Yifeng Gu

Influence of silicon on the thermally-induced crystallization process of Si-Sb4Te phase change materials

Si-Sb4Te phase change thin films with different silicon contents have been investigated by in situ heating technique in transmission electron microscopy (TEM). The studies show that Si-doping can significantly improve the thermal stability of Si-Sb4Te thin films, refine their grain size, and change the nucleation characters with the increase of silicon content. By in situ annealing in TEM, the crystalline phase of Si-Sb4Te thin films can be indexed as hexagonal Sb structure, and Si is still holding amorphous state which is considered as the reason for the change, by destroying the long-range order lattice of crystal grains.Si-Sb4Te phase change thin films with different silicon contents have been investigated by in situ heating technique in transmission electron microscopy (TEM). The studies show that Si-doping can significantly improve the thermal stability of Si-Sb4Te thin films, refine their grain size, and change the nucleation characters with the increase of silicon content. By in situ annealing in TEM, the crystalline phase of Si-Sb4Te thin films can be indexed as hexagonal Sb structure, and Si is still holding amorphous state which is considered as the reason for the change, by destroying the long-range order lattice of crystal grains.

Electrical transport behavior of Ca3MnxCo4−xO9 (0⩽x⩽1.28) at low temperatures

Electrical transport behavior of Ca3MnxCo4−xO9 (0⩽x⩽1.28), prepared by sol-gel process, was investigated at temperatures from 300K down to 5K. The results indicate that dc conductivity σ and carrier concentration decay exponentially with decreasing temperature below ∼75K and ∼100K for Ca3MnxCo4−xO9 with x=0 and 0.03, respectively. The increase of resistivity (T≲100K) of Ca3Mn0.03Co3.97O9 with decreasing temperature originates mainly from reduction in its carrier concentration due to weakening of thermal activation. Nevertheless, the temperature behavior of resistivity at T≲75K for Ca3Co4O9 is mainly governed by two factors: (1) reduction in its carrier concentration; and (2) rise of carrier mobility resulting presumably from reduced optical phonon scattering. In contrast, the temperature dependence of the resistivity for the heavily substituted Ca3MnxCo4−xO9 (x=0.57,0.9,1.28) displays semiconductor-like behavior in the whole temperature range investigated, but does not obey an exponential law. Instead, a ...

SixSb2Te materials with stable phase for phase change random access memory applications

The physical and electrical properties of SixSb2Te system materials with various Si contents have been systemically studied with the aim of finding the most suitable composition for the phase change random access memory (PCRAM) applications. SixSb2Te shows better thermal stability than Ge2Sb2Te5 due to no Te separation under high annealing temperatures. The increase of Si content can enhance the data retention ability of SixSb2Te materials. When the value of x is larger than 0.44, the 10-year data retention temperature for SixSb2Te will exceed 110 °C, which meets the long-term data retention requirement. Furthermore, Si-rich SixSb2Te materials exhibit the improvement on thickness change after annealing compared with Ge2Sb2Te5. In addition, the PCRAM devices based on SixSb2Te (x = 0.31, 0.44) were fabricated and the electrical operations were carried out. Both of them show the outstanding performances with long-term operations.

Phase-change material Ge0.61Sb2Te for application in high-speed phase change random access memory

Compared with Ge2Sb2Te5, Ge0.61Sb2Te has higher crystallization temperature (∼200.5 °C), larger crystallization activation energy (∼3.28 eV), and better data retention (∼120.8 °C for 10 yr). The switching between amorphous and crystalline state could be triggered by the electric pulse of as short as 10 ns. With the resistance ratio of two orders of magnitude, the endurance test was up to 106 cycles. Ge0.61Sb2Te material is a promising candidate for the trade-off between programming speed and data retention.

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.

Characterization of the thermal properties for Si-implanted Sb2Te3 phase change material

The microstructure and thermal properties of Si-implanted Sb2Te3 were investigated. Crystalline Si-implanted Sb2Te3 film with relatively uniform composition depth profile was fabricated, which consists of Si4−x (0 < x < 1) ions and crystalline Sb2Te3. No separated Te phase was found. The crystallization activation energies of crystallization increase with Si dose from 5 × 1015 to 2.16 × 1016 Si-ions/cm2. The crystallization temperatures of the films are 149, 168, and 174 °C with 5 × 1015, 1 × 1016, and 2.16 × 1016 Si-ions/cm2 ion implantation, respectively. Furthermore, the Sb2Te3 film implanted with the dose of 1 × 1016 Si-ions/cm2 can maintain the data for 10 yr at 85 °C, which is comparable to Ge2Sb2Te5. The results indicate that the Si-implanted Sb2Te3 is a promising candidate for phase change memory application.

Investigation of Al-Sb-Se alloy for long data retention and low power consumption phase change memory application

Te-free phase-change material Al-Sb-Se is investigated and considered to be a promising candidate of storage medium for phase change memory (PCM) application. Al0.49Sb2.19Se exhibits a higher crystallization temperature (∼222.7 °C), a larger crystallization activation energy (∼4.17 eV), and a better data retention (∼146.5 °C for 10 yr) in comparison with those of Ge2Sb2Te5. The uniformity of material distribution for crystalline film improves the reliability of phase change memory. Al0.49Sb2.19Se-based memory cell significantly shows lower power consumption for SET/RESET reversible switching than that of Ge2Sb2Te5-based one. Furthermore, PCM based on Al0.49Sb2.19Se shows endurance up to 3.5 × 103 cycles with stability resistance of about two orders of magnitude on/off ratio.

Mechanism of Oxidation on Si2Sb2Te5 Phase Change Material and Its Application

Uniformly oxygen doping into phase change materials (PCMs) will increase their crystallization temperatures, but natural oxidation of PCMs has an opposite effect. Mechanism of oxidation for Si2Sb2Te5 was studied by in situ X-ray photoelectron spectroscopy, employing an oxygen content gradient sample. During an oxidation process, oxygen preferentially reacts with Si due to its smallest electronegativity value among Si, Sb, and Te elements. Models have been proposed for the different mechanisms of oxidation. O-doping into Si–Sb–Te was proposed to prepare nano-crystal PCM based on the discovering. Performance of nano-crystal material and memory device was effectively promoted.

Simulation of novel phase change memory cell with Titanium Nitride heating layer

A novel phase-change memory cell with a heating layer structure (HLS) was proposed in this work. By having an additional Titanium Nitride (TiN) layer under the bottom electrode, the heat loss can be effectively prevented. A three-dimensional finite element model for phase change memory (PCM) is established to simulate thermal and electrical behaviors. Compared with the traditional structure (TS) PCM cell, the simulation results indicate that the HLS has advantages of increasing the heating efficiency and reducing the heat loss. The thermal effect of the access device of PCM is also simulated and the HLS offers a higher driving current because of a higher temperature in the device region. Therefore, the HLS cell will be propitious for developing the PCRAM with low power consumption and high integration.

Phase Change Line Memory Cell Based on Ge2Sb2Te5 Fabricated Using Focused Ion Beam

Using focused-ion-beam-deposited SiO2 as a hard mask, a new phase change line memory cell fabrication method was developed in this work. A phase change line memory cell with a length of 150 nm and width of 100 nm was fabricated and electrical characterization was performed. Reversible phase transition between amorphous (RESET) and polycrystalline (SET) states was realized. Threshold current of the device is only 0.5 µA, which is considerably low owning to the line structure. The reset operation was achieved by applying a voltage pulse with a magnitude of 3.5 V and duration of 50 ns, and the set operation was achieved by applying 1.6 V for 1000 ns. The dynamic resistance switching ratio (off/on ratio) was nearly 104. The device fabrication method enables simplified scaling to ultrasmall phase change device dimensions.