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The recent synthesis and characterization of violet phosphorus (VP) demonstrate that it is the most stable allotrope of phosphorus, which could become a promising 2D layered semiconductor with potential applications in optoelectronic and electronic devices. However, experiments exploring the complex refractive index or permittivity have yet to be performed, hindering further application access. Due to the small size of the VP crystal sample available by the current growth method, the measurement for the complex refractive index is challenging. In this work, coherent acoustic phonon spectroscopy was performed first in the a–b plane using polarization-resolved pump-probe spectroscopy, revealing negligible anisotropy in the refractive index in the VIS–NIR regime (400–1000 nm). Then, the non-polarized reflectance spectra of VP films with different thicknesses on Si in the VIS–NIR regime were collected. The complex refractive index spectra were obtained by fitting the reflectance spectra with Fresnel's law considering multiple reflections in thin films. It was found that VP has high refractive indices and low extinction coefficients in the spectral range of 560–1000 nm, based on which a metasurface with VP nanodisks could achieve a high reflection band in the VIS–NIR regime by simulation. These results unveil the striking potential of VP for nanophotonics applications. [ABSTRACT FROM AUTHOR]

Designing an optoelectronic device requires adequate information about the photo-response of the active material. Here, we have studied the optical and optoelectrical properties of the solution-processed inverse spinel structured CoFe2O4 (CFO) thin film and correlated it with the photovoltaic performance of CFO active material-based all-oxide solar cell. The optical absorption spectra of spin-coated CFO films show the existence of a direct bandgap of 2.64 eV with a maximum absorption coefficient >105/cm. A simple spin-coated CFO/TiO2p–n heterojunction shows an open circuit voltage of over 0.95 V under 1 sun illumination. However, photo-induced oxygen vacancy formation and the desorption of surface oxygen lead to additional electron generation and hole capture, respectively, in the p-type CFO, resulting in an anomalous photocurrent decay under white light illumination, resulting in a low short-circuit current density. This study provides a fundamental understanding of photo-carrier dynamics in solution-processed CFO thin films and apprehends the photophysics of designing spinel-ferrite-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Elastic strain engineering in the GeSn bandgap structure is an attractive area for designing novel material properties. The linear interpolation of the elastic constants of Ge and Sn is commonly used to estimate their respective values for Ge1−xSnx alloys. This work reveals that Young's modulus of Ge1−xSnx epitaxial layers has a non-monotonic dependence on Sn composition. It is shown that the decrease in the elastic modulus correlates with the increase in Sn content in pseudomorphically grown Ge1−xSnx-epilayers with Sn concentration in the range of 1–5 at. % and subcritical thicknesses. An anomalous increase in the elastic modulus is observed with the further increase in Sn content (12 at. %), which is also accompanied by an increase in in-plane tensile strain. Phase separation and a decrease in the elastic modulus are observed for Ge1−xSnx-epilayers grown above the critical thickness with Sn concentration ≥ 12 at. %. A correlation between the experimental elastic moduli and calculated elastic energies explains the complexity of strain-driven anomalous elastic properties of Ge1−xSnx-epilayers. The observed anomalous behavior of the Young's modulus for these GeSn epitaxial layers appears to be related to their recently predicted and observed short-range atomic order. [ABSTRACT FROM AUTHOR]

In recent years, with the fast development of magnetic devices for information technology, the demands of magnetic thin films with both high functional stability and integration flexibility rapidly increase. It is believed that building a magnet with complementary advantages of van der Waals (vdW) and non-vdW magnets, which can be described as a "quasi-vdW magnet," will be highly appreciated. One may expect a quasi-vdW magnet to have chemical bonding between the neighboring sublayers for strong magnetic coupling, but to preserve clean surfaces with vdW feature for flexible interface engineering. For this purpose, an intercalation of magnetic atoms into the interlayer gaps of vdW magnets, as a powerful method for tuning the interlayer coupling, can be a practical approach. In this work, using the first-principles calculations, we study the potential to utilize the Fe-intercalation to transform the vdW magnets Fe3GeTe2 (FGT) into quasi-vdW magnets. As two extreme cases, it is revealed that: (i) the Fe-intercalated FGT bilayer Fe-[Fe3GeTe2]2 (Fe-[FGT]2) does have remarkable interlayer ferromagnetic coupling based on covalent bonding between the intercalated Fe atom and FGT monolayers and retains low exfoliation energy with vdW feature, suggesting that the Fe-[FGT]2 bilayer can be regarded as a quasi-vdW magnet; and (ii) the Fe-intercalation can transform the vdW FGT bulk into a non-vdW Fe-Fe3GeTe2 (Fe-FGT) bulk magnet. Accordingly, as for the intermediate cases, it is suggested that Fe-intercalated FGT multilayers (Fen−1-[FGT]n, n > 2) can also be potential quasi-vdW magnets, forming a family of magnetic thin films that provide alternative building blocks for microminiaturized magnetic devices. [ABSTRACT FROM AUTHOR]

The saturated magnetization (mtot) and the Gilbert damping constant (α) are the two key factors that determine the critical current density of the magnetization reversal in the spin-transfer-torque magnetic memory devices. Here, this study demonstrates the efficient modulation of these two parameters by tunning the composition of the Heusler Co3–xFexAl thin films, utilizing the x-ray magnetic circular dichroism technique and ferromagnetic resonance measurements. With the increase in Fe concentration, the mtot shows a downward trend mainly resulting from the decrease in Fe local magnetic moment instead of Co. On the other hand, the ultralow α decreases from 0.004 to 0.0012. This has been attributed to the reduction in the spin–orbit coupling, which is corroborated by the decrease in the orbit-to-spin moment ratio. Our findings add a building block for the Heusler compounds with tunable Gilbert damping and appropriate magnetization and show great potential in spintronic applications. [ABSTRACT FROM AUTHOR]

In this work, we investigate the physical and electrical properties of WS2 thin films grown by a plasma-enhanced atomic layer deposition process, both before and after device fabrication. The WS2 films were deposited on thermally oxidized silicon substrates using the W(NMe2)2(NtBu)2 precursor and a H2S plasma at 450 °C. The WS2 films were approximately 8 nm thick, measured from high-resolution cross-sectional transmission electron imaging, and generally exhibited the desired horizontal basal-plane orientation of the WS2 layers to the SiO2 surface. Hall analysis revealed a p-type behavior with a carrier concentration of 1.31 × 1017 cm−3. Temperature-dependent electrical analysis of circular transfer length method test structures, with Ni/Au contacts, yielded the activation energy (Ea) of both the specific contact resistivity and the WS2 resistivity as 100 and 91 meV, respectively. The similarity of these two values indicates that the characteristics of both are dominated by the temperature dependence of the WS2 hole concentration. Change in the material, such as in sheet resistance, due to device fabrication is attributed to the chemicals and thermal treatments associated with resist spinning and baking, ambient and UV exposure, metal deposition, and metal lift off for contact pad formation. [ABSTRACT FROM AUTHOR]

The application of strongly correlated oxides as electronic devices is crucial issues to both modern physics and industry. In this work, the strongly correlated oxides EuNiO3 epitaxial thin film on SrTiO3 is synthesized, and the metal–insulator transition at 193 °C is identified. By injecting proton into EuNiO3 via a Pt catalyst, the huge resistance modulation up to six orders of magnitude is achieved. Moreover, the resistance switching is also observed in a prototype device built by protonated EuNiO3 and Nb-SrTiO3. Due to the intrinsic low activation energy of proton diffusion, comparing with oxygen-driven devices with the same structure, the switching speed of the proton-driven prototype device is 102–103 faster and might be further enhanced by optimizing the device design. [ABSTRACT FROM AUTHOR]

Predicting nanoscale thermal transport in dielectrics requires models, such as the Boltzmann transport equation (BTE), that account for phonon boundary scattering in structures with complex geometries. Although the BTE has been validated against several key experiments, its computational expense limits its applicability. Here, we demonstrate the use of an analytic reduced-order model for predicting the thermal conductivity in dimensionally confined materials, i.e., monolithic and porous thin films, and rectangular and cylindrical nanowires. The approach uses the recently developed "Ballistic Correction Model," which accounts for materials' full distribution of phonon mean-free-paths. The model is validated against BTE simulations for a selection of base materials, obtaining excellent agreement. By furnishing a precise yet easy-to-use prediction of thermal transport in nanostructures, our work strives to accelerate the identification of materials for energy-conversion and thermal-management applications. [ABSTRACT FROM AUTHOR]

We reported a peculiar thickness dependence of perpendicular magnetic anisotropy (PMA), stronger in thinner films, in spinel NiCo2O4 (NCO) films deposited epitaxially on MgAl2O4 (001) single crystalline substrates. The effective PMA field is about 3 T in 23.0 nm-thick films but increases dramatically to about 7 T in 4.5 nm-thick films. X-ray absorption spectra reveal an inhomogeneous distribution of Co2+ cations along the film's normal with more Co2+ adjacent to the film/substrate interface. X-ray linear dichroism spectra reveal that the PMA in NCO films is associated with electrons occupying the x2–y2 orbital of Co cations at the tetrahedral sites. This work demonstrates the potential of ultrathin NCO films in all-oxide spintronic devices. [ABSTRACT FROM AUTHOR]

Thin film epsilon-near-zero (ENZ) supports guided modes analogous to surface plasmon modes in the infrared, where surface plasmons are difficult to excite. Leveraging these modes can enhance the signal of surface-enhanced infrared absorption (SIERA) sensors by a factor of three, enabling the detection of minute amounts of materials. However, material losses can significantly impact sensor performance. To address this issue, we establish a condition that limits the impact of losses while still preserving the advantage of ENZ-surface waves. Our findings may pave the way for the identification and exploration of particularly suitable materials for highly efficient photonic SIERA sensors. [ABSTRACT FROM AUTHOR]

Exploring and revealing the influence of the pyroelectric thin film array element size on its structure and pyroelectric performance is crucial for designing integrated pyroelectric infrared detectors. In this work, the transverse size effect on the piezoelectric, dielectric, and especially the pyroelectric properties for a new-generation relaxor ferroelectric material Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIMNT) was studied by a finite element method. The lateral size-dependent piezoelectric and dielectric properties of the PIMNT thin film indicated that with the decrease in the transverse size, the piezoelectric constant d33 and relative dielectric constant ε r increased substantially. The piezoelectric constant d33 and relative dielectric constant ε r along ⟨001⟩ and ⟨011⟩ orientation increased faster than those along ⟨111⟩. A critical aspect ratio (in this paper, it was defined as radius/thickness) was found around 1:1 for three directions. We further discovered that the pyroelectric coefficient for PIMNT thin film along the ⟨111⟩ direction (the best crystallographic orientation for pyroelectric performance) decreased from 8.5 × 10−4 to 8.0 × 10−4 C/(m2·K) with the aspect ratio down to 0.01. The variation of the piezoelectric, dielectric, and pyroelectric properties originated from the declamping of the PIMNT thin film from the substrate. This finding gives insight into the transverse size effect on the electrical properties of new-generation relaxor PIMNT thin film and provides a guidance for designing high-performance infrared array detectors. [ABSTRACT FROM AUTHOR]

We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes. At low temperatures, we observe not only a large unsaturated magnetoresistance and Shubnikov–de Haas oscillations with current in the in-plane direction but also a saturated vertical longitudinal magnetoresistance and quantum oscillations with current in the out-of-plane direction in a thin film of WTe2. Our work provides insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics. [ABSTRACT FROM AUTHOR]

The thermal conductivity measurement of films with submicrometer thicknesses is difficult due to their exceptionally low thermal resistance, which makes it challenging to accurately measure the temperature changes that occur as heat flows through the film. Thus, specialized and sensitive measurement techniques are required. 3ω method is a widely used and reliable tool for measuring the thermal conductivity of films. However, the high in-plane thermal conductivity in thin films results in rapid heat dissipation across the thin film, resulting in poor measurement sensitivity and making it difficult to accurately measure the temperature gradient with the traditional 3ω method. Also, the traditional 3ω method requires cross-plane thermal conductivity to derive the in-plane counterpart. Here, we introduce a dual-domain 3ω method that adopts AC-modulated heating and electrode arrays facilitating surface temperature profiling: (1) the sensitivity was significantly improved due to the employment of low-thermal-conductivity-substrate, and (2) cross-plane thermal conductivity is not required for the analysis of in-plane counterpart. This measurement platform allows us to control heat penetration in depth via varied heating frequencies as well as spatial temperature detection through laterally distributed electrodes on the thin film surface. By utilizing the described method, we have determined the in-plane thermal conductivity of a copper film, having a thickness of 300 nm, which was found to be 346 Wm−1K−1 and validated by the Wiedemann–Franz law. [ABSTRACT FROM AUTHOR]

We report transparent quantum dot light-emitting diodes with a current focusing structure. By depositing a SiO2 thin film to form the current focusing structure, the DC density and luminance significantly increased to over 8700 mA/cm2 and 360 000 cd/m2, respectively. The emission spectra and current densities as functions of SiO2 thickness and aperture width have been investigated and discussed. This current focusing design is proved effective and can be further applied to other planar light-emitting diode devices. [ABSTRACT FROM AUTHOR]

We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes. At low temperatures, we observe not only a large unsaturated magnetoresistance and Shubnikov–de Haas oscillations with current in the in-plane direction but also a saturated vertical longitudinal magnetoresistance and quantum oscillations with current in the out-of-plane direction in a thin film of WTe2. Our work provides insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics. [ABSTRACT FROM AUTHOR]

Controlling the charge density in low-dimensional materials with an electrostatic potential is a powerful tool to explore and influence their electronic and optical properties. Conventional solid gates impose strict geometrical constraints to the devices and often absorb electromagnetic radiation in the infrared (IR) region. A powerful alternative is ionic liquid (IL) gating. This technique only needs a metallic electrode in contact with the IL, and the highest achievable electric field is limited by the electrochemical interactions of the IL with the environment. Despite the excellent gating properties, a large number of ILs are hardly exploitable for optical experiments in the mid-IR region because they typically suffer from low optical transparency and degradation in ambient conditions. Here, we report the realization of two electrolytes based on bromide ILs dissolved in polymethyl methacrylate (PMMA). We demonstrate that such electrolytes in the form of thin films can induce state-of-the-art charge densities as high as 20 × 10 15 cm − 2 with an electrochemical window of [−1V, 1V] in vacuum. Thanks to the low water absorption of PMMA, they work both in vacuum and in ambient atmosphere after a simple vacuum curing. Furthermore, our electrolytes can be spin-coated into flat thin films with optical transparency in the range from 600 to 4000 cm–1. Thanks to these properties, these electrolytes are excellent candidates to fill the gap as versatile gating layers for electronic and mid-IR optoelectronic devices. [ABSTRACT FROM AUTHOR]

This paper reports the retention behavior for Al0.93B0.07N thin films, a member of the novel family of wurtzite ferroelectrics. Our experiments suggest that bipolar cycling of metal (Pt/W)/Al0.93B0.07N/W/Al2O3 film stacks first induced wake-up and then a region of constant switchable polarization. The films showed excellent retention of the stored polarization state. As expected, data retention was slightly inferior in the opposite state (OS) measurements. However, it is noted that even after 3.6 × 106 s (1000 h) at 200 °C, the OS signal margin still exceeded 200 μC/cm2. The predicted OS retention is 82% after 10 yr baking at 200 °C. [ABSTRACT FROM AUTHOR]

The anomalous Hall effect measurements are used to probe the magnetization reversal of terbium iron garnet (TbIG) thin films at different temperatures. The compensation temperature (Tcomp) of TbIG thin films is revealed, and the film thickness effect on the Tcomp is studied. The results indicate a rise of Tcomp along with decreasing film thickness. We postulate two possible origins for the observed behavior, namely interfacial element diffusion and strain effects between TbIG films and Gd3Ga5O12 substrates. The results have implications for the study of spintronic devices based on ultrathin rare-earth iron garnet thin films. [ABSTRACT FROM AUTHOR]

Transverse vibrations can induce the non-linear compression of a thin film of air to levitate objects, via the squeeze-film effect. This phenomenon is well captured by the Reynolds' lubrication theory; however, the same theory fails to describe this levitation when the fluid is incompressible. In this case, the computation predicts no steady-state levitation, contradicting the documented experimental evidence. In this Letter, we uncover the main source of the time-averaged pressure asymmetry in the incompressible fluid thin film, leading the levitation phenomenon to exist. Furthermore, we reveal the physical law governing the steady-state levitation height, which we confirm experimentally. [ABSTRACT FROM AUTHOR]

In this Letter, the role of background carbon in metalorganic chemical vapor deposition (MOCVD) β-Ga2O3 growth using trimethylgallium (TMGa) as the Ga precursor was investigated. The quantitative C and H incorporations in MOCVD β-Ga2O3 thin films grown at different growth rates and temperatures were measured via quantitative secondary ion mass spectroscopy (SIMS). The SIMS results revealed both [C] and [H] increase as the TMGa molar flow rate/growth rate increases or growth temperature decreases. The intentional Si incorporation in MOCVD β-Ga2O3 thin films decreases as the growth rate increases or the growth temperature decreases. For films grown at relatively fast growth rates (GRs) (TMGa > 58 μmol/min, GR > 2.8 μm/h) or relatively low temperature (<950 °C), the [C] increases faster than that of the [H]. The experimental results from this study demonstrate the previously predicted theory—H can effectively passivate the compensation effect of C in n-type β-Ga2O3. The extracted net doping concentration from quantitative SIMS {[Si]-([C]-[H])} agrees well with the free carrier concentration measured from Hall measurement. The revealing of the role of C compensation in MOCVD β-Ga2O3 and the effect of H incorporation will provide guidance on designing material synthesis for targeted device applications. [ABSTRACT FROM AUTHOR]

A ferroelectric memristor has attracted much attention due to convenient controlling by polarization switching, but the resistive switching has been attributed to the drift or charge trapping of defects. To distinguish the resistive switching mechanism between ferroelectric polarization switching and the normal resistive switching mechanism such as the drift or charge trapping of defects, BaTiO3 (BTO) thin films were grown on a (001) Nb:SrTiO3 single crystal substrate by pulsed laser deposition with high and low laser energy density. Based on a piezoelectric force microscope, ferroelectricity is found in BTO thin films grown at high laser energy density. X-ray photoelectron spectroscopy further confirms the existence of defects in the BTO films grown at low laser energy density. The high energy sample with low density of defects exhibits a resistance hysteresis loop but little current hysteresis loop, while the low energy sample with high density of defects shows a significant resistance and current hysteresis loop simultaneously. These results provide a deep understanding about the resistive switching from ferroelectric polarization switching and the drift or charge trapping of defects. [ABSTRACT FROM AUTHOR]

The distinguished properties of hexagonal boron nitride (h -BN), specifically its atomically smooth surface, large critical electric field, and large electronic bandgap, make it ideal for thin film microelectronics and as an ultrawide bandgap semiconductor. Owing to weak van der Waals interactions between layers, h -BN exhibits a significant degree of anisotropic thermal conductivity. The in-plane thermal conductivity of h -BN has extensively been studied, yet the only measured data of cross-plane thermal conductivity ( k ⊥ ) are for exfoliated h -BN films. Exfoliation from bulk crystals is not a sustainable method for scalable production of h -BN due to its low repeatability, low yield, poor control of sample thickness, and limitation to small areas. Thus, it is necessary to investigate the thickness-dependence of k ⊥ for thin films grown by a practical growth method, such as pulsed laser deposition (PLD), which enables the production of reliable and large-area h -BN films with a control of film thickness. We grew h -BN using PLD at 750 °C and observed a decreasing trend of k ⊥ as thickness increases from 30 to 300 nm, varying from ∼1.5 to ∼0.2 W/(m K). We observed a relatively high k ⊥ value for h -BN at a thickness of 30 nm, providing insight into the k ⊥ of PLD-grown films suitable for electronics applications. [ABSTRACT FROM AUTHOR]

Ferrell and Berreman modes are absorption resonances in thin metal films and polar-dielectric media that arise from radiative bulk plasmon-polariton and phonon-polariton excitations. Compared to surface polaritons, Ferrell and Berreman modes occur due to volume charge oscillations across the medium and provide a unique pathway for light–matter interactions. Though the resonances are studied individually, stringent polarization and material requirements have prevented their observation in one host medium. Here, we show simultaneous excitation of Ferrell and Berreman absorption resonances in refractory epitaxial TiN/Al0.72Sc0.28N plasmonic metal/polar-dielectric hyperbolic metamaterials in the visible and far-infrared spectral ranges. The nanoscale periodicity of the superlattices enables the coupling of bulk plasmons (and longitudinal optical phonons) across different TiN (and Al0.72Sc0.28N) layers and allows polarization matching with free-space light that results in Ferrell (and Berreman) mode excitations. Ferrell and Berreman absorption resonances can be used for strong light confinement in radiative cooling, thermophotovoltaics, and other dual-band applications. [ABSTRACT FROM AUTHOR]

A special wake-up effect from antiferroelectric-like to ferroelectric (AFE-FE) characteristics in HfxZr1−xO2 thin films has been discussed intensively in terms of endurance performance enhancement. However, its physical origin and general impact on endurance remain unclear. In this work, the influence of various process parameters on the AFE-FE wake-up effect as well as on endurance performance and the material changes during AFE-FE wake-up are systematically studied. It is found that various parameters induce the AFE-FE wake-up effect and enhance endurance performance in the same way with enhancing tetragonal phase formation in HfxZr1−xO2 films, and the cycles of wake-up are universally associated with those of total endurance. In addition, via synchrotron-based grazing incidence x-ray diffraction, a tetragonal-orthorhombic-monoclinic phase transition is observed during AFE-FE wake-up. On the basis of these results, a correlation among crystalline composition, the AFE-FE wake-up effect, and the endurance performance of HfxZr1−xO2 thin films is established. This provides a clear guideline to a viable solution for the high endurance of HfxZr1−xO2 FE memory devices via crystal phase engineering. [ABSTRACT FROM AUTHOR]

Ultra-wide bandgap (UWBG) semiconductors are promising for many applications, such as power electronics and deep-ultraviolet photonics. In this research, UWBG β-phase magnesium gallium oxide (MgGaO) thin films with a bandgap of 5.1 eV were grown using low-temperature homo-buffer layers in a plasma-assisted molecular beam epitaxy system. The role of the growth temperature and thickness of low-temperature buffer layer on the quality of the active layer was studied using x-ray diffraction and transmission electron microscopy and by analyzing the properties of metal–semiconductor–metal photodetector devices based on these films. It is found that lower buffer growth temperature at 300 °C leads to higher crystal quality of active layer. For the same low buffer growth temperature, different crystal quality in the active layer is attained with different buffer layer thickness. A buffer layer thickness at 40 nm has the best active layer quality with the highest photo current under 265 nm illumination and long decay time as a result of reduced recombination of photo-generated carriers through fewer defects in the active layer. [ABSTRACT FROM AUTHOR]

Metal oxides are promising photoanode materials for photoelectrochemical reactions due to their chemical stability and low-cost synthesis. However, their high photocarrier recombination rate limits their commercial applications. One strategy to promote photocarrier separation efficiency in metal oxides is building an inner electric field through band engineering techniques. In this study, a gradient band structure was prepared in (Ga, Fe)2O3 thin films on F:SnO2 (FTO) coated glass substrate by varying the Ga/Fe ratio. The performance of the gradient photoanode was evaluated by measuring its photocatalytic activity with photocurrent. The forward gradient photoanode, with increasing Ga/Fe ratio from bottom substrate to photoanode surface, exhibited a higher photocurrent density (0.44 mA / cm 2 at 1.23 V vs reversible hydrogen electrode (RHE) under AM 1.5G illumination) compared to the reverse gradient photoanode with decreasing the Ga/Fe ratio in the same situation. The forward gradient band structure, which facilitates photocarrier separation and transportation, was believed to play a dominant role in improving the photocatalytic performance. This work demonstrates an effective strategy to improve photocarrier separation and photocatalytic performance of metal oxide photocatalysts through band engineering. [ABSTRACT FROM AUTHOR]

Manipulating magnetic properties by purely electrical means is a key challenge to improve information technology. In the three-dimensional ferromagnetic (FM) metal thin films, for example, the 3d-transitonal metal thin films Fe, Co, and Ni, the electric-field effect is restricted within the surface around 2–3 Å due to the screening effect, and it is hard to separate the bulk and surface states. In the present investigation, through first-principles calculations, we explore the electric-field effect on the two-dimensional (2D) FM metal monolayers Fe3GeTe2, Fe3GeSe2, and Fe3GeTeSe. It is found that magnetic anisotropy energy (MAE) of these three materials follows the sequence: Fe3GeTe2 > Fe3GeTeSe > Fe3GeSe2, because the atomic spin–orbital coupling of Te is much stronger than that of Se. The most interesting finding is that the electric-field effect on MAE is obviously enhanced in Fe3GeTeSe, which is nearly ten times of Fe3GeTe2 and Fe3GeSe2, and we find the large difference results from its asymmetric structure. Our investigation reveals the magnetoelectric (ME) mechanism and obtains the large ME effect, which is of great significance in the 2D magnetoelectronics. [ABSTRACT FROM AUTHOR]

Despite its long history, synthesizing n-type polycrystalline Ge layers with high-electron mobility on insulating substrates has been difficult. Based on our recently developed solid-phase crystallization technology, here, we have demonstrated the highest recorded electron mobility (450 cm2 V−1 s−1) for Ge-based polycrystalline thin films on insulating substrates. The underlayer type and small amount of Sn addition were the key parameters controlling both the density and barrier height of the grain boundaries in the P-doped polycrystalline Ge layers. The low growth temperature (≤400 °C) allowed us to develop a GeSn layer on a heat-resistant polyimide film, which exhibited the highest electron mobility (200 cm2 V−1 s−1), as a semiconductor thin film synthesized directly on a flexible substrate. These achievements herald the development of high-performance polycrystalline Ge-based devices on inexpensive glass and flexible plastic substrates. [ABSTRACT FROM AUTHOR]

Niobium thin films are key components of superconducting microwave resonators. Interest in these devices has increased dramatically because of their application in quantum systems. Despite tremendous effort to improve their performance, loss mechanisms are still not well understood. Nb/substrate and Nb/air interfaces are likely culprits in contributing to decoherence and ultimately limiting the performance of superconducting devices. Here, we investigate the Nb/substrate interface by studying the effect of hydrogen-passivated H:Si(111) substrates on the local superconducting properties of ∼40 nm thick Nb films compared to Nb films grown on typical Si(001) substrates. Specifically, low-temperature scanning tunneling microscopy and spectroscopy are employed to compare nanoscale material properties. The atomically flat monohydride H:Si(111) substrates are found to yield a smoother and less defective interface with the Nb film. Correspondingly, the Nb films grown on H:Si(111) substrates present more uniform superconducting properties and exhibit less quasiparticle broadening. [ABSTRACT FROM AUTHOR]

Bismuth sulfide possesses an extremely high absorption coefficient, a relatively small bandgap, and high charge carrier mobility, which are favorable for photovoltaics and photodetection. However, the device performance of binary Bi2S3 is still limited by the poor charge transport and complicated trap features caused by the stoichiometric imbalance, as both sulfur vacancy and bismuth vacancy could result in tremendous trap states. In this work, we incorporate a small amount of extrinsic elements in the solution-processed Bi2S3 thin films and systematically investigate the influence of extrinsic doping on the charge transport properties of Bi2S3 thin films via time-resolved microwave conductivity and field-effect transistors. We also fabricate photodetectors based on these Zn2+ incorporated Bi2S3 thin films and achieve state-of-the-art device performance, including relatively high on/off ratio, high responsivity, and extremely low dark current and noise, which is promising for next-generation solution-processed photodetection. [ABSTRACT FROM AUTHOR]

The excellent field-effect passivation provided by aluminum oxide (Al2O3) on germanium surfaces relies on the high negative fixed charge present in the film. However, in many applications, a neutral or a positive charge would be preferred. Here, we investigate the surface passivation performance and the charge polarity of plasma-enhanced atomic layer deposited (PEALD) silicon oxide (SiO2) on Ge. The results show that even a 3 nm thick PEALD SiO2 provides a positive charge density (Qtot, ∼2.6 × 1011 cm−2) and a relatively good surface passivation (maximum surface recombination velocity SRVmax ∼16 cm/s). When the SiO2 thin film is capped with an ALD Al2O3 layer, the surface passivation improves further and a low midgap interface defect density (Dit) of ∼1 × 1011 eV−1 cm−2 is achieved. By varying the SiO2 thickness under the Al2O3 capping, it is possible to control the Qtot from virtually neutral (∼2.8 × 1010 cm−2) to moderately positive (∼8.5 × 1011 cm−2) values. Consequently, an excellent SRVmax as low as 1.3 cm/s is obtained using optimized SiO2/Al2O3 layer thicknesses. Finally, the origin of the positive charge as well as the interface defects related to PEALD SiO2 are discussed. [ABSTRACT FROM AUTHOR]

Suppression of ferroelectricity in ultrathin films of improper ferroelectric hexagonal ferrites or manganites has been attributed to the effect of interfacial clamping; however, the quantitative understanding and related phenomenological model are still lacking. In this work, we report on the paraelectric-to-ferroelectric phase transition of epitaxial h-ScFeO3 films with different thicknesses through in situ reflection high-energy electron diffraction. Based on the interfacial clamping model and the Landau theory, we show that the thickness-dependence of the ferroelectric Curie temperature can be understood in terms of the characteristic length of an interfacial clamping layer and the bulk Curie temperature. Furthermore, we found that the critical thickness of improper ferroelectricity is proportional to the characteristic length of the interfacial clamping layer. These results reveal the essential role of mechanical clamping from interface on the improper ferroelectricity of hexagonal ferrites or manganites and could serve as the guidance to achieve robust improper ferroelectricity in ultrathin films. [ABSTRACT FROM AUTHOR]

As spin caloritronic measurements become increasingly common techniques for characterizing material properties, it is important to quantify potentially confounding effects. We report measurements of the Nernst–Ettingshausen response from room temperature to 5 K in thin film wires of Pt and W, metals commonly used as inverse spin Hall detectors in spin Seebeck characterization. Johnson–Nyquist noise thermometry is used to assess the temperature change in the metals with heater power at low temperatures, and the thermal path is analyzed via finite-element modeling. The Nernst–Ettingshausen response of W is found to be approximately temperature-independent, while the response of Pt increases at low temperatures. These results are discussed in the context of theoretical expectations and the possible role of magnetic impurities in Pt. [ABSTRACT FROM AUTHOR]

The emerging semiconductor spintronics has offered a practical routine for developing high-speed and energy-efficient electronic and optoelectronic devices. GaN holds broad prospects for room-temperature spintronic applications due to its weak spin scattering and moderate spin–orbit coupling. However, the development of GaN-based spintronic devices is still hindered by the relatively low spin injection efficiency and gate controllability. In this study, gate-modulated spin transport was achieved in a highly spin-polarized GaN-based non-local spin valve. A maximum spin diffusion length of 510 nm and a high spin polarization of 14.1% was obtained with the CoFeB/MgO tunnel spin injector. By applying gate voltages from −3 to +3 V, the spin-dependent magnetoresistance can be tuned in the range of 1.6–3.9 Ω. The modulation is attributed to the controllable spin relaxation of electrons by the gate electric field. This work has demonstrated high spin polarization and exceptional electric controllability in GaN, pushing forward the research in spin field-effect transistors. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is attracting considerable attention for applications in power electronics and deep ultraviolet (DUV) optoelectronics owing to the ultra-wide bandgap of 4.85 eV and amendable n-type conductivity. In this work, we report the achievement of Si-doped β-Ga2O3 (Si:β-Ga2O3) thin films grown on vicinal α-Al2O3 (0001) substrates with high electrical conductivity and DUV transparency of promising potential as transparent electrodes. The use of Al2O3 substrates with miscut angles promotes step-flow growth mode, leading to substantial improvement of crystalline quality and electrical properties of the Si:β-Ga2O3 films. A high conductivity of 37 S·cm−1 and average DUV transparency of 85% have been achieved for 0.5% Si-doped film grown on a 6° miscut substrate. High-resolution x-ray and ultraviolet photoemission spectroscopy were further used to elucidate the surface electronic properties of the grown Si:β-Ga2O3 films. An upward surface band bending was found at the surface region of Si:β-Ga2O3 films. Interestingly, all the Si:β-Ga2O3 films have a very low work function of approximately 3.3 eV, which makes Si:β-Ga2O3 suitable materials for efficient electron injection. The present Si:β-Ga2O3 films with high conductivity, DUV transparency, and low work function would be useful as the DUV transparent electrode to develop advanced DUV optoelectronic devices. [ABSTRACT FROM AUTHOR]

We report a thickness-dependent flexoresistance effect caused by strain gradient in SrTiO3 (STO) epitaxial thin films. The strain gradient in STO was induced by using an atomic force microscope diamond-coated tip. The depolarization field caused by the strain gradient changes the electrical state of the film, resulting in a decrease in the resistance of the films by 4–6 orders of magnitude. By testing the current–voltage (I–V) curves of STO films with different thicknesses, it is found that the flexoresistance behavior of STO films is obvious thickness dependent: the thicker the film, the higher the force threshold to trigger the film to enter a highly conductive state. The comparison of the surface morphology before and after the I–V test with loading shows that only when the thickness of the STO film is below 8 nm, the film can enter the conductive state from the insulating state without damaging the surface. During the electrical state switching process, the strain gradient caused by tip loading play an important role in the modification of the film barrier. [ABSTRACT FROM AUTHOR]

We have studied the effect of He+ irradiation on the dynamics of chiral domain walls in Pt/Co/AlOx trilayers in the creep regime. Irradiation leads to a strong decrease in the depinning field and a non-monotonous change of the effective pinning barriers. The variations of domain wall dynamics result essentially from the strong decrease in the effective anisotropy constant, which increases the domain wall width. The latter is found to present a perfect scaling with the length-scale of the interaction between domain wall and disorder, ξ. On the other hand, the strength of the domain wall–disorder interaction, fpin, is weakly impacted by the irradiation, suggesting that the length-scales of the disorder fluctuation remain smaller than the domain wall width. [ABSTRACT FROM AUTHOR]

Bulk or anomalous photovoltaic effect in ferroelectrics has recently sparked interest due to the generation of switchable photovoltages that are not limited by the bandgap of the material. The development of strategies to tune its magnitude is a key for the development of light-driven devices, e.g., photostrictive actuators, photostrictive sensors, or reconfigurable waveguides. In this paper, the bulk photovoltaic effect of polycrystalline solution-deposited bismuth ferrite thin films is studied under different stress conditions induced by different substrates and quantified using a direct strain assessment via x-ray diffraction. An increase in the short-circuit bulk photovoltaic current of 150% is observed with a change from a compressive stress of 0.54 GPa to a tensile stress of 0.93 GPa. This change is attributed to intrinsic piezophotovoltaic effect, demonstrating the potential to strain engineer the bulk photovoltaic effect in thin films. [ABSTRACT FROM AUTHOR]

Surface plasmons resonance is a promising way to improve the luminescence efficiency of light-emitting materials including InGaN/GaN-based quantum wells; however, it has rarely been used for GaN epilayer with metal nanoparticles (NPs). We demonstrated in enhancing the bandgap emission from GaN epilayer by localized surface plasmon resonance (LSPR) in a quadrupole oscillation mode using Ag NPs fabricated on the GaN. The shapes of the Ag NPs fabricated on the GaN substrates gradually changed over time and were eventually destroyed completely. This should be the reason why previous studies of enhancement of GaN-based materials by using Ag NPs have not performed as good results as expected. This problem was solved by employing oxide thin films to protect the Ag NPs from destruction or aggregation and obtain enhanced photoluminescence of the GaN epilayer. The localized and enhanced electric fields have been calculated by the finite difference time domain (FDTD) method to elucidate the enhancement mechanism. However, it has been still difficult to use this method to reproduce the enhanced emissions caused by the quantum interaction between the surface plasmons and excitons. We reproduced the highly efficient emissions attributed to the Purcell effect by evaluating the nanoantenna effect using the FDTD method. This technique confirmed that an oxide thin film between the GaN substrate and Ag NPs played an effective role in the local enhancement of the electromagnetic field and light extraction from the LSPR mode when several types of oxide thin films were considered. [ABSTRACT FROM AUTHOR]

Topological insulators are considered new states of quantum matter that cannot be systematically related to conventional insulators and semiconductors. Among them, Bi2Se3 has attracted an increasing interest due to a simple surface band structure and due to a strong contribution of the surface to transport. While the dc electric transport properties have been extensively studied, intrinsic fluctuations and their effect on the surface conduction have received less attention. In order to better investigate these aspects, a detailed characterization of the low-frequency noise, also known as noise spectroscopy, has been made in Bi2Se3 thin films. The experimental results have been obtained for different samples thickness and geometry, in a temperature range from 300 down to 8 K, and as a function of dc bias current and gate voltage. While the observed spectral noise shows a typical thermal and shot noise part, an unusual reduction of the 1/f noise component is found, especially in the low-temperature region. A correlation of this behavior with structural and dc electric transport investigations suggests that it could be an indication of the occurrence of the topological regime. Flicker noise measurements, therefore, could be considered as a valid alternative technique to standard topological surface state spectroscopy. [ABSTRACT FROM AUTHOR]

We report deterministic control over a moiré superlattice interference pattern in twisted bilayer graphene by implementing designable device-level heterostrain with process-induced strain engineering, a widely used technique in industrial silicon nanofabrication processes. By depositing stressed thin films onto our twisted bilayer graphene samples, heterostrain magnitude and strain directionality can be controlled by stressor film force (film stress × film thickness) and patterned stressor geometry, respectively. We examine strain and moiré interference with Raman spectroscopy through in-plane and moiré-activated phonon mode shifts. Results support systematic C3 rotational symmetry breaking and tunable periodicity in moiré superlattices under the application of uniaxial or biaxial heterostrain. Experimental results are validated by molecular statics simulations and density functional theory based first principles calculations. This provides a method not only to tune moiré interference without additional twisting but also to allow for a systematic pathway to explore different van der Waals based moiré superlattice symmetries by deterministic design. [ABSTRACT FROM AUTHOR]

Low-cost, stable, and easy-to-fabricate resistive switching memory (RSM) devices are highly desirable for next-generation nonvolatile memories. Spinel-structured CuFe2O4 (CFO), composed of earth-abundant, environmentally friendly elements, is a multifunctional material mainly featuring super-paramagnetism. This work explores the potential of spin-coated CFO thin films as an active material in RSM. A simple fluorine-doped tin oxide (FTO)/CFO/Ag device shows a bipolar resistive switching behavior with the problems like scattered SET and RESET voltages and endurance deterioration. Adding a TiO2 layer in the conventional device, we overcame the above limitations and achieved additional advantages of multilevel switching and improved temperature stability. The solution-processed FTO/TiO2/CFO/Ag bilayer device shows stable endurance with a maximum ON/OFF ratio of 100 with the ability to have multiple high resistance states and exhibits excellent temperature stability up to 250 °C. Our results further enhance the multifunctionality of CFO with the potential of being low-cost multilevel RSM. [ABSTRACT FROM AUTHOR]

Near-infrared luminescence phosphors are key material basis to potential applications for light sources and optoelectronic devices. In particular, it is vital to tune the luminescent properties of these phosphors in a flexible and controllable manner. Here, we demonstrate that a flexural strain originated from bending can be used to modulate the photoluminescence of freestanding Ni2+ doped SrTiO3 membranes. The bent membranes show remarkable red-shift emissions, arising from the variations of the symmetry of host materials and the local crystal fields around the Ni2+ ions. In addition, the phosphor films show a reversible and stable wavelength modulation with remarkable anti-fatigue characteristics after 104 bending cycles. These results provide a potential routine to develop flexible strain-tunable devices for applications in optical amplifiers and other optoelectronics. [ABSTRACT FROM AUTHOR]

Despite its scalability and CMOS process compatibility, the limited endurance and sub-optimal stress response of ferroelectric Zr-substituted hafnia [(Hf,Zr)O2] have been one of the key impediments toward its integration into practical device and technology applications. Here, using electrical measurements complemented by photoluminescence spectroscopy, we investigate the underlying mechanisms behind this behavior in 10 nm thick W/Hf0.5Zr0.5O2/W capacitors. Analyzing the evolution of leakage current with stress cycles and the spectroscopic response of the stress-induced leakage current, we attribute the behavior to defect levels, which lie at 0.6 eV from the conduction band edge of the ferroelectric. Photoluminescence spectroscopy, in turn, further corroborates the defect level's position within the bandgap while enabling its attribution to the presence of oxygen vacancies. This work helps to identify oxygen vacancies as the key factor responsible for the degraded endurance and stress response in (Hf,Zr)O2 and subsequently motivates the exploration of methods to reduce the oxygen vacancy concentrations without destabilizing the ferroelectric orthorhombic phase. [ABSTRACT FROM AUTHOR]

Self-powered ultraviolet (UV) photodetectors (PDs) based on GaN have been of great importance in the application of UV communication, thanks to its wide direct bandgap and strong resistance to irradiation. However, current self-powered GaN-based heterojunction UV photodetectors could not meet the requirement of fast photoresponse. Herein, type-II pn heterojunction GaS/GaN-based self-powered PDs have been proposed with a naturally p-type doping GaS thin film grown on n-type GaN via chemical vapor deposition. The electronic and optical properties of GaS/GaN heterojunction were investigated via experiments and the density functional theory. Afterward, as-prepared GaS/GaN-based PDs reveal an excellent self-powered photosensitivity/detectivity of 6.26 mA W−1/8.29 × 109 Jones at 0 V at 365 nm, ultrafast response speed with a rise/fall time of 48/80 μs as well as an amazing rejection ratio (R365 nm/R500 nm) of 3.42 × 104, and a fine rectification ratio of 105.9. This work provides a feasible method to synthesize high-performance GaS/GaN heterojunctions and demonstrates their enormous potential in ultrafast response self-powered UV photodetection. [ABSTRACT FROM AUTHOR]

One hotspot of integrated optics is how to realize a highly integrated and high-gain on-chip amplification system in a thin film of lithium niobate on insulator (TFLNOI). Here, a low erbium-doped TFLNOI waveguide amplifier with shorter length is demonstrated using the photolithography-assisted oblique-reactive ion etching technique. A maximum net internal gain of 5.4 dB in the small-signal-gain regime is measured at the peak emission wavelength of 1531.35 nm for a waveguide length of 1.5 mm and an erbium-doped concentration of 0.1 mol. %, indicating a gain per unit length of 36 dB cm−1. This work paves the way for the monolithic integration of diverse active and passive photonic components on the TFLNOI platform. [ABSTRACT FROM AUTHOR]

Artificial superlattice thin films of lead zirconate titanate (PZT) were epitaxially grown on silicon substrates, and the influence of superlattice strain on their piezoelectric properties was investigated. The c-axis oriented PZT superlattice thin film consists of two different PZT layers, Pb(Zr0.65Ti0.35)O3 (PZT-65) and Pb(ZrxTi1−x)O3 (PZT-X: x = X/100 = 0.3–0.9), with a 4 nm period. Satellite peaks were clearly observed in x-ray diffraction patterns, and cross-sectional composition measurements confirmed the superlattice structure with good interfaces, showing an alternate change in Zr and Ti compositions. Ferroelectric properties varied significantly depending on the PZT-X composition, and in particular, the PZT-65/PZT-30 superlattice thin film showed nearly the same ferroelectricity as the tetragonal phase under a large compressive strain of PZT-65 from the PZT-30 layer. For the PZT-65/PZT-X (X = 30–58) superlattice thin films, the PZT-65 layers received a compressive strain, and a relatively large piezoelectric coefficient, which did not depend on the PZT-X composition, was obtained. However, a decrease in piezoelectricity was observed in PZT-65/PZT-X (X = 70 and 90), where the PZT-65 layers experienced tensile strain. This result indicates that the piezoelectric properties of PZT superlattice thin films can be controlled by the interlayer strain. [ABSTRACT FROM AUTHOR]

Spin–orbit torque (SOT) switching in chiral antiferromagnets, such as D019-Mn3Sn, has opened prospects in antiferromagnetic spintronics. In this paper, we investigated the critical switching voltage as a function of the voltage pulse width in order to explore the thermal stability factor Δ in the D019-Mn3Sn. Although the critical switching voltage is found to decrease with increasing the voltage pulse width as expected by the thermally activated switching process with Δ of 131.1 at 300 K, our detailed examinations reveal that the Joule heating due to the voltage pulse could induce the magnetic phase transition in the D019-Mn3Sn, which significantly alters the energy landscape relevant to the SOT switching. This assisting mechanism for the SOT switching inhibits an accurate determination of Δ but could deliver a smaller Δ than the actual. Our study provides cautionary note in estimating the thermal stability of the chiral antiferromagnets. [ABSTRACT FROM AUTHOR]

In this research, β-Ga2O3/NiO heterostructures were grown directly on CeO2 buffered Hastelloy flexible substrates. With pulsed laser deposition under high temperatures, as-grown β-Ga2O3 and NiO thin films have a preferred out-of-plane orientation along the ⟨−201⟩ and ➎111➉ directions. This is due to the ideal epitaxial ability of the CeO2 buffer layer, which serves as a perfect template for the epitaxial growth of single-oriented NiO and β-Ga2O3 by creating a constant gradient from CeO2 (2.7 Å along ➎001➉) to NiO (2.9 Å along ➎110➉), and eventually to β-Ga2O3 (3.04 Å along ➎010➉). The Hastelloy substrates endow photodetectors with good deformability and mechanical robustness. Moreover, owing to the type-II band alignment of β-Ga2O3/NiO heterostructures, the photodetectors have a good photocurrent at zero bias under 284 nm of light illumination. In addition, the photocurrent is significantly higher than when using an analogous heterostructure (as described in some previous reports), because the β-Ga2O3 and NiO thin films are crystalized along a single orientation with fewer defects. [ABSTRACT FROM AUTHOR]

The whispering-gallery-mode microcavities are important in fundamental science and practical applications, which also play an essential role in integrated photonic devices. Here, we prepared a 50 μm diameter thin film lithium niobate on insulator (TFLNOI) microdisk with a Q value of 1.88 × 105 through photolithography-assisted dry-wet etching. Then, we utilize the strong thermo-optic birefringence of TFLNOI to demonstrate the second-harmonic generation (SHG) and highly temperature-tunable SHG in a TFLNOI microdisk resonator, wherein the normalized conversion efficiency of the second harmonic is 140% W−1. Moreover, the tuning slope of spectral tuning is 1.68 nm K−1 for a telecom-band pump. Our device shows great promise for efficient on-chip wavelength conversion to produce highly tunable coherent visible light for broad applications. [ABSTRACT FROM AUTHOR]