous → Sb2Te crystalline phase if the Zn content in the film is less than ~10 at. % and amorphous → Sb crystalline phase if the Zn content is more than ~10 at. %. Among the films, the Zn28.62Sb53.69Te17.69 film was found to show the highest crystallization temperature (~255 °C), best 10 year data retention (~165.9 °C), and shortest crystallization time of ~58 ns at 70 mW with a stable rhombohedral Sb phase; thus, it is considered promising for phase-change memory applications.

Characterization of Physical Properties for Zn-Doped Sb3Te Films

Compared with an undoped Sb3Te film, a Zn26.28(Sb3Te)73.72 film exhibits high crystallization temperature (~202 °C) and activation energy of crystallization (~3.28 eV), resulting in stability at high temperatures with an extrapolated life-time of 10 years at 130 °C. The crystalline resistance increases and the amorphous/crystalline resistance ratio is about 104, which is helpful to achieve a high On/OFF ratio. The optical band gap of a Zn26.28(Sb3Te)73.72 film decreases as it transforms from the amorphous phase to the crystalline phase. The average kinetic exponent (n) of the film is 2.44, indicating that its crystallization mechanism is of the nucleation-dominated type.

Controllable formation of nano-crystalline in Sb4Te films by Zn doping

We investigated the optical, electrical, and thermal properties of Zn-doped Sb4Te films for application in phase change memory. Together with well-documented results of Zn-doped Sb2Te3, Sb2Te, Sb7Te3, and Sb3Te systems, we plotted the ternary amorphous-phase forming-region of Zn-Sb-Te. Zn-doping increased the crystallization temperature and data retention ability of Sb4Te films. We identified the optimal composition as Zn28.6(Sb4Te)71.4, which presents reversible optical performance between the amorphous and crystalline states. The minimum time for onset crystallization was 15 ns and the required pulse width for complete crystallization was 165 ns at 70 mW. Furthermore, in all of the Zn-doped Sb-Te films, it was confirmed that Zn-doping can effectively control the growth of nano-crystalline grains and allows only a single phase to form during crystallization.

Fast crystallization and low-power amorphization of Mg-Sb-Te reversible phase-change films

We prepared Mg-doped Sb2Te films and investigated their structural, optical and electrical properties. It was found that Mg doping can increase the crystallization temperature, suppress the crystal growth and shorten the crystallization time of Sb2Te. Compared to Ge2Sb2Te5, the optimal composition of Mg21.5(Sb2Te)78.5 exhibited a higher crystallization temperature (~183 °C), larger crystallization activation energy (~3.86 eV), and better data retention ability (maintaining the amorphous state at ~121 °C for ten years), indicating improved amorphous state stability due to the formation of Mg–Sb bonds. A reversible, repetitive optical switching behavior was realized in the Mg21.5(Sb2Te)78.5 film, with a fast crystallization speed of 52 ns and a low amorphization power of 35 mW.

Fast reversible laser-induced crystallization of Sb-rich Zn-Sb-Se phase change material with excellent stability

We present a new reversible phase-change medium Sb-rich Zn-Sb-Se film, which possesses a large difference in both optical and electrical constant. The doped-ZnSb, sub-formed Zn-Se, and exhausted Sb-Se3/2 co-influence the physical properties. Typically, there is ∼105 resistance ratio and ∼14% relative reflectivity change in Zn19Sb45.7Se35.3 film when switched by electricity or laser pulses between amorphous and crystalline states. The higher Tc (∼250°C), larger Ea (∼8.57eV), better 10-yr data retention (∼200.2°C), higher crystallization resistance (∼3 × 103Ω/□ at 300°C-annealled) and relative lower melting temperature (∼550.2°C) are exhibited in Zn19Sb45.7Se35.3 film. Importantly, a short crystalline time (∼80ns at 70mW) of the ideal Zn19Sb45.7Se35.3 film can be obtained without sacrificing room-temperature stability.

X-ray photoelectron spectra of Ge-As-Te glasses

Ternary Ge10AsxTe90-x glasses with a mean coordination number (MCN) from 2.3 to 2.8 were prepared, and their physical and structural properties were characterized. It was found that, the density of the glass decreases but glass transition temperature Tg increases, and the near infrared transmission edge shifts to shorter wavelength with increasing As content. The Ge, As, and Te 3d spectra were decomposed into different doublets that correspond to different structural units and the results showed that, the numbers of Te-Te-Te trimmers and Te-Te-As(Ge) structural units decrease and finally disappear, while the perfect AsTe3/2 pyramidal and GeTe4/2 tetrahedral structure in Te-rich samples gradually transferred to defect structures including As-As and Ge-Ge homopolar bonds with increasing As concentration. No threshold behaviour can be found in the structural evolution of Ge10AsxTe90-x glasses due to a large atomic contrast between As and Te, and no any change in the chemical coordination of Te can be observed even in Te-poor glasses.Ternary Ge10AsxTe90-x glasses with a mean coordination number (MCN) from 2.3 to 2.8 were prepared, and their physical and structural properties were characterized. It was found that, the density of the glass decreases but glass transition temperature Tg increases, and the near infrared transmission edge shifts to shorter wavelength with increasing As content. The Ge, As, and Te 3d spectra were decomposed into different doublets that correspond to different structural units and the results showed that, the numbers of Te-Te-Te trimmers and Te-Te-As(Ge) structural units decrease and finally disappear, while the perfect AsTe3/2 pyramidal and GeTe4/2 tetrahedral structure in Te-rich samples gradually transferred to defect structures including As-As and Ge-Ge homopolar bonds with increasing As concentration. No threshold behaviour can be found in the structural evolution of Ge10AsxTe90-x glasses due to a large atomic contrast between As and Te, and no any change in the chemical coordination of Te can be observe...