Yuefeng Gong

Programming voltage reduction in phase change memory cells with tungsten trioxide bottom heating layer/electrode

A phase change memory cell with tungsten trioxide bottom heating layer/electrode is investigated. The crystalline tungsten trioxide heating layer promotes the temperature rise in the Ge(2)Sb(2)Te(5) layer which causes the reduction in the reset voltage compared to a conventional phase change memory cell. Theoretical thermal simulation and calculation for the reset process are applied to understand the thermal effect of the tungsten trioxide heating layer/electrode. The improvement in thermal efficiency of the PCM cell mainly originates from the low thermal conductivity of the crystalline tungsten trioxide material.

Phase change memory cell using tungsten trioxide bottom heating layer

Phase change memory (PCM) cell with tungsten trioxide (WO3) bottom heating layer is investigated. The crystalline WO3 heating layer promotes the temperature rise in Ge2Sb2Te5 layer that causes the reduction in reset voltage compared to a conventional PCM cell. The theoretical thermal simulation and calculation for reset process are applied to understand the thermal effect of WO3 heating layer. The improvement in thermal efficiency of PCM cell originates from the low electrical resistivity and low thermal conductivity of crystalline WO3 material.

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.

Density functional theoretical determinations of electronic and optical properties of nanowires and bulks for CdS and CdSe

The authors present density functionaltheoretical results of electronic and optical properties of wurtzite-type CdS and CdSesemiconductornanowires and bulks. The results show that quantum confinement will increase the energy gap, decrease the dielectric function, change the sign of birefringence, and increase the absolute value of birefringence at low-frequency region, as the dimensions of CdS and CdSe are reduced from the bulk to a nanowire with diameter of several nanometers. The optical spectral peaks of CdSenanowires show redshifts as compared with the corresponding ones of CdSnanowires.

Phase change memory based on Ge2Sb2Te5 capped between polygermanium layers

The effects of polygermanium interlayer, acting mainly as heating and thermal-resisting one, on performance of phase change memory are studied. Thermal simulation shows that the maximum temperature in Ge2Sb2Te5 layer is remarkably promoted when Ge2Sb2Te5 is capped between polygermanium layers. The maximum temperature location moves closer to the interface between Ge2Sb2Te5 and tungsten plug, significantly avoiding the parallel resistance in the reset state, which makes the memory more reliable. Electrical measurement shows that the polygermanium capped structure has a much lower reset current and larger resistance contrast compared to the one without polygermanium layers.

Three-Dimensional Numerical Simulation of Phase-Change Memory Cell with Probe like Bottom Electrode Structure

A new device structure of phase-change memory (PCM) cell with a Probe like bottom electrode (PBE) was proposed and its electrical-thermal characteristics were investigated by three-dimensional finite element analysis. The programming region of the definition (GST) layer in the PBE cell is much smaller than that in a conventional normal-bottom-contact (NBC) cell after the RESET operation. The high concentrations of electric-field density and electric-current density in the small programming region of GST layer in the PBE cell have the advantages of reducing the power consumption and increasing the heating efficiency of PCM devices. Compared with the NBC cell, the RESET threshold current of the PBE cell is reduced from 1.2 to 0.45 mA and the heating efficiency increases from 28.7 to 44.1%. Therefore, the lower programming current, the smaller molten region of GST and the higher heating efficiency in the PBE cell will be propitious for developing the PCM with low power consumption and high integration density.

A theoretical investigation of hyperpolarizability for small GanAsm (n+m=4–10) clusters

In this paper, the second and third order polarizabilities of small Ga(n)As(m) (n + m=4-10) clusters are systematically investigated using the time dependent density functional theory (TDDFT)6-311+G* combined with the sum-over-states method (SOSTDDFT6-311+G*). For the static second order polarizabilities, the two-level term (beta(vec.2)) makes a significant contribution to the beta(vec) for all considered Ga(n)As(m) clusters except for the Ga3As4 cluster. And, for the static third order polarizabilities, the positive channel (gamma(II)) makes a larger contribution to gamma(tot) than the negative channel (gamma(I)). Similar to the cubic GaAs bulk materials, the small Ga(n)As(m) cluster assembled materials exhibit large second order (1 x 10(-6) esu) and third order susceptibilities (5 x 10(-11) esu). The dynamic behavior of beta(-2omega; omega, omega) and gamma(-3omega; omega, omega, omega) show that the small Ga(n)As(m) cluster will be a good candidate of nonlinear optical materials due to the avoidance of linear resonance photoabsorption.

TDHF-SOS treatments on linear and nonlinear optical properties of III-V semiconductor clusters (Ga3As3, Ga3Sb3, In3P3, In3As3, In3Sb3)

Abstract We employ an ab initio time-dependent Hartree–Fock (TDHF) formalism combined with sum-over-states (SOS) method to calculate the linear and nonlinear optical properties of direct semiconductor clusters. The obtained electronic absorption spectra predict a variety of the spectral shapes and peak positions and an increase of the second-order nonlinear optical response with an increase of V-group ionic radius in the title clusters. Charge transfers from the π bonding to π antibonding orbitals between III and V group atoms make a significant contribution to the second-order polarizability.

Multistate storage through successive phase change and resistive change

Combination of phase change and resistive change was proposed to achieve multistate storage in one cell and Ge2Sb2Te5 (GST)/tungsten oxide (WOx) stack was fabricated. In reset process, three resistance staircases were observed, corresponding to the device transition from low-resistance-state (LRS) to a medium-resistance-state (MRS) first, and then transition from the MRS to high-resistance-state (HRS). In set process, two S-shaped negative-differential-resistances were observed, corresponding to the device transition from HRS to MRS first, and then transition from MRS to LRS. The GST/WOx architecture with successive phase change and resistive change is considered to have potential for multistate storage.

Improved performances of Ge2Sb2Te5 based phase change memory by W doping

W-doped Ge2Sb2Te5 materials have been investigated for phase change memory applications. W0.08(Ge2Sb2Te5)0.92 phase change film exhibits a higher crystallization temperature (255 °C), better data retention of 10 years at 183 °C, and higher crystalline resistance compared with traditional Ge2Sb2Te5. The Wx(Ge2Sb2Te5)1−x films crystallize into a stable face-centered cubic phase with uniform crystal grains. Phase-change memory device based on W0.08(Ge2Sb2Te5)0.92 shows short operation time (10 ns) and good endurance (6 × 105 cycles). Wx(Ge2Sb2Te5)1−x material is a promising candidate for phase-change memory applications that require good data retention and fast operation speed.