Rms Surface Roughness Research Articles

Structural, optical and photoresponse characteristics of metal-insulator-semiconductor (MIS) type Au/Ni/CeO2/GaN Schottky barrier ultraviolet photodetector

GaN based metal-insulator-semiconductor (MIS) type ultraviolet photodetector was fabricated and investigated using high-k dielectric CeO2 as an insulating oxide layer. Using XRD analysis, the phase formation of the as-deposited CeO2 films on GaN was found to be cubic fluorite. Non-contact mode atomic force microscopy technique was utilized and explored the surface morphology of CeO2 films on GaN composed of prearranged clusters of spherical shape with an average rms surface roughness of 0.428 μm. XPS analysis has revealed the existence of two oxidation states such as Ce3+ and Ce4+ in the Ce3d spectral envelop. Using absorbance versus wavelength data, the Tauc's plot was plotted and calculated a direct optical bandgap of 3.52 eV. The current-voltage (I–V) characteristics extracted from the device revealed the symmetric behavior or formation of back-to-back Schottky barrier at the metal-semiconductor (MS) interface. Photoresponsivity of the device at +10 V bias was calculated as 28.99 A/W and it is higher compared to the values extracted from metal-semiconductor-metal (MSM) type UV PDs reported in the literature. Furthermore, the transient response characteristics of the prepared device showed good stability with almost same rise time and fall time of ~2.73 s and ~5.35 s, respectively. Based on the device performance, the proposed MIS type structure could be a suitable for the development of ultraviolet photodetectors.

Preparation of graphene oxide/silica hybrid composite membranes and performance studies in water treatment

Water shortages have become a major problem facing the world today. Membrane separation technology is commonly used in water treatment applications. The development of new materials for water treatment can reduce the energy required for water treatment, reduce cost, and improve efficiency. The unique structure and properties of graphene and silica make them attractive materials for preparation of nanofiltration membranes for water treatment. We have successfully prepared a graphene oxide/silica (GO/SiO2) hybrid composite materials by in situ hydrolysis, using tetraethyl orthosilicate (TEOS) as a silicon source in an alkaline environment. The chemistry and structure of these materials are characterized by TEM, FTIR, Raman, and XPS of GO and the GO/SiO2 composite. BET porosimetry reveals that the total pore volume of the composite is 2.84 cm3 g−1, the specific surface area is 2897 m2 g−1, and the average pore diameter is 3.97 nm. We prepared GO membranes and GO/SiO2 composite membranes by vacuum suction filtration. The morphology of the membrane was observed by FESEM and AFM. The composite membrane has a larger surface roughness (Rms = 9.39). We also tested the thermal stability by TGA, and hydrophilicity by water contact angle measurements. The water permeance of the composite membrane is up to 229.15 L m−2 h−1 bar−1, and the rejection of the rhodamine B dye molecules is as high as 99%. The GO/SiO2 hybrid composite membrane has good hydrophilicity and thermal stability, high water permeance and rejection, and can be developed as a high-performance material for water treatment.

Effect of precursor ratios on the structural, morphological and optical properties of CBD-CdS films

Cadmium sulfide (CdS) films were deposited on fluorine doped tin oxide (FTO) glass substrates by chemical bath deposition (CBD). X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), photoluminescence (PL), and Fourier transform infrared (FT-IR) analyses were employed to compare the structural, morphological, and optical properties of deposited films. The XRD studies confirmed the cubic crystalline nature of the deposited films, and the intensities of the peaks were found to vary for different precursor ratios. The results of SEM study show that the films were smooth and uniform. The RMS surface roughness values of the CdS films were found to be in the range of 25.42nm to 35.33nm. Energy band-gap values of the CdS films were found in the range of 2.32eV to 2.39eV. Two distinct bands (centred at 489nm and 556nm) were found in the PL emission spectra. S-C-N, Cd-S, and C-S vibrations were observed to dominate in the FT-IR spectra.

Scanning Probe Microscopy Applications in Failure Analysis of Semiconductor Devices

Abstract Scanning probe microscopy (SPM) is widely used for fault isolation as well as diagnosing leakage current, detecting open circuits, and characterizing doping related defects. In this article, the author presents two SPM applications that are fairly uncommon but no less important in the scope of failure analysis. The first case involves the discovery of nano-steps on the surface of high-voltage NFETs, a phenomenon associated with stress-induced crystalline shift along the (111) silicon plane. In the second case, the author uses an AFM probe in the conductive mode to correlate tunneling current distribution with hot spots in high-k gate oxide films, which is shown to be a better indicator of oxide quality than rms surface roughness.

Influence of Silicon Layers on the Growth of ITO and AZO in Silicon Heterojunction Solar Cells

In this article, we report on the properties of indium tin oxide (ITO) deposited on thin-film silicon layers designed for the application as carrier selective contacts for silicon heterojunction (SHJ) solar cells. We find that ITO deposited on hydrogenated nanocrystalline silicon (nc-Si:H) layers presents a significant drop on electron mobility ${\mu} _{{\rm{e}}}$ in comparison to layers deposited on hydrogenated amorphous silicon films (a-Si:H). The nc-Si:H layers are not only found to exhibit a larger crystallinity than a-Si:H, but are also characterized by a considerably increased surface rms roughness. As we can see from transmission electron microscopy (TEM), this promotes the growth of smaller and fractured features in the initial stages of ITO growth. Furthermore, secondary ion mass spectrometry profiles show different penetration depths of hydrogen from the thin film silicon layers into the ITO, which might both influence ITO and device passivation properties. Comparing ITO to aluminum doped zinc oxide (AZO), we find that AZO can actually exhibit superior properties on nc-Si:H layers. We assess the impact of the modified ITO R sh on the series resistance $R_{s}$ of SHJ solar cells with >23% efficiency for optimized devices. This behavior should be considered when designing solar cells with amorphous or nanocrystalline layers as carrier selective contacts.

Effects of nitrogen/oxygen on the electrical and optical properties and microstructure of triple layer AZO/Ag/AZO thin films

Aluminum-doped zinc oxide (AZO)/Ag/aluminum-doped zinc oxide (AAA) specimens are prepared by varying O2 and N2 flow rates during the AZO deposition to investigate the gas effects on surface morphology, microstructure, conductivity type and optical and electrical properties. The AZO specimens are found to have decreases in grain size and RMS surface roughness (SRq), and an increase in compressive residual stress when the N2 flow rate increases higher than 2.5 sccm. The addition of O2 can contribute a similar tendency to grain size, SRq and residual stress; the grain size and SRq are always comparatively smaller, while the residual stress is higher than that prepared with N2. The grain size and SRq are obtained to be proportional to the AZO thickness, whereas they are inversely proportional to the compressive residual stress of AZO. Specimen D prepared with (N2: 0, O2: 2.5) sccm and having 60-nm AZO thickness possesses the strongest anti-reflection effect of the AAA structure, and therefore impedes the reflection from the Ag interlayer significantly. P-type conductivity is achieved by introducing the N2 into the AZO layers and (N2: 15, O2: 0) sccm is used to achieved the highest carrier concentration (CC) and mobility (Mb), and thus the lowest resistivity (R) of all specimens in this study. Increasing the O2 flow rate leads to decreases in Mb and CC, but can obtain the highest average transmittance ( T ¯ ) as (N2: 0, O2: 2.5) sccm is applied. The O2 flow rate becomes the dominant factor for the electrical and optical properties and microstructure as AZO films are deposited in the N2 and O2 mixed atmosphere. (N2: 15, O2: 0) sccm is used to achieve the highest CC and the lowest R of the oxide/metal/oxide specimens reported in the literatures.

Influence of argon pressure on the crystalline phase formation of RF sputtered zinc sulfide thin films

Zinc Sulphide thin films on quartz substrates, were deposited using RF sputtering with variable argon pressures, to assess its effect on the crystallinity and surface morphology. Structural analysis by XRD reveals the formation of crystalline ZnS films, with preferential growth along hexagonal (1 0 3) direction, at a low argon pressure of 0.02 mbar and preferential growth along cubic (1 1 1) direction, at an argon pressure greater than 0.02 mbar. It has been found that crystallinity, is improved at argon pressure of 0.1 mbar. An exponential decrease in film thickness & RMS surface roughness is observed, increasing argon pressure.

Sputtering Growth of Low-Damping Yttrium-Iron-Garnet Thin Films

This letter reports the development of low-damping yttrium-iron-garnet (YIG) thin films via sputtering. The films were deposited by sputtering at room temperature first and were then annealed in O2 at high temperature. It is found that the annealing temperature critically affects the structural properties of the YIG films and thereby dictates the static and dynamic properties of the films. A 75 nm thick YIG film annealed at 900 °C shows an rms surface roughness of 0.08 nm, a coercivity of only 14 A/m (or 0.18 Oe), a saturation induction of 0.1778 T (or 1778 G), which is very close to the bulk value, a gyromagnetic ratio of 2.82 × 104 MHz/T (or 2.82 MHz/Oe), which almost matches the standard value, and a Gilbert damping constant of α a 5.2 × 10–5, which is the lowest among the values reported so far for magnetic films in the nanometer thickness range. Frequency-dependent ferromagnetic resonance measurements with different field orientations confirmed that two-magnon scattering, if present, is very weak, and the measured damping value represents the actual damping of the YIG film.

Effects of thickness, impurity, surface roughness, and temperature on transport and thermoelectric coefficients of ultra-thin silicon nanowires: An analytic study

Abstract A comprehensive study of electronic, thermal, and thermoelectric coefficients of ultra-thin Si nanowires based on the analytic procedure is presented, and the effects of temperature and doping density variation that were not addressed in the previous simulated work, are reported. The quantum-wire electronic subbands and chemical potential are obtained via effective mass and envelope function approximation. To determine the low-field electron mobility and lattice thermal conductivity, relaxation time approximation within linear response theory is employed. The electron mobility is found to reach a peak value at 8 nm wire thickness, and reduces asymptotically to its bulk value with the increasing wire cross-section. Thermoelectric properties are studied over a wide range of thicknesses (3–14 nm), doping densities ( 3 × 1 0 15 – 1 . 6 × 1 0 19 cm−3), and temperatures (200–700 K). The thermoelectric figure of merit ZT increases modestly with the temperature in the range investigated, while its increase with the doping density is more distinct. The ZT variation with quantum confinement or nanowire cross-section reaches a peak value of 0.53 at 4 nm thickness at room temperature for rms surface roughness height Δ = 0 . 5 nm and doping density 1 . 6 × 1 0 19 cm−3, which is consistent with the previous work. The results of this comprehensive analytic study are found to be consistent with the available experimental data and in excellent agreement with elaborate Monte Carlo simulations, where available, while the method also allows for better physical understanding of the effects of different factors involved.

Thermal atomic layer deposition of ruthenium metal thin films using nonoxidative coreactants

Atomic layer deposition (ALD) of ruthenium metal films is presented using (η4-2,3-dimethylbutadiene)(tricarbonyl)ruthenium [Ru(DMBD)(CO)3] with the coreactants 1,1-dimethylhydrazine, hydrazine, or tert-butylamine. The dependence of growth rate on precursor pulse lengths at 200 °C showed a saturative, self-limited behavior at ≥3.0 s for Ru(DMBD)(CO)3 and ≥0.1 s for 1,1-dimethylhydrazine. An ALD window was observed from 200 to 210 °C, with a growth rate of 0.42 Å/cycle. Films grown at 200 °C showed rms surface roughnesses of <1 nm. X-ray photoelectron spectroscopy of a 42 nm thick film grown at 200 °C revealed 90.6% ruthenium, 7.0% nitrogen, and 2.0% oxygen. Ruthenium films were deposited on patterned substrates with TiN surfaces using various treatments at 200 °C with 250 cycles. 42 nm thick ruthenium films grown at 200 °C were subjected to annealing studies under hydrogen and ammonia atmospheres at 400 °C, followed by rapid thermal annealing at 600 °C. These annealing procedures led to higher purity, more crystalline, and lower resistivity ruthenium films. The coreactants hydrazine and tert-butylamine were evaluated in ruthenium ALD trials using Ru(DMBD)(CO)3. Hydrazine gave a growth rate of 0.42 Å/cycle within a 200–205 °C ALD window, whereas tert-butylamine gave a growth rate of 0.25 Å/cycle at 200 °C.

Reduction of process temperature for Si surface flattening utilizing Ar/H2 ambient annealing and its application to SOI-MISFETs with bilayer HfN high-k gate insulator

We have investigated the reduction of the process temperature for the Si surface flattening process by annealing in Ar/H2 ambient and its application to Si-on-insulator (SOI) metal-insulator-semiconductor field-effect transistors (MISFETs) with bilayer HfN high-k gate insulator. The surface rms roughness of 0.057 nm was realized for the p-Si(100) substrates by the annealing at 925 °C/10 min in Ar/1.0%H2 ambient. Although slip-line defects were observed in the isolated SOI region after the optimized flattening process, the device characteristics of the fabricated SOI-MISFETs with HfN1.3/HfN1.1/Si(100) bilayer gate insulator were found to have been improved by the surface flattening utilizing Ar/1.0%H2 annealing.

Effect of Electron Beam Annealing Duration on the Properties of Zno Thin Films

The effect of electron beam (EB) annealing duration on the properties of pure ZnO thin films was studied. Pure ZnO precursor films prepared by sol-gel spin coating method were directly annealed by EB for different duration from 20 s to 480 s. The structural, optical and electrical properties were investigated. XRD patterns show (002) preferential growth for all the ZnO films. SEM results reveal that the average grain size changes from ∼10 nm to ∼70 nm with the increase of annealing duration and AFM results show the surface RMS roughness is less than 3 nm. The optical transmittance spectra present the absorption edge of the films is at about 380 nm and the average transmittance is more than 85% in visible region. It’s exciting to find that the ZnO film annealed for only 120 s exhibits resistivity 2×10−2 Ωcm and carrier concentration as high as 5.5×1019 cm−3, which shows an advantage of EB annealing in the preparation of ZnO transparent conductive films.

Characteristics of Diamond-Like Carbon Films Fabricated by Using a Filtered Vacuum Arc System for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSC) based on diamond-like carbon (DLC) anti-reflection coating (ARC) thin films are investigated. The DLC film was proposed to improve the cell performance of DSSC devices. In this work, the DLC films were fabricated by using the FVA (filtered vacuum arc) method, the structural, optical, and physical properties of the fabricated DLC films with various film thicknesses were experimentally investigated. All DLC films showed smoother and more uniform surfaces with increasing film thickness. Furthermore, the values of rms surface roughness, friction coefficient, the refractive index, and the absorption coefficient increased with increasing film thickness, the hardness and the elastic modulus of the DLC films were improved. These results are associated with the increased sp3 contents in the amorphous carbon film due to the effects of the energetic ion bombardment during film deposition.

Hyperelastic characteristics of graphene natural rubber composites and reinforcement and toughening mechanisms at multi-scale

In order to meet the increasing demand of damping and shock absorption materials, graphene-based rubber composites are considered as a candidate material because of their excellent strength and toughness. Here, we propose a small and excessive amount of 3-aminopropyl triethoxy silane (APTS) modified graphene oxide as a functional composite material for filling and reinforcing natural rubber, and we also prepare the corresponding natural rubber composites containing graphene oxide (go) and reduced graphene oxide (rgo). Compared with natural rubber filled with carbon black, the crosslinking density and mechanical properties of the composites containing graphene prepared in this paper improved significantly. The strengthening and toughening mechanism of APTS modified graphene oxide filled composites revealed at the micron, nano and even molecular level by FTIR, AFM and SEM characterisation. Notably, by comparing the surface RMS roughness of graphene oxide edited with a small and excessive amount of APTS, we find that the higher the surface roughness of graphene, the more obvious the effect of strengthening and toughening after compounding with natural rubber. These results will be helpful for the application of graphene composites in rubber damping materials.

Phase transformation and modifications in high-k ZrO2 nanocrystalline thin films by low energy Kr5+ ion beam irradiation

In present communication, zirconium oxide (ZrO2) thin films of thickness (155 nm) were grown by using RF sputtering technique. To address and systematically understand the effects of low energy (800 keV) irradiation, high-k ZrO2 thin films were irradiated at room temperature by Kr ions with a range of fluence 1E15 to 1E17 ions. cm−2. The polymorphous zirconia pristine and irradiated thin films were characterized through X-ray diffraction (XRD) technique which confirms the coexistence of monoclinic and tetragonal phases. Monoclinic to tetragonal structural phase transformation was observed for Kr5+ ions irradiated samples at a fluence of 5E15, 1E16, 1E17 ions. cm−2. All samples were characterized by UV–Vis spectroscopy to study the mechanism of variation in optical energy band gap (4.42–4.52 eV), Urbach energy (402–449 meV) and refractive index (1.518–1.523) with increasing ion fluence. Photoluminescence (PL) spectroscopy was done at room temperature to record the significant PL broad prominent emission peaks at 350 nm and 430 nm and the peak intensity of these peaks significantly varies due to the annihilation of primary defects or generation of new defects centres by the influence of Kr ion irradiation. To determine the significant variation in grain size (50–85 nm) and RMS surface roughness (0.54–1.04 nm), pristine and irradiated sample were characterized by atomic force microscopy (AFM). A complete mechanism of the evolution of surface roughness was analyzed by determining the power spectral density (PSD). Further surface roughness component in the range of 0.4–1.7 was calculated using the slope of the tail of PSD. The FTIR spectra of pristine and low energy ion irradiated ZrO2 thin films were recorded in the range of 1250–3200 cm−1. The Rutherford backscattering spectrometry (RBS) was carried out in channeling condition and the experimental results were simulated to evaluate the elemental composition of Zr and O (⁓1:2) and film thickness (155 nm) of the samples. The changes of the chemical binding energy of ZrO2 thin films of Zr 3d and O 1s region before and after ion irradiation were investigated by x-ray photoelectron spectroscopy (XPS).

Electrodeposited MoS2 Thin Films for Stable Immobilization of Thiolated Biomolecules

We report a robust new combination of substrate materials and immobilization chemistry for biosensing that addresses one of the oldest challenges in this field, the stability and reproducibility of biomolecule immobilization. The substrate materials are electrodeposited MoS2 and Cu-doped MoS2 thin films atop glassy carbon electrodes (GCE) created by a two-step process of cathodic electrodeposition, followed by high temperature annealing. Electrodeposition atop a GCE is preceded by an electrochemical pre-treatment process to increase the surface roughness and introduce oxygen functional groups, dramatically improving film adhesion. MoS2 thin films are electrodeposited from 10 mM (NH4)2MoS4 and 0.2 M KCl at pH 6.8, while Cu-doped MoS2 thin films are electrodeposited from 10.0 mM (NH4)2MoS4, 5.0 mM CuSO4, 0.10 M KCl, and 0.50 M KSCN at pH 6.95. Since MoS2 is an active hydrodesulfurization (HDS) catalyst for sulfur removal from hydrocarbons, both thin film materials immediately react with and desulfurize the electrolyte from which they are deposited. The desired MoS2 stoichiometry, which includes coordinately unsaturated Mo sites, can be recovered by high temperature annealing in a reducing environment. High temperature annealing removes the excess sulfur from the as-deposited film, and also partially recrystallizes and partially orients the MoS2 thin films, as suggested by x-ray diffraction (XRD) studies. This annealing procedure regenerates the coordinately unsaturated and highly sulfiphilic Mo sites atop HDS catalyst materials, which can then form strong bonds to thiolated biomolecules. We demonstrate this by disulfide bond reduction at the hinge region of polyclonal antibodies utilizing tris (2-carboxyethyl) phosphine (TCEP) as the reducing agent. Unlike other reducing agents for disulfide bond cleavage, which contain thiol groups, TCEP probably does not compete with antibody fragments for thiol binding sites atop the MoS2 surface. The attached figure illustrates the biosensing platform described here. This new biosensing platform is demonstrated by disulfide bond reduction within polyclonal antibodies to 3-phenoxybenzoic acid (3-PBA), binding of these antibody fragments to newly created MoS2 and Cu-doped MoS2 thin films, and detection of the 3-PBA antigen using electrochemical impedance spectroscopy (EIS). Control experiments demonstrate that only the annealed MoS2 and Cu-doped MoS2 thin films are active towards binding of antibody fragments. In both cases, the charge transfer resistance (Rct) is the equivalent circuit element from the Randles circuit that is most sensitive to antigen binding. Atop un-doped MoS2, the sensitivity for 3-PBA detection by EIS is 2.7x108 Ω-cm2-M-1, and the detection limit is 2.5x10-6 M. Atop Cu-doped MoS2, the sensitivity for 3-PBA detection is 5.9x108 Ω-cm2-M-1, and the detection limit is 3.8x10-6 M. The variation of Rct with antigen concentration atop MoS2 is consistent with the Langmuir adsorption isotherm, with an initial linear relationship, and Rct eventually saturating at high concentrations when all binding sites are occupied. However, two plateau regions are reproducibly observed atop Cu-doped MoS2. The rms surface roughness obtained by atomic force microscopy (AFM) measurements atop MoS2 and Cu-doped MoS2 substrates ranges from 60–140 nm, so these methods are not limited to ultra-smooth substrates. Figure 1

Reverse biased nanocrystalline graphite (NCG)/p-Si schottky junction for methane gas sensor

An investigation on the effect of the reverse biased operation of NCG/p-Si Schottky contact during methane gas exposure at room temperature has been presented. The experimental results show the larger current shift at the reverse bias operation, compared to the forward bias by exposing to methane gas. This can be attributed to the adsorption of methane gas into the metal surface layer and produces a negative charge at the junction, thus reduces the barrier height of the device. The reverse barrier height was calculated under the reverse bias condition, demonstrated the value decreased from 0.58-0.53eV towards a higher concentration of methane gas. The Schottky junction also affected by the increase in a free carrier when exposure to the reducing gas such as methane. Raman spectra are reported to be detected at G, D and 2D band with the grain size 1.88nm to exhibit single crystallite graphite properties. The results correlate well with the 3D AFM scans reveal the RMS surface roughness of 1.1 to 2.8nm.

Effects of anodic layer thickness on overall performance of all-solid-state electrochromic device

In the present study, the effects of the anodic electrochromic layer employed in the all-solid-state ECD in the structure of ITO/NiO/Ta2O5/WO3/ITO/glass on the optical and electrochromic performances of the device are investigated. For this purposes, the nickel oxide films with various thicknesses were deposited onto glass and ITO-coated glass substrates by RF magnetron sputtering technique using a NiO target in pure Ar atmosphere at room temperature. The optical, structural and electrochromic properties of the NiO films with various thicknesses (140 nm, 230 nm, 310 nm and 480 nm) were investigated. The optical transmission of the films decreased as the thickness of the films increased. The X-ray diffraction patterns showed that the films were polycrystalline phase and the peak density increased with the film thickness. The optical band gap was found to decrease from 3.71 to 3.50 eV with increasing film thickness. The root-mean-square (rms) surface roughnesses of the films increased (from 1.43 nm to 2.94 nm) with increased film thickness. Coloration efficiencies and optical modulations of the NiO films were obtained for a wide spectral range. The amount of inserted/extracted charges into/from the films during bleaching/coloring were obtained as a function of the film thickness. The highest coloration efficiency (40.7 cm2/C) and optical modulation (27.8%) at a wavelength of 550 nm were obtained for the NiO film with a thickness of 480 nm. Finally, all-solid-state electrochromic devices were fabricated by using the thinnest and thickest NiO films with a configuration of ITO/NiO/Ta2O5/WO3/ITO/glass. The ECD-1 device with a thinner NiO layer showed a high optical modulation (36.4%). On the other hand, the ECD-2 with a thicker NiO layer showed a very low optical modulation (1.5%) at 550 nm. Our results indicated that the thicker NiO film exhibited better electrochromic properties in the half-cell configuration, while the thinner one showed higher optical modulation in electrochromic device fabrication with a complete-cell configuration. Therefore, the effect of the thickness of a NiO layer on the electrochromic performance of both the half-cell and complete cell configuration electrochromic coatings were investigated in detail.

Effect of annealing temperature on the structural, morphological and optical properties of ThO2 thin films grown by photochemical metal–organic deposition

In this work, we report the influence of annealing temperature on the structural, morphological and optical properties of ThO2 thin films deposited on fused quartz and (100) silicon substrates by photochemical metal–organic deposition (PMOD) using hinokitiolate thorium (IV) complex as the precursor. X-ray photoelectron spectroscopy (XPS) confirmed the deposition of the ThO2 films. The effect of thermal annealing (from 300 °C to 1100 °C) on the structural properties of the ThO2 films was evaluated with X-ray diffraction (XRD), UV–Vis transmittance spectroscopy, spectroscopic ellipsometry and atomic force microscopy (AFM). XRD patterns of the films annealed above 300 °C revealed that all diffraction peaks belong to a cubic ThO2 structure without preferential orientation. The average crystallite size increased from 2.3 nm to 3.7 nm as the annealing temperature increased from 300 °C to 750 °C. Annealing at 1100 °C promoted the formation of huttonite (β-ThSiO4) in the ThO2 layer. Films deposited on fused quartz showed 80% transmittance in the visible range, and the annealing temperature did not impact the band gap energy. The AFM studies revealed that as the annealing temperature increased, the film surface roughness decreased. Between 750 °C and 950 °C, the film surface was relatively smooth with an of RMS surface roughness in the range 2.3–5.0 nm. The values of the refractive index of the films were in the range of n = 1.6 for the as-deposited film to n = 2.1 for the films annealed at 750 °C. This result suggests the potential use of ThO2 films as a transparent reflector material for short wavelengths.

Layer-Dependent Interfacial Transport and Optoelectrical Properties of MoS2 on Ultraflat Metals.

Layered materials based on transition-metal dichalcogenides (TMDs) are promising for a wide range of electronic and optoelectronic devices. Realizing such practical applications often requires metal-TMD connections or contacts. Hence, a complete understanding of electronic band alignments and potential barrier heights governing the transport through metal-TMD junctions is critical. However, it is presently unclear how the energy bands of a TMD align while in contact with a metal as a function of the number of layers. In pursuit of removing this knowledge gap, we have performed conductive atomic force microscopy (CAFM) of few-layered (1 to 5 layers) MoS2 immobilized on ultraflat conducting Au surfaces [root-mean-square (rms) surface roughness < 0.2 nm] and indium-tin oxide (ITO) substrates (rms surface roughness < 0.7 nm) forming a vertical metal (CAFM tip)-semiconductor-metal device. We have observed that the current increases with the number of layers up to five layers. By applying Fowler-Nordheim tunneling theory, we have determined the barrier heights for different layers and observed how this barrier decreases as the number of layers increases. Using density functional theory calculations, we successfully demonstrated that the barrier height decreases as the layer number increases. By illuminating TMDs on a transparent ultraflat conducting ITO substrate, we observed a reduction in current when compared to the current measured in the dark, hence demonstrating negative photoconductivity. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of the TMD-metal junction, which depends on the numbers of TMD layers and may pave an avenue toward developing nanoscale electronic devices with tailored layer-dependent transport properties.