Nonvolatile Isomorphic Valence Transition in SmTe Films.

The burgeoning field of optoelectronic devices necessitates a mechanism that gives rise to a large contrast in the electrical and optical properties. A SmTe film with a NaCl-type structure demonstrates significant differences in resistivity (over 105) and band gap (approximately 1.45 eV) between as-deposited and annealed films, even in the absence of a structural transition. The change in the electronic structure and accompanying physical properties is attributed to a rigid-band shift triggered by a valence transition (VT) between Sm2+ and Sm3+. The stress field within the SmTe film appears closely tied to the mixed valence state of Sm, suggesting that stress is a driving force in this VT. By mixing the valence states, the formation energy of the low-resistive state decreases, providing nonvolatility. Moreover, the valence state of Sm can be regulated through annealing and device-operation processes, such as applying voltage and current pulses. This investigation introduces an approach to developing semiconductor materials for optoelectrical applications.

Lithium-Induced Reorientation of Few-Layer MoS2 Films.

Molybdenum disulfide (MoS2) few-layer films have gained considerable attention for their possible applications in electronics and optics and also as a promising material for energy conversion and storage. Intercalating alkali metals, such as lithium, offers the opportunity to engineer the electronic properties of MoS2. However, the influence of lithium on the growth of MoS2 layers has not been fully explored. Here, we have studied how lithium affects the structural and optical properties of the MoS2 few-layer films prepared using a new method based on one-zone sulfurization with Li2S as a source of lithium. This method enables incorporation of Li into octahedral and tetrahedral sites of the already prepared MoS2 films or during MoS2 formation. Our results discover an important effect of lithium promoting the epitaxial growth and horizontal alignment of the films. Moreover, we have observed a vertical-to-horizontal reorientation in vertically aligned MoS2 films upon lithiation. The measurements show long-term stability and preserved chemical composition of the horizontally aligned Li-doped MoS2.

NbTe4 Phase-Change Material: Breaking the Phase-Change Temperature Balance in 2D Van der Waals Transition-Metal Binary Chalcogenide.

2D van der Waals (vdW) transition metal di-chalcogenides (TMDs) have garnered significant attention in the nonvolatile memory field for their tunable electrical properties, scalability, and potential for phase engineering. However, their complex switching mechanism and complicated fabrication methods pose challenges for mass production. Sputtering is a promising technique for large-area 2D vdW TMD fabrication, but the high melting point (typically Tm > 1000 °C) of TMDs requires elevated temperatures for good crystallinity. This study focuses on the low-Tm 2D vdW TM tetra-chalcogenides and identifies NbTe4 as a promising candidate with an ultra-low Tm of around 447 °C (onset temperature). As-grown NbTe4 forms an amorphous phase upon deposition that can be crystallized by annealing at temperatures above 272 °C. The simultaneous presence of a low Tm and a high crystallization temperature Tc can resolve important issues facing current phase-change memory compounds, such as high Reset energies and poor thermal stability of the amorphous phase. Therefore, NbTe4 holds great promise as a potential solution to these issues.

Discovery of a metastable van der Waals semiconductor via polymorphic crystallization of an amorphous film.

Here we report on the growth of thin crystalline films of the metastable phase GeTe2. Direct observation by transmission electron microscopy revealed a Te-Ge-Te stacking with van der Waals gaps. Moreover, electrical and optical measurements revealed the films exhibted semiconducting properties commensurate with electronics applications. Feasibility studies in which device structures were fabricated demonstrated the potential application of GeTe2 as an electronic material.

Dimensional transformation of chemical bonding during crystallization in a layered chalcogenide material.

Two-dimensional (2D) van der Waals (vdW) materials possess a crystal structure in which a covalently-bonded few atomic-layer motif forms a single unit with individual motifs being weakly bound to each other by vdW forces. Cr2Ge2Te6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications. Here, we provide evidence for a dimensional transformation in the chemical bonding from a randomly bonded three-dimensional (3D) disordered amorphous phase to a 2D bonded vdW crystalline phase. A counterintuitive metastable "quasi-layered" state during crystallization that exhibits both "long-range order and short-range disorder" with respect to atomic alignment clearly distinguishes the system from conventional materials. This unusual behavior is thought to originate from the 2D nature of the crystalline phase. These observations provide insight into the crystallization mechanism of layered materials in general, and consequently, will be useful for the realization of 2D vdW material-based functional nanoelectronic device applications.

Lithium-Doping Effects in Cu(In,Ga)Se2 Thin-Film and Photovoltaic Properties.

The beneficial effects of heavy alkali metals such as K, Rb, and Cs in enhancing Cu(In,Ga)Se2 (CIGS) photovoltaic efficiencies are widely known, though the detailed mechanism is still open for discussion. In the present work, the effects of the lightest alkali metal, Li, on CIGS thin-film and device properties are focused upon and compared to the effects of heavy alkali metals. Till date, the beneficial effects of elemental Li on Cu2ZnSnS4 photovoltaic devices in enhancing efficiencies have been reported. On the other hand, it is shown in the present work that the beneficial effects of Li on CIGS are not so significant. In contrast to the effects of Na or Rb in enhancing CIGS(112) growth orientation, Li was revealed not to affect CIGS growth orientation. The most distinctive feature observed between Li and other alkali metals was the elemental depth profile in CIGS films. Namely, Na and heavier alkali metals show a concentration peak near the surface (relatively Cu-poor) region of CIGS films, whereas elemental Li showed no such trend, suggesting that Li has no significant effect on CIGS surface modification. Nonetheless, Li was found to have some effect in enhancing the PL peak intensity and photovoltaic performance of CIGS, though the effect is relatively small in comparison to that obtained with other alkali metals.

Cr-Triggered Local Structural Change in Cr2Ge2Te6 Phase Change Material.

Cr2Ge2Te6 (CrGT) is a phase change material with higher resistivity in the crystalline phase than in the amorphous phase. CrGT exhibits an ultralow operation energy for amorphization. In this study, the origin of the increased resistance in crystalline CrGT compared to amorphous CrGT and the underlying phase change mechanism were investigated in terms of both local structural change and associated change in electronic state. The density of states at the Fermi level in crystalline CrGT decreased with increasing annealing temperature and became negligible upon annealing at 380 °C. Simultaneously, the Fermi level shifted from the vicinity of the valence band to the band gap center, leading to an increase in resistance. The phase change from amorphous to crystalline CrGT occurred through a metastable crystalline phase with a local structure similar to that of the amorphous phase. Cr nanoclusters were confirmed to exist in both the amorphous and crystalline phases. The presence of Cr nanoclusters induced Cr vacancies in the crystalline phase. These Cr vacancies generated hole carriers, leading to p-type conduction. Photoelectron spectroscopy of the Cr 2s core level clearly indicated a decrease in the fraction of Cr-Cr bonds and an increase in the fraction of Cr-Te bonds in crystalline CrGT upon annealing. Meanwhile, the coordination number of the Cr nanoclusters decreased as the number of Cr-Cr bonds was reduced. Together, these results imply that the origin of the increased resistance in crystalline CrGT is the filling of Cr vacancies by Cr atoms diffusing from Cr nanoclusters.

Photon energy dependence of Kerr rotation in GeTe/Sb2Te3 chalcogenide superlattices.

We report on pump-probe based helicity dependent time-resolved Kerr measurements under infrared excitation of chalcogenide superlattices, consisting of alternately stacked GeTe and Sb2Te3 layers. The Kerr rotation signal consists of the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE), both of which are found to monotonically increase with decreasing photon energy over a sub-eV energy range. Although the dependence of the SIFE can be attributed to the response function of direct third-order nonlinear susceptibility, the magnitude of the SOKE reflects cascading second-order nonlinear susceptibility resulting from electronic transitions between bulk valence/conduction bands and interface-originating Dirac states of the superlattice.

Terahertz generation measurements of multilayered GeTe-Sb2Te3 phase change materials.

Multilayered structures of GeTe and Sb2Te3 phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measurements. Three- and 20-layer iPCM samples were irradiated with a femtosecond laser, and the generated THz radiation was observed. The emitted THz pulse was found to be always p polarized independent of the polarization of the excitation pulse. Based on the polarization dependence as well as the flip of the THz field from photoexcited Sb2Te3 and Bi2Te3, the THz emission process can be attributed to the surge current flow due to the built-in surface depletion layer formed in p-type semiconducting iPCM materials.

Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses.

We have systematically investigated the spatial and temporal dynamics of crystallization that occur in the phase-change material Ge_{2}Sb_{2}Te_{5} upon irradiation with an intense terahertz (THz) pulse. THz-pump-optical-probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This fact causes the large enhancement of electric field associated with the THz pulses only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer-scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.

Topological Phase Buried in a Chalcogenide Superlattice Monitored by Helicity-Dependent Kerr Measurement.

Chalcogenide superlattices (SLs), formed by the alternate stacking of GeTe and Sb2Te3 layers, also referred to as interfacial phase-change memory (iPCM), are a leading candidate for spin-based memory device applications. Theoretically, the iPCM structure has been predicted to form a three-dimensional topological insulator or Dirac semimetal phase depending on the constituent layer thicknesses. Here, we experimentally investigate the topological insulating nature of chalcogenide SLs using a helicity-dependent time-resolved Kerr measurement. The helicity-dependent Kerr signal is observed to exhibit a four-cycle oscillation with π/2 periodicity, suggesting the existence of a Dirac-like cone in some chalcogenide SLs. Furthermore, we found that increasing the thickness of the GeTe layer dramatically changed the periodicity, indicating a phase transition from a Dirac semimetal into a trivial insulator. Our results demonstrate that thickness-tuned chalcogenide SLs can play an important role in the manipulation of topological states, which may open up new possibilities for spintronic devices based on chalcogenide SLs.

A cascading nonlinear magneto-optical effect in topological insulators.

Topological insulators (TIs) are characterized by possessing metallic (gapless) surface states and a finite band-gap state in the bulk. As the thickness of a TI layer decreases down to a few nanometers, hybridization between the top and bottom surfaces takes place due to quantum tunneling, consequently at a critical thickness a crossover from a 3D-TI to a 2D insulator occurs. Although such a crossover is generally accessible by scanning tunneling microscopy, or by angle-resolved photoemission spectroscopy, such measurements require clean surfaces. Here, we demonstrate that a cascading nonlinear magneto-optical effect induced via strong spin-orbit coupling can examine such crossovers. The helicity dependence of the time-resolved Kerr rotation exhibits a robust change in periodicity at a critical thickness, from which it is possible to predict the formation of a Dirac cone in a film several quintuple layers thick. This method enables prediction of a Dirac cone using a fundamental nonlinear optical effect that can be applied to a wide range of TIs and related 2D materials.

Compositional tuning in sputter-grown highly-oriented Bi-Te films and their optical and electronic structures.

Growth of Bi-Te films by helicon-wave magnetron sputtering is systematically explored using alloy targets. The film compositions obtained are found to strongly depend on both the sputtering and antenna-coil powers. The obtainable film compositions range from Bi55Te45 to Bi43Te57 when a Bi2Te3 alloy target is used, and from Bi42Te58 to Bi40Te60 (Bi2Te3) for a Te-rich Bi30Te70 target. All films show strong orientation of the van der Waals layers (00l planes) parallel to the substrate. The atomic level stacking of Bi2Te3 quintuple and Bi bi-layers has been directly observed by high resolution transmission electron microscopy. Band structure simulations reveal that Bi-rich Bi4Te3 bulk is a zero band gap semimetal with a Dirac cone at the Gamma point when spin-orbit coupling is included. Optical measurements also confirm that the material has a zero band gap. The tunability of the composition and the topological insulating properties of the layers will enable the use of these materials for future electronics applications on an industrial scale.

Si-Doping Effects in Cu(In,Ga)Se2 Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells.

We found that elemental Si-doped Cu(In,Ga)Se2 (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure buffer-free solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition alkali halide treatments such as KF or RbF, techniques known to boost CIGS device performance. The results obtained offer a new approach based on a new concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials for better device performance.

Pressure-Induced Phase Transitions in GeTe-Rich Ge-Sb-Te Alloys across the Rhombohedral-to-Cubic Transitions.

We demonstrate that pressure-induced amorphization in Ge-Sb-Te alloys across the ferroelectric-paraelectric transition can be represented as a mixture of coherently distorted rhombohedral Ge8Sb2Te11 and randomly distorted cubic Ge4Sb2Te7 and high-temperature Ge8Sb2Te11 phases. While coherent distortion in Ge8Sb2Te11 does not prevent the crystalline state from collapsing into its amorphous counterpart in a similar manner to pure GeTe, the pressure-amorphized Ge8Sb2Te11 phase begins to revert to the crystalline cubic phase at ∼9 GPa in contrast to Ge4Sb2Te7, which remains amorphous under ambient conditions when gradually decompressed from 40 GPa. Moreover, experimentally, it was observed that pressure-induced amorphization in Ge8Sb2Te11 is a temperature-dependent process. Ge8Sb2Te11 transforms into the amorphous phase at ∼27.5 and 25.2 GPa at room temperature and 408 K, respectively, and completely amorphizes at 32 GPa at 408 K, while some crystalline texture could be seen until 38 GPa (the last measurement point) at room temperature. To understand the origins of the temperature dependence of the pressure-induced amorphization process, density functional theory calculations were performed for compositions along the (GeTe)x - (Sb2Te3)1-x tie line under large hydrostatic pressures. The calculated results agreed well with the experimental data.

Manipulating the Bulk Band Structure of Artificially Constructed van der Waals Chalcogenide Heterostructures.

The bulk band structures of a variety of artificially constructed van der Waals chalcogenide heterostructures IVTe/V2VI3 (IV: C, Si, Ge, Sn, Pb; V: As, Sb, Bi; VI: S, Se, Te) have been systematically examined using ab initio simulations based on density functional theory. The crystal structure and the electronic band structure of the heterostructures were found to strongly depend on the choice of elements as well as the presence of van der Waals corrections. Furthermore, it was found that the use of the modified Becke-Johnson local density approximation functional demonstrated that a Dirac cone is formed when tensile stress is applied to a GeTe/Sb2Te3 heterostructure, and the band gap can be controlled by tuning the stress. Based on these simulation results, a novel electrical switching device using a chalcogenide heterostructure is proposed.

Atomic Reconfiguration of van der Waals Gaps as the Key to Switching in GeTe/Sb2Te3 Superlattices.

Nonvolatile memory, of which phase-change memory (PCM) is a leading technology, is currently a key element of various electronics and portable systems. An important step in the development of conceptually new devices is the class of van der Waals (vdW)-bonded GeTe/Sb2Te3 superlattices (SLs). With their order of magnitude faster switching rates and lower energy consumption compared to those of alloy-based devices, they are widely regarded as the next step in the implementation of PCM. In contrast to conventional PCM, where the SET and RESET states arise from the crystalline and amorphous phases, in SLs, both the SET and RESET states remain crystalline. In an earlier work, the superior performance of SLs was attributed to the reduction of entropic losses associated with the one-dimensional motion of interfacial Ge atoms located in the vicinity of Sb2Te3 quintuple layers. Subsequent experimental studies using transmission electron microscopy revealed that GeTe and Sb2Te3 blocks strongly intermix during the growth of the GeTe phase, challenging the original proposal but at the same time raising new fundamental issues. In this work, we propose a new approach to switching in SLs associated with the reconfiguration of vdW gaps accompanied by local deviation of stoichiometry from the GeTe/Sb2Te3 quasibinary alloys. The model resolves in a natural way the existing controversies, explains the large conductivity contrast between the SET and RESET crystalline states, is not compromised by Ge/Sb intermixing, and provides a new perspective for the industrial development of memory devices based on such SLs. The proposed concept of vdW gap reconfiguration may also be applicable to designing a broad variety of engineered two-dimensional vdW solids.

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials.

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived (>100 ps) transient metastable state of Ge2Sb2Te5 with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies.

Anisotropic lattice response induced by a linearly-polarized femtosecond optical pulse excitation in interfacial phase change memory material.

Optical excitation of matter with linearly-polarized femtosecond pulses creates a transient non-equilibrium lattice displacement along a certain direction. Here, the pump and probe pulse polarization dependence of the photo-induced ultrafast lattice dynamics in (GeTe)2/(Sb2Te3)4 interfacial phase change memory material is investigated under obliquely incident conditions. Drastic pump polarization dependence of the coherent phonon amplitude is observed when the probe polarization angle is parallel to the c-axis of the sample, while the pump polarization dependence is negligible when the probe polarization angle is perpendicular to the c-axis. The enhancement of phonon oscillation amplitude due to pump polarization rotation for a specific probe polarization angle is only found in the early time stage (≤2 ps). These results indicate that the origin of the pump and probe polarization dependence is dominantly attributable to the anisotropically-formed photo-excited carriers which cause the directional lattice dynamics.

Coherent gigahertz phonons in Ge2Sb2Te5 phase-change materials.

Using ≈40 fs ultrashort laser pulses, we investigate the picosecond acoustic response from a prototypical phase change material, thin Ge2Sb2Te5 (GST) films with various thicknesses. After excitation with a 1.53 eV-energy pulse with a fluence of ≈5 mJ cm(-2), the time-resolved reflectivity change exhibits transient electronic response, followed by a combination of exponential-like strain and coherent acoustic phonons in the gigahertz (GHz) frequency range. The time-domain shape of the coherent acoustic pulse is well reproduced by the use of the strain model by Thomsen et al 1986 (Phys. Rev. B 34 4129). We found that the decay rate (the inverse of the relaxation time) of the acoustic phonon both in the amorphous and in the crystalline phases decreases as the film thickness increases. The thickness dependence of the acoustic phonon decay is well modeled based on both phonon-defect scattering and acoustic phonon attenuation at the GST/Si interface, and it is revealed that those scattering and attenuation are larger in crystalline GST films than those in amorphous GST films.