Film Growth Process Research Articles

Influence of helium incorporation on growth process and properties of aluminum thin films deposited by DC magnetron sputtering

The effect of helium content on the morphology, crystallinity, and composition of aluminum films was investigated by depositing He-loaded Al films onto Si substrates via direct current (DC) magnetron sputtering in different Ar/He plasma mixtures. Three different plasma regimes were identified depending on the percentage of He in the gas phase. For a low He to total gas ratio (ΓHe ≤ 70%), the plasma is dominated by argon, where Ar+ ions contribute to sputter out the target atoms. The films deposited in this regime exhibited the classical dense columnar structure and contain very low amount of He (below 2%). Then, as ΓHe increases, helium ions begin to be formed and more fast He neutrals reach the substrate, affecting the film growth. As He amount increased in the gas phase up to 95%, the proportion of He inserted in the films rised up to ⁓15 at. %. Moreover, bubbles/porosity were formed inside the films; those obtained in pure He plasma presented a highly porous fiberform nanostructure. All results confirmed that the modification of the film characteristics was related to the change of the deposition conditions when Ar was replaced by He and to the insertion/release mechanisms of He during the growth.

CONDENSATION PHASE CHANGE BEHAVIORS ON A ROUGH SURFACE CHARACTERIZED BY FRACTAL CANTOR

Understanding the fundamental mechanisms of vapor condensation on rough surfaces is crucial to a wide range of industrial applications. A hybrid thermal lattice Boltzmann model of the condensation heat transfer process on downward-facing rough surfaces characterized by the Cantor fractal is developed and numerically analyzed to investigate the condensation phase change behaviors on rough hydrophobic and hydrophilic surfaces. The dynamic behaviors of vapor condensation, including the evolutions of vapor–liquid interface, heat flux, condensate mass, and temperature distribution, on the hydrophilic and hydrophobic rough surfaces are presented and compared with corresponding smooth surfaces. The results indicate that the rough surface preferred a filmwise condensation under hydrophilic conditions but a hybrid dropwise–filmwise condensation under hydrophobic conditions. On the rough hydrophobic surface, the liquid film can rapidly adsorb droplets, maintaining a high-efficiency dropwise condensation. The absorption of droplets accelerates the liquid film growth and detachment process on the rough hydrophobic surface, which reduces the time-averaged thermal resistance of the filmwise region. These two behaviors together enhance condensation heat transfer on the downward-facing rough hydrophobic surface. Besides, stable dropwise condensation could also be formed on smooth hydrophilic surfaces and has better heat transfer performance than corresponding hydrophobic surfaces under the same heat transfer condition.

Effects of Substrate Bias Voltage on Structure of Diamond-Like Carbon Films on AISI 316L Stainless Steel: A Molecular Dynamics Simulation Study.

With the recent significant advances in micro- and nanoscale fabrication techniques, deposition of diamond-like carbon films on stainless steel substrates has been experimentally achieved. However, the underlying mechanism for the formation of film microstructures has remained elusive. In this study, the growth processes of diamond-like carbon films on AISI 316L substrate are studied via the molecular dynamics method. Effects of substrate bias voltage on the structure properties and sp3 hybridization ratio are investigated. A diamond-like carbon film with a compact structure and smooth surface is obtained at 120 V bias voltage. Looser structures with high surface roughness are observed in films deposited under bias voltages of 0 V or 300 V. In addition, sp3 fraction increases with increasing substrate bias voltage from 0 V to 120 V, while an opposite trend is obtained when the bias voltage is further increased from 120 V to 300 V. The highest magnitude of sp3 fraction was about 48.5% at 120 V bias voltage. The dependence of sp3 fraction in carbon films on the substrate bias voltage achieves a high consistency within the experiment results. The mechanism for the dependence of diamond-like carbon structures on the substrate bias voltage is discussed as well.

Spin reorientation in tetragonally distorted spinel oxideNiCo2O4epitaxial films

We experimentally investigated the magnetic properties of NiCo$_2$O$_4$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magneto-torque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature ($T_{\rm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although $T_{\mathrm{SR}}$ is dependent on the growth method as well as the conditions. The cone angles measured from the $c$-axis increased successively at $T_{\mathrm{SR}}$ and approached a maximum of 45-50 degrees at the lowest measurement temperature of 5 K. Calculation with the cluster model suggests that the Ni$^{3+}$ ions occupying the $T_d$ site could be the origin of the ECMA. Both the magnetic properties and the results of the calculation based on the cluster model indicate that the ECMA is attributable to the cation anti-site distribution of Ni$^{3+}$, which is possibly formed during the growth process of the thin films.

High-Performance Perovskite Betavoltaics Employing High-Crystallinity MAPbBr3 Films.

Long-life and self-powered betavoltaic batteries are extremely attractive for many fields that require a long-term power supply, such as space exploration, polar exploration, and implantable medical technology. Organic lead halide perovskites are great potential candidate materials for betavoltaic batteries due to the large attenuation coefficient and the long carrier diffusion length, which guarantee the scale match between the penetration depth of β particles and the carrier diffusion length. However, the performance of perovskite betavoltaics is limited by the fabrication process of the thick and high-crystallinity perovskite film. In this work, we demonstrated high-performance perovskite betavoltaic cells using thick, high-quality, and wide-band-gap MAPbBr3 polycrystalline films. The solvent annealing method was adopted to improve the crystallinity and eliminate the pinholes in the MAPbBr3 film. The optimal MAPbBr3 betavoltaic cell achieved a power conversion efficiency (PCE) of 5.35% and a maximum output power of 1.203 μW under radiation of electrons of 15 keV with an equivalent activity of 253 mCi. These results are a nearly 50% improvement from previous reports. Effects of the MAPbBr3 perovskite layer thickness on the device performance were also discussed. The mechanisms of film-growth processes and device physics could provide insights for the research community of perovskites and betavoltaics.

Large-area CsPbBr3 perovskite films grown with effective one-step RF-magnetron sputtering

In this work, we demonstrated the large-area film growth of all-inorganic perovskite CsPbBr3 with semiconductor industry compatible radio frequency magneton sputtering technology. This approach avoids the use of toxic organic solvents, which are required extensively for the solution growth process of these perovskite films. Even on ordinary glass substrates, the CsPbBr3 film of 100 cm2 with a single phase and ultra-smooth surface can be grown under optimized sputtering conditions, among which the substrate temperature is proved very important. Temperature-dependent photoluminescence spectra and femtosecond transient absorption spectra studies show that no subband defect levels are observed in such perovskite films. The temperature-dependent resistivity measurement exhibits a typical thermally activated electrical behavior, and a bandgap of 2.24 eV is obtained, which is consistent with the optical measurements. Therefore, both the optical and electrical studies indicate that the CsPbBr3 film with high optical quality can be grown with effective one-step sputtering technology.

Suspended nanocrystalline diamond ridge waveguides designed for the mid-infrared

A comprehensive study and design of air-clad suspended ridge diamond waveguides for operation across the 2.5–16 µm spectral range is presented, specifically targeting nanocrystalline diamond (NCD) thin films directly grown on silicon substrates. Three film thicknesses of 520, 1000 and 2000 nm are considered, to cover overlapping sub-bands of 2.5–5, 4–9 and 8–16 µm, respectively. Within each sub-band, the waveguide dimensions for single mode quasi-TE operation are found and the waveguide material losses and bending losses are estimated at each design point. In addition, in each case the minimum required undercut depth and etch hole placement for optical isolation of the waveguide mode from the silicon substrate is also quantified. We also estimate the losses associated with scattering from surface roughness, which is an unavoidable byproduct of the NCD thin film growth process. Our results indicate that despite the relatively low film thickness-to-wavelength ratio, mechanically stable waveguides with good optical confinement and low material and bending losses can be realised to cover the full 2.5–16 µm range. In addition, scattering loss estimations predict a drastic drop in roughness-induced scattering losses above 6 µm, even for relatively rough films. In addition to highlighting the utility of suspended NCD as a versatile platform for mid-infrared integrated photonics, the approaches and results presented here can be used to inform the design of suspended air-clad waveguides in other material platforms.

Scalable growth of vertically oriented graphene nanosheets with high rate by a high-flux mesoplasma chemical vapor deposition

The Vertically Oriented Graphene Nanosheets (VGNs) possesses many promising properties and attracts intense interests in various applications due to its unique nanoarchitecture and intrinsic properties of graphene. However, a scalable growth of VGNs film with high rate is still an open challenge that hampers both the scientific research and mass applications. Herein, we report for the first time a large area growth of VGNs film with high rate via a high-density and high-flux mesoplasma chemical vapor deposition by rotating the substrate. A uniform VGNs film on a large area with a high growth rate on the order of ten micrometer per minute has been obtained. A continues growth process of VGNs film with high rate is also predicted by sweeping the substrate, which gives a new route for the scalable growth of VGNs film with high rate for the future mass applications.

Growth of black arsenic phosphorus thin films and its application for field-effect transistors

Black arsenic phosphorus single crystals were grown using a short-way transport technique resulting in crystals up to 12 × 110 and ranging from 200 nm to 2 thick. The reaction conditions require tin, tin (IV) iodide, gray arsenic, and red phosphorus placed in an evacuated quartz ampule and ramped up to a maximum temperature of 630 °C. The crystal structure and elemental composition were characterized using Raman spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy, cross-sectional transmission microscopy, and electron backscatter diffraction. The data provides valuable insight into the growth mechanism. A previously developed b-P thin film growth technique can be adapted to b-AsP film growth with slight modifications to the reaction duration and reactant mass ratios. Devices fabricated from exfoliated bulk-b-AsP grown in the same reaction condition as the thin film growth process are characterized, showing an on-off current ratio of 102, a threshold voltage of −60 V, and a peak field-effect hole mobility of 23 cm2 V−1 s−1 at V d = −0.9 V and V g = −60 V.

Innovative Infrared-pulsed laser assisted RF sputtered β-Ga2O3 thin film at low temperature process

Innovative new method for the pulsed laser assisted radio-frequency sputtering at low temperature process was successfully applied for the β phase Ga2O3 thin film as wide band gap materials. Thin film of wide bandgap material, β-Ga2O3, was successfully deposited on SiO2/Si and quartz substrates by employing radio-frequency sputtering system with assisting process of ytterbium-laser irradiation. Both sputtering system and infrared pulsed laser irradiation process were employed at the same time to increase the acquired energy densities for the adatoms on the thin film growth process. X-ray diffraction patterns (XRD) showed the prominent peaks of unique crystalline properties of Ga2O3 thin films in β phase. The optical energy bandgap of Ga2O3 thin films derived from spectral transmittance was measured in the range of 4.90–5.06 eV for a variation of laser power. High resolution X-ray photon spectroscopy (XPS) analysis showed that O 1s and Ga 3d position in core level. We confirm that infrared optical energy was effectively added to fabricate β-Ga2O3 thin film by employing the Yb-laser-assisted RF sputtering process.

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.

Effects of substrate temperature and intermediate layer on adhesion, structural and mechanical properties of coaxial arc plasma deposition grown nanodiamond composite films on Si substrates

Low-temperature fabrication of nanodiamond films on untreated silicon substrates, which are preferred substrates and widely employed in industry, can extend nanodiamond film applications, for instance in biosensing and miniaturization technologies. For its realization, the weak adhesion of nanodiamond films on silicon substrates due to insufficient atomic interdiffusion at the interfaces is a serious issue that needs to be overcome. In the present study, coaxial arc plasma deposition (CAPD) grown nanodiamond films were fabricated at low temperatures on silicon substrates. The effects of the substrate temperature, which is an important parameter in the CAPD nanodiamond film growth process, were structurally and physically investigated. It was found that the films softened with increasing substrate temperature and atomic interdiffusions at interfaces between the films and substrates were activated, which resulted in the formation of SiC at the interfaces. On the other hand, the films fabricated directly on silicon at room temperature were exfoliated due to the poor adhesion and high residual stress. Consequently, intermediate layers that were fabricated at elevated substrate temperatures were employed to improve the adhesion strength of nanodiamond films on silicon substrates. This way, the atomic interdiffusion at the interfaces could be promoted and the residual stress in the films was relieved. In the second stage, the films were fabricated on the intermediate layers at different temperatures. It has shown that the top layer films fabricated at low temperature gained distinctive mechanical and structural properties. By applying this strategy, several micrometres of hard nanodiamond films can be fabricated on Si substrates without easily being peeled off.

The utilization of boron nitride (BN) for granular L1-FePt HAMR media fabrication

In this experimental study, we present a dual-layer structure of FePt-boron nitride (BN)/FePt-SiOx granular media for the heat-assisted magnetic recording. The boron nitride (BN) was deposited together with FePt at 700 °C for the first 2.5 nm in the initial film growth. After the initial growth, SiOx was used to replace the BN to serve as the grain boundary material for the rest of film growth process. The transmission electron microscopy study on the fully grown FePt-BN/FePt-SiOx film shows well-isolated FePt grains that are fully encircled by the well-defined grain boundaries. The areal density of FePt grains is distinctively higher than that of the conventional granular FePt-C-based film at similar film thickness. The FePt-BN/FePt-SiOx bilayer granular media also demonstrate excellent magnetic properties, which are comparable to that of FePt-C-SiOx granular media in terms of the hysteresis characteristics.

The Challenge of Ambient Air–Processed Organometallic Halide Perovskite: Technology Transfer From Spin Coating to Meniscus Blade Coating of Perovskite Thin Films

The development of perovskite solar cells (PSCs) has been extensively studied in the past decade, and the power conversion efficiency (PCE) has reached a record of 25.2%. Despite impressively high PCE, the fabrication process mainly relied on a well-controlled environment, an inert gas–filled glovebox, and devices of small areas were demonstrated. This impedes the technology transfer from laboratory scale spin coating to manufacturing ambient air scalable processes. Furthermore, the nucleation and crystal growth processes of the perovskite thin films are different when the films are prepared in different environmental conditions. In this review, we summarize the recent advances of ambient air–processed organometallic halide perovskite thin films. Focuses are made on the impact of ambient air conditions, typically adventitious moisture, on the crystallization of perovskites thin films. The challenges and strategies in the technology transfer from the glovebox or ambient air spin coating to scalable meniscus blade coating are also discussed to shed light on the manufacture of ambient air–processed PSCs.

Growth and piezoelectric properties of amorphous and crystalline (K1−xNax)NbO3−based thin films

(K1−xNax)NbO3 (KNN)-based piezoelectric thin films have been extensively studied for application in micro-electromechanical systems. However, growing homogenous crystalline (K1−xNax)NbO3 (CKNN) films with good piezoelectric properties is difficult because Na2O and K2O evaporate during the growth process at high temperatures. Recently, amorphous (K1−xNax)NbO3 (AKNN) films containing KNN nanocrystals with good piezoelectric properties have been fabricated at low temperatures. Furthermore, [001]-oriented crystalline (K1−xNax)NbO3 (OCKNN) films with excellent piezoelectric properties have been grown at low temperatures (≤ 350 °C) using a metal-oxide nanosheet seed layer. This novel method is excellent for the growth of homogeneous KNN thin films. These films were deposited on a polymer substrate; thus, they can be utilized in future flexible electronic devices. In this study, the structural and piezoelectric properties of AKNN- and CKNN-based films fabricated at high temperatures, along with the growth process and application of OCKNN-based thin films at low temperatures, are reviewed.

Dislocations at coalescence boundaries in heteroepitaxial GaN/sapphire studied after the epitaxial layer has completely coalesced

We have performed cross-sectional scanning capacitance microscopy (SCM), cathodoluminescence (CL) microscopy in the scanning electron microscope (SEM) and transmission electron microscopy (TEM) all on the same few-micron region of a GaN/sapphire sample. To achieve this, it was necessary to develop a process flow which allowed the same features viewed in a cleaved cross-section to be traced from one microscope to the next and to adapt the focused ion beam preparation of the TEM lamella to allow preparation of a site-specific sample on a pre-cleaved cross-section. Growth of our GaN/sapphire samples involved coalescence of three-dimensional islands to form a continuous film. Highly doped marker layers were included in the sample so that coalescence boundaries formed late in the film growth process could be identified in SCM and CL. Using TEM, we then identified one or more dislocations associated with each of several such late-coalescing boundaries. In contrast, previous studies have addressed coalescence boundaries formed earlier in the growth process and have shown that early-stage island coalescence does not lead to dislocation formation.

Solvent coordination engineering for high-quality hybrid organic-inorganic perovskite films

Identifying the structure of iodoplumbate complexes in various solutions is essential for linking the coordination environment of the perovskite precursor to its final properties. In this study, we investigate the structure of methylammonium lead iodide (MAPbI3) perovskite precursor solutions as a function of the volume ratio of N,N-dimethylformamide and dimethyl sulfoxide (DMSO) using X-ray absorption fine structure spectroscopy. It was demonstrated that DMSO modifies the precursor structure at the atomic level. X-ray diffraction, scanning electron microscopy, and UV–Vis spectroscopy analyses showed that the coordination environment of iodoplumbate complexes in precursor solutions is strongly correlated with the morphology of perovskite films. The appropriate amount of DMSO in the precursor solution modifies the Pb-O and Pb-I coordination of iodoplumbate complexes, which could change the growth process of high-crystallization perovskite films. This study sheds light on the formation mechanism of perovskite films from precursor solutions, which will lead to the development of high-quality films and ultimately high-efficiency devices.

In-situ RHEED analysis of reactively sputtered epitaxial FeN thin films

The properties of any material can be easily manipulated by employing an epitaxial thin film growth on a desired substrate due to the dependence of its 3D structure on the template used as a substrate as well as the effect of interface produced between the film and the substrate. However, to understand the modified properties, it becomes necessary to obtain a complete knowledge about the resulting atomic structure. In-situ reflection high energy electron diffraction (RHEED) has proven to be a very useful technique to monitor the structure of a surface as well as its crystallographic orientations. However, in-situ RHEED is generally used in film growth processes taking place under ultra-high or high vacuum conditions. Magnetron sputtering on the other hand, is performed at relatively higher pressures and the stray magnetic field generated from the magnetron source, interferes with the incident or reflected electron beam originating from the RHEED source. In this work, by overcoming these challenges, we employed the usage of in-situ RHEED technique in a magnetron sputtering system and have successfully demonstrated its ability to determine the growth mechanism of epitaxial iron mononitride (FeN) thin films.

Lateral growth of xenon hydrate films on mica

<abstract> <p>In this paper, we report an <italic>in situ</italic> optical microscopy study of lateral growth of xenon (Xe) hydrate thin films on mica at sub-zero temperatures. The interactions between a solid surface and water molecules can strongly affect the alignment of water molecules and induce ice-like ordered structures within the water layer at the water-surface interface. Mica was chosen as a model surface to study the surface effect of hydrophilic sheet silicates on the lateral growth of Xe hydrate films. Under the experimental conditions, the lateral growth of Xe hydrate films was measured to be at an average rapid rate of ~200 μm/s and 400 μm/s under two different pressures of Xe. Mass transfer estimation of the Xe-water system revealed that the increasing trend of lateral film growth rates followed the increase in the net mass flux and aqueous solubility of Xe. However, as the supercooling temperature increased, the trend of lateral film growth rates attained a plateau region where little change in the rate was observed. This unique feature in the lateral film growth trend, the fast lateral growth kinetics, and the short induction time for hydrate film growth hinted at the assistance of the mica surface to aid the lateral growth process of Xe hydrate films at low Xe mass flux and at a low degree of subcooling. A mechanism based on the reported structured water layer at the interface on mica was proposed to rationalize a postulated surface-promotional effect of mica on the nucleation and lateral growth kinetics of Xe hydrate films.</p> </abstract>

The effect of small addition of copper on the growth process, structure, surface charge and adsorption properties of ZnO films in the pyrolysis of dithiocarbamates

Highly textured ZnO films are fabricated, the outer polar surface of which is decorated with copper complexes. The segregation of copper on the surface during growth is attributed to the manifestation of the Jahn–Teller effect.