Dongning Yao

Carbon-doped Ge2Sb2Te5 phase change material: A candidate for high-density phase change memory application

Carbon-doped Ge2Sb2Te5 material is proposed for high-density phase-change memories. The carbon doping effects on electrical and structural properties of Ge2Sb2Te5 are studied by in situ resistance and x-ray diffraction measurements as well as optical spectroscopy. C atoms are found to significantly enhance the thermal stability of amorphous Ge2Sb2Te5 by increasing the degree of disorder of the amorphous phase. The reversible electrical switching capability of the phase-change memory cells is improved in terms of power consumption with carbon addition. The endurance of ∼2.1 × 104 cycles suggests that C-doped Ge2Sb2Te5 film will be a potential phase-change material for high-density storage application.

Uniform Ti-doped Sb2Te3 materials for high-speed phase change memory applications

Compared with pure Sb2Te3, Ti0.32Sb2Te3 (TST) phase change material has larger resistance ratio, higher crystallization temperature and better thermal stability. The sharp decrease in mobility is responsible for the increasing amorphous and crystalline sheet resistance. The uniform crystalline structure of TST film is very benefit for the endurance characteristic. The Set and Reset operation voltages for TST-based phase change memory device are much lower than those of conventional Ge2Sb2Te5-based one. Remarkably, the device presents extremely rapid Set operation speed (∼6 ns). Furthermore, up to 1 × 106 programming cycles are obtained with stable Set and Reset resistances.

Al1.3Sb3Te material for phase change memory application

Comparing with Ge2Sb2Te5, Al1.3Sb3Te is proved to be a promising candidate for phase-change memory use because of its higher crystallization temperature (∼210 °C), larger crystallization activation energy (3.32 eV), and better data retention ability (124 °C for 10 yr). Furthermore, Al1.3Sb3Te shows fast phase change speed and crystallizes into a uniformly embedded crystal structure. As short as 10 ns width, voltage pulse can realize reversible operations for Al1.3Sb3Te based phase-change memory cell. Moreover, phase-change memory cell based on Al1.3Sb3Te material also has good endurance (∼2.5 × 104 cycles) and an enough resistance ratio of ∼102.

Investigation of CuSb4Te2 alloy for high-speed phase change random access memory applications

The thermal stability of amorphous Sb2Te film can be significantly improved by the addition of Cu. CuSb4Te2 alloy is considered to be a potential candidate for phase change random access memory (PCRAM), as evidenced by a higher crystallization temperature, a better data retention ability, and a faster switching speed in comparison with those of Ge2Sb2Te5. A reversible switching between set and reset states can be realized by an electric pulse as short as 7 ns for CuSb4Te2-based PCRAM cell. In addition, CuSb4Te2 shows endurance up to 1.5 × 105 cycles with a resistance ratio of about two orders of magnitude.

Ga-Sb-Se material for low-power phase change memory

In this paper, Te-free Ga-Sb-Se material is considered to be a storage medium for phase change memory. Compared with Ge2Sb2Te5, Ga1Sb6Se3 exhibits a better thermal stability, which leads to a brilliant performance for data retention. Ga1Sb6Se3-based phase change memory cell shows reversible switching between reset and set states with a resistance ratio of two orders of magnitude. The minimum reset/set voltages are significantly lower than those of Ge2Sb2Te5-based one. Meanwhile, Ga1Sb6Se3 film possesses a faster switching speed than Ge2Sb2Te5. Thermal simulation confirms the improvement of cell performance originating from the low thermal conductivity and low melting point of Ga1Sb6Se3.

Sb-rich Si-Sb-Te phase change material for multilevel data storage: The degree of disorder in the crystalline state

The phase change memory with monolayer chalcogenide film (Si18Sb52Te30) is investigated for the feasibility of multilevel data storage. During the annealing of the film, a relatively stable intermediate resistance can be obtained at an appropriate heating rate. The transmission electron microscopy in situ analysis reveals a conversion of crystallization mechanism from nucleation to crystal growth, which leads a continuous reduction in the degree of disorder. It is indicated from the electrical properties of the devices that the fall edge of the voltage pulse is the critical factor that determines a reliable triple-level resistance state of the phase change memory cell.

High thermal stability and low density variation of carbon-doped Ge2Sb2Te5 for phase-change memory application

Carbon-doped Ge2Sb2Te5 (GSTC) film has been experimentally studied as a thermal stable material for high temperature applications. The 10-yr data retention temperature is remarkably increased through C doping. Furthermore, GSTC films have better interface properties after annealing at 410 °C for 30 min. The density variation of GSTC film is significantly improved, which is very important to device reliability. X-ray photoelectron spectroscopy results reveal that the thermal stability enhancement of GSTC film attributes to the forming of C-Ge, C-Sb, and C-Te bonds. The perfect thermal stability makes GSTC materials a good candidate in the actual production of phase-change memory.

Si2Sb2Te6 Phase Change Material for Low-Power Phase Change Memory Application

Novel Si2Sb2Te6 phase change material for low-power chalcogenide random access memory is prepared by sputtering of Si2Sb2Te6 alloy target. The motion of Te atom was confirmed from the X-ray diffraction patterns. Phase change random access memory device based on the Si2Sb2Te6 alloy was successfully fabricated. Threshold current (Ith) of the device is only 1 µA. SET and RESET voltage pulse values are 1.2 and 2.4 V, respectively, with voltage pulse width of 150 ns. Up to 5×105 cycles of endurance had been achieved with a resistance ratio of 100.

The effect of titanium doping on the structure and phase change characteristics of Sb4Te

As a growth-dominated phase change material, Sb4Te (ST) has fast crystallization speed while thermal stability is very poor, which makes it unsuitable for application in phase change random access memory (PCRAM). After doping Ti, the crystallization temperature is greatly improved to 210.33 °C, which is much higher than that of conventional Ge2Sb2Te5 (∼150 °C), and the melting point is reduced to 540.27 °C. In addition, grain size of crystalline Ti-doped Sb4Te (TST) film is significantly decreased to nanoscale. Ti atom is believed to occupy the lattice site of Sb atom in TST. With good thermal stability, TST-based PCRAM cell also has fast crystallization rate of 6 ns. Furthermore, the energy consumption is also lower than that of Ge2Sb2Te5-based one. Endurance of exceeding 2E5 cycles is obtained with a resistance ratio of one order of magnitude. Therefore, Ti doping seems to be a good way to solve the contradiction between thermal stability and fast crystallization speed of Sb-Te alloys.

Performance improvement of phase-change memory cell using AlSb3Te and atomic layer deposition TiO2 buffer layer

A phase change memory (PCM) cell with atomic layer deposition titanium dioxide bottom heating layer is investigated. The crystalline titanium dioxide heating layer promotes the temperature rise in the AlSb3Te layer which causes the reduction in the reset voltage compared to a conventional phase change memory cell. The improvement in thermal efficiency of the PCM cell mainly originates from the low thermal conductivity of the crystalline titanium dioxide material. Among the various thicknesses of the TiO2 buffer layer, 4 nm was the most appropriate thickness that maximized the improvement with negligible sacrifice of the other device performances, such as the reset/set resistance ratio, voltage window, and endurance.