Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application.

Superlattice-like Ge50Te50/Ge8Sb92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

Excitation mechanism of A1g mode and origin of nonlinear temperature dependence of Raman shift of CVD-grown mono- and few-layer MoS2 films.

MoS2 films are grown on SiO2/Si substrates by chemical vapor deposition. The vibrational properties of optical phonons of mono-, bi- and multilayer MoS2 are studied by Raman scattering spectroscopy over temperature range from 90 to 540 K with 514.5 nm and 785 nm lasers. The Raman peaks of E2g1 and A1g modes are observed simultaneously for mono-, bi- and multilayer MoS2 with 514.5 nm laser, but only the Raman peak of E2g1 mode is seen for monolayer MoS2 as 785 nm laser is used, revealing electron-phonon exchange excitation mechanism of A1g mode for the first time. The Raman shifts of E2g1 and A1g modes present obvious nonlinear temperature dependence. A semi-quantitative model is used to fit the nonlinear temperature dependence of Raman shifts which matches well to experimental data. Meanwhile, the fitting results reveal that the nonlinear temperature dependence of Raman shifts of E2g1 mode mainly originates from three-phonon anharmonic effect, while one of A1g mode is contributed by stronger three- and weaker four-phonon anharmonic effects cooperatively but two contributions are comparable in intensity.

Localized surface modes in parity-time-symmetric potentials.

We investigate light-beam propagation along the interface between linear and nonlinear media with parity-time symmetry. A novel class of two-dimensional localized surface modes (LSMs) is found analytically and numerically. If the potential is parity-time invariant along the direction parallel to the interface between the two media, stable LSMs can exist. Otherwise, if the potential is parity-time invariant along the direction perpendicular to the interface between the two media, there are no stable LSMs.

Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl2 Monolayer.

The development of two-dimensional (2D) magnetism is driven not only by the interest of low-dimensional physics but also by potential applications in high-density miniaturized spintronic devices. However, 2D materials possessing a ferromagnetic order with a relatively high Curie temperature (Tc) are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl2 on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl2, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature (Tc) of ∼147 K is revealed for monolayer FeCl2, based on our static magneto-optical Kerr effect measurements. Furthermore, temperature-dependent magnetization dynamics is investigated by the time-resolved magneto-optical Kerr effect. A transition from one- to two-step demagnetization occurs as the lattice temperature approaches Tc, which supports the Elliott-Yafet spin relaxation mechanism. The findings contribute to a deeper understanding of the underlying mechanisms governing ultrafast magnetization in 2D ferromagnetic materials.

Femtosecond laser-irradiated crystallization of amorphous Si2Sb2Te3 films and its in-situ characterization by coherent phonon spectroscopy.

Femtosecond laser-irradiation-induced phase change of a new amorphous Si(2)Sb(2)Te(3) film with a good thermal stability and low reset current is studied by coherent phonon spectroscopy. New coherent optical phonons (COP) occur as laser irradiation fluence reaches some threshold, implying laser-induced phase change emerged. The compositions in phase-changed area revealed by COP modes agree well with ones in reported annealed crystallized film, implying laser-induced phase change as crystallization. Pump fluence dependence of COP dynamics reveals good crystallization quality of the phase-changed film, exhibiting promising application of Si(2)Sb(2)Te(3) films in optical phase change memory. Acoustic phonons are also found and identified.

Study on the crystallization process of GaSb-Sb2Te3 pseudobinary films for phase-change random access memory.

Non-isothermal change in electrical resistance was used to investigate the crystallization process of GaSb-Sb2Te3 pseudobinary films prepared by co-sputtering using GaSb and Sb2Te3 targets. The crystallization parameters were determined directly by in-situ electrical resistance-temperature measurements. The activation energy of crystallization and rate factor were deduced from the Kissinger's plot. The kinetics exponent was calculated using the Ozawa's method. The crystallization temperature (185-228 degrees C) and activation energy (2.01-5.65 eV) increased monotonically with increasing Ga concentration from 5 to 34 mol%, while the average kinetics exponent decreased from 1.63 to 1.02. The crystallization mechanism of the compositions with Ga concentration more than 10 mol% was one-dimensional growth from the nuclei due to the average kinetics exponent smaller than 1.5. Crystallization time of the studied compositions was estimated theoretically by the Johnson-Mehl-Avrami equation and measured experimentally by the reflectivity change induced by the laser pulse. It is shown that Ga27Sb47Te26 film exhibited the shortest crystallization time, suggesting a potential candidate for phase-change random access memory application.

A transmission-grating-modulated pump-probe absorption spectroscopy and demonstration of diffusion dynamics of photoexcited carriers in bulk intrinsic GaAs film.

A transmission-grating-modulated time-resolved pump-probe absorption spectroscopy is developed and formularized. The spectroscopy combines normal time-resolved pump-probe absorption spectroscopy with a binary transmission grating, is sensitive to the spatiotemporal evolution of photoinjected carriers, and has extensive applicability in the study of diffusion transport dynamics of photoinjected carriers. This spectroscopy has many advantages over reported optical methods to measure diffusion dynamics, such as simple experimental setup and operation, and high detection sensitivity. The measurement of diffusion dynamics is demonstrated on bulk intrinsic GaAs films. A carrier density dependence of carrier diffusion coefficient is obtained and agrees well with reported results.

Femtosecond laser-induced crystallization of amorphous Ga-Sb-Se films and coherent phonon dynamics.

Femtosecond laser-irradiation-induced phase change of new environment friendly Te-free amorphous Ga-Sb-Se films is studied by coherent phonon spectroscopy. New coherent optical phonons (COP) occur when laser irradiation power reaches some threshold, implying laser-induced phase change taken place. Pump power dependence of COP dynamics reveals the phase change as crystallization and crystallization quality is comparable to one of annealing crystallization, showing application potential of Ga-Sb-Se films in optical phase change memory. The laser-irradiated crystallization of different component Ga-Sb-Se films is studied. It is found crystallization threshold power depends on Sb content, implying Sb-content control of the crystallization temperature of Ga-Sb-Se films.

Transmission-grating-photomasked transient spin grating and its application to measurement of electron-spin ambipolar diffusion in (110) GaAs quantum wells.

A circular dichromatic transient absorption difference spectroscopy of transmission-grating-photomasked transient spin grating is developed and formularized. It is very simple in experimental setup and operation, and has high detection sensitivity. It is applied to measure spin diffusion dynamics and excited electron density dependence of spin ambipolar diffusion coefficient in (110) GaAs quantum wells. It is found that the spin ambipolar diffusion coefficient of (110) and (001) GaAs quantum wells is close to each other, but has an opposite dependence tendency on excited electron density. This spectroscopy is expected to have extensive applicability in the measurement of spin transport.

Ultrafast Dynamics of Demagnetization in FeMn/MnGa Bilayer Nanofilm Structures via Phonon Transport.

Superdiffusive spin transport has been proposed as a new mechanism of ultrafast demagnetization in layered magnetic nanostructures and demonstrated experimentally. However, it is unknown if it is possible for phonon transport to occur and manipulate ultrafast demagnetization. Here, we explore the ultrafast dynamics of demagnetization of an antiferromagnet/ferromagnet bilayer nanostructure, of a FeMn/MnGa bilayer film prepared by molecular beam epitaxy. Ultrafast dynamics of a two-step demagnetization were observed through the time-resolved magneto-optical Kerr effect. The first-step fast component of the two-step demagnetization occurred within ~200 fs, while the second-step slow component emerged in a few tens of picoseconds. For a single MnGa film, only the ultrafast dynamics of the first-step fast demagnetization were observed, revealing that the second-step slow demagnetization originates from interlayer phonon transport. A four-temperature model considering phonon transport was developed and used to effectively reproduce the observed ultrafast dynamics of two-step demagnetization. Our results reveal the effect of phonon transport on demagnetization for the first time and open up a new route to manipulate ultrafast demagnetization in layered magnetic structures.

Characterization of Femtosecond laser-irradiation crystallization and structure of multiple periodic Si/Sb80Te20 nanocomposite films by coherent phonon spectroscopy.

Multiple parameters of nanocomposite Si/Sb80Te20 multilayer films are possibly optimized simultaneously to satisfy the development of ideal phase-change memory devices by adjusting chemical composition and physical structure of multilayer films. The crystallization and structure of the films are studied by coherent phonon spectroscopy. Laser irradiation power dependence of coherent optical phonon spectroscopy reveals laser-induced crystallization of the amorphous multilayer film, while coherent acoustic phonon spectroscopy reveals the presence of folded acoustic phonons which suggests a good periodic structure of the multilayer films. Laser irradiation-induced crystallization shows applicable potentials of the multilayer films in optical phase change storage.

Mechanism of Nano-Structuring Manipulation of the Crystallization Temperature of Superlattice-like [Ge8Sb92/Ge]3 Phase-Change Films.

Superlattice-like (SLL) phase-change film is considered to be a promising phase-change material because it provides more controllabilities for the optimization of multiple performances of phase-change films. However, the mechanism by which SLL structure affects the properties of phase-change films is not well-understood. Here, four SLL phase-change films [Ge8Sb92(15 nm)/Ge (x nm)]3 with different x are fabricated. Their behaviors of crystallization are investigated by measuring sheet resistance and coherent phonon spectroscopy, which show that the crystallization temperature (TC) of these films increases anomalously with x, rather than decreases as the interfacial effects model predicted. A new stress effect is proposed to explain the anomalous increase in TC with x. Raman spectroscopy reveals that Raman shifts of all phonon modes in SLL films deviate from their respective standard Raman shifts in stress-free crystalline films, confirming the presence of stress in SLL films. It is also shown that tensile and compressive stresses exist in Ge and Ge8Sb92 layers, respectively, which agrees with the lattice mismatch between the Ge and Ge8Sb92 constituent layers. It is also found that the stress reduces with increasing x. Such a thickness dependence of stress can be used to explain the increase in crystallization temperature of four SLL films with x according to stress-enhanced crystallization. Our results reveal a new mechanism to affect the crystallization behaviors of SLL phase-change films besides interfacial effect. Stress and interfacial effects actually coexist and compete in SLL films, which can be used to explain the reported anomalous change in crystallization temperature with the film thickness and cycle number of periods in SLL phase-change films.

The cavity dispersion noncoaxiality effects on broadband few-cycle pulse generation in Ti:sapphire laser.

The cavity dispersion noncoaxiality (CDN) effects on broadband few-cycle pulse generation of a Kerr-lens mode-locked Ti:sapphire laser is investigated theoretically and experimentally. It is found that the influence of CDN is comparable with that of self-focusing and self-phase-modulation in the frequency-dependent mode size (FDMS) effects. Spectra extending from 680 nm to 1020 nm with pulse duration shorter than three optical cycles are favorably generated under the minimum CDN in the vicinity of the coaxial point of the sub-cavity.

High thermal stability and low power dissipation PCM with nanoscale oxygen-doped SS thin film.

To improve thermal stability and reduce power dissipation of phase-change memory (PCM), the oxygen-doped Sn15Sb85 (SS) thin film is proposed by magnetron sputtering in this study. Comparing to undoped Sn15Sb85(SS), the oxygen-doped-SS thin film has superior thermal stability and better data retention. Meanwhile, the electrical conductivity of crystallisation oxygen-doped-SS thin film is also lower than that of SS, which means its less power consuming in PCM. The electrical conductivity ratio between amorphous and crystalline states for oxygen-doped SS reaches up to two orders of magnitude. After oxygen doping, the root-mean-square surface roughness from amorphous (0.29 nm) to crystalline (0.46 nm) state for oxygen-doped-SS thin films becomes smaller. The switching time of amorphisation process for the oxygen-doped-SS thin film (∼2.07 ns) is shorter than Ge2Sb2Te5 (GST) (∼3.05 ns). X-ray diffractometer is recorded to investigate the change of crystalline structure. Thus, the authors infer that oxygen-doped SS is a promising phase-change thin film for PCM.

Spin relaxation dynamics of holes in intrinsic GaAs quantum wells studied by transient circular dichromatic absorption spectroscopy at room temperature.

Spin relaxation dynamics of holes in intrinsic GaAs quantum wells is studied using time-resolved circular dichromatic absorption spectroscopy at room temperature. It is found that ultrafast dynamics is dominated by the cooperative contributions of band filling and many-body effects. The relative contribution of the two effects is opposite in strength for electrons and holes. As a result, transient circular dichromatic differential transmission (TCD-DT) with co- and cross-circularly polarized pump and probe presents different strength at several picosecond delay time. Ultrafast spin relaxation dynamics of excited holes is sensitively reflected in TCD-DT with cross-circularly polarized pump and probe. A model, including coherent artifact, thermalization of nonthermal carriers and the cooperative contribution of band filling and many-body effects, is developed, and used to fit TCD-DT with cross-circularly polarized pump and probe. Spin relaxation time of holes is achieved as a function of excited hole density for the first time at room temperature, and increases with hole density, which disagrees with a theoretical prediction based on EY spin relaxation mechanism, implying that EY mechanism may be not dominant hole spin relaxation mechanism at room temperature, but DP mechanism is dominant possibly.

Simultaneous laser excitation of backward volume and perpendicular standing spin waves in full-Heusler Co2FeAl0.5Si0.5 films.

Spin-wave dynamics in full-Heusler Co2FeAl0.5Si0.5 films are studied using all-optical pump-probe magneto-optical polar Kerr spectroscopy. Backward volume magnetostatic spin-wave (BVMSW) mode is observed in films with thickness ranging from 20 to 100 nm besides perpendicular standing spin-wave (PSSW) mode, and found to be excited more efficiently than the PSSW mode. The field dependence of the effective Gilbert damping parameter appears especial extrinsic origin. The relationship between the lifetime and the group velocity of BVMSW mode is revealed. The frequency of BVMSW mode does not obviously depend on the film thickness, but the lifetime and the effective damping appear to do so. The simultaneous excitation of BVMSW and PSSW in Heusler alloy films as well as the characterization of their dynamic behaviors may be of interest for magnonic and spintronic applications.

Multilayer SnSb4-SbSe Thin Films for Phase Change Materials Possessing Ultrafast Phase Change Speed and Enhanced Stability.

A multilayer thin film, comprising two different phase change material (PCM) components alternatively deposited, provides an effective means to tune and leverage good properties of its components, promising a new route toward high-performance PCMs. The present study systematically investigated the SnSb4-SbSe multilayer thin film as a potential PCM, combining experiments and first-principles calculations, and demonstrated that these multilayer thin films exhibit good electrical resistivity, robust thermal stability, and superior phase change speed. In particular, the potential operating temperature for 10 years is shown to be 122.0 °C and the phase change speed reaches 5 ns in the device test. The good thermal stability of the multilayer thin film is shown to come from the formation of the Sb2Se3 phase, whereas the fast phase change speed can be attributed to the formation of vacancies and a SbSe metastable phase. It is also demonstrated that the SbSe metastable phase contributes to further enhancing the electrical resistivity of the crystalline state and the thermal stability of the amorphous state, being vital to determining the properties of the multilayer SnSb4-SbSe thin film.

Quantitative Analysis of Temperature Dependence of Raman shift of monolayer WS2.

We report the temperature-dependent evolution of Raman spectra of monolayer WS2 directly CVD-grown on a gold foil and then transferred onto quartz substrates over a wide temperature range from 84 to 543 K. The nonlinear temperature dependence of Raman shifts for both and A1g modes has been observed. The first-order temperature coefficients of Raman shifts are obtained to be -0.0093 (cm(-1)/K) and -0.0122 (cm(-1)/K) for and A1g peaks, respectively. A physical model, including thermal expansion and three- and four-phonon anharmonic effects, is used quantitatively to analyze the observed nonlinear temperature dependence. Thermal expansion coefficient (TEC) of monolayer WS2 is extracted from the experimental data for the first time. It is found that thermal expansion coefficient of out-plane mode is larger than one of in-plane mode, and TECs of and A1g modes are temperature-dependent weakly and strongly, respectively. It is also found that the nonlinear temperature dependence of Raman shift of mode mainly originates from the anharmonic effect of three-phonon process, whereas one of A1g mode is mainly contributed by thermal expansion effect in high temperature region, revealing that thermal expansion effect cannot be ignored.

Effect of periodic number of [Si/Sb80Te20]x multilayer film on its laser-induced crystallization studied by coherent phonon spectroscopy.

The periodic number dependence of the femtosecond laser-induced crystallization threshold of [Si(5nm)/Sb80Te20(5nm)]x nanocomposite multilayer films has been investigated by coherent phonon spectroscopy. Coherent optical phonon spectra show that femtosecond laser-irradiated crystallization threshold of the multilayer films relies obviously on the periodic number of the multilayer films and decreases with the increasing periodic number. The mechanism of the periodic number dependence is also studied. Possible mechanisms of reflectivity and thermal conductivity losses as well as the effect of the glass substrate are ruled out, while the remaining superlattice structure effect is ascribed to be responsible for the periodic number dependence. The sheet resistance of multilayer films versus a lattice temperature is measured and shows a similar periodic number dependence with one of the laser irradiation crystallization power threshold. In addition, the periodic number dependence of the crystallization temperature can be fitted well with an experiential formula obtained by considering coupling exchange interactions between adjacent layers in a superlattice. Those results provide us with the evidence to support our viewpoint. Our results show that the periodic number of multilayer films may become another controllable parameter in the design and parameter optimization of multilayer phase change films.

Synchronized time- and high-field-resolved all-optical pump-probe magneto-optical setup based on a strong alternating magnetic field and its application in magnetization dynamics of high coercivity magnetic medium.

An alternating magnetic field (AMF) apparatus is developed and composed of an electromagnet and driving power supply. The structure of the electromagnet and configuration of the driving supply are described in detail. The apparatus can produce a peak magnetic field up to 9000 Oe and above under a small driving power at its resonance frequency of 1.14 kHz. Based on synchronization between the AMF and the femtosecond laser pulse train, a photomagnetic synchronized time- and high-field-resolved all-optical pump-probe magnetic-optical setup is developed. This setup has the ability to reinitialize any magnetic states between two successive laser pulses so that irreversible magnetization reversal dynamics can be studied. Dynamic Kerr hysteresis loops and magnetization reversal dynamics of high coercivity ferromagnetic TbFeCo and FePt films are demonstrated using this setup, showing importance of this synchronized time- and high-field-resolved all-optical pump-probe magnetic-optical setup in the research of ultrafast magnetization dynamics.