Volmer-Weber Mode Research Articles

Thickness-Dependent Relative Dielectric Constant of Organic Ultrathin Films.

In formulas employed for analysis of organic electronic devices, the relative dielectric constant value of the semiconductor organic films is often assumed rather than measured, even though it is a fundamental parameter for a correct interpretation. This is particularly true for ultrathin films made of discrete molecular layers. In this work, Spectroscopy Ellipsometry and Scanning Capacitance Microscopy were used to study thin films made of N,N'-bis(n-octyl)-x:y,dicyanoperylene-3,4 : 9,10-bis(dicarboximide). The relative dielectric constant presents a non-monotonic trend with thickness: it is equal to 2.1 for one molecular layer, saturating at 3.2 for increasing thickness. This maximum value, equivalent to the bulk one, occurs when the coverage is in between the third to the fourth layer. In this range, the growth switches from a Frank-Van der Merwe (2D growth) to a Volmer-Weber mode (3D growth); in addition, the molecular configuration assumes a bent/distorted geometry with respect to the initial edge-on one. These results establish a morphological dependence of the dielectric constant, especially in the vicinity of the substrate interface, that disappears at a certain distance from it.

Investigating the growth behavior of titanium dioxide film prepared with direct current pulsed–magnetron sputtering technology

The direct current pulsed magnetron sputtering technology is employed to deposit TiO2 film onto the glass and Si(100) substrates. After annealing treatment, TiO2 film displays anatase (001) preferential orientation. The growth behavior of deposited and annealed TiO2 films is investigated. The results show that in the original stage, TiO2 film growth behavior is in Volmer–Weber mode, which is dominated by the diffusion effect. Considering the rivalry between TiO2 crystal nuclei, both crystallographic orientation and diffusion effects jointly dominate the film growth behavior in the rest stage. More importantly, the annealing treatment resulted in the coalescence of TiO2 crystal nuclei, as well as the growth of TiO2 crystal nuclei. Most importantly, after coalescence and growth of TiO2 nuclei, the anatase [001] crystal orientation of TiO2 grains remains the same as the normal direction of TiO2 film. Finally, after annealing treatment, TiO2 films comprise anatase (001) preferential orientation.

A Stranski–Krastanov to Volmer–Weber transition in the growth mode of self-assembled quantum dots

We present recent experiments for tensile-strained Ge QDs on InAlAs(111)A where we observe that growth proceeds via the Stranski–Krastanov (SK) growth mode at lower temperatures and transitions to the Volmer–Weber (VW) growth mode at higher temperatures. We discuss atomistic kinetic Monte Carlo simulations that show how the SK to VW growth mode transition is affected by model parameters. We find that systems are more likely to transition from a SK to a VW mode when the bond energy between substrate atoms is weaker than the bond energy between atoms of the deposited material. We also show how entropic effects due to intermixing can stabilize a layer-by-layer growth mode in certain parameter regimes.

Secondary Phases, Morphology and Band Tails of Third Generation Photovoltaic Absorber Layer CZTS Annealed at Different Temperatures

Abstract: In this work, we have successfully fabricated Cu2ZnSnS4 (CZTS) thin films using the sol-gel spin-coating technique. Thin films were annealed in an argon atmosphere at different temperatures of 350, 375, 400, 425, and 450 ℃ for an hour. The influence of annealing temperature on structural, morphological, and optical properties was investigated. x-ray diffraction measurement confirmed the formation of secondary phases in as-deposited and annealed thin films at 350 and 375 ℃. A pure kesterite phase was obtained at an annealing temperature of 400 ℃, but degradation occurred at higher temperatures. The morphology study of the surface structure showed that thin films have homogeneous surfaces. The Volmer-Weber mode was the dominating growth mode. The as-deposited and annealed CZTS thin films exhibited a high absorption coefficient of the order of 104 cm-1, and the optical energy band gap Eg was red-shifted with increasing annealing temperature. The optical study showed a decrease in Urbach tail energy with an increase in annealing temperature and reached 0.39 eV at 400 ℃. The refractive index and dielectric constants of CZTS thin films were calculated. Keywords: Cu2ZnSnS4 thin films, Sol-Gel technique, Secondary phases, Roughness parameters, Band gap, Urbach tails.

Energetically Favored 2D to 3D Transition: Why Silicene Cannot Be Grown on Ag(111).

Silicene, a two-dimensional (2D) Si monolayer with properties similar to those of graphene, has attracted considerable attention because of its compatibility with existing technology. Most growth efforts to date have focused on the Ag(111) substrate, with a 3 × 3 phase widely reported below one monolayer (ML). As the coverage increases, a √3 × √3 pattern frequently emerges, which has been proposed by various experimental investigations as a reconstructed structure. We report first-principles calculations to understand this series of observations. A major finding from our energetics studies is that Si growth on Ag(111) beyond one ML will switch to the Volmer-Weber mode, forming three-dimensional sp3 films. Combining with the condition that the 3 × 3 monolayer on Ag(111) does not have the correct buckling pattern of freestanding silicene, we conclude that silicene cannot be grown on Ag(111) and that a 2D to 3D transition is energetically favored beyond one ML.

Исследование процесса вторичной конденсации LiF в ходе реактивного ионного травления тонкопленочного ниобата лития во фторсодержащей плазме

This paper investigates LiF redeposition during reactive ion etching (RIE) of X-cut thin-film lithium niobate (TFLN) with Ar/SF6 ICP/CCP plasma. RIE of TFLN is a topochemical reaction (TCR), described by the Kolmogorov-Avrami-Erofeev equation, during which there occurs LiF redeposition from the supersaturated vapor phase onto the sample surface. It is shown that the redeposition process is characterized by an induction period, which depends on the etch rate and the TFLN crystal structure imperfections. To study the LiF redeposition process, we used spectroscopic reflectometry, energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The dislocation density of TFLN was investigated with the help of etch pit method and X-ray diffraction. This study is the first to show that the duration of the induction period of TCR on the TFLN surface is inversely related to the etch rate. The RIE process is characterized by critical etch rates, above which a thin LiF film grows according to the two-dimensional Volmer-Weber mode. It was experimentally shown that the critical etch rates decrease with increasing the process pressure. For the first time, the dislocation density of TFLN was experimentally evaluated by means of the etch pit method. An experimental comparison of the induction periods of TCR on the surface of bulk and TFLN was carried out. It has been revealed that the increased dislocation density of TFLN significantly reduces the duration of the induction period. The impact of the modification of the TFLN structure with proton exchange on the kinetics of RIE was studied. It was demonstrated that the replacement of Li+ with H+ leads to decrease in the degree of LiF vapor saturation above the TFLN surface and, as a consequence, to a significant increase in the induction period of TCR and an increase in the etch rates by 5–9 times compared to unmodified TFLN.

Insight into uniform filming of LiF‐rich interphase via synergistic adsorption for high‐performance lithium metal anode

AbstractMulti‐scale simulation is an important basis for constructing digital batteries to improve battery design and application. LiF‐rich solid electrolyte interphase (SEI) is experimentally proven to be crucial for the electrochemical performance of lithium metal batteries. However, the LiF‐rich SEI is sensitive to various electrolyte formulas and the fundamental mechanism is still unclear. Herein, the structure and formation mechanism of LiF‐rich SEI in different electrolyte formulas have been reviewed. On this basis, it further discussed the possible filming mechanism of LiF‐rich SEI determined by the initial adsorption of the electrolyte‐derived species on the lithium metal anode (LMA). It proposed that individual LiF species follow the Volmer–Weber mode of film growth due to its poor wettability on LMA. Whereas, the synergistic adsorption of additive‐derived species with LiF promotes the Frank‐Vander Merwe mode of film growth, resulting in uniform LiF deposition on the LMA surface. This perspective provides new insight into the correlation between high LiF content, wettability of LiF, and highperformance of uniform LiF‐rich SEI. It disclosed the importance of additive assistant synergistic adsorption on the uniform growth of LiF‐rich SEI, contributing to the reasonable design of electrolyte formulas and high‐performance LMA, and enlightening the way for multi‐scale simulation of SEI.

Magnetic coercivity control via buffer layer roughness in Pt/Co multilayers

Magnetic properties of thin metallic films are strongly dependent on the growth parameters. Therefore, fine-tuning these parameters is necessary to produce multilayers with specific physical properties required by various applications. Here, we present an efficient approach to tune the coercive field of magnetron sputtered Co/Pt multilayers using Au underlayer with variable thickness. We were able to control the interfacial roughness of the layers in the stack via the change of the Au underlayer thickness due to its island-like growth mechanism (Volmer-Weber mode). As the nominal thickness of Au increased, the islands grew in larger lateral size, resulting in higher surface roughness of the underlayer, which was transferred to subsequent interfaces. Magnetization measurements showed additional magnetic anisotropy promoted by this mechanism. Variating the thickness of the Au layer up to 20 nm, we changed the coercive field in the range from ∼ 200 Oe to ∼ 1100 Oe while maintaining a nearly constant saturation magnetization. Presented results demonstrate the possibility to precisely design magnetic properties of metallic multilayers for applications via buffer layer roughness engineering.

Ultra-thin silver films grown by sputtering with a soft ion beam-treated intermediate layer

Silver thin films have wide-ranging applications in optical coatings and optoelectronic devices. However, their poor wettability to substrates such as glass often leads to an island growth mode, known as the Volmer–Weber mode. This study demonstrates a method that utilizes a low-energy ion beam (IB) treatment in conjunction with magnetron sputtering to fabricate continuous silver films as thin as 6 nm. A single-beam ion source generates low-energy soft ions to establish a nominal 1 nm seed silver layer, which significantly enhances the wettability of the subsequently deposited silver films, resulting in a continuous film of approximately 6 nm with a resistivity as low as 11.4 µΩ.cm. The transmittance spectra of these films were found to be comparable to simulated results, and the standard 100-grid tape test showed a marked improvement in adhesion to glass compared to silver films sputter-deposited without the IB treatment. High-resolution scanning electron microscopy images of the early growth stage indicate that the IB treatment promotes nucleation, while films without the IB treatment tend to form isolated islands. X-ray diffraction patterns indicate that the (111) crystallization is suppressed by the soft IB treatment, while growth of large crystals with (200) orientation is strengthened. This method is a promising approach for the fabrication of silver thin films with improved properties for use in optical coatings and optoelectronics.

Influence of Sputtering Pressure on the Micro-Topography of Sputtered Cu/Si Films: Integrated Multiscale Simulation

In this work, an integrated multiscale simulation of magnetron sputtering epitaxy was conducted to study the effect of sputtering pressure on the surface micro-topography of sputtered Cu/Si films. Simulation results indicated that, as the sputtering pressure increased from 0.15 to 2 Pa, the peak energy of the incident energy distribution gradually decreased from 2 to 0.2 eV, which might be mainly due to the gradual decrease in the proportion of deposited Cu atoms whose energy ranged from 2 to 30 eV; the peak angle of the incident polar angle distribution increased from 25° to 35°, which might be attributed to the gradual thermalization of deposited Cu atoms; the growth mode of Cu film transformed from the two-dimensional layered mode to the Volmer-Weber mode. The transformation mechanism of growth mode was analyzed in detail. A comprehensive analysis of the simulation results indicated that incident energy ranging from 2 to 30 eV and incident angle between 10° and 35° might be conducive to the two-dimensional layered growth of sputtered Cu films. This work proposes an application-oriented modeling approach for magnetron sputtering epitaxy.

Growth dynamics of selective-area-grown rutile-type SnO2 on TiO2 (110) substrate

We demonstrated selective-area growth of r-SnO2 on a SiO2-masked r-TiO2 (110) substrate. The heteroepitaxy on a window started with a Volmer–Weber mode to grow islands with {100}-, {11̄0}-, and {011}-faceted sidewalls, whose growth shapes were consistent with the rutile structure’s equilibrium shape. The islands coalesced each other to make a flat (110) top surface on a striped window, and lateral overgrowth started after the complete coverage of the window. Cross-sectional transmission-electron-microscopy observation of the stripe revealed that misfit dislocations propagated perpendicularly to the facet planes by the image force effect and that the dislocation density reduced substantially in the wing regions.

Heteroepitaxial Growth of Colloidal Crystals: Dependence of the Growth Mode on the Interparticle Interactions and Lattice Spacing.

Epitaxial growth is one of the most important techniques for the control of crystal growth, especially for growing thin-film semiconductor crystals. Similarly, colloidal epitaxy, a template-assisted self-assembly method, is a powerful technique for controlling the structure of colloidal crystals. In this study, heteroepitaxial growth, which differs from homoepitaxial growth of conventional colloidal epitaxy, using foreign colloidal crystals as a substrate, was used to grow single-component colloidal crystal films. The Frank-van der Merwe (FM), Stranski-Krastanov (SK), and Volmer-Weber (VW) modes were observed, and the mode varied with the lattice-misfit ratio and interparticle interactions between the substrate and epitaxial phase. The transition of the growth mode (from SK to VW) and the coexistence of different growth modes (FM and VW) were observed, and their processes were revealed by in situ observation. Colloidal heteroepitaxy was confirmed to be useful for controlling structure, which will enable exploration of novel colloidal self-assembly structures.

Thickness Impact on the Morphology, Strain Relaxation and Defects of Diamond Heteroepitaxially Grown on Ir/Al2O3 Substrates.

This paper investigates the formation and propagation of defects in the heteroepitaxial growth of single-crystal diamond with a thick film achieving 500 µm on Ir (001)/Al2O3 substrate. The growth of diamond follows the Volmer–Weber mode, i.e., initially shows the islands and subsequently coalesces to closed films. The films’ strain imposed by the substrate gradually relaxed as the film thickness increased. It was found that defects are mainly located at the diamond/Ir interface and are then mainly propagated along the [001] direction from the nucleation region. Etching pits along the [001] direction formed by H2/O2 plasma treatment were used to show defect distribution at the diamond/Ir/Al2O3 interface and in the diamond bulk, which revealed the reduction of etching pit density in diamond thick-film surface. These results show the evident impact of the thickness on the heteroepitaxially grown diamond films, which is of importance for various device applications.

Continuity of thin layers of an organic semiconductor induced by the modification of the gate insulator

The change in the crystallization process of an organic semiconductor layer in the presence of a small amount of TiO2 nanoparticles from the Volmer–Weber mode to the Stranski–Krastanov mode is reported and explained.

Strain-Modulated Seeded Growth of Highly Branched Black Au Superparticles for Efficient Photothermal Conversion.

Creating highly branched plasmonic superparticles can effectively induce broadband light absorption and convert light to heat regardless of the light wavelength, angle, and polarization. However, their direct synthesis in a controllable manner remains a significant challenge. In this work, we propose a strain modulation strategy to produce branched Au nanostructures that promotes the growth of Au on Au seeds in the Volmer-Weber (island) mode instead of the typical Frank-van der Merwe (layer-by-layer) mode. The key to this strategy is to continuously deposit polydopamine formed in situ on the growing surface of the seeds to increase the chemical potential of the subsequent deposition of Au, thus achieving continuous heterogeneous nucleation and growth. The branched Au superparticles exhibit a photothermal conversion efficiency of 91.0% thanks to their small scattering cross-section and direction-independent absorption. Even at a low light power of 0.5 W/cm2 and a low dosage of 25 ppm, these particles show an excellent efficacy in photothermal cancer therapy. This work provides the fundamental basis for designing branched plasmonic nanostructures and expands the application scope of the plasmonic photothermal effect.

The formation mechanism of (001) preferred orientation for anatase TiO2 film prepared by DC pulsed magnetron sputtering

TiO2 films were deposited by DC pulsed magnetron sputtering with different sputtering power density (SPD). Then, as-deposited TiO2 films were annealed at 600 °C and the anatase TiO2 films were obtained. XPS results showed Ti/O atom ratio was kept at 1/(1.94 ± 0.04) for all as-deposited and annealed TiO2 films. XRD results showed that the annealed TiO2 films displayed obvious anatase (001) preferred orientation, which was deposited with SPD in 0.83 W/cm2. The cross-section TEM results showed that annealed TiO2 film with anatase (001) preferred orientation consisted of a small quantity of columnar crystals and a majority of equiaxed crystals, which [001] crystallographic axis was parallel to the normal direction of film. AFM results revealed that with SPD in 0.83 W/cm2, the dominated diffusion effect controlled TiO2 film growth process with Volmer-Weber mode, which contained the nano-size anatase crystal nuclei. The [001] crystal orientation of anatase crystal nuclei was uniform parallel to the normal direction of TiO2 film growth surface. During the annealing process, the nano-size anatase crystal nuclei grew and coalesced into the large anatase grains without the change of [001] crystal orientation, which resulted in TiO2 film with obvious (001) preferred orientation.

Growth mechanism for zinc coatings deposited by vacuum thermal evaporation

The vacuum thermal evaporation technique was used to simultaneously deposit zinc coatings onto interstitial free steel plates and single-crystal silicon wafers in a high vacuum environment. The effect of substrate temperature on the morphology and crystal orientation of zinc coatings was investigated. When the substrate temperature was 25 and 50 °C, the zinc crystallites were plate-like and grew under a particular angle to the substrate surface. After the substrate was heated to 100 °C, the zinc crystallites were regular hexagonal and arranged almost parallel to the substrate surface. In addition, observation of pure zinc coatings with different thicknesses showed that the growth of zinc coating was mainly in the Volmer–Weber mode. When the process parameters were appropriate, the zinc coating was composed of closely arranged columnar crystallites, and the crystallites grew preferentially along [0001] direction.

Growth of single crystalline films on lattice-mismatched substrates through 3D to 2D mode transition

Regarding crystalline film growth on large lattice-mismatched substrates, there are two primary modes by which thin films grow on a crystal surface or interface. They are Volmer-Weber (VW: island formation) mode and Stranski-Krastanov (SK: layer-plus-island) mode. Since both growth modes end up in the formation of three-dimensional (3D) islands, fabrication of single crystalline films on lattice-mismatched substrates has been challenging. Here, we demonstrate another growth mode, where a buffer layer consisting of 3D islands initially forms and a relaxed two-dimensional (2D) layer subsequently grows on the buffer layer. This 3D-2D mode transition has been realized using impurities. We observed the 3D-2D mode transition for the case of ZnO film growth on 18%-lattice-mismatched sapphire substrates. First, nano-sized 3D islands grow with the help of nitrogen impurities. Then, the islands coalesce to form a 2D layer after cessation of the nitrogen supply, whereupon an increase in the surface energy may provide a driving force for the coalescence. Finally, the films grow in 2D mode, forming atomically flat terraces. We believe that our findings will offer new opportunities for high-quality film growth of a wide variety of materials that have no lattice-matched substrates.

Analytical validation of the Young–Dupré law for epitaxially-strained thin films

We present here an analysis of the regularity of minimizers of a variational model for epitaxially strained thin-films. The regularity of energetically-optimal film profiles is studied by extending previous methods and by developing new ideas based on transmission problems. The achieved regularity results relate to both the Stranski-Krastanow and the Volmer-Weber modes, the possibility of different elastic properties between the film and the substrate, and the presence of the surface tensions of all three involved interfaces: film/gas, substrate/gas, and film/substrate. Finally, geometrical conditions are provided for the optimal wetting angle, i.e. the angle formed at the contact point of films with the substrate. In particular, the Young–Dupré law is shown to hold, yielding what appears to be the first analytical validation of such law for a thin-film model in the context of Continuum Mechanics.

Thermal Conductivity and Phonon Scattering Processes of ALD Grown PbTe–PbSe Thermoelectric Thin Films

AbstractThis work studies the thermal conductivity and phonon scattering processes in a series of n‐type lead telluride‐lead selenide (PbTe–PbSe) nanostructured thin films grown by atomic layer deposition (ALD). The ALD growth of the PbTe–PbSe samples in this work results in nonepitaxial films grown directly on native oxide/Si substrates, where the Volmer–Weber mode of growth promotes grains with a preferred columnar orientation. The ALD growth of these lead‐rich PbTe, PbSe, and PbTe–PbSe thin films results in secondary oxide phases, along with an increase microstructural quality with increased film thickness. The compositional variation and resulting point and planar defects in the PbTe–PbSe nanostructures give rise to additional phonon scattering events that reduce the thermal conductivity below that of the corresponding ALD‐grown control PbTe and PbSe films. Temperature‐dependent thermal conductivity measurements show that the phonon scattering in these ALD‐grown PbTe–PbSe nanostructured materials, along with ALD‐grown PbTe and PbSe thin films, are driven by extrinsic defect scattering processes as opposed to phonon–phonon scattering processes intrinsic to the PbTe or PbSe phonon spectra. The implication of this work is that polycrystalline, nanostructured ALD composites of thermoelectric PbTe–PbSe films are effective in reducing the phonon thermal conductivity, and represent a pathway for further improvement of the figure of merit (ZT), enhancing their thermoelectric application potential.