Atomic Layer Deposition of Sb2 Te3 /GeTe Superlattice Film and Its Melt-Quenching-Free Phase-Transition Mechanism for Phase-Change Memory.

Atomic layer deposition (ALD) of Sb2 Te3 /GeTe superlattice (SL) film on planar and vertical sidewall areas containing TiN metal and SiO2 insulator is demonstrated. The peculiar chemical affinity of the ALD precursor to the substrate surface and the 2D nature of the Sb2 Te3 enable the growth of an in situ crystallized SL film with a preferred orientation. The SL film shows a reduced reset current of ≈1/7 of the randomly oriented Ge2 Sb2 Te5 alloy. The reset switching is induced by the transition from the SL to the (111)-oriented face-centered-cubic (FCC) Ge2 Sb2 Te5 alloy and subsequent melt-quenching-free amorphization. The in-plane compressive stress, induced by the SL-to-FCC structural transition, enhances the electromigration of Ge along the [111] direction of FCC structure, which enables such a significant improvement. Set operation switches the amorphous to the (111)-oriented FCC structure.

An ab initio approach on the asymmetric stacking of GaAs 〈111〉 nanowires grown by a vapor-solid method.

This study provides an ab initio thermodynamics approach to take a step forward in the theoretical modeling on the growth of GaAs nanowires. In order to understand the effects of growth conditions on the involvement of stacking faults and polytypism, we investigated the vapor-phase growth kinetics under arbitrary temperature-pressure conditions by combining the atomic-scale calculation with the thermodynamic treatment of a vapor-solid system. Considering entropy contribution and electronic energy, the chemical potential and surface energies of various reconstructions were calculated as a function of temperature and pressure, leading to the prediction of the change in Gibbs free energy at each stage of nucleation and growth. This enabled us to predict the temperature-pressure-dependent variation in nucleation rate and formation probability of possible stacking sequences: zinc blende, stacking faults, twin, and wurtzite. As a result, the formation probabilities of stacking faults and polytypism were found to decrease with increasing temperature or decreasing pressure, which agreed well with available experiments. In addition, by showing that the formation probability of the stacking defects in GaAs nanowires grown along the 〈111〉B direction is about ten times higher than that along the 〈111〉A direction, the intriguing asymmetric stacking behavior during the growth along the polar direction and its dependence on growth conditions were fundamentally elucidated. The proposed ab initio approach bridges the gap between atomic-scale static calculation at zero-temperature and kinetic growth process under arbitrary vapor-phase conditions, and thus will contribute to the nanoscale growth not only for GaAs nanowires but also for other materials.

Ferroelectric switching in bilayer 3R MoS2 via interlayer shear mode driven by nonlinear phononics.

We theoretically investigate the mechanism of ferroelectric switching via interlayer shear in 3R MoS2 using first principles and lattice dynamics calculations. First principle calculations show the prominent anharmonic coupling of the infrared inactive interlayer shear and the infrared active phonons. The nonlinear coupling terms generates an effective anharmonic force which drives the interlayer shear mode and lowers the ferroelectric switching barrier depending on the amplitude and polarization of infrared mode. Lattice dynamics simulations show that the interlayer shear mode can be coherently excited to the switching threshold by a train of infrared pulses polarized along the zigzag axis of MoS2. The results of this study indicate the possibility of ultrafast ferroelectricity in stacked two-dimensional materials from the control of stacking sequence.

Optical control of the layer degree of freedom through Wannier-Stark states in polar 3R MoS2.

Electrons in two-dimensional layered crystals gain a discrete positional degree of freedom over layers. We propose the two-dimensional transition metal dichalcogenide homostructure with polar symmetry as a prototypical platform where the degrees of freedom for the layers and valleys can be independently controlled through an optical method. In 3R MoS2, a model system, the presence of the spontaneous polarization and built-in electric field along the stacking axis is theoretically proven by the density functional theory. The K valley states under the electric field exhibit Wannier-Stark type localization with atomic-scale confinement driven by double group symmetry. The simple interlayer-dynamics-selection rule of the valley carriers in 3R homostructure enables a binary operation, upward or downward motion, using visible and infrared light sources. Together with the valley-index, a 2 [Formula: see text] 2 states/cell device using a dual-frequency polarized light source is suggested.

Equilibrium crystal shape of GaAs and InAs considering surface vibration and new (111)B reconstruction: ab-initio thermodynamics.

This work reports on the theoretical equilibrium crystal shapes of GaAs and InAs as a function of temperature and pressure, taking into account the contribution of the surface vibration, using ab-initio thermodynamic calculations. For this purpose, new (111)B reconstructions, which are energetically stable at a high temperature, are suggested. It was found that there was a feasible correspondence between the calculated equilibrium shapes and the experimental shapes, which implied that the previous experimental growth was performed under conditions that were close to equilibrium. In this study, GaAs and InAs were selected as prototype compound semiconductors, but the developed calculation methodology can also be applied to other III-V compound semiconductor materials.

Surface reconstruction of InAs (001) depending on the pressure and temperature examined by density functional thermodynamics.

A detailed understanding of the atomic configuration of the compound semiconductor surface, especially after reconstruction, is very important for the device fabrication and performance. While there have been numerous experimental studies using the scanning probe techniques, further theoretical studies on surface reconstruction are necessary to promote the clear understanding of the origins and development of such subtle surface structures. In this work, therefore, a pressure-temperature surface reconstruction diagram was constructed for the model case of the InAs (001) surface considering both the vibrational entropy and configurational entropy based on the density functional theory. Notably, the equilibrium fraction of various reconstructions was determined as a function of the pressure and temperature, not as a function of the chemical potential, which largely facilitated the direct comparison with the experiments. By taking into account the entropy effects, the coexistence of the multiple reconstructions and the fractional change of each reconstruction by the thermodynamic condition were predicted and were in agreement with the previous experimental observations. This work provides the community with a useful framework for such type of theoretical studies.