Minghua Li

Band alignment between Ta2O5 and metals for resistive random access memory electrodes engineering

Band alignment of resistive random access memory (RRAM) switching material Ta2O5 and different metal electrode materials was examined using high-resolution X-ray photoelectron spectroscopy. Schottky and hole barrier heights at the interface between electrode and Ta2O5 were obtained, where the electrodes consist of materials with low to high work function (Φm,vac from 4.06 to 5.93 eV). Effective metal work functions were extracted to study the Fermi level pinning effect and to discuss the dominant conduction mechanism. An accurate band alignment between electrodes and Ta2O5 is obtained and can be used for RRAM electrode engineering and conduction mechanism study.

Dependence of the properties of phase change random access memory on nitrogen doping concentration in Ge2Sb2Te5

The dependence of the electrical properties of Ge2Sb2Te5 on nitrogen doping concentration was investigated, which was explained based on the trends in the materials properties of nitrogen-doped Ge2Sb2Te5. The effect of nitrogen doping in Ge2Sb2Te5 on the crystallization temperature and changes upon annealing with various nitrogen concentrations were thus exploited to explain the trends. X-ray diffraction analysis corroborates the necessity to transform to the metastable face-centered-cubic phase, and showed that direct conversion to the stable hexagonal-close-packed phase which occurs at higher nitrogen concentrations could adversely affect device performance. Approaches for enhancement of thermal stability and reduction in reset current in phase change memory devices were also discussed.

Novel selector for high density non-volatile memory with ultra-low holding voltage and 10 7 on/off ratio

We present a novel selector made of doped-chalcogenide material. This selector not only achieves low holding voltage (0.2 V) and large on/off ratio (>107), but also exhibits the high on-current density (>1.6 MA/cm2) and large hysteresis window (1.2 V). Besides, excellent selector performances with ultra-low off-state leakage current (10 pA), high switching speed (<;10 ns), high endurance (>109), good thermal stability (up to 180°C) have been demonstrated. Furthermore, the device exhibits good scalability which is suitable for 3D array integrations.

Crystallization-induced stress in thin phase change films of different thicknesses

We have studied crystallization-induced stress in phase change films (Ge2Sb2Te5) as a function of thickness and with and without a capping layer, by measuring the deflection of microcantilevers. The stress is found to increase with decreasing film thickness. A thin dielectric capping layer leads to a further increase in stress compared to uncapped films. This observation can be explained by the suppression of stress relaxation in the phase change film in the presence of a capping layer. High stress will affect device performance as the size of phase change memory cells decreases.

Fermi-level pinning and charge neutrality level in nitrogen-doped Ge2Sb2Te5: Characterization and application in phase change memory devices

We study the dependence of the hole barrier height at the metal/α-Ge2Sb2Te5 interface as a function of nitrogen doping in Ge2Sb2Te5 as well as the vacuum work function of the metal. Materials parameters such as the band gap, dielectric constant, and electron affinity values of these nitrogen-doped films were also determined. All Ge2Sb2Te5 films studied in this work are amorphous. Following further physical analysis, the effective work functions of metals on nitrogen-doped Ge2Sb2Te5 films were obtained and found to differ from that of their values in vacuum. This led to the extraction of the slope parameter Sx and charge neutrality level ΦCNL which characterizes Ge2Sb2Te5. Appreciable metal Fermi-level pinning to the charge neutrality level of Ge2Sb2Te5, which is located near the valence band edge, was observed. We then demonstrate application of the extracted parameters to obtain the band alignment of α-Ge2Sb2Te5 with various other materials such as SiO2, giving good agreement with experimental results.

Band alignment between GeTe and SiO2/metals for characterization of junctions in nonvolatile resistance change elements

GeTe materials were characterized using x-ray photoelectron spectroscopy in both the amorphous and crystalline states. Valence and conduction band alignments relative to a SiO2 reference were measured to allow the GeTe band diagram, work function, and electron affinity to be inferred. Hole barrier heights was also studied for several metal/GeTe systems (metal=Al,Ni,W) to extract the charge neutrality level of these interfaces for GeTe in both the crystalline and amorphous states. Near perfect Fermi-level pinning was observed for crystalline GeTe in contact with all of the metals with much less pinning observed for amorphous GeTe.

Dependence of energy band offsets at Ge2Sb2Te5/SiO2 interface on nitrogen concentration

The electronic property for a series of nitrogen-doped Ge2Sb2Te5 phase change material was characterized using high-resolution x-ray photoelectron spectroscopy. The Te 3d5/2 and Si 2p core-level spectra as well as valence band spectra were used in the analysis. As the nitrogen content increases, the valence band offset also decreases, while that of the conduction band increases. Our results show that the valence band and conduction band offsets of nitrogen-doped Ge2Sb2Te5 on silicon oxide exhibit a linear dependence on nitrogen content in the film, for nitrogen content of up to 8.4 at. %.

TiWOx interfacial layer for current reduction and cyclability enhancement of phase change memory

TiWOx interfacial layer was proposed and implemented to act as both heater and inter-diffusion barrier for phase change memory through a complementary metal-oxide semiconductor compatible oxidization process. Significant reduction of RESET current was obtained due to more efficient Joule heating and better thermal confinement. About one order of magnitude endurance increase was achieved for the device with TiWOx due to suppression of inter-diffusion between Ge2Sb2Te5 and TiW. The change of the minimum RESET voltage against cycling was reduced by TiWOx layer with shorter RESET pulse, which would benefit device cyclability.

ZrO 2 doped GeTe for aerospace applications

Varying ZrO2 doped GeTe phase change material of atomic percent greater than 10% were deposited and characterized. It was discovered that the crystallization of amorphous doped GeTe is suppressed by the incorporation of ZrO2 at lower concentration but the crystallization improves as the concentration increases as depicted by the activation energy for different concentration. Thus it resulted in an optimum concentration for highest activation energy for better stability. ZrO2 concentration at 11% which has the highest activation energy of 3.64 eV and crystallization temperature of 210 °C with 10 years retention of 135 °C was fabricated, tested and compared with GeTe. Doped GeTe achieved power reduction of 55% as compared to GeTe and achieved endurance of 104 cycles.

X-ray Stress Evaluation in Phase Change GeSbTe Material and TiW Electrodes

Stress generation and relaxation upon Ge2Sb2Te5 (GST) crystallization were studied by X-ray diffraction technique, which allows the stress in either GST or Ti3W7 (TiW) to be evaluated independently within TiW/GST/TiW multilayer film. The GST crystallization results in tensile stress in the GST film and additional compressive stress in the TiW film, due to volume shrinkage of the GST film. Moreover, the tensile stress in the GST film is significantly increased when it is sandwiched between TiW films. Interfacial effect is proposed to attribute the dependence of stress on the capping layer thickness.