Spontaneous sparse learning for PCM-based memristor neural networks.

Neural networks trained by backpropagation have achieved tremendous successes on numerous intelligent tasks. However, naïve gradient-based training and updating methods on memristors impede applications due to intrinsic material properties. Here, we built a 39 nm 1 Gb phase change memory (PCM) memristor array and quantified the unique resistance drift effect. On this basis, spontaneous sparse learning (SSL) scheme that leverages the resistance drift to improve PCM-based memristor network training is developed. During training, SSL regards the drift effect as spontaneous consistency-based distillation process that reinforces the array weights at the high-resistance state continuously unless the gradient-based method switches them to low resistance. Experiments show that the SSL not only helps the convergence of network with better performance and sparsity controllability without additional computation in handwritten digit classification. This work promotes the learning algorithms with the intrinsic properties of memristor devices, opening a new direction for development of neuromorphic computing chips.

Evolution of crystal structures in GeTe during phase transition.

We investigated changes in the crystal structure of GeTe during its phase transition. Using density functional theory (DFT) calculations, four possible crystal structures were identified: R3m, P1, Cm, and Fm3m. Among these, P1 and Cm were examined here for the first time. By calculating the internal energy of the crystal volume change, we verified that P1, R3m, and Cm can coexist in crystalline GeTe. The X-ray diffraction spectra of annealed and laser-irradiated GeTe films revealed coexisting P1 or R3m and Cm. In addition, we confirmed that Cm transforms into P1 or R3m after laser irradiation. The presence of these new structures was revealed in the crystal Raman spectra. Many of the Raman peaks in the crystalized GeTe could be explained by the coexistence of various structures. By calculating the band gaps of these structures, we also found that a structural transformation induces a change in the crystal resistance, owing to differences in the band gaps of individual structures. The generation of new crystal structures suggests a facile phase change and instability during the structural transformation.

Thermal and Electrical Conduction of Single-crystal Bi2Te3 Nanostructures grown using a one step process.

Single-crystal Bi2Te3 nanowires (NWs) and nanoribbons (NRs) were synthesized by a vapor-liquid-solid (VLS) method from Bi2Te3 powder. To investigate the thermal properties of the Bi2Te3 nanostructure, a nondestructive technique based on temperature dependent Raman mapping was carried out. The Raman peaks were red shifted with increasing temperature. In addition, the fraction of the laser power absorbed inside the Bi2Te3 nanostructures was estimated by optical simulation and used to calculate the thermal conductivity value (κ). The thermal conductivity value obtained for the Bi2Te3 NW and NR was 1.47 Wm(-1)K(-1) and 1.81 Wm(-1)K(-1) at 300 K, respectively. The electrical conductivity of the Bi2Te3 nanostructure was also measured. In particular, an excellent electrical conductivity value of 1.22 * 10(3 )Ω(-1) cm(-1) was obtained for the Bi2Te3 NW at 300 K. This result can be attributed to topological insulator surface states. As a result of our study, the figure of merit (ZT) for the Bi2Te3 NW and NR can be significantly improved.

Evolution of the surface state in Bi2Se2Te thin films during phase transition.

Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation techniques. The high-purity growth of uniform Bi2Se2Te1 thin films has not yet been achieved using a thermal evaporation method. However, as a result of a self-ordering process during annealing, an as-grown amorphous film with p-type polarity could transform into single crystalline Bi2Se2Te1 with n-type polarity. Using THz-time domain spectroscopy (THz-TDS) and ultraviolet photoemission spectroscopy (UPS), we concluded that the conductivity is dominated by the Drude contribution, suggesting the presence of a quantum well state and surface states. Moreover we demonstrated that the emission of terahertz waves from the (001) surface of the single crystalline Bi2Se2Te1 thin film would be possible under the excitation of 790 nm femtosecond optical pulses, indicating the presence of a Dirac-fermion, a photo-Dember effect at the surface state and the transient current within the surface depletion region. The results reported herein provide useful information regarding a valuable deposition method that can be useful in studies of the evolution of surface state electrons in topological insulators.

Interface-modified unipolar resistive random access memory (RRAM) structure for low-power application.

An interface-engineered resistive random access memory (RRAM) using bilayer transition metal oxide (TMO) is presented for improving unipolar resistive-switching characteristics. The experiment and simulation data show that better resistive switching characteristics and superb uniformity can be realized by inserting a thin AIOx insertion layer between the Ir/NiO interface. To elucidate the uniformity improvement of our bilayer structure, the conducting-defect effects in the resistive cell were also investigated using a random circuit breaker (RCB) simulation model. It has been verified that the forming and set characteristics are more effectively improved because the conducting-defect ratio in the insertion layer region is low, therefore making it more advantageous for a filament path controllability. Using the optimal oxygen contents in both the insertion layer and the resistive cell, it was confirmed that a significant reduction of up to 0.15 mA of the reset current (I(RESET)) is possible compared to the conventional cell. These results indicate that new AI insertion has a large contribution to the reset and forming processes.

Dimensional effect of non-polar resistive random access memory (RRAM) for low-power memory application.

The relationships between the resistive cell dimension and the related analytical parameters such as the forming voltage, set voltage, and reset current were investigated to implement high-density and low-power unipolar RRAM. It was shown that the formation process in unipolar switching is strongly related to the cell dimension in the sub-nm region, not only in terms of its vertical thickness but also of its horizontal length, using the numerical simulation method. With the optimal cell size having sufficient initial resistance and a low forming voltage, the achievement of the greatest feasibility of the high-density low-power RRAM will be further accelerated. A numerical simulation was performed using a random circuit breaker (RCB) simulation model to investigate the optimal resistive switching condition. The on/off resistance ratio increases as the cell area decreases at the sub-nm level, and these phenomena are explained in terms of the relatively large set resistance change in a very small area due to the conductive defect (CD) amount effect in the RCB network model.

Electronic structure of Te/Sb/Ge and Sb/Te/Ge multi layer films using photoelectron spectroscopy.

Te/Sb/Ge and Sb/Te/Ge multilayer films with an atomically controlled interface were synthesized using effusion cell and e-beam techniques. The layers interacted during the deposition, resulting in films composed of Sb-Te+Sb-Sb/Ge and Sb/Sb-Te/Ge-Te/Ge respectively. Atomic diffusion and chemical reactions in films during the annealing process were investigated by photoemission spectroscopy. In the case of Te/Sb/Ge, Ge diffused into the Sb-Te region released Sb in Sb-Te bonds and interacted with residual Te, resulting in a change in valence band line shape, which was similar to that of a Ge(1)Sb(2)Te(4) crystalline phase. The Ge-Sb-Te alloy underwent a stoichiometric change during the process, resulting in a 1.2:2:4 ratio, consistent with the most stable stoichiometry value calculated by ab initio density-functional theory. The experimental results strongly suggest that the most stable structure is generated through a reaction process involving the minimization of total energy. In addition, Ge in the Sb/Te/Ge film diffused into Sb-Te region by thermal energy. However, Ge was not able to diffuse to the near surface because Sb atoms of the high concentration at the surface were easily segregated and hindered the diffusion of other elements.