Atomistic phase transition dynamics of In 2Se 3 semiconductor

Modeling and simulation of low power ferroelectric non-volatile memory tunnel field effect transistors using silicon-doped hafnium oxide as gate dielectric

The two-dimensional van der Waals layered semiconductor In2Se3 has emerged as a promising candidate for non-volatile ferroelectric memory, optoelectronic devices, and polymorphic phase engineering. Polymorphic In2Se3 typically stabilizes into three distinct phases: α-, β′-, and β *-In2Se3, each predominant within specific temperature ranges. Although the crystal structures and ferroelectric properties of these phases have been widely studied, the unambiguous assignment of their in-plane and out-of-plane ferroelectric behaviors, as well as the mechanisms driving their phase transitions, remains actively debated. Here, we investigate the evolution of atomic and electronic structure in molecular beam epitaxy (MBE)-grown ultrathin In2Se3 films through correlated microstructural and macroscopic physical properties analysis. By employing scanning tunneling microscopy/spectroscopy (STM/STS), temperature-dependent Raman spectroscopy, and piezoresponse force microscopy (PFM), we demonstrate a reversible temperature-induced phase transition between the in-plane ferroelectric β * and antiferroelectric β' phases. Furthermore, we confirm robust out-of-plane ferroelectric polarization of as-grown films and achieve an electric-field-driven transition from the β * to β' phase. Our findings not only deepen the understanding of phase transitions and polarization evolution in two-dimensional semiconductors, but also open new avenues for developing tunable non-volatile ferroelectric memory devices.

The current attempt is aimed to honor the first centennial of Johannes Diderik van der Waals (VDW) awarding Nobel Prize in Physics. The VDW theory of ordinary fluids is reviewed in the first part of the paper, where special effort is devoted to the equation of state and the law of corresponding states. In addition, a few mathematical features involving properties of cubic equations are discussed, for appreciating the intrinsic beauty of the VDW theory. A theory of astrophysical fluids is shortly reviewed in the second part of the paper, grounding on the tensor virial theorem for two-component systems, and an equation of state is formulated with a convenient choice of reduced variables. Additional effort is devoted to particular choices of density profiles, namely a simple guidance case and two cases of astrophysical interest. The related macroisothermal curves are found to be qualitatively similar to VDW isothermal curves below the critical threshold and, for sufficiently steep density profiles, a critical macroisothermal curve exists, with a single horizontal inflexion point. Under the working hypothesis of a phase transition (assumed to be gas-stars) for astrophysical fluids, similar to the vapour-liquid phase transition in ordinary fluids, the location of gas clouds, stellar systems, galaxies, cluster of galaxies, on the plane scanned by reduced variables, is tentatively assigned. A brief discussion shows how van der Waals' two great discoveries, namely a gas equation of state where tidal interactions between molecules are taken into account, and the law of corresponding states, related to microcosmos, find a counterpart with regard to macrocosmos. In conclusion, after more than a century since the awarding of the Nobel Prize in Physics, van der Waals' ideas are still valid and helpful to day for a full understanding of the universe.

The investigation carried out shows that in the low-symmetry compouds La(BH4)3 and La(BH4)3·3THF, as in the high-symmetry MBH4, phase transitions occur. An unexpected result is the fact that the symmetry of the environment of the boron ions for these phase transitions practically does not change and remains close to cubic. The activation energy for reorientation of the BH − 4 ion (1.2 kcal/mole) is somewhat overstimated, if we start from the difference in the electronegativity of the boron and hydrogen ions (2.0 and 2.15). This may mean that the nature of the barrier to reorientation of the BH − 4 anion in lanthanum tetrahydroborates is connected with the van der Waals interaction of the tetrahedral BH − 4 groups with the environment. From the139La NMR data it follows that the coordination sphere of the triply-charged lanthanum is characterized by low-symmetry of the arrangement of the BH4− and THF (OC4H8) ligands, and the La3+-(BH4)− bond has predominantly ionic character. Consequently, the potential energy of the BH − 4 anions in the lattice is the sum of the strong isotropic ionic component and the substantially weaker anisotropic van der Waals component. In the low-temperature phases, for both compounds we observe structurally nonequivalent positions for the BH − 4 anions, retained in the high-temperature phase. The phase transitions in the studied compounds are connected with a change in the nature of the hydrogen atom distribution. Due to the low height of the barriers, the reorientational mobility sets in even at low temperatures, and in the phase transition region the value of υc reaches 1010–1011 Hz. For such a high reorientational frequency, there may be substantial effects from correlation rotation of the anions, leading to a change in the hydrogen atom distribution in the structure and an order-disorder type phase transition.

Driving Force of Molecular/Ionic Superfluid Formation

A quantitative understanding of the microscopic mechanisms responsible for damping in van der Waals nanomechanical resonators remains elusive. In this work, we investigate van der Waals magnets, where the thermal expansion coefficient exhibits an anomaly at the magnetic phase transition due to magnetoelastic coupling. Thermal expansion mediates the coupling between mechanical strain and heat flow and determines the strength of thermoelastic damping (TED). Consequently, variations in the thermal expansion coefficient are reflected directly in TED, motivating our focus on this mechanism. We extend existing TED models to incorporate anisotropic thermal conduction, a critical property of van der Waals materials. By combining the thermodynamic properties of the resonator material with the anisotropic TED model, we examine dissipation as a function of temperature. Our findings reveal a pronounced impact of the phase transition on dissipation, along with transitions between distinct dissipation regimes controlled by geometry and the relative contributions of in-plane and out-of-plane thermal conductivity. These regimes are characterized by the resonant interplay between strain and in-plane or through-plane heat propagation. To validate our theory, we compare it to experimental data of the temperature-dependent mechanical resonances of ${\mathrm{FePS}}_{3}$ resonators.

LiNbO 3 -type Mg 2.98(2) Al 1.99(2) Si 3.02(2) O 12 (py-LN) was synthesized by recovering a run product from 2000 K and 45 GPa to ambient conditions using a large volume press. Rietveld structural refinements were carried out using the one-dimensional synchrotron XRD pattern collected at ambient conditions. The unit-cell lattice parameters were determined to be a = 4.8194(3) A, c = 12.6885(8) A, V = 255.23(3) A 3 , with Z = 6 (hexagonal, R 3 c ). The average A -O and B -O distances of the A O 6 and B O 6 octahedra have values similar to those that can be obtained from the sum of the ionic radii of the averaged A - and B -site cations and oxygen (2.073 and 1.833 A, respectively). The present compound has the B -site cations at the octahedral site largely shifted along the c axis compared with other LiNbO 3 -type phases formed by back-transition from perovskite (Pv)-structure, and as a result, the coordination number of this site is better described as 3+3. It appears therefore that the B -site cation in the octahedral position cannot be completely preserved during the back-transition because of the small size of Si and Al, which occupy usually a tetrahedral site at ambient conditions. The formation of py-LN can be explained by the tilting of B O 6 octahedra of the perovskite structure having the pyrope composition and formed at high P-T conditions. The tilting is driven by the decrease in ionic radius ratio between the A -site cation and oxygen during decompression. This also explains why there is no back-transition from the Pv-structure to the ilmenite-structure during decompression, since this is a reconstructive phase transition whose activation energy cannot be overcome at room temperature. Py-LN may be formed in shocked meteorites by the back-transformation after the garnet-bridgmanite transition, and will indicate shock conditions around 45 GPa and 2000 K.

Liquid-gas phase transition of finite nucleus is studied by means of microscopic reaction theory antisymmetrized molecular dynamics with momentum fluctuations. By calculating the time development of hot system in a potential well, thermodynamic variables, such as temperature and pressure and their relationship to the excitation energy of the system, are obtained. We see clearly the existence of three regions in calculated caloric curve, namely, liquid-dominant, plateau, and gas regions. The transition from liquid-dominant phase to gas phase begins with the cracking of hot liquid nucleus and disintegration into fragments. The property of plateau region strongly depends on the pressure of the system. The gas phase is well reproduced by van der Waals' equation.

Due to the voltage driven switching at low voltages combined with nonvolatility of the achieved polarization state, ferroelectric materials have a unique potential for low power nonvolatile electronic devices. The competitivity of such devices is hindered by compatibility issues of well-known ferroelectrics with established semiconductor technology. The discovery of ferroelectricity in hafnium oxide changed this situation. The natural application of nonvolatile devices is as a memory cell. Nonvolatile memory devices also built the basis for other applications like in-memory or neuromorphic computing. Three different basic ferroelectric devices can be constructed: ferroelectric capacitors, ferroelectric field effect transistors and ferroelectric tunneling junctions. In this article first the material science of the ferroelectricity in hafnium oxide will be summarized with a special focus on tailoring the switching characteristics towards different applications.The current status of nonvolatile ferroelectric memories then lays the ground for looking into applications like in-memory computing. Finally, a special focus will be given to showcase how the basic building blocks of spiking neural networks, the neuron and the synapse, can be realized and how they can be combined to realize neuromorphic computing systems. A summary, comparison with other technologies like resistive switching devices and an outlook completes the paper.

In situ X-ray diffraction studies of rare-earth metals Sc- and Y-doped AlN nanoprisms were carried out using angle-dispersive synchrotron radiation technique in a diamond anvil cell up to approximately 40 and 35 GPa, respectively. Pressure-induced wurtzite-to-rocksalt phase transitions start at 18.6 and 16.2 GPa and complete at 29.8 and 26.5 GPa, showing lower phase-transition pressures compared with bulk AlN and AlN nanowires while slightly higher than that of AlN nanocrystals. The effects of volume expansion, volume collapse, and crystal defects on phase transition have been discussed. The distinct high-pressure behaviors can be explained in terms of the doping-induced Al vacancy defects along with substitute ions sit at the cation sites, which lead to the distortion of the crystal structure that reduce the structural stability of the doped AlN nanoprisms.

Acoustic studies of sodium bismuth tungstate, NaBi(WO4)2, single crystal were carried out in the temperature range from 290 K to 340 K. Strong anomalies in ultrasound velocity and attenuation were observed above room temperature for both longitudinal and transverse waves. The results obtained suggested a phase transition at 319.2 K. The order parameter responsible for the phase transition was found to be coupled linearly with strains in transverse acoustic waves and nonlinearly with those in longitudinal waves. It was suggested that the phase transition is related with substitutional ordering of the Na+ and Bi3+ ions at the cation sites. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Where the atoms are: Cation disorder and anion displacement in D IIX VI–A IB IIIX VI2 semiconductors

Ferroelectric-based nonvolatile memory (FeRAM) is now a mainstream product category supported by Ramtron, Fujitsu, Texas Instruments, IBM, and Matsushita. The internal array of ferroelectric capacitors in FeRAMs consumes absolutely no energy when the chip is powered. This lack of energy consumption combined with SRAM-like operation at SRAM speeds constitutes a unique advantage for FeRAM memory in low-power embedded applications. Because the ferroelectric technology is buried deep inside the IC package surrounded by digital interface circuits, the true nature of ferroelectric memory capacitor operation is obscured from the user. It is possible to operate a simple ferroelectric nonvolatile memory from the I/O pins of a microprocessor using discrete ferroelectric capacitors in a Sawyer-Tower circuit configuration. Designing and operating a ferroelectric memory with discrete capacitors provides insight into the nature of FeRAMs, their internal operation, and their reliability. The read and write times of the discrete memory will be fast, limited only by the current capacity of the I/O pins. The discrete ferroelectric capacitors will retain their written state without concern for whether the microcontroller is powered or not. Unlike EEPROM or FLASH transistors, discrete ferroelectric capacitors can be physically handled during retention without corrupting their stored states.

We propose low-voltage driven method of ferroelectric memory device by using optical polarization measurement for detecting slightly difference of the polarized state as the difference in fast axis of birefringence. Non-volatile ferroelectric memory has a necessity to apply a voltage higher than coercive electric field for inverting spontaneous polarizations. Furthermore, repeated operation of these memory devices causes fatigue of ferroelectric ceramics due to the strain of a crystal structure which arise from the changing an electric polarization. For confirming the amount of memory effect as a function of the applied electric field, the hysteresis curve in the optical polarization property of PLZT (Lead Lanthanum Zirconate Titanate) has been measured. The coercive electric field Ec and Emax which is an electric field to align directions of all electric polarizations of a sample PLZT-8.9/65/35 are 14 and 3 kV/cm, respectively. Azimuth angle exhibited the amount of memory effect of 5.4° at the maximum electric field 4.5 kV/cm. In addition, in the range from 0 kV/cm to 3.0 kV/cm, the amount of memory effect becomes small values less than 1°. These results mean to allow us to use the memory effect in a driving electric field less than Emax by using the optical polarization property.

Here, we demonstrate a ferroelectric memory device using a substitute ferroelectric polymer, poly(vinylidene-co chlorotrifluoroethylene) (PVDF-CTFE) as an insulator which shows reliable ferroelectric dipole polarity upon different heat annealing condition. As a charge transport layer, one of representative organic semiconductor oligothiophene is used and to enhance the charge carrier mobility, single crystal of oligothiophene is fabricated by drop casting method with controlling the evaporation rate.