Enhanced Photocharacteristics by Fermi Level Modulating in Sb2 Te3 /Bi2 Se3 Topological Insulator p-n Junction.

Topological insulators have recently received attention in optoelectronic devices because of their high mobility and broadband absorption resulting from their topological surface states. In particular, theoretical and experimental studies have emerged that can improve the spin generation efficiency in a topological insulator-based p-n junction structure called a TPNJ, drawing attention in optospintronics. Recently, research on implementing the TPNJ structure is conducted; however, studies on the device characteristics of the TPNJ structure are still insufficient. In this study, the TPNJ structure is effectively implemented without intermixing by controlling the annealing temperature, and the photocharacteristics appearing in the TPNJ structure are investigated using a cross-pattern that can compare the characteristics in a single device. Enhanced photo characteristics are observed for the TPNJ structure. An optical pump Terahertz probe and a physical property measurement system are used to confirm the cause of improved photoresponsivity. Consequently, the photocharacteristics are improved owing to the change in the absorption mechanism and surface transport channel caused by the Fermi level shift in the TPNJ structure.

Measurement of dielectric function and bandgap of germanium telluride using monochromated electron energy-loss spectroscopy.

Using a monochromator in transmission electron microscopy, a low-energy-loss spectrum can provide inter- and intra-band transition information for nanoscale devices with high energy and spatial resolutions. However, some losses, such as Cherenkov radiation, phonon scattering, and surface plasmon resonance superimposed at zero-loss peak, make it asymmetric. These pose limitations to the direct interpretation of optical properties, such as complex dielectric function and bandgap onset in the raw electron energy-loss spectra. This study demonstrates measuring the dielectric function of germanium telluride using an off-axis electron energy-loss spectroscopy method. The interband transition from the measured complex dielectric function agrees with the calculated band structure of germanium telluride. In addition, we compare the zero-loss subtraction models and propose a reliable routine for bandgap measurement from raw valence electron energy-loss spectra. Using the proposed method, the direct bandgap of germanium telluride thin film was measured from the low-energy-loss spectrum in transmission electron microscopy. The result is in good agreement with the bandgap energy measured using an optical method.

Forming Stable van der Waals Contacts between Metals and 2D Semiconductors.

High-performing 2D electrical and optical devices can be realized by forming an ideal van der Waals (vdW) metal contact with weak interactions and stable interface states. However, the methods for applying metal contacts while avoiding damage from metal deposition present challenges in realizing a uniform, stable vdW interface. To overcome this problem, this study develops a method for forming vdW contacts using a sacrificial Se buffer layer. This study explores this method by investigating the difference in the Schottky barrier height between the vdW metal contact deposited using a buffer layer, a transferred metal contact, and a conventional directly deposited metal contact using rectification and photovoltaic characteristics of a Schottky diode structure with graphite. Evidently, the Se buffer layer method forms the most stable and ideal vdW contact while preventing Fermi-level pinning. A tungsten diselenide Schottky diode fabricated using these vdW contacts with Au and graphite as the top and bottom electrodes, respectively, exhibits excellent operation with an ideality factor of ≈1, an on/off ratio of > 107 , and coherent properties. Additionally, when using only the vdW Au contact, the electrical and optical properties of the device can be minutely modulated by changing the structure of the Schottky diode.

Funnel Devices Based on Asymmetrically Strained Transition Metal Dichalcogenides.

The strain applied to transition metal dichalcogenides (TMDs) reduces their energy bandgap, and local strains result in a funnel-like band structure in which funneled excitons move toward the most strained region. Herein, a funnel device based on asymmetrically strained WS2 and MoS2 is reported. Asymmetric strains are induced by transferring the TMD flakes onto a fork-shaped SU-8 microstructure. Raman and photoluminescence spectra peaks are shifted according to the morphology of the SU-8 microstructure, indicating the application of asymmetric strains to the TMDs. To investigate whether funneled excitons can be converted to electrical currents, various devices are constructed by depositing symmetric and asymmetric electrodes onto the strained TMDs. The scanning photocurrent mapping images follow a fork-shaped pattern, indicating probable conversion of the funneled excitons into electrical currents. In the case of the funnel devices with asymmetric Au and Al electrodes, short-circuit current (ISC ) of WS2 is enhanced by the strains, whereas ISC of MoS2 is suppressed because the Schottky barrier lowers with increasing strain for the MoS2 . These results demonstrate that the funnel devices can be implemented using asymmetrically strained TMDs and the effect of strains on the Schottky barrier is dependent on the TMD used.

Conversion between Metavalent and Covalent Bond in Metastable Superlattices Composed of 2D and 3D Sublayers.

Reversible conversion over multimillion times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, an extended category of crystal structures from stable states via redistribution of vacancies, research on kinetic behavior of the vacancies is highly in demand. However, it remains lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies of Sb2Te3 gradually occurs with diverged energy barriers owing to their own activation code: the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Studies on the behavior can be further applied to multiphase superlattices composed of Sb2Te3 (2D) and GeTe (3D) sublayers, which represent superior memory performances, but their operating mechanisms were still under debate due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb2Te3 (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could only be achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies toward vacancy engineering are expected in versatile materials.

Topological Surface-Dominated Spintronic THz Emission in Topologically Nontrivial Bi1- x Sbx Films.

Topological materials have significant potential for spintronic applications owing to their superior spin-charge interconversion. Here, the spin-to-charge conversion (SCC) characteristics of epitaxial Bi1- x Sbx films is investigated across the topological phase transition by spintronic terahertz (THz) spectroscopy. An unexpected, intense spintronic THz emission is observed in the topologically nontrivial semimetal Bi1- x Sbx films, significantly greater than that of Pt and Bi2 Se3 , which indicates the potential of Bi1- x Sbx for spintronic applications. More importantly, the topological surface state (TSS) is observed to significantly contribute to SCC, despite the coexistence of the bulk state, which is possible via a unique ultrafast SCC process, considering the decay process of the spin-polarized hot electrons. This means that topological material-based spintronic devices should be fabricated in a manner that fully utilizes the TSS, not the bulk state, to maximize their performance. The results not only provide a clue for identifying the source of the giant spin Hall angle of Bi1- x Sbx , but also expand the application potential of topological materials by indicating that the optically induced spin current provides a unique method for focused-spin injection into the TSS.

Ferroelastic-Ferroelectric Multiferroicity in van der Waals Rhenium Dichalcogenides.

2D multiferroics with combined ferroic orders have gained attention owing to their novel functionality and underlying science. Intrinsic ferroelastic-ferroelectric multiferroicity in single-crystalline van der Waals rhenium dichalcogenides, whose symmetries are broken by the Peierls distortion and layer-stacking order, is demonstrated. Ferroelastic switching of the domain orientation and accompanying anisotropic properties is achieved with 1% uniaxial strain using the polymer encapsulation method. Based on the electron localization function and bond dissociation energy of the Re-Re bonds, the change in bond configuration during the evolution of the domain wall and the preferred switching between the two specific orientation states are explained. Furthermore, the ferroelastic switching of ferroelectric polarization is confirmed using the photovoltaic effect. The study provides insights into the reversible bond-switching process and potential applications based on 2D multiferroicity.

Switching to Hidden Metallic Crystal Phase in Phase-Change Materials by Photoenhanced Metavalent Bonding.

Metavalent bonding is crucial for the determination of phase transition and improvement of device performance in phase-change materials, which are attracting interest for use in memory devices. Although monitoring dielectric and phononic parameters provides a direct measure of the metavalent bonding, the control of phase-change phenomena and metavalent bonding in the dynamical regime has yet to be demonstrated. This study reports the photoenhanced metavalent bonding and resulting hidden metallic crystalline state of Ti-doped Sb2Te3, a representative phase-change material with ultralong sustainability. Using ultrafast terahertz spectroscopy, Ti0.4Sb2Te3 was discovered to possess ultralong pump-probe dynamics, which is retained over hundreds of picoseconds, unlike the short-lived state of undoped Sb2Te3. Moreover, for Ti0.4Sb2Te3 during the long-lived transmission change, the infrared-active phonon is highly softened, even more than the amount of a thermal phonon shift, indicating the photoenhancement of lattice anharmonicity. Such a long-lived relaxation implies photoinduced transition into a crystalline state of ultrastrong metavalent bonding in Ti0.4Sb2Te3, on the basis of comparisons of the dynamical dielectric constant and temporal phonon shift. Our results show the realization of photoengineering of phase-change materials by tuning electron sharing or transferring.

Enhanced Spin-to-Charge Conversion Efficiency in Ultrathin Bi2Se3 Observed by Spintronic Terahertz Spectroscopy.

Owing to their remarkable spin-charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin-orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi2Se3 was enhanced by up to ∼2.4 times.

Optical characteristics of type-II hexagonal-shaped GaSb quantum dots on GaAs synthesized using nanowire self-growth mechanism from Ga metal droplet.

We report the growth mechanism and optical characteristics of type-II band-aligned GaSb quantum dots (QDs) grown on GaAs using a droplet epitaxy-driven nanowire formation mechanism with molecular beam epitaxy. Using transmission electron microscopy and scanning electron microscopy images, we confirmed that the QDs, which comprised zinc-blende crystal structures with hexagonal shapes, were successfully grown through the formation of a nanowire from a Ga droplet, with reduced strain between GaAs and GaSb. Photoluminescence (PL) peaks of GaSb capped by a GaAs layer were observed at 1.11 eV, 1.26 eV, and 1.47 eV, assigned to the QDs, a wetting-like layer (WLL), and bulk GaAs, respectively, at the measurement temperature of 14 K and excitation laser power of 30 mW. The integrated PL intensity of the QDs was significantly stronger than that of the WLL, which indicated well-grown GaSb QDs on GaAs and the generation of an interlayer exciton, as shown in the power- and temperature-dependent PL spectra, respectively. In addition, time-resolved PL data showed that the GaSb QD and GaAs layers formed a self-aligned type-II band alignment; the temperature-dependent PL data exhibited a high equivalent internal quantum efficiency of 15 ± 0.2%.

Enhanced reliability of phase-change memory via modulation of local structure and chemical bonding by incorporating carbon in Ge2Sb2Te5.

In this study, we investigated the effect of phase-change characteristics on the device performance of carbon-incorporated Ge2Sb2Te5 (CGST) to understand the origin of the enhanced reliability and stabilization of the device. Macroscopic and microscopic measurements confirmed that the structural stability significantly increased with the incorporation of as much as 10% carbon. After the completion of bond formation between C and Ge, the excess C (>5 atomic%) engages in bonding with Sb in localized regions because of the difference in formation energy. These bonds of C with Ge and Sb induce non-uniform local charge density of the short-range order. Finally, because the strong bonds between Ge and C shorten the short Ge-Te bonds, the high thermal stability of CGST relative to that of GST can be attributed to intensified Peierls distortion. The formation of strong bonds successfully underpins the local structures and reduces the stochastic effect. Moreover, extension of the C bonding to Sb enhances the structural reliability, resulting in highly stable CGST in the amorphous phase. Finally, the device stability of CGST in the reset state of the amorphous structure during the device switching process was significantly improved.

Quasicrystalline phase-change memory.

Phase-change memory utilizing amorphous-to-crystalline phase-change processes for reset-to-set operation as a nonvolatile memory has been recently commercialized as a storage class memory. Unfortunately, designing new phase-change materials (PCMs) with low phase-change energy and sufficient thermal stability is difficult because phase-change energy and thermal stability decrease simultaneously as the amorphous phase destabilizes. This issue arising from the trade-off relationship between stability and energy consumption can be solved by reducing the entropic loss of phase-change energy as apparent in crystalline-to-crystalline phase-change process of a GeTe/Sb2Te3 superlattice structure. A paradigm shift in atomic crystallography has been recently produced using a quasi-crystal, which is a new type of atomic ordering symmetry without any linear translational symmetry. This paper introduces a novel class of PCMs based on a quasicrystalline-to-approximant crystalline phase-change process, whose phase-change energy and thermal stability are simultaneously enhanced compared to those of the GeTe/Sb2Te3 superlattice structure. This report includes a new concept that reduces entropic loss using a quasicrystalline state and takes the first step in the development of new PCMs with significantly low phase-change energy and considerably high thermal stability.

Ultra-low Energy Phase Change Memory with Improved Thermal Stability by Tailoring the Local Structure through Ag Doping.

Although Sb2Te3, as a candidate material for next-generation memory devices, has attractive properties such as higher operation speed and lower power consumption than Ge2Sb2Te5, its poor stability prevents its application to commercial memory devices. Transition metal dopants provide enhancements in its phase change characteristics, improving both thermal stability and operation energy. However, the enhancement mechanism remains to be sufficiently investigated, and standard properties need to be achieved. Herein, the phase change properties of Sb2Te3 are confirmed to be enhanced by the incorporation of a heavy transition metal element such as Ag. The crystallization temperature increases by nearly 40%, and the operation energy is reduced by approximately 60%. These enhancements are associated with the changes in the local Sb2Te3 structure caused by Ag incorporation. As the incorporated Ag atoms substitute Sb in the Sb-Te octahedron, this turns into a Ag-Te defective tetrahedron with a strong Ag-Te bond that induces distortion in the crystal lattice. The formation of this bond is attributed to the electron configuration of Ag and its fully filled d orbital. Thus, Ag-doped Sb2Te3 is a promising candidate for practical phase change memory devices with high stability and high operation speed.

Enhanced Photoinduced Carrier Generation Efficiency through Surface Band Bending in Topological Insulator Bi2Se3 Thin Films by the Oxidized Layer.

Topological insulators (TIs) have become popular in the field of optoelectronic devices because of their broadband and high-sensitivity properties, which are attributed to the narrow band gap of the bulk state and high mobility of the Dirac surface state. Although perfectly grown TIs are known to exhibit strong stability against oxidation, in most cases, the existence of vacancy defects in TIs reacts to air and the characteristics of TIs is affected by oxidation. Therefore, changes in the band structure and electrical characteristics by oxidation should be considered. A significant change occurs because of the oxidation; however, the dependence of the photoresponse of TIs on oxidation has not been studied in detail. In this study, the photoresponsivity of oxidized Bi2Se3 films is enhanced, rather than degraded, after oxidation in air for 24 h, resulting in a maximum responsivity of 140 mA W-1. This responsivity is substantially higher than previously reported values for Bi2Se3. Furthermore, a change in the photoresponse time of Bi2Se3 due to air exposure is systematically observed. Based on variations in the Fermi level and work function, using photoelectron spectroscopy, it is confirmed that the responsivity is improved from the junction effect of the Bi-based surface oxidized layer.

Topological Phase Control of Surface States in Bi2Se3 via Spin-Orbit Coupling Modulation through Interface Engineering between HfO2-X.

The direct control of topological surface states in topological insulators is an important prerequisite for the application of these materials. Conventional attempts to utilize magnetic doping, mechanical tuning, structural engineering, external bias, and external magnetic fields suffer from a lack of reversible switching and have limited tunability. We demonstrate the direct control of topological phases in a bismuth selenide (Bi2Se3) topological insulator in 3 nm molecular beam epitaxy-grown films through the hybridization of the topological surface states with the hafnium (Hf) d-orbitals in the topmost layer of an underlying oxygen-deficient hafnium oxide (HfO2) substrate. The higher angular momentum of the d-orbitals of Hf is hybridized strongly by topological insulators, thereby enhancing the spin-orbit coupling and perturbing the topological surface states asymmetry in Bi2Se3. As the oxygen defect is cured or generated reversibly by external electric fields, our research facilitates the complete electrical control of the topological phases of topological insulators by controlling the defect density in the adjacent transition metal oxide. In addition, this mechanism can be applied in other related topological materials such as Weyl and Dirac semimetals in future endeavors to facilitate practical applications in unit-element devices for quantum computing and quantum communication.

Phase-change like process through bond switching in distorted and resonantly bonded crystal.

Although some methods to improve phase-change memory efficiency have been proposed, an effective experimental approach to induce a phase-change like process without external heat energy has not yet been reported. Herein we have shown that GeTe is a prototype phase-change material, which can exhibit a non-thermal phase-change-like process under uniaxial stress. Due to its structural characteristics like directional structural instability and resonance bonding under 1% uniaxial stress, we observed that bond switching in the GeTe film between short and long bonds is possible. Due to this phase change, GeTe displays the same phase-change as crystal layer rotation. Crystal layer rotation has not been observed in the conventional phase change process using intermediate states, but it is related to the structural characteristics required for maintaining local coordination. Moreover, since the resonance bonding characteristics are effectively turned off upon applying uniaxial stress, the high-frequency dielectric constant can be significantly decreased. Our results also show that the most significant process in the non-thermal phase transition of phase-change materials is the modulation of the lattice relaxation process after the initial perturbation, rather than the method inducing the perturbation itself. Finally, these consequences suggest that a new type of phase-change memory is possible through changes in the optical properties under stress.

Closing the Surface Bandgap in Thin Bi2Se3/Graphene Heterostructures.

Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (Bi2Se3) is the most spotlighted three-dimensional TI material; it has a Dirac cone at each top and bottom surface and a relatively wide bandgap. For application, suppression of the bulk effect is crucial, but in ultrathin TI materials, with thicknesses less than 3 QL, the finite size effect works on the linear dispersion of the surface states, so that the surface band has a finite bandgap because of the hybridization between the top and bottom surface states and Rashba splitting, resulting from the structure inversion asymmetry. Here, we studied the gapless top surface Dirac state of strained 3 QL Bi2Se3/graphene heterostructures. A strain caused by the graphene layer reduces the bandgap of surface states, and the band bending resulting from the charge transfer at the Bi2Se3-graphene interface induces localization of surface states to each top and bottom layer to suppress the overlap of the two surface states. In addition, we verified the independent transport channel of the top surface Dirac state in Bi2Se3/graphene heterostructures by measuring the magneto-conductance. Our findings suggest that the strain and the proximity effect in TI/non-TI heterostructures may be feasible ways to engineer the topological surface states beyond the physical and topological thickness limit.

P-N Junction Diode Using Plasma Boron-Doped Black Phosphorus for High-Performance Photovoltaic Devices.

This study used a spatially controlled boron-doping technique that enables a p-n junction diode to be realized within a single 2D black phosphorus (BP) nanosheet for high-performance photovoltaic application. The reliability of the BP surface and state-of-the-art 2D p-n heterostructure's gated junctions was obtained using the controllable pulsed-plasma process technique. Chemical and structural analyses of the boron-doped BP were performed using X-ray photoelectron spectroscopy, transmission electron microscopy, and first-principles density functional theory (DFT) calculations, and the electrical characteristics of a field-effect transistor based on the p-n heterostructure were determined. The incorporated boron generated high electron density at the BP surface. The electron mobility of BP was significantly enhanced to ∼265 cm2/V·s for the top gating mode, indicating greatly improved electron transport behavior. Ultraviolet photoelectron spectroscopy and DFT characterizations revealed the occurrence of significant surface charge transfer in the BP. Moreover, the pulsed-plasma boron-doped BP p-n junction devices exhibited high-efficiency photodetection behavior (rise time: 1.2 ms and responsivity: 11.3 mA/W at Vg = 0 V). This study's findings on the tunable nature of the surface-transfer doping scheme reveal that BP is a promising candidate for optoelectronic devices and advanced complementary logic electronics.

Interface engineering for a stable chemical structure of oxidized-black phosphorus via self-reduction in AlOx atomic layer deposition.

We evaluated the change in the chemical structure between dielectrics (AlOx and HfOx) grown by atomic layer deposition (ALD) and oxidized black phosphorus (BP), as a function of air exposure time. Chemical and structural analyses of the oxidized phosphorus species (PxOy) were performed using atomic force microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, first-principles density functional theory calculations, and the electrical characteristics of field-effect transistors (FETs). Based on the combined experiments and theoretical investigations, we clearly show that oxidized phosphorus species (PxOy, until exposed for 24 h) are significantly decreased (self-reduction) during the ALD of AlOx. In particular, the field effect characteristics of a FET device based on Al2O3/AlOx/oxidized BP improved significantly with enhanced electrical properties, a mobility of ∼253 cm2 V-1 s-1 and an on-off ratio of ∼105, compared to those of HfO2/HfOx/oxidized BP with a mobility of ∼97 cm2 V-1 s-1 and an on-off ratio of ∼103-104. These distinct differences result from a significantly decreased interface trap density (Dit ∼ 1011 cm-2 eV-1) and subthreshold gate swing (SS ∼ 270 mV dec-1) in the BP device caused by the formation of stable energy states at the AlOx/oxidized BP interface, even with BP oxidized by air exposure.

Electronic Structure of C60/Zinc Phthalocyanine/V2O5 Interfaces Studied Using Photoemission Spectroscopy for Organic Photovoltaic Applications.

The interfacial electronic structures of a bilayer of fullerene (C60) and zinc phthalocyanine (ZnPc) grown on vanadium pentoxide (V2O5) thin films deposited using radio frequency sputtering under various conditions were studied using X-ray and ultraviolet photoelectron spectroscopy. The energy difference between the highest occupied molecular orbital (HOMO) level of the ZnPc layer and the lowest unoccupied molecular orbital (LUMO) level of the C60 layer was determined and compared with that grown on an indium tin oxide (ITO) substrate. The energy difference of a heterojunction on all V2O5 was found to be 1.3~1.4 eV, while that on ITO was 1.1 eV. This difference could be due to the higher binding energy of the HOMO of ZnPc on V2O5 than that on ITO regardless of work functions of the substrates. We also determined the complete energy level diagrams of C60/ZnPc on V2O5 and ITO.