Current Controlled Relaxation Oscillations in Ge_2Sb_2Te_5-Based Phase Change Memory Devices

The relaxation oscillation of the phase change memory （PCM） devices based on the Ge2Sb2Te5 material is investigated by applying square current pulses. The current pulses with different amplitudes could be accurately given by the independently designed current testing system. The relaxation oscillation across the PCM device could be measured using an oscilloscope. The oscillation duration decreases with time, showing an inner link with the shrinking threshold voltage Vth. However, the relaxation oscillation would not terminate until the remaining voltage Von reaches the holding voltage Vh. This demonstrates that the relaxation oscillation might be controlled by Von. The increasing current amplitudes could only quicken the oscillation velocity but not be able to eliminate it, which indicates that the relaxation oscillation might be an inherent behavior for the PCM cell.

Thermal effect of Ge_2Sb_2Te_5 in phase change memory device

In the fabrication of phase change random access memory （PRAM） devices, high temperature thermal processes are inevitable. We investigate the thermal stability of GezSb2Te5 （GST） which is a prototypical phase change material. After high temperature process, voids of phase change material exist at the interface between Ge2Sb2Te5 and substrate in the initial open memory cell. This lower region of GezSb2Te5 is found to be a Te-rich phase change layer. Phase change memory devices are fabricated in different process conditions and examined by scanning electron microscopy and energy dispersive X-ray. It is found that hot-chuck process, nitrogen-doping process, and lower temperature inter-metal dielectric （IMD） deposition process can ease the thermal impact of line-GST PRAM cell.

Characteristics of Sb6Te4/VO2 Multilayer Thin Films for Good Stability and Ultrafast Speed Applied in Phase Change Memory

The Sb6 Te4/VO2 multilayer thin films are prepared by magnetron sputtering and the potential application in phase change memory is investigated in detail. Compared with Sb6 Te4, Sb6 Te4/VO2 multilayer composite thin films have higher phase change temperature and crystallization resistance, indicating better thermal stability and less power consumption. Also, Sb6 Te4/VO2 has a broader energy band of 1.58 eV and better data retention （125℃ for 103/）. The crystallization is suppressed by the multilayer interfaces in Sbf Te4/VO2 thin film with a smaller rms surface roughness for Sbf Te4/VO2 than monolayer Sb4Te6. The picosecond laser technology is applied to study the phase change speed. A short crystallization time of 5.21 ns is realized for the Sb6Te4 （2nm）/VO2 （8nm） thin film. The Sb6 Te4/VO2 multilayer thin film is a potential and competitive phase change material for its good thermal stability and fast phase change speed.

Structural Evolution and Phase Change Properties of C-Doped Ge_2Sb_2Te_5 Films During Heating in Air

We elucidate the importance of a capping layer on the structural evolution and phase change properties of carbondoped Ge2 Sb2 Te5（C-GST） films during heating in air. Both the C-GST films without and with a thin SiO2 capping layer（C-GST and C-GST/SiO2） are deposited for comparison. Large differences are observed between C-GST and C-GST/SiO2 films in resistance-temperature, x-ray diffraction, x-ray photoelectron spectroscopy,Raman spectra, data retention capability and optical band gap measurements. In the C-GST film, resistancetemperature measurement reveals an unusual smooth decrease in resistance above 110℃ during heating. Xray diffraction result has excluded the possibility of phase change in the C-GST film below 170℃. The x-ray photoelectron spectroscopy experimental result reveals the evolution of Te chemical valence because of the carbon oxidation during heating. Raman spectra further demonstrate that phase changes from an amorphous state to the hexagonal state occur directly during heating in the C-GST film. The quite smooth decrease in resistance is believed to be related with the formation of Te-rich GeTe4-n Gen（n = 0, 1） units above 110℃ in the C-GST film. The oxidation of carbon is harmful to the C-GST phase change properties.

Nonvolatile waveguide transmission tuning with electrically-driven ultra-small GST phase-change material

Low-power reconfigurable optical circuits are highly demanded to satisfy a variety of different applications. Conventional electro-optic and thermo-optic refractive index tuning methods in silicon photonics are not suitable for reconfiguration of optical circuits due to their high static power consumption and volatility. We propose and demonstrate a nonvolatile tuning method by utilizing the reversible phase change property of GST integrated on top of the silicon waveguide. The phase change is enabled by applying electrical pulses to the lm-sized GST active region in a sandwich structure. The experimental results show that the optical transmission of the silicon waveguide can be tuned by controlling the phase state of GST.

Si_2Sb_2Te_5 phase change material studied by an atomic force microscope nano-tip

The Si2Sb2Te5 phase change material has been studied by applying a nano-tip（30 nm in diameter） on an atomic force microscopy system.Memory switching from a high resistance state to a low resistance state has been achieved,with a resistance change of about 1000 times.In a typical I-V curve,the current increases significantly after the voltage exceeds～4.3 V.The phase transformation of a Si2Sb2Te5 film was studied in situ by means of in situ X-ray diffraction and temperature dependent resistance measurements.The thermal stability of Si2Sb2Te5 and Ge2Sb2Te5 was characterized and compared as well.

Direct observation of metastable face-centered cubic Sb2Te3 crystal

Although phase change memory technology has developed drastically in the past two decades, the cognition of the key switching materials still ignores an important member, the face-centered cubic Sb2Te3. Apart from the well-known equilibrium hexagonal Sb2Te3 crystal, we prove the metastable face-centered cubic Sb2Te3 phase does exist. Such a metastable crystal contains a large concentration of vacancies randomly occupying the cationic lattice sites. The face-centered cubic to hexagonal phase transformation of Sb2Te3, accompanied by vacancy aggregation, occurs at a quite lower temperature compared to that of Ge2Sb2Te5 alloy. We prove that the covalent-like bonds prevail in the metastable Sb2Te3 crystal, deviating from the ideal resonant features. If a proper doping technique is adopted, the metastable Sb2Te3 phase could be promising for realizing reversibly swift and low-energy phase change memory applications. Our study may offer a new insight into commercialized Ge-Sb-Te systems and help in the design of novel phase change materials to boost the performances of the phase change memorv device.