Weihua Wu

TixSb100-x thin films as candidates for phase-change memory application

Tellurium-free TixSb100−x phase-change materials were systematically investigated. The amorphous-to-crystalline transition was studied by in situ resistance measurements. The crystallization temperature, crystalline resistance, and optical bandgap of TixSb100−x thin films were enhanced significantly with the increase in the titanium concentration. The phase structure and microstructure were confirmed by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM), respectively. The chemical state of the elements was detected by X-ray photoelectron spectroscopy (XPS). The phase transition speed between the amorphous and crystalline states was obtained by picosecond laser pulses. Phase change memory cells based on the Ti27Sb73 thin film were fabricated to evaluate the electrical characteristics as well. The results indicate that the TixSb100−x thin films have the great potentiality in phase change memory applications.

Study on the physical properties and structure of titanium antimony thin films for phase change memory application

The phase change behaviors of titanium antimony thin films were examined as a function of Ti concentration by in situ electrical measurement. With the increment in titanium content, the crystallization temperature enhanced, while the electrical conductivity reduced. The sharp decline in carrier density is responsible for the drop of the electrical conductivity before and after crystallization. The amorphous and polycrystalline state was confirmed by means of selected area electron diffraction. The shift of Raman modes associated with Sb upon the phase transition was observed. The surface morphology and density fluctuation were obtained from atomic force microscopy and X-ray reflectivity. The interfacial adhesion strength between the titanium antimony thin films and silicon dioxide was implemented by Nano Indenter®. Based on Arrhenius plot and Johnson–Mehl–Avrami model, the crystallization kinetics including the crystallization activation energy, the crystallization mechanism and the Avrami coefficient were investigated as well. The obtained values of Avrami indexes illustrate one-dimensional growth-dominated mechanism of titanium antimony thin films.

Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application

Superlattice-like Ge50Te50/Ge8Sb92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

Understanding the crystallization behavior and structure of titanium addition in germanium antimony phase change thin films

The effects of a titanium dopant on the phase transition behavior and crystallization mechanism of Ge8Sb92 films were systematically investigated. The crystallization behavior induced by heat was studied by in situ resistance measurements. With the incorporation of titanium atoms, both the crystallization activation energy and electrical resistance increase, resulting in a higher amorphous thermal stability and a lower of the programming energy consumption. A broadening of the optical bandgap causes the enhancement in the amorphous resistance. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy analysis reveal that a small amount of titanium addition can retard the grain growth and refine the crystal size, owing to the formation of amorphous Ge and Sb components. The shift in the Raman modes associated with Sb upon crystallization was observed. X-ray reflectivity and atomic force microscopy results illustrate that the volume fluctuation becomes smaller and the surface morphology becomes smoother after titanium doping. A reversible phase transition can be achieved by picosecond laser pulses. Phase change memory cells based on a titanium-doped Ge8Sb92 film were also fabricated to evaluate the electrical characteristics. The results indicate that the suitable incorporation of titanium in Ge8Sb92 thin films is an effective way to tune and optimize the crystallization performance for phase change memory applications.

Investigation of multilayer SnSb4/ZnSb thin films for phase change memory applications

Multilayer SnSb4/ZnSb (SS/ZS) thin films have been investigated for phase change applications. The composition [SS (4 nm)/ZS (10 nm)]4 exhibits a high crystallization temperature (T c ~ 230 °C), high data retention temperature for 10 years (T 10-yr ~ 152 °C), small density change, and low thermal conductivity. A cell based on [SS (4 nm)/ZS (10 nm)]4 achieves fast SET/RESET switching speed (~10 ns) and low reset power consumption (the energy for RESET operation = 9.6 × 10−13 J).

Fast switching and low power of superlattice-like SnSe2/Sb thin films for phase change memory application

Two non-promising phase change materials (SnSe2 and Sb) were prepared through the superlattice-like (SLL) method to explore the suitable phase change layer for phase change memory (PCM) application. The crystallization temperature, activation energy, and 10-year data retention of the SLL [SnSe2(10 nm)/Sb(2 nm)]4 ([SS(10)/S(2)]4) thin film are 185 °C, 3.03 eV, and 116 °C, respectively. The volume change of the SLL [SS(10)/S(2)]4 thin film during the crystallization is as small as 3.5%. The phase transition speed of the SLL [SS(10)/S(2)]4 thin film for crystallization is only about 11.9 ns. PCM cell based on the SLL [SS(10)/S(2)]4 thin film shows high operation speed (20 ns for SET/RESET) and lower power consumption (2.75 × 10−11 J for RESET operation).