Transition metal nitrides such as CrN are promising for thermoelectric applications due to their high stability and tunable electronic properties, yet they have been largely limited to n-type conduction, restricting device design. Here, we report the successful synthesis of p-type CrN films via controlled residual oxygen incorporation during RF magnetron sputtering regulated through N2 gas flow adjustment, without introducing any additional oxygen source. A low N2 gas flow rate (fN2) produces N-deficient CrN1-delta(O) films with n-type conduction dominated by nitrogen vacancies, while higher fN2 (>= 4 sccm) stabilizes Cr vacancies in over-stoichiometric Cr1-delta N(O) films, resulting in p-type hole conduction. Cr K-edge X-ray absorption fine structure (XAFS) reveals Cr-Cr bond elongation, reduced coordination, and enhanced Cr3d-O/N2p hybridization, indicative of localized hole states. Temperature-dependent transport measurements confirm the mechanisms, leading to a room-temperature power factor (PF) up to 0.105 mW m-1 K-2 (n-type) and 0.053 mW m-1 K-2 (p-type). The structural similarity between the n- and p-type films enables the creation of p-n homojunctions, highlighting a straightforward route for CrN-based thermoelectric devices.

The high contact resistivity at metal/semiconductor interfaces in transition metal dichalcogenides, particularly MoS2, severely limits device performance and remains an urgent challenge. Recently, semimetal contacts have been reported to reduce contact resistivity, however, they suffer from poor thermal stability. Here, we propose metallic amorphous chalcogenide as a solution and demonstrate amorphous TiS2 (a-TiS2) as a promising electrode material. Molecular dynamics simulations using a machine learned potential reveal the spontaneous formation of a van der Waals (vdW) gap at the a-TiS2/MoS2 interface. Subsequent density functional theory calculations show that the quasi-vdW interface suppresses the formation of metal-induced gap states (MIGS). The suppression of MIGS has the potential to reduce contact resistivity. In parallel, experimental characterization of sputtered a-TiS2 thin films by X-ray diffraction confirms thermal stability up to 500 degrees C. The amorphous nature facilitates omnidirectional contact formation, a critical attribute for next-generation Gate-All-Around (GAA) transistors.

Quasi-one-dimensional (quasi-1D) van der Waals (vdWs) materials, such as ZrTe5, exhibit unique electrical properties and quantum phenomena, making them attractive for advanced electronic applications. However, large-scale growth of ZrTe5 thin films presents challenges. We address this by employing sputtering, a common semiconductor industry technique. The as-deposited ZrTe5 film is amorphous, and post-annealing induces a crystallization process akin to transition-metal dichalcogenides. Our study investigates the electrical and optical properties during this amorphous-to-crystalline transition, revealing insights into the underlying mechanism. This work contributes to the fundamental understanding of quasi-1D materials and introduces a scalable fabrication method for ZrTe5 which offers the possibility of fabricating unique future electronic and optical devices.

The integration of light and materials technology is key to the creation of innovative sensing technologies. Sensing of electric and magnetic fields, and temperature with high spatio-temporal resolution is a critical task for the development of the next-generation of nanometer-scale quantum devices. Color centers in diamonds are attractive for potential applications owing to their characteristic quantum states, although they require metallic contacts for the introduction of external microwaves. Here, we build an ultrafast diamond nonlinear photonic sensor to assess the surface electric field; an electro-optic sensor based on nitrogen-vacancy centers in a diamond nanotip breaks the spatial-limit of conventional pump-probe techniques. The 10-fs near-infrared optical pulse modulates the surface electric field of a 2D transition metal dichalcogenide and we monitor the dynamics of the local electric field at nanometer-femtosecond spatio-temporal resolutions. Our nanoscopic technique will provide new horizons to the sensing of advanced nano materials.

Ovonic threshold switching (OTS) is a type of volatile resistive switching primarily observed in amorphous chalcogenides. The switching process involves an abrupt transition from a high-resistance state to a low-resistance state when a voltage above a specific threshold (V th) is applied. OTS materials serve as selectors in nonvolatile memories with 3D XPoint-type structures, in combination with phase-change materials (PCMs), which exhibit threshold-type nonvolatile resistive switching. Despite the existence of transport models that can explain the OTS behavior, the role of the metal-OTS interface has been underexplored. This study employs angle-resolved hard X-ray photoelectron spectroscopy to investigate the interfacial electronic structure of Ge-Te-based OTS materials with different metal electrodes. The results indicate that V th varies with the work function of the contact metal because the onset voltage for impact ionization is affected by band bending at the interface. Our findings reveal that interfacial properties significantly influence OTS behavior, offering a novel method for controlling V th. This study underscores the importance of selecting appropriate metal contacts for optimizing the performance of OTS devices.

Wide-bandgap chalcopyrite materials are attractive candidates for a wide variety of energy conversion devices such as the top cell of tandem-type photovoltaic devices and photoelectrochemical water splitting hydrogen evolution devices. Nevertheless, simultaneous realization of high open circuit voltage (VOC) and high fill factor (FF) values has been challenging, and thus, the photovoltaic performance has been limited. In this article, high VOC and high FF values of wide-gap chalcopyrite CuGaSe2 thin-film solar cells are simultaneously demonstrated using an aluminum-induced back-surface field effect. An independently certified photovoltaic efficiency of 12.25% was obtained from an elemental In-free CuGaSe2:Al (bandgap energy ∼1.7 eV) solar cell (VOC: 0.959 V, short circuit current density: 17.64 mA cm-2, FF: 72.5%). In addition, an over 1 V-VOC CuGaSe2 cell (FF: 74.5%) was obtained even with the use of a conventional CdS buffer layer. Although incorporation of aluminum often leads to degradation of the chalcopyrite solar cell performance, the addition of a small amount of aluminum is found to be effective in enhancing wide-gap chalcopyrite photovoltaic performance.

We investigate the ultrafast spin dynamics of the prototypical topological insulator Bi2Se3 using time-resolved Kerr rotation (polarization-change) measurements across near-infrared wavelengths. The Kerr rotation angle ΔθK of Bi2Se3 was found to significantly depend on the photon energy around a resonance transition (∼1.0 eV) of bulk states, as well as the ellipticity of the pump light, in the presence of spin excitation. The observed photon-energy dependence of ΔθK can be well simulated by assuming spin-dependent refractive-index changes in the presence of bandgap renormalization and state-filling effect upon photoexcitation. Our study delivers comprehensive insights into the opto-spintronic properties of bulk Bi2Se3 and the fundamental physical processes underlying polarization changes. These findings are expected to be crucial in developing ultrafast magneto-optical memory devices, which can perform read-and-write operations in the terahertz regime.

Polarons can control carrier mobility and can also be used in the design of quantum devices. Although much effort has been directed into investigating the nature of polarons, observation of defect-related polarons is challenging due to electron-defect scattering. Here we explore the polaronic behavior of nitrogen-vacancy (NV) centers in a diamond crystal using an ultrafast pump-probe technique. A 10-fs optical pulse acts as a source of high electric field exceeding the dielectric breakdown threshold, in turn exerting a force on the NV charge distribution and polar optical phonons. The electronic and phononic responses are enhanced by an order of magnitude for a low density of NV centers, which we attribute to a combination of cooperative polaronic effects and scattering by defects. First-principles calculations support the presence of dipolar Fröhlich interaction via non-zero Born effective charges. Our findings provide insights into the physics of color centers in diamonds.

The successful prevention of interfacial reactions between ferromagnetic materials and topological insulators (TIs) is crucial for the realization of reliable spintronic devices using TIs. To this end, we investigated the magnetic properties and interfacial reaction behavior of a bilayer structure composed of ferromagnetic CoFeB and TI Sb2Te3. The effects of including an MgO interlayer was also investigated. Ferromagnetic resonance (FMR) studies showed a remarkably weak resonance peak for the sample annealed at 400°C for the 2 nm-thick interlayer, and finally, no resonance peak for a film below 1 nm in thickness was observed. X-ray diffraction (XRD) results demonstrated that the Sb2Te3 peak intensities started to decrease upon annealing at 200°C and completely disappeared for annealing temperatures >400°C. Hard X-ray photoelectron spectroscopy (HAXPES) results also support that the core-level peaks of Sb and Te split upon annealing at temperatures >200°C, suggesting the dissociation of Sb2Te3. These results indicate that Sb2Te3 and CoFeB react at the interface during annealing, resulting in a loss of the ferromagnetic properties of the CoFeB layer. Meanwhile, a sample containing a 3-nm-thick MgO layer retained its original Sb2Te3/MgO/CoFeB structure even after annealing at 400°C, as evidenced by its unchanged XRD peak intensity and FMR spectrum. We expect that our findings will be highly valuable in developing TI-based spintronic devices.

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.

Phase-change materials such as Ge-Sb-Te (GST) exhibiting amorphous and crystalline phases can be used for phase-change random-access memory (PCRAM). GST-based PCRAM has been applied as a storage-class memory; however, its relatively low ON/OFF ratio and the large Joule heating energy required for the RESET process (amorphization) significantly limit the storage density. This study proposes a phase-change nitride, CrN, with a much wider programming window (ON/OFF ratio more than 10(5)) and lower RESET energy (one order of magnitude reduction from GST). High-resolution transmission electron microscopy revealed a phase-change from the low-resistance cubic CrN phase into the highly resistive hexagonal CrN2 phase induced by the Soret-effect. The proposed phase-change nitride could greatly expand the scope of conventional phase-change chalcogenides and offer a strategy for the next-generation of PCRAM, enabling a large ON/OFF ratio (similar to 10(5)), low switching energy (similar to 100 pJ), and fast operation (similar to 30 ns).

Layered transition-metal dichalcogenides (TMDs) are model systems to explore ultrafast many-body interactions and various nonlinear optical phenomena. For the application of TMD-based optoelectronic devices capable of ultrafast response, it is essential to understand how characteristic electron–hole and electron–phonon couplings modify ultrafast electronic and optical properties under photoexcitation. Here, we investigate the sub-picosecond optical responses of layered semiconductor 2H–MoTe2 in the presence of an electron–hole (e–h) plasma and a long-lived coherent phonon. Transient reflectivity measurements depending on photon energy reveal that the optical response for short-time delays (< 1ps) was significantly modified by band-gap renormalization and state filling due to the presence of the e–h plasma. Furthermore, octave, sum, and difference phonon frequencies transiently appeared for the early time delays (< 2ps). The emergent multiple phonon frequencies can be described as higher-order optical modulations due to deformation-potential electron–phonon coupling under resonant photoexcitation conditions. This work provides comprehensive insights into fundamental physics and the application of non-equilibrium quasiparticle generations on TMDs under time-periodic phonon driving forces.

Electronic dynamics in the excited state of transition-metal dichalcogenides (TMDs) has attracted great interest. To understand the ultrafast intraband scattering process of excited electrons in the conduction band, we demonstrated ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation and ultrafast transient reflectivity measurements of a TMD material, 2H-MoTe2. Due to the saturable absorption (or Pauli blocking) effect present in 2H-MoTe2, the system does not absorb the second excitation pulse until the excited electrons generated by the first excitation pulse with a specific fluence are scattered in the conduction band. By exploiting the Pauli blocking effect in ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation, we found that the excited electrons were scattered within 100 fs comparable to the excitation optical pulse duration. Furthermore, the excited electrons were relaxed to the lowest energy level of the conduction band (K- or Σ-valley) within 1–2 ps.

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 T m > 1000 °C) of TMDs requires elevated temperatures for good crystallinity. This study focuses on the low‐ T m 2D vdW TM tetra‐chalcogenides and identifies NbTe 4 as a promising candidate with an ultra‐low T m of around 447 °C (onset temperature). As‐grown NbTe 4 forms an amorphous phase upon deposition that can be crystallized by annealing at temperatures above 272 °C. The simultaneous presence of a low T m and a high crystallization temperature T c can resolve important issues facing current phase‐change memory compounds, such as high Reset energies and poor thermal stability of the amorphous phase. Therefore, NbTe 4 holds great promise as a potential solution to these issues.

The non-linear threshold-type current-voltage behavior that characterizes selector devices for three-dimensional (3D) crossbar-type nonvolatile memory devices relies upon a phenomenon known as Ovonic threshold switching (OTS). Because current practical OTS materials are based on toxic elements, such as Se and As, Te-based OTS materials are expected to offer a more environmentally friendly option. However, the electronic structure that determines the OTS behavior of Te-based OTS materials is not well understood. In this paper, the electronic structure of amorphous Si0.29Te0.71, has been explored using hard X-ray photoelectron spectroscopy (HAXPES) in conjunction with density functional theory (DFT) calculations. The HAXPES results show that the Si0.29Te0.71 amorphous network of the simulated amorphous structure is based upon Te-Te, Te-Si, and Si-Si bonding. DFT calculations revealed that Si3p and Te5p states contribute to bonding, whereas occupied non-bonding Te5p states form the top of the valence state. A projected local density of states analysis shows that the Si site forms conduction-tail states, whereas the Te site forms both conduction- and valence-tail states. Furthermore, Te-Te dimers contribute significantly to the midgap states that characterize the OTS behavior. Finally, the valence-tail state extension within the mobility gap of Si0.29Te0.71 was experimentally demonstrated.

Two-dimensional (2D) layered semiconductors such as molybdenum (Mo)- and tungsten (W)-based transition-metal dichalcogenides (TMDCs) have been model systems to explore various quantum many-body processes owing to the strong confinement of carriers and phonons in each atomic layer. The confinement effect in semiconductor TMDCs strongly modifies the electronic and optical properties via the generation of strongly bound excitons or large band-gap renormalization (BGR) in the presence of unbound electron-hole (e-h) plasma under irradiation of ultrashort intense laser pulses [1]. The generation of the e-h pairs and electron-phonon coupling in semiconductors generally appears immediately after photoexcitation within < 1 ps [2], [3]. Therefore, it is crucial to investigate the ultrafast optical response to gain insight into non-equilibrium quasiparticle dynamics in 2D semiconductors.

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.

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.

Through interband photoexcitation, a representative transitionmetal dichalcogenide (TMD) material, MoTe2, can undergovarious phenomena such as photothermal conversion, phase transition,nonlinear optical effects, and laser ablation depending on the excitationlevel. However, a comprehensive study of the photoinduced structuraldynamics of MoTe2 has yet to be performed because someof these phenomena interfere in a complex manner. In the present study,the photoinduced structural dynamics of 2H-MoTe2 was investigatedunder various excitation levels at a wavelength of 400 nm using ultrafasttime-resolved electron diffraction and transient reflection measurements.Photoexcitation induced coherent phonons for 1-2 ps, whichsubsequently decayed into isotropic thermal vibrations at & SIM;10ps. The amplitudes of the generated coherent phonon and thermal vibrationswere found to linearly increase as the incident fluence approached3-4 mJ/cm(2); however, the amplitudes remained nearlyconstant when the incident fluence ranged from 4-14 mJ/cm(2) due to saturable absorption. Multiphoton absorption processesmight be dominant above a fluence of 15 mJ/cm(2). Photoexcitationat high fluence (20-30 mJ/cm(2)) permanently damagedthe sample through laser ablation and tellurium segregation. The insightsin this study are critical for the further applicability and fundamentaloptical properties of photodevices based on TMD materials.

The formation kinetics of the 2D-layered InSe crystallinephasein thin amorphous InSe films was studied by means of differentialscanning calorimetry, Raman spectroscopy, X-ray diffraction analysis,and X-ray absorption spectroscopy. All measurements were performedon as-deposited films (thicknesses ranging between 200 and 1500 nm),with Kapton foil used as a substrate. In the films with thickness & GE; 1000 nm, the formation was found to proceed via two steps:the exothermic re-organization of the amorphous phase (activationenergy of 420 kJ & BULL;mol(-1)), followed by the exothermicformation of the crystalline phase (activation energy of 227 kJ & BULL;mol(-1)). For InSe films with thickness & LE; 500 nm,the two processes merged, but their reaction mechanism was still foundto consist of two sequential phase transformations, with the reorganizationin the amorphous phase being the necessary preliminary step. Bothkinetic processes were found to be slowed down by the presence ofmechanical defects and by the surface/interface imperfections. Thepossibility of two-step preparation of ideally layered flexible 2DInSe ceramics is suggested based on the separated high-temperatureamorphous phase reorganization, followed by a low-temperature crystalgrowth step (during which excellent 2D InSe layering could be achieved).