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We demonstrated the epitaxial lateral overgrowth (ELO) of (1¯012) (r-plane) α-Ga2O3 using a striped mask pattern along ⟨1¯21¯0⟩. α-Ga2O3 stripes with an asymmetric cross-sectional shape were formed selectively on the windows at the initial growth stage. They grew vertically and laterally to coalesce with each other, and a compact film was obtained. The film surface exhibited wave-like morphology with macro-scale inclined terraces and steps because of the asymmetric cross-sectional stripe shape. The macrosteps moved laterally like traveling waves as the growth proceeded. Transmission electron microscopy revealed that a domain on a window grew toward the inclined c-axis direction to cover the adjacent domain after the coalescence. As a result, the dislocations, which propagated into the α-Ga2O3 stripe from the seed layer through the window, bent toward the c axis and concentrated in a narrow area. This concentration should enhance the pair annihilation. Therefore, the dislocation density was markedly reduced on the top surface including the areas above the windows and coalesced boundaries in contrast to the cases of conventional c-, a-, and m-plane ELO.

Remote epitaxy is a recently discovered type of epitaxy, wherein single-crystalline thin films can be grown on graphene-coated substrates following the crystallinity of the substrate via remote interaction through graphene. Although remote epitaxy provides a pathway to form freestanding membranes by controlled exfoliation of grown film at the graphene interface, implementing remote epitaxy is not straightforward because atomically precise control of interface is required. Here, we unveil the role of the graphene–substrate interface on the remote epitaxy of GaAs by investigating the interface at the atomic scale. By comparing remote epitaxy on wet-transferred and dry-transferred graphene, we show that interfacial oxide layer formed at the graphene–substrate interface hinders remote interaction through graphene when wet-transferred graphene is employed, which is confirmed by an increase of interatomic distance through graphene and also by the formation of polycrystalline films on graphene. On the other hand, when dry-transferred graphene is employed, the interface is free of native oxide, and single-crystalline remote epitaxial films are formed on graphene, with the interatomic distance between the epilayer and the substrate matching with the theoretically predicted value. The first atomic layer of the grown film on graphene is vertically aligned with the top layer of the substrate with these atoms having different polarities, substantiating the remote interaction of adatoms with the substrate through graphene. These results directly show the impact of interface properties formed by different graphene transfer methods on remote epitaxy.

The electrodeposition of Cu onto epitaxial Ru(0001) seed layers was investigated from a sulfuric acid-based solution containing dilute copper(II) sulfate and chloride ions. Using galvanostatic deposition at −350 μA/cm2, Cu was deposited epitaxially onto a 30 nm-thick Ru(0001) seed layer, despite a compressive misfit strain between −6.9% and −8.3%, depending on the extent of strain relaxation of the Ru layer. However, rather than depositing as a single crystal, Cu grew as a bicrystal having a common out-of-plane orientation of Cu(111) and two equivalent in-plane orientations. The Cu grain size was large, on the order of micrometers, and the grain boundaries were identified as incoherent ∑3{211} twin boundaries. The Cu initially grew as isolated islands, coalescing into a contiguous film at thicknesses around 50 nm. The Cu film was rough, and thickness and coverage varied over the electrodeposited region. After the initial island growth, Cu void fraction and film roughness both decreased with thickness as the deposit transitioned into a planar film with nanometric islands growing on the film surface. However, at thicknesses exceeding 200 nm, anisotropic growth of large, faceted Cu islands on the planar Cu film again increased the surface roughness. The epitaxially deposited Cu bicrystal showed an improvement in resistivity when compared with polycrystalline Cu similarly electrodeposited onto a polycrystalline Ru seed.

Surface passivation of germanium is vital for optimal performance of Ge based optoelectronic devices especially considering their rapidlyincreasing surface-to-volume ratios. In this work, we have investigated the surface passivation of Ge by a stack consisting of a thin layer ofhydrogenated amorphous silicon (a-Si:H) and an aluminum oxide (Al2O3) capping layer. Plasma-enhanced chemical vapor deposition wasused to deposit the a-Si:H (0–10 nm), while thermal and plasma-enhanced atomic layer deposition (ALD) were employed for the Al2O3films (0–22 nm). Transient photoconductance decay measurements revealed a recombination velocity as low as 2.7 cm s−1 for an a-Si:Hlayer as thin as 1.8 nm and an Al2O3 film of only ∼6 nm. In this state-of-the-art passivation scheme, the plasma-enhanced ALD process forthe Al2O3 capping layer proved superior to the thermal ALD process since it resulted in an exceptionally high negative fixed charge density(Qf ∼ 1013 cm−2), which proved a key factor for the low surface recombination velocity. Transmission electron microscopy and energy x-raydispersion revealed that a thin SiOx layer (∼1.4 nm) forms between a-Si:H and Al2O3 during the ALD process, which is thought to be theorigin of this high negative fixed charge density. This passivation stack is regarded as highly interesting for applications such as solar cells,nanolasers, and nano-LEDs based on p-type Ge.

The two-dimensional (2D) materials provide an excellent platform for the study of the dimensional effect. The richer the types of 2D materials, the broader the unknown field we can explore. However, among the large number of 2D materials manufactured by humans, true single-crystalline (SC) atomically thin 2D metals are rare. The instability of SC 2D metal materials puts high demands on its fabrication process. By implementing molecular dynamics (MD) simulations, we proved that the SC biatomic-layer (BL) gallium film can be formed at the interface between two graphene layers. The Ga atoms deposited on the surface of the graphene on the copper substrate will spontaneously evolve into independent liquid nano-islands, and then cover the nano-island with a monolayer graphene. When the Ga nano-islands confined under the graphene layer are heated to 500 °C, they will expand into a BL Ga film, and finally, the entire system is cooled to room temperature to obtain the SCBL Ga film. It is found that these nano-islands are in the liquid state at ∼400 °C, but they undergo a phase transition and evolve into the solid state at ∼500°C. At the same time, the nano-islands also drop from 3D to 2D. In addition, the vertical heterostructure with moire superstructure is formed between the SCBL Ga and the top layer graphene. The calculations of the electronic properties show that the Dirac conical point of the graphene in the heterostructure is shifted below the Fermi level, which proves that SCBL Ga is able to induce semimetallic to metallic conversion in graphene, indicating SCBL Ga can be used for metal contacts in 2D devices.

When a nominally flat surface is bombarded with a broad ion beam at oblique incidence, nanoscale ripples often develop on the surface. For high angles of incidence, surfaces typically develop into a terraced form at the late stages of their time evolution. In the present work, this process is exploited to prevent unwanted smoothing of ordered terraced substrates during the deposition of thin films. A Si surface prepatterned with a 500 nm pitch binary grating structure was bombarded at oblique incidence by a low energy Xe+ ion beam to establish an ordered terraced topography. Subsequently, Si/SiO2 bilayers were deposited on the surface, and further oblique incidence Xe+ bombardment was performed following the deposition of each Si layer to re-establish the ordered terraced topography. Self-organized processes, such as in the present work, that only require exposure of a surface to a plasma or ion source have the potential to provide a simple and inexpensive route for fabricating large-area nanostructured surfaces. The presented procedure has potential applications in the fabrication of multilayer blazed gratings for use in the extreme ultraviolet or soft x-ray regimes.

In this article, the surface formation mechanism of single crystalline diamond (SCD) by a nanosecond pulsed laser is performed, where surface cracking, phase transformation, and typical surface morphology are involved. The nanometric surface characteristics of the ablated micro-grooves on the diamond caused by the thermal effect are first studied, where the crack generation and propagation are discussed by the thermoelastic bending model for the laser irradiation process. In addition, the composed phases and the formation mechanism of the induced metamorphic layer (IML) and deposited metamorphic layer (DML) near the ablated groove are investigated, where the diamond/graphite transition layer is the coexistence of the distributed crystalline diamond and the oriented graphite, the direction of which is at an angle of 60°–80° to the laser scanning direction and toward the center of the ablated groove. Based on the subsurface exploration and the grain growth kinetics analysis, the formation process of the DML is proposed as follows: the nucleation → the formation of carbon islands → the combination of the carbon islands → the formation of the deposited metamorphic layer.

In SiN/AlGaN/GaN heterostructures, the evaluation of interface charges at the SiN/AlGaN and AlGaN/GaN interfaces is crucial since they both rule the formation of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. In this paper, we conducted a thorough analysis of the gate-to-channel capacitance CGC(VG) and of the drain current ID(VG) over a gate voltage VG range enabling the depletion of the 2DEG and the formation of the electron channel at the SiN/AlGaN interface. This work includes the establishment of analytical equations for VTH1 (formation of the 2DEG) and VTH2 (formation of the electron channel at the SiN/AlGaN interface) as a function of interface charges and of the p-doping below the 2DEG. The inclusion of the p-doped layer below the 2DEG and the use we made of VTH2 have not been reported in previous studies. Our analysis allows a reliable estimate of the interface charges at the AlxGa1−xN/GaN and SiN/AlxGa1−xN interfaces for various Al concentrations x as well as to demonstrate that the polarization charge at the SiN/AlxGa1−xN interface is compensated, which confirms previous findings. Moreover, this compensation is found to be induced by the AlGaN layer rather than the SiN layer.

Atomic layer deposition is a powerful technique for achieving atomic-level control in the deposition of thin films and nanoparticles. The ultrathin noble metal films can be applied in many functional devices, but it is a challenge to obtain such films since the island growth mode generally happens instead of the layer growth mode. In this work, the nucleation and growth of platinum on Si, Al2O3, Au, and Ni substrates were studied using (MeCp)PtMe3 and O2 plasma as a precursor and a co-reactant, respectively. The evolution of Pt coverage on different surfaces was observed and discussed based on the experimental results by x-ray photoelectron spectra. The chemical adsorption of the precursor and the following processes like metal atom diffusion on substrate surfaces and up-stepping onto the existing metal islands were considered to dominate the growth before continuous films formed. The chemisorption determined the metal coverage on bare substrate surfaces, and the total coverage was influenced by metal atom diffusion and up-stepping behaviors that are determined by surface energy relationships between the deposited metal and substrate surfaces. Pt films were easier to form on Al2O3 and Ni surfaces compared with on Si and Au surfaces, respectively. A model was proposed to help to understand the mechanisms in the nucleation and growth processes, involving the chemisorption, diffusion, and up-stepping, which contributed to prepare ultrathin continuous Pt films on different substrates.

Active tunability of phonon dispersion and spontaneous emission (SE) still remain open owing to its exciting application potential. In this study, multilayer graphene and hexagonal boron nitride (hBN) combined with the Ge2Sb2Te5 (GST) functional substrate are proposed to obtain tunable hyperbolic plasmon–phonon dispersion and SE. Results indicate that GST phase change has a profound impact on hyperbolic phonon polaritons in the hBN layer and can obtain an actively tunable SE. Moreover, when graphene is introduced into the hBN/GST system, hyperbolic plasmon–phonon polaritons inside the two reststrahlen (RS) bands of hBN and surface plasmon–phonon polaritons outside the two RS bands can be achieved, both of which can be actively tuned by adjusting the GST phase state and graphene chemical potential. In addition, the coupled polariton branches between hBN phonon polaritons and graphene plasmons become stronger and corresponding SE enhancement regions obviously broaden with increasing the graphene/hBN layer numbers for the fixed thickness multilayer graphene/hBN on GST substrate. These characteristics may provide an available route to realize active tunability of both phonon dispersion and SE enhancement of natural or artificial hyperbolic materials.

The influence of the facet trace region in the 4H-SiC substrate on the glide and propagation behaviors of basal plane dislocations (BPDs) in a 4H-SiC homoepitaxial layer was investigated using x-ray topography, high-resolution x-ray diffraction, and micro-Raman scattering spectroscopy. The facet trace region in the substrate, which has a higher nitrogen doping concentration than the other regions in the substrate, is of great interest since it could influence the glide and propagation behaviors of BPDs during 4H-SiC homoepitaxial growth through enhanced nitrogen doping concentration in the region. It was found that the epitaxial layer grown on the facet trace region in the substrate significantly suppressed glide motion of BPDs and exhibited a reduced conversion probability of BPDs to threading edge dislocations during the homoepitaxial growth process. Based on these results, the mechanisms that the epilayer grown on a substrate region with a slightly higher nitrogen concentration showed significantly different glide and propagation behaviors of BPDs are discussed.

In this study, SiC and Ga2O3 substrates were bonded under atmospheric conditions using an extremely thin amorphous layer (∼7 A). Conventional wafer-bonding techniques employ an oxidizing treatment for surface functionalization, resulting in the formation of an oxide layer at the bonding interface. This study demonstrates the bonding of a hydrofluoric-acid-treated SiC surface with a plasma-activated Ga2O3 substrate, where the –OH groups on the SiC and Ga2O3 surfaces form direct bonding via a dehydration reaction at 250 °C. The interfacial analysis indicates that bonding using the reduction treatment instead of oxidization reduces the thickness of the intermediate layer at the SiC/Ga2O3 interface, which is a thermal and electrical barrier. In addition, it is remarkable that the SiC and Ga2O3 substrates are directly bonded by generally used surface cleaning processes, contacting the surfaces under atmospheric conditions, and annealing at 250 °C. We believe that the bonding process using the reduction process can contribute to future heterogeneous devices based on integrated dissimilar substrates.

Defect morphology plays a crucial role in determining the properties of the system and can harness new functionalities. One of the widely studied layered materials is semiconducting molybdenum disulfide (MoS 2) with interesting electronic, optical, and spin-valley properties strongly dependent on the stacking order. The defects on this material are extensively studied but limited to individual layers. Here, we provide a systematic study of a defect in a multi-layer MoS 2 sample grown by pulsed laser deposition using transmission electron microscopy in cross-sectional form and first-principles calculation to explore their electronic properties. The various dislocations in the system, such as ripple, kink, peak, and edge dislocation, change the inter-layer distance. The observed inversion domain boundaries introduce 3R stacking in the system with deviation from straight layer nature. These stacking defects add richness to existing defect structures and open new opportunities for novel device applications beyond a single-layer limit.

Ultra-thin silver films of thicknesses of the order of 10 nm and less were prepared in different ambient conditions (vacuum, Ar, and N2) by pulsed laser deposition on glass and fused silica substrates. The in situ monitoring of electrical resistance of deposited films and optical emission spectroscopy of plasma were implemented as real-time analysis techniques. Change in the growth mechanism of the Ag layer in N2 ambient is expressed by an acceleration of the coalescence process, which shifts the percolation point toward lower mass thicknesses. The films prepared in vacuum and Ar ambient were found to be unstable for a final resistance in the range from 1 to 100 MΩ while the films deposited in N2 revealed stable electrical resistance. The percolation point was further lowered by introducing a sublayer of AgxOy for the film deposited in N2 gas. Based on data provided by AFM, SEM, and spectroscopic ellipsometry, different film formation mechanisms are discussed in relation to surface morphology and optical properties. Optical emission spectroscopy was used to monitor the deposition process and identify the species presented during the deposition process. The energy of the ejected particle is affected by the addition of ambient gas and depends on the properties of the working gas.

The electronic transport and optical properties of high quality multilayers of NbTiN/AlN with ultrathin NbTiN layers were characterized. The anisotropy of the dielectric function of the multilayers confirmed their hyperbolic metamaterial properties. The superconductive transition temperature, Tc, of these engineered superconductors was enhanced up to 32% compared to the Tc of a single ultrathin NbTiN layer while the resistivity per NbTiN layer remained unchanged. We have demonstrated that this Tc increase can be attributed to enhanced electron–electron interaction in superconducting hyperbolic metamaterials. The measured critical fields are high and have an anomalous temperature dependence on the direction perpendicular to the magnetic field. These results demonstrate that the metamaterial engineering approach can be used to enhance Hc2.

In this study, we propose a new piezoelectric measurement method using unimorph microcantilevers to evaluate precise values of direct and converse piezoelectric coefficients for piezoelectric microelectromechanical system applications. We fabricated unimorph microcantilevers comprising 3-μm-thick polycrystalline Pb(Zr,Ti)O3 (PZT) thin films on a 20-μm-thick silicon (Si) layer. The converse piezoelectric coefficient was measured as |e31,f| = 3.9–5.8 C/m2, which was in good agreement with the measurement results of PZT thin films without microfabrication. The direct piezoelectric properties of the PZT microcantilevers were measured at the resonance frequency of each microcantilever, and |e31,f| was ∼2 C/m2, which was consistent with previous reports on the piezoelectric properties of a PZT thin film on Si. Since the measurement results included the effects of the stress condition and microfabrication damage, the measurement method of this study can evaluate the actual piezoelectric coefficient of piezoelectric properties in microdevice structures.

(Ta2O5)1−x–(TiO2)x, with x = 0.035, thin films were deposited onto p-type silicon and quartz substrates following two different routes: first, by the sputtering of the ceramic target and, second, by sputtering of mosaic (Ta, Ti) metal target in the presence of oxygen (hereafter referred to as CT and MT, respectively). The deposited films were found to crystallize on annealing at and above 700 °C. The dielectric constant of the prepared films was found to increase with increasing annealing temperature, up to 700 °C, and on annealing at 800 °C, it was found to decrease. The dielectric constant of the CT was observed to be higher than that of the MT film structures at each annealing temperature. From the transmittance measurements, different optical parameters of the deposited crystalline films were calculated. The leakage current density of the CT films was found to increase with the annealing temperature, whereas in the MT films, it drastically decreased by an order of ∼3 when the annealing temperature was increased from 700 to 800 °C. Different conduction mechanisms were observed in the different applied field regions in the prepared film structure. The observed electrical properties of the prepared film structure seem to depend on the status of the growing interfacial oxide layer on annealing.

To improve the performance of the conventional p-GaN gate AlGaN/GaN high electron mobility transistors (HEMTs), we propose an improved design for e-mode AlGaN/GaN HEMT with a gate stack β-Ga2O3/p-GaN structure. The simulated results show that the proposed device increases the threshold voltage and the gate breakdown voltage in comparison with the conventional p-GaN gate HEMT due to the use of the β-Ga2O3 layer, which has an additional β-Ga2O3/p-GaN heterojunction and decreases the strength of electric field in the gate region. Moreover, the proposed device exhibits a lower off-state leakage current, which can be attributed to the less donor ionized density. In addition, the impacts of the β-Ga2O3 layer thickness are investigated. There is a trade-off between β-Ga2O3 layer thickness and the performance of the proposed device, including threshold voltage, gate breakdown, and saturation drain current.

Lead sulfide (PbS) nano-domain (ND) thin layers were grown by a chemical bath deposition technique on a heavily doped n-type GaAs substrate forming a heterojunction. The PbS NDs' absorption was blue shifted from its original wavelength due to the quantum confinement effect (by controling their sizes below the Bohr radius) to sense short wavelength infrared (SWIR) light (ranging between 1200 and 1800 nm). In order to assess the electrical properties of the PbS ND based layers to be used as a SWIR detector, we performed electrical measurements that included current–voltage (I–V), capacitance–voltage (C–V), electron impedance spectroscopy (EIS), and transmission electron microscopy (TEM). The I–V measurements showed hysteresis behavior that was found to be imposed by the high trap concentration within the PbS NDs and at the layer interfaces. The hysteresis that was demonstrated during the electrical measurement is explained by the slow filling of the trap within the PbS ND thin layer. In order to reduce the trap density, we performed a thermal treatment at 150 ° C for 30 min prior to the electrical measurements. Using C–V measurements, we expected to find a reduction in the capacitance due to the decrease of the trap density after the thermal treatment. However, we discovered an increase in the capacitance, where temperature-dependent I–V measurements revealed an additional slow charging mechanism in series with the heterojunction. We found that this can be attributed to the generation of a thin isolation layer at the interface between the PbS-NDs layer and the GaAs substrate that also contains interface traps induced by the thermal treatment that was performed in an air atmosphere. We corroborated our assumption by performing EIS measurements and TEM analysis, which showed the generation of a thin isolation layer at the PbS/GaAs interface.

We studied the effect of the (Ni0.8Fe0.2)100−xCrx seed layer on the current-in-plane giant magnetoresistance (CIP-GMR) in Fe16Co66Ni18/Cu multilayer films. Both the GMR ratio (ΔR/R) and interlayer exchange coupling (IEC) between the FeCoNi layers showed strong dependences on the Cr concentration x in the seed layer. The highest ΔR/R of ∼30% was obtained for x = 38 (at. %) due to a comparatively strong antiparallel IEC resulting from a strong [111]-out-of-plane-texture of the [FeCoNi/Cu] multilayer. This confirms that a strong [111] texture is essential for the GMR in antiferromagnetically coupled multilayers as reported in previous works. However, we found that the strong [111] texture is not sufficient to obtain large ΔR/R. For x = 48, the [FeCoNi/Cu] multilayer showed a strong [111] texture as that for x = 38; however, the observed ΔR/R and antiparallel IEC for x = 48 were much smaller than those for x = 38. Detailed microstructural and chemical analyses by scanning transmission electron microscopy revealed that the fluctuation of the thickness of the Cu spacer layer can explain the reduction of antiparallel IEC that gave rise to the decrease in ΔR/R in the case of x = 48. We also discuss the effect of IEC energies on the sensitivity of GMR sensors.

We investigate the magneto-optical (MO) polar Kerr enhancement of Fabry–Perot cavities with perpendicular magnetic anisotropy for chemical and biological sensing applications. The MO cavities consist of stacked films with a half-mirrored surface layer, a dielectric interference layer, a total-reflection mirror, and a magnetic metal layer. Co80Pt20 and Al-doped ZnO (AZO) thin films were utilized as the magnetic and interference layers, respectively. The cavities produce two types of MO enhancement depending on the thickness of the magnetic layer. The sample with a thick magnetic layer as a total-reflection mirror generates a single peak in the polar Kerr spectrum. Contrarily, inserting a several-nanometer-thick CoPt layer into the AZO layer produces a resonant-type spectrum. Although both samples generate large MO enhancement factors (>50), the magnetic properties significantly differ. The AZO/CoPt/AZO interference layer controls the crystalline and interface conditions and produces a square-shaped out-of-plane hysteresis loop with a large Kerr rotation angle. Moreover, the use of an ultrathin CoPt film significantly reduces the saturation magnetization field. Thus, Fabry–Perot cavities with a CoPt/AZO interference layer may serve as high-sensitivity, low-energy-consumption sensor elements.

Atom Probe Tomography (APT) is used to explore Si into which a high concentration of Ti has been incorporated through ion implantation and pulsed laser melting. Ti shows abundant segregation out of the Si, with regions near the surface showing evidence of the classic “cellular breakdown” morphology characteristic of constitutional supercooling. Ti concentrations in excess of the nominal Mott limit have previously been reported, but these concentrations have relied on secondary ion mass spectrometry (SIMS) measurements, which are susceptible to artifacts. The APT method provides improvements over the SIMS method and shows that Ti concentrations are below the Mott limit everywhere outside of the broken-down regions below the surface SiO layer. The data confirm that Ti behaves as would be expected under a conventional rapid solidification theory. This has implications for how Ti in Si concentration data produced by non-atomistic techniques are interpreted and also indicates that the use of the conventional solidification apparatus can be used to predict Ti concentrations that may be achievable using implantation and laser melting techniques.

The enhanced surface mobility in metallic glasses (MGs) has been a constant source of fascination due to its unique mechanical properties. We show experimentally that the mobile surface layer of MGs functions as a lubricating layer in friction experiments, which is evidenced by a reduction of a friction coefficient of 50% or less and suppression of dissipative stick-slip behavior with decreasing scratch depth down to nanoscale in the various MGs. The lubrication mechanism could be attributed to easier shearing of the mobile surface layer induced by homogeneous plastic flow. Importantly, the thickness of the self-lubricating layer is inversely proportional to glass transition temperature with a higher homologous temperature yielding a larger thickness. These results extend the fundamental understanding of the ubiquitous MG surface and present a path for the rational design of self-lubricating materials.

Nitrogen doped diamond-like carbon (NDLC) is a candidate protective coating in state-of-the-art heat assisted magnetic recording (HAMR) media. To ensure the robustness of the media particularly at higher temperature applications, mechanical properties of ultra-thin sub 5-nm NDLC coatings are of great interest. Due to instrument limitations and very shallow films, it is very challenging to accurately measure sub-5 nm NDLC films and other HAMR components from experiments without substrate effects. In this study, very shallow nanoindentations were performed, and the results were fitted with finite element analysis using a modified indenter geometry to predict the elastic modulus and yield strength of NDLC films of two different thicknesses (3.5 and 4.5 nm) and other components without any substrate effect. Results showed that higher NDLC film thickness led to better elastic modulus and yield strength at 25 °C before and after heating and at 300 °C. Hardness to yield strength ratio (H/Y) for NDLC films was also determined and found within the range of 2.2–2.8, which is higher than the H/Y ratio of DLC films from earlier studies. This implied the dependence of H/Y ratio on the thickness, temperature conditions, and chemical structure of NDLC films. Results also showed that the yield strength of FeCo metal layer and glass substrate in HAMR media decreased at 300 °C, but almost fully recovered to their initial properties after removal of heat.

In recent years, poly-Si based passivated contacts elevated the conversion efficiencies of crystalline Si solar cells to levels of 26%abs due to their outstanding electrical surface passivation performance and current transport characteristics. A major associated challenge, however, is the large parasitic light absorption within the doped poly-Si, regardless if the contacts are applied on the front and/or on the rear side of the solar cell. It, therefore, might be beneficial to confine the passivated contacts to local regions underneath the metal contacts. We present an effective and flexible laser-based approach to structure the poly-Si layer after its full-area deposition. Laser pulses with a pulse duration of 9 ps and a wavelength of 355 nm trigger an amorphization of the poly-Si surface. The minimum threshold fluence for amorphization is between 89 and 129 mJ/cm2. The a-Si layer, which is laterally homogeneous and up to (33 ± 4) nm in thickness, works as an etch barrier in an alkaline solution. The most robust barrier corresponding to the maximum thickness of the a-Si layer is found for a fluence of (270 ± 30) mJ/cm2. Besides the impact of the laser fluence on the etch resistiveness of the modified poly-Si layer, we study the role of oxygen during the laser process. We find that oxygen becomes incorporated into the material for certain laser fluences, which results in a more robust etch barrier. The amount of oxygen incorporated is below 3 wt. %. Eventually, we present a phenomenological model of our findings.

Microellipsometric investigations of metal–insulator–metal (MIM) structures with regular square lattices of nanocylinders as one metallic layer are presented together with a theoretical description of all dipolar interactions in such a system. A comparison between the theoretical and experimental results is made, which clearly demonstrates the influence of surface plasmon on the optical properties of such MIM systems. The hybridization of localized and surface plasmon modes is revealed.

We report on the large-area and high-quality growth of single- to few-monolayer thick MoS2 thin films on oxidized Si (100) substrates via the pulsed laser deposition method. Our Raman, x-ray photoelectron spectroscopic, and FE-SEM measurements confirmed that atomically thin MoS2 layers are highly uniform and are stoichiometric. We found a type-I band alignment at the MoS2/SiO2 heterointerfaces through photoemission spectroscopic valence-band measurements. The valence- and conduction band offset (VBO and CBO) at the 1L MoS2/SiO2 interface was observed to be 3.91 and 2.96 ± 0.05 eV, respectively. The values of VBO and CBO increase up to 4.15 and 3.56 ± 0.05 eV, respectively, with an increase in the MoS2 layer number. This observation can be attributed to the shift of the Mo-4dz2 orbitals due to interlayer coupling for thicker MoS2 films, reducing its bandgap, resulting in an increment in VBO and CBO values.

We have systematically investigated highly mismatched AlInSb photodiodes grown on GaAs substrates operating in the mid-infrared range. A novel characterization method was introduced to analyze the recombination mechanism within an active layer of the devices, which revealed a high conductance stemming from the leaky behavior of dislocations. The introduction of a dislocation filter layer successfully reduced threading dislocations and improved resistance area product of photodiodes, leading to high detectivity at room temperature.

Ternary dielectrics with varying composition formed by alloying two binary oxides can enable tunable optical and electrical properties for advanced technological applications. Atomic layer deposition (ALD) gives precise control over ternary dielectric composition through the ability to finely tune the precursor pulsing ratio. This work presents ALD development of Si-rich SixTi1−xO2 dielectrics with varying composition (x), along with spectroscopic and electrical characterization of their properties. Stoichiometry of the SixTi1−xO2 films was determined using X-ray photoelectron spectroscopy. Their composition-dependent refractive index, energy bandgap, and reflectance show promise for diverse optical applications ranging from anti-reflective coatings in photovoltaics to optical waveguides. This work also reports a first comparative study of SixTi1−xO2 films prepared by thermal (T-) and plasma-enhanced (PE-) ALD with varying Si composition and deposition temperatures. Deposition rates of 0.67–0.92 A/cycle were obtained for SixTi1−xO2 films with x = 0.5–0.91 deposited using PE-ALD at 250 °C, which were higher than that of T-ALD at 200 °C (0.42–0.05 A/cycle). PE-ALD also exhibited a high deposition rate of 0.81 A/cycle for SixTi1−xO2 film with x = 0.91 at a low growth temperature of 150 °C. The PE-ALD Si-rich silica–titania films show substantially lower (100×) leakage current densities than the thermally deposited films, along with higher breakdown fields for decreasing deposition temperature. A dielectric constant as low as ∼5 was achieved for PE-ALD SixTi1−xO2 films with high Si (x = 0.91) content.

This paper deals with the investigation of the discharge in the solution of 0.9% NaCl in water for the conditions, when the voltage at the gap is in a vicinity of the so-called critical voltage at which the discharge in a thin vapor layer at the surface of active electrode appears. In the experiments, the critical voltage is at a level of (180–200) V. Most of the data correspond to powering the discharge by the bipolar voltage pulses of 5 s duration. The main attention is centered on the nonsteady (transient) processes and on the forms of the discharge operation. For the voltage pulses of negative polarity, when the metal electrode is the cathode, the discharge is sustained in the glow regime with the occasional glow-to-spark transitions. At the positive polarity of the voltage, the surface of liquid inside the vapor layer plays a role of cathode in the gas-discharge gap. In these conditions, the process of the glow-to-spark transition is suppressed and the discharge is sustained in the glow regime.

The effects of different ammonium sulfide [(NH4)2Sx]-based treatments on the surface passivation of dry-etched and thermally recovered GaN epilayer have been investigated. A two-step treatment using HCl followed by the mixture of (NH4)2Sx and isopropyl alcohol demonstrated a more effective removal of intrinsic oxide layer and formation of sulfide passivation. The compensating centers on both the p- and n+-type GaN surfaces were significantly reduced. The fabricated base-floating GaN/Al0.1GaN heterojunction phototransistors (HPTs) with the surface passivation showed improved I–V and enhanced optical gain due to the effective suppression of surface recombination current. A maximum gain of 1.3 × 105 was obtained at 5 V bias in the HPTs using the two-step treatment.

The carbon nanotube (CNT) is a compelling and promising material for industrial applications requiring high strength and rigidity. For a multi-walled CNT (MWCNT), the nominal tensile strength and Young's modulus (considering the whole cross-sectional area of the specimen) are key mechanical factors for the practical application of macroscopic fibers and composites. However, the nominal tensile strength and Young's modulus of MWCNTs are much lower than their effective tensile strength and Young's modulus (considering the fracture cross-sectional area) because the outermost graphite layer always fractures first due to the low cross-link between graphite layers. In this paper, we fabricated the carbon nanofibers (CNFs) by epitaxial growth on super-aligned MWCNT film template and conducted in situ uniaxial tensile tests on individual carbon-nanotube-templated CNFs. The individual CNFs show improved nominal mechanical performance than previously reported MWCNTs. The nominal mechanical properties enhancement of the CNFs is attributed to the effective control on load transfer between interwalls.

The carrier lifetime control in p-type 4H-SiC epilayers with intentional aluminum (Al) and boron (B) doping is demonstrated as part of work to develop a p-type “recombination-enhancing layer” for n-channel insulated gate bipolar devices fabricated on p-type substrates. The (Al + B)-doped epilayers (Al: 5 × 1017, B: 4 × 1016 cm−3) showed a very short minority carrier lifetime of less than 20 ns at 293 K, resembling that of highly Al-doped epilayers (Al: 1 × 1019 cm−3). Besides, the minority carrier lifetimes in (Al + B)-doped epilayers are stable against post-annealing in Ar and H2 ambient, while that of Al-doped epilayers varied considerably. PiN diodes with a 10 μm-thick (Al + B)-doped buffer layer inserted on p-type substrates showed no evident degradation after a stress test under a pulse current density of 2000 A/cm2.

Amorphous films of Nb2O5 composition were deposited by thermal atomic layer deposition on (001) Si substrates and subsequently crystallized by annealing in forming gas at temperatures ranging from 550 °C to 1000 °C. After subjecting these films to an 850 °C anneal, cross-sectional transmission electron microscopy revealed the presence of B-Nb2O5 and T-Nb2O5 phases in the matrix, as well as reduced R-NbO2 in floret-shaped regions. Annealing at 1000 °C completed the reduction process, resulting in the insulator-to-metal transition (IMT) capable T-NbO2 phase throughout the film. ALD films of composition Nb2O5 were also deposited on electron-transparent SiN membranes and then subjected to 550 °C and 1000 °C anneals. Here, the 550 °C anneal induced the B-Nb2O5 and T-Nb2O5 phases without inducing the R-NbO2 phase. The 1000 °C anneal of the films deposited on SiN membranes retained B-Nb2O5 while inducing the R-NbO2 phase, but without bringing the process to completion and inducing the T-NbO2 phase. The effectiveness of the 1000 °C reducing annealing treatment to induce the T-NbO2 phase for Nb2O5 films deposited on (001) Si substrates, while stopping short of this transition for films deposited on SiN membranes, suggests the importance of the SiO2 layer on the Si substrate in contributing to the reduction reaction that results in the technologically important insulator-to-metal transition (IMT)-capable T-NbO2 phase.

Using vertical spin-valve devices, we experimentally investigate the room-temperature spin diffusion length in an undoped p-Ge layer grown on ferromagnetic Fe 3Si. Because low-temperature molecular beam epitaxy techniques enable us to grow vertically stacked and all-epitaxial CoFe/Ge/Fe 3Si trilayers on Si(111), we can intentionally vary the thickness ( t Ge) of the intermediate undoped p-Ge layer during the growth. With decreasing t Ge, the magnitude of the spin signals gradually increases at room temperature. From the analysis based on the model by Fert and Jaffres, the room-temperature spin diffusion length in the undoped p-Ge grown on Fe 3Si is experimentally estimated to be ∼ 8.4 nm, much shorter than those reported in previous works on commercial p-Ge substrates.

On a single-walled carbon nanotube (SWCNT) surface, water forms a peculiar adsorption layer comprising two monolayers, and the physical properties of this adsorption layer remain unclear. We studied the changes that occurred in this water adsorption layer from room temperature down to 140 K using photoluminescence (PL) spectroscopy of suspended SWCNTs. The PL emission energy exhibited complex changes with temperature depending on the chirality of the SWCNTs. These changes were described quantitatively on the basis of changes in the bandgap, the dielectric constant of the adsorption layer, and the strain imposed by the adsorption layer. The results suggested that the adsorption layer might be a two-dimensional disordered solid rather than a liquid.

Compared to the bulky ceramic counterparts, antiferroelectric (AFE) thin films exhibit higher energy-storage performance. It has been demonstrated that the performance of the AFE thin films is strongly affected by the thickness. However, possible changes in the phase structures and microstructures accompanied by varying thicknesses have been less known, which makes it hard to fully understand the physical insight of the thickness effect. Herein, we fabricate a series of PbZrO3 (PZ) thin films by chemical solution deposition whose thickness is approximately proportional to the number of deposition layers, i.e., one to four layers with each layer of about 60 nm in thickness. The detailed structural characterization has been performed by using x-ray diffraction, a scanning electron microscope, and a transmission electron microscope. The films are composed of nanorods oriented normal to the films in which each nanorod maintains single crystalline though segmented by the horizontal interfaces between each of the two neighboring layers, suggesting nearly perfect epitaxy during multilayer growth. The multidomain state is found in three-/four-layer PZ thin films while the one-/two-layer PZ films always present a monodomain state. Meanwhile, incommensurate phases with different modulation periods have been observed in the four-layer PZ films. By comparing the as-observed structural features with the measured electrical properties of different films, it seems that microstructural evolution with the film thickness should not be negligible in evaluating the structure–property relation of PZ-based thin films.

Dislocations in a homoepitaxial diamond film grown by chemical vapor deposition (CVD) were characterized by confocal Raman imaging. The diamond film was grown on a high-pressure high-temperature (HPHT) substrate using an oxygen-adding condition to minimize dislocation formation during diamond growth. The dislocation densities in the CVD thin film and substrate were found to be 3 × 104 and 2 × 104 cm−2, respectively, from large-area Raman imaging. The spatial variation of stress around dislocations in CVD diamond was analyzed three-dimensionally using in-plane images of peak positions of diamond first-order lines taken at different depths from the diamond film surface. The variational directions of the stresses depended on the types of dislocation: ⟨110⟩ variational direction corresponds to edge [001] dislocation, whereas the ⟨100⟩ variational direction corresponds to 45° mixed [001] dislocation. Most dislocations passed through the CVD thin film from the substrate. Some dislocations were newly generated at the CVD layer–substrate interface. The dislocations in CVD diamond propagate along the [001] direction of diamond crystal with the same Burgers vector in the HPHT substrate.

There is a growing necessity to develop revolutionary neutron detectors for nuclear energy, nuclear physics, medical physics, astrophysics, biological imaging, nonproliferation, and national security. The often-used Helium-3 (He-3) neutron detector is becoming increasingly difficult to obtain due to He-3 shortages. As an emerging oxide semiconductor material, Ga2O3 exhibits excellent physical properties. These physical merits enable Ga2O3’s potential as a high-performance semiconductor neutron detector for extreme condition applications. Here, two approaches are explored, i.e., applying an exterior conversion layer of boron-10 (B-10) on Ga2O3 and directly doping B-10 into Ga2O3 to demonstrate Ga2O3’s capability for neutron detection. Using Monte Carlo simulation, we show the distinct difference in neutron detection efficiency of Ga2O3 when applying direct doping of B-10 into Ga2O3 vs applying a uniform B-10 conversion layer on top of Ga2O3. Our results exhibit that the theoretically predicted maximum doping level of B-10 in Ga2O3 does not lead to the same detection efficiency as that of a simple B-10 conversion layer when detecting 480 keV gammas. Except for the most thermalized neutrons at 0.01 eV, direct doping simulations are not able to achieve comparable results to that of the conversion layer method.

The use of an indium surfactant considerably alters the composition and morphology of low-temperature non-polar m-plane ( 10 1 ¯ 0 ) AlxGa1−xN (x ∼ 0.2) and of silicon-doped AlGaN/InGaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy. This paper compares heterostructures grown with indium surfactant with those grown under conventional stoichiometric and gallium-rich conditions at the relatively low temperature necessary for growth of In0.16Ga0.84N quantum wells (565 °C). Stoichiometric growth results in rough, inhomogeneous AlGaN layers that are unsuitable for optical devices. Gallium-rich growth produces a smoother AlGaN layer, reduced inhomogeneities, and sharper interfaces as compared to stoichiometric growth. However, due to the low temperature, gallium-rich growth leads to the formation of an unintentional GaN layer on top of each AlGaN barrier, reducing the energies of confined electronic states in the quantum wells. An indium surfactant enables two-dimensional AlGaN growth at low temperature, producing atomically flat surface morphology and sharp heterostructure interfaces. Indium surfactant assisted epitaxy also eliminates the high aluminum alloy inhomogeneities observed with conventional stoichiometric and gallium-rich growth. Even though partial indium incorporation into the AlGaN layer is found at the studied temperatures, the high-quality, uniform non-polar In0.055Al0.19Ga0.755N/In0.16Ga0.84N quantum wells grown with indium surfactant display bright and narrow photoluminescence that is essential for device applications.

In this work, we report nearly single oriented {001}-textured ferroelectric PbZr 0.52 Ti 0.48 O 3 thin films grown by pulsed laser deposition onto AISI 304 stainless steel substrates. Pt, Al 2 O 3, and LaNiO 3 buffer layers promote the PbZr 0.52 Ti 0.48 O 3 {001} texture and protect the substrate against oxidation during deposition. The dominant {001} texture of the PbZr 0.52 Ti 0.48 O 3 layer was confirmed using x-ray and electron backscatter diffraction. Before poling, the films exhibit a permittivity of about 350 at 1 kHz and a dielectric loss below 5%. The films display a remanent polarization of about 16.5 μ C cm − 2 and a high coercive field of up to E c = 135.9 kV cm − 1. The properties of these PbZr 0.52 Ti 0.48 O 3 thin films on stainless steel are promising for various MEMS applications such as transducers or energy harvesters.

Tailoring the magnetic properties of interfaces with light element materials is very promising for achieving energy-efficient spintronic devices. Here, the magnetic properties of SiC/Fe4N(111) interfaces with different stacking patterns and interlayer distances are investigated by first-principles calculations. It is found that the perpendicular magnetic anisotropy of SiC/Fe4N(111) interfaces decreases when compared with the clean Fe4N(111) surface, where it decreases by 28.5% in the model where the C atom is directly above the corner Fe atom. The change in magnetic anisotropy energy (MAE) can be mainly ascribed to the surface and subsurface Fe atomic layers of the Fe4N substrate, while the deep atomic layers show little contribution. Moreover, the interlayer distance can reverse the sign of MAE and the Dzyaloshinskii–Moriya interaction at the interfacial Fe atomic layer. The MAE of the face-centered Fe (FeB) atom is sensitive to the interlayer distance, indicating that FeB atoms play a key role in the interfacial properties. These results indicate that interlayer distance engineering is an effective method to manipulate the magnetic properties of interfaces.

Few-layer InSe draws tremendous research interests owing to the superior electronic and optical properties. It exhibits high carrier mobility up to more than 1000 cm2/Vs at room temperature. The strongly layer-tunable band gap spans a large spectral range from near-infrared to the visible. In this perspective, we systematically review the optical properties of few-layer InSe. Firstly, the intrinsic optical and electronic properties are introduced. Compared to other two-dimensional (2D) materials, the light-matter interaction of few-layer InSe is unusual. The band gap transition is inactive or extremely weak for in-plane polarized light, and the emission light is mainly polarized along the out-of-plane direction. Secondly, we will present several schemes to tune the optical properties of few-layer InSe such as external strain, surface chemical doping and van der Waals (vdW) interfacing. Thirdly, we survey the applications of few-layer InSe in photodetection and heterostructures. Overall, few-layer InSe exhibits great potential not only in fundamental research, but also in electronic and optoelectronic applications., A review article, 42 pages, 12 figures

We investigated the relation between the domain wall (DW) configuration and the strength of the interlayer exchange coupling (IEC) in multilayered magnetic wires, where a Ru spacer layer mediates the IEC between two ferromagnetic layers, and a Pt cap layer contributes to the interfacial Dzyaloshinskii–Moriya interaction. It was found that the antiferromagnetic IEC stabilizes the Neel-type DWs in the wires and that the stability varies with the strength of the antiferromagnetic IEC. The micromagnetic simulations imply that the Neel-type DWs are stabilized because the rotation of the magnetic moments in the DWs is restricted in the antiferromagnetically coupled ferromagnetic layers.

Microchannel plates (MCPs) are widely utilized as key device components in various photomultipliers; however, the performance of MCPs cannot be further improved by traditional processing. Atomic layer deposition (ALD) is a promising route to prepare a composite conductive layer and secondary electron emission (SEE) layer structure on the inner wall of the MCP. Moreover, ZnO is an essential component of a composite conductive layer, which is located at the bottom of the SEE layer and significantly influences the SEE coefficient, which, in turn, affects the gain performance of MCPs. Herein, ALD is used to deposit different thicknesses of ZnO films (1–50 nm) on an Si substrate, resulting in an ZnO/Si double-layer film structure. The relationship between the SEE coefficient and the primary electron energy of ZnO films with different thicknesses was established. The maximum secondary electron yield value of 2.04 is achieved at a film thickness of 30 nm. Moreover, Dionne's SEE model and theory of semiconductors are used to simulate and verify the experimental results. These results provide useful guidelines for the development of ALD-MCPs.

Porous nanoparticles are very popular because of their high surface/volume ratio; moreover, they have stronger plasmonic properties than their solid counterparts. Due to these properties, these are potential candidates in optical, or even in ophthalmological applications. We prepared porous gold nanoparticles on SiO 2 / Si as well as on sapphire substrates with solid-state dewetting–dealloying methods. In this work, we studied the morphological and optical properties of porous gold nanoparticles coated with a thin ( ∼ 7 nm) TiO 2 layer using the plasma-enhanced atomic layer deposition method. We show that heat treatments can be used to tune the optical properties of titania coated porous gold hybrid nanoparticles in a wide range of wavelengths. The change in the optical properties is induced by the TiO 2 phase transformation, which also initiates a change in the local refractive index, and assisted by the decrease of the melting point of Au on the nanoscale.

The lifetime of GaAs photocathodes in polarized electron guns is limited due to the delicate activation layer. An atomically clean and smooth GaAs surface is needed to deposit a robust activation layer, such as Cs 2 Te, with longer lifetime compared to traditional (Cs,O) activation. A previous experiment with Cs 2 Te activation on GaAs used heat cleaning temperatures around 400 ° C to avoid an increase in surface roughness [Bae et al., Appl. Phys. Lett. 112, 154101 (2018)]. High-temperature heat cleaning around 580 ° C, which results in a relatively contamination-free surface, could be one possible way to improve quantum efficiency. However, one should be cautious about surface roughness degradation during high-temperature heat cleaning. In this paper, we report results of surface roughness measurements on native, heat cleaned, and (Cs,O) activated GaAs photocathodes under vacuum. The results, measured by ultrahigh vacuum scanning tunneling microscopy, show that the surface roughness improves as the heat cleaning temperature is increased, by at least a factor of three for 580 ° C heat cleaning, compared to the native sample. Activation with (Cs,O) is shown to increase surface roughness by a factor of four compared to a 580 ° C heat cleaned sample. This confirms that high-temperature heat cleaning can be useful for depositing good quality robust activation layers on GaAs. We also report chemical analysis for each step of preparation for p-doped GaAs photocathodes using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and ultraviolet photoelectron spectroscopy. Our results indicate that the (Cs,O) activation layer forms a sandwich structure consists of Cs and oxygen. We found no formation of any specific compound such as Cs 2 O or Cs 11 O 3.

This work focuses on the stability against the oxidation of the chemical, structural, and magnetic properties of the system consisting of a layer of Co tetra-phenyl porphyrins grown on the top of Fe(001) – p(1 × 1)O. Such a system is characterized by a very high degree of structural order and the existence of magnetic coupling between the molecules and the substrate, even at room temperature, as we recently reported [Jagadeesh et al., Appl. Phys. Lett. 115, 082404 (2019)]. We highlight, by using x-ray photoemission spectroscopy, the effect of porphyrins in screening the substrate from oxidation. The coupling between the magnetic response of the system and the order of the molecular layer is investigated by means of spin-resolved UV photoemission spectroscopy and low-energy electron diffraction, respectively. As a result, a link is eventually found between this response and the chemical and structural stability of the interface.