Film-substrate Interface Research Articles

Elastic property characterization of soft substrate-supported thin films using multiscale digital image correlation

We present an original experimental approach to characterizing the Young’s modulus of nanoscale thin films on a soft substrate. A microtester is developed to permit pre-stretching of the polymer substrate and symmetric film deposition on both sides of its half surface. During compression and tensile tests related to the as-deposited residual stress state, the full-field macrostrains of the film-substrate composite and the uncoated substrate can be measured simultaneously with the film microstrains using multiscale digital image correlation. The macrostrain difference allows extraction of the elastic properties of thin films. Combined with microstrains, a perfect strain transfer at the film-substrate interface, minus buckling of the composite, is evidenced. No film delamination or cracking phenomena are observed, and the measured Young’s modulus agrees well with the literature. Our approach could potentially be used to explore the mechanical properties and behaviors of emerging functional thin films, both crystalline and amorphous.

Epitaxial Growth of Iron and Indium Oxide Thin Films on Sapphire R-Plane with a Regular Relief

The formation of a terrace-step surface nanostructure disoriented with respect to the R-plane on sapphire crystals is studied. The possibility of the formation of atomically smooth steps with a height about 0.34 nm is shown. This size corresponds to the interplanar distance of 0.34 nm along the direction [012] in a sapphire crystal. The substrates prepared in such a way are used in the course of the epitaxy of Fe2O3 and In2O3 films. A method of a high-temperature oxidation of previously deposited iron and indium films under atmospheric conditions is used. It is shown that the structural and geometrical similarity of Fe2O3 and sapphire crystal lattices promotes the growth of epitaxial films aligned with respect to the substrate. It is determined that the stress occurring in the growing In2O3 films in the course of oxidation and solid-phase epitaxy, as well as owing to a discrepancy between the lattice parameters at the film–substrate interface, is insufficient for the formation of the metastable rhombohedral In2O3 phase. The influence of terrace-step nanostructure of the surface of substrates at the initial stages of the growth of epitaxial films is discussed.

Effect of additive ethylene glycol on morphology and mechanical hardness of anodic oxide film formed on AA7075

This paper reports the effect of additive ethylene glycol (EG) on the morphology and mechanical hardness of anodic oxide films formed on AA7075 by hard anodizing technique. Anodizing was conducted in 2 M H2SO4 solution with the addition of 0, 10, 20, and 30% EG at a constant current 300 A/m2 at 10°C. The cross-sectional morphology was studied by FE-SEM and the elemental composition was analyzed by EDX. The mechanical hardness was measured by a Vickers Hardness machine. The results indicated that the addition of EG improved the films adherent on the substrates by preventing the formation of cracks and the enrichment of the intermetallic phases at the film-substrate interface. The presence of EG accelerated oxidation reaction of the intermetallic phase. The competing oxidation reactions led to the significant decrease in film thickness from 38 µm down to below 20 µm with the addition of EG. The film formed in EG containing electrolyte composed not only aluminum oxide but also copper and zinc oxides. The film formed in EG containing electrolytes exhibited microvoids as a result of oxygen gas evolution which accompanied the oxidation reaction of the intermetallic phase. As a consequence, the mechanical hardness decreased with increasing EG concentration.

Studies on solid state reactions of atomic layer deposited thin films of lithium carbonate with hafnia and zirconia

In this paper, results on the solid state reactions of atomic layer deposited Li2CO3 with HfO2 and ZrO2 are reported. An Li2CO3 film was deposited on top of hafnia and zirconia, and the stacks were annealed at various temperatures in air to remove the carbonate and facilitate lithium diffusion into the oxides. It was found that Li+ ions are mobile in hafnia and zirconia at high temperatures, diffusing to the film–substrate interface and forming silicates with the Si substrate during heating. Based on grazing incidence x-ray diffraction experiments, no changes in the oxide phases take place during this process. Field emission scanning electron microscopy images reveal that some surface defects are formed on the transition metal oxide surfaces during lithium diffusion. The authors also show that lithium can diffuse through hafnia and react with a potential lithium-ion battery electrode material TiO2 residing below the HfO2 layer, forming Li2TiO3.

Influence of trimethylaluminum predoses on the growth morphology, film-substrate interface, and microstructure of MOCVD-grown AlN on (1 1 1)Si

Aluminum nitride (AlN) was deposited on (111) silicon by metalorganic chemical vapor deposition after varied trimethylaluminum “predoses”. Growth morphologies, film-substrate interfaces, and film microstructures were examined using scanning electron microscopy, atomic force microscopy, X-ray diffraction, and transmission electron microscopy. In samples grown with predoses, lateral growth was observed over faceted “patches” formed during the predose. Three-dimensional growth was observed to seed from small islands on the surface of these patches and eventually overgrow them. The three-dimensional growth mode was similar to that observed when AlN was grown without a predose, resulting in similar morphologies in all films, regardless of predose, after the islands coalesced. The AlN-silicon interface was found to be predominantly amorphous when no predose was used. However, narrow regions were observed over which the film was in atomic registry with the substrate. This indicates AlN nucleates in epitaxy with the substrate and amorphous silicon nitride forms between nucleation sites due to ammonia exposure. Films grown with predoses had structurally abrupt interfaces, suggesting aluminum within the observed patch features inhibits the reaction between ammonia and silicon at the onset of growth. A structure distinct from both wurtzite AlN and diamond cubic silicon was observed at the substrate interface in films grown with a predose, consistent with either zinc blende AlN or a strained Si/Al alloy. A mosaic microstructure was observed in all films, grown with or without predoses, which consisted of sub-boundaries formed by clusters of threading dislocations. Threading dislocations, separated by hundreds of nanometers, were found to be tilted along common directions, providing evidence for a dislocation bending mechanism possibly enhanced by the predose.

Thin dysprosium oxide films formed by rapid thermal annealing on porous SiC substrate

In this paper, we consider the effect of rapid thermal annealing (RTA) on the properties of Dy2O3 film formed on the surface of a substrate with a por-SiC/SiC structure. The atomic composition of the films under study was analyzed as a function of the RTA time. It is shown that the RTA method makes it possible to obtain thin Dy oxide films with a composition close to the stoichiometric one. In this case, an increase in the RTA time leads to improving the quality of film-substrate interface and increasing the optical transmission of Dy2O3/por-SiC/SiC structure

Tunable magnetic domains and depth resolved microstructure in Gd-Fe thin films

This paper reports the tunability of magnetic domains and its correlation with depth-resolved microstructure in 100 nm thick Gd-Fe films after rapid thermal processing. The as-deposited films are amorphous in nature and display elongated periodic stripe domains, signifying the development of perpendicular magnetic anisotropy. Rapid thermal processing at 550oC for different time intervals viz. 5, 10, 15 and 20 min induces the crystallization of Fe over the amorphous Gd-Fe matrix and the presence of wider reaction zone has been evidenced at the film-substrate interface. While the magnetization measurements reveal a spin re-orientation along the plane of the film surface, the stripe domains are no longer observed for the processed films. 3D micromagnetic simulations have been performed to complement the experimental findings. The consideration of in-plane tilt of anisotropy axis along with anisotropy reduction successfully demonstrate the experimentally observed changes in magnetic properties and domain structure.

Electrostatic potential and valence modulation in La0.7Sr0.3MnO3 thin films

The Mn valence in thin film La0.7Sr0.3MnO3 was studied as a function of film thickness in the range of 1–16 unit cells with a combination of non-destructive bulk and surface sensitive X-ray absorption spectroscopy techniques. Using a layer-by-layer valence model, it was found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface. These results were supported by first principles calculations. The surface valence increases to up to Mn3.7+, whereas the interface valence reduces down to Mn2.5+. The change in valence from the expected bulk value is consistent with charge redistribution due to the polar discontinuity at the film-substrate interface. The comparison with theory employed here illustrates how this layer-by-layer valence evolves with film thickness and allows for a deeper understanding of the microscopic mechanisms at play in this effect. These results offer insight on how the two-dimensional electron gas is created in thin film oxide alloys and how the magnetic ordering is reduced with dimensionality.

Diamond film growth by HFCVD on Q-carbon seeded substrate

While hot-filament assisted chemical vapor deposition (HFCVD) is a well-established technique to synthesize diamond thin films using microdiamond seeds, the quality of grown diamond thin films is often compromised due to the presence of contaminants, i.e. graphitic entities and the eroded tungsten filament remnants, at the film-substrate interface. Here, we present a novel approach to form high-quality, contamination-free diamond thin films with HFCVD using Q-carbon precursor. The Q-carbon is a metastable phase which is formed by nanosecond laser melting of amorphous carbon and rapid quenching from the superundercooled state and consists of ∼75% sp3 and rest sp2 hybridized carbon. Using Q-carbon seeds in HFCVD, we demonstrate the growth of polycrystalline diamond film with a clean interface without any tungsten filament impurities. With large-area vibrational Raman mode analysis, we also observe a significant reduction in the presence of overall graphitic entities in the diamond film. With the realization of such a high-quality interface, we present a pathway to fabricate significantly improved diamond coatings and solid-state devices.

Ultrafast laser generated strain in granular and continuous FePt thin films

We employ ultrafast X-ray diffraction to compare the lattice dynamics of laser-excited continuous and granular FePt films on MgO (100) substrates. Contrary to recent results on free-standing granular films, we observe in both cases a pronounced and long-lasting out-of-plane expansion. We attribute this discrepancy to the in-plane expansion, which is suppressed by symmetry in continuous films. Granular films on substrates are less constrained and already show a reduced out-of-plane contraction. Via the Poisson effect, out-of-plane contractions drive in-plane expansion and vice versa. Consistently, the granular film exhibits a short-lived out-of-plane contraction driven by ultrafast demagnetization which is followed by a reduced and delayed expansion. From the acoustic reflections of the observed strain waves at the film-substrate interface, we extract a 13% reduction of the elastic constants in thin 10 nm FePt films compared to bulk-like samples.

Structure, properties and growth mechanism of a self-assembled nanocylindrical MoS2/Mo-S-C composite film

A self-assembled nanocylindrical MoS2/M-S-C composite film with excellent tribological properties was synthesized by simultaneously sputtering of MoS2 and graphite targets. The HRTEM observation of the polished surface and cross-sections reveals that this nanocylindrical structure with MoS2-rich phase encircled by carbon matrix arranging alternately in vertical patterns is initiated from the film-substrate interface and continues to the film surface. The XRD results show that this film exhibits dominant (1 0 0) basal planes. In comparison with pure MoS2 film, this nanocylindrical MoS2/Mo-S-C composite film exhibits largely enhanced hardness and higher elastic recovery ratio (delastic/dmax) under the same load. Surprisingly, the pin-on-disk tribotests in vacuum on it display a low friction-coefficient and long wear lives (up to 5 × 105 revolutions). The HRTEM observation of wear tracks finds that the MoS2 texture was reassembled on the film surface, and a tribofilm with preferential (0 0 2) basal planes in thickness about 100 nm is quickly formed in sliding. Based on the XRD results and TEM images of this film, this clear nanocylindrical structure was attributed to atomic transport occurring preferentially along the edge sites of MoS2 crystallites.

Imprint behavior and polarization relaxation of PLZT thin films

Thickness dependence of imprint and polarization dynamics of Pb1-xLax(Zr1-yTiy)1-x/4O3 (PLZT) thin films is reported in this work. Asymmetries in the histograms of the local piezoresponse reveal an imprint effect in the studied films whose origin could be associated to Schottky barriers near the film-substrate interface. Time-resolved spectroscopic measurements shows that the local polarization relaxation follows the exponential dependence . A maximum relaxation time τ ≈ 2.18 s was observed for the 350 nm thick film. A similar thickness dependence between grain size, correlation length (ξ) and relaxation time (τ) suggest an intrinsic relationship between them.

Mechanical properties and reliability of aluminum nitride thin films

Knowledge of the mechanical properties and fatigue behavior of thin films is important for the design and reliability of microfabricated devices. This study uses the bulge test to measure the residual stress, Young's modulus, and fracture strength of aluminum nitride (AlN) thin films with different microstructures prepared by sputtering, metalorganic vapor phase epitaxy (MOVPE), and atomic layer deposition (ALD). In addition, the fatigue behavior is studied under cyclic loading. The results indicate that the fracture strength and Young's modulus of AlN are mainly determined by the film microstructure, which is consecutively influenced by the deposition method and conditions. A microstructure with a higher order of crystallinity has increased fracture strength and Young's modulus. Additionally, the strength limiting defects are located at the film-substrate interface. The measured residual stresses were 249, 876, 1,526, and 272 MPa for two sputtered films of different thicknesses, MOVPE and ALD films, respectively. The fracture strengths were 1.42, 1.54, 2.76, and 0.61 GPa, and Young's moduli were 335, 343, 346, and 272 GPa. No clear signs of fatigue were observed after 10,000 cycles at a load corresponding to 83% of the fracture strength.

The evidence of the role of surface plasmon polaritons in formation of femtosecond highly-regular laser-induced periodic structures on Cr films

Cr films of controlled thickness deposited on glass substrates were irradiated by a high number of low-intensity femtosecond laser pulses below the Cr single-pulse damage threshold, producing periodic surface structures (LIPSS) of very high regularity via metal oxidation. To address the multiplicity of electromagnetic modes allowed for thin films, a rigorous numerical approach for modeling surface plasmon polaritons (SPP) in thin-film geometry has been developed. Three types of modes are predicted: the classical SPP with periodicities Λ ~ λ (λ is laser wavelength) at air-film interface, and Λ ~ λ/n at film-substrate interface (n is refractive index of substrate), and a propagation mode Λ ~ λ/nCr where nCr is refractive index of film material. Experimentally observed LIPSS periods match well the predicted modes for the extreme cases, Λ ~ λ/n at film thickness h ≤ 30 nm and Λ ~ λ at h ≥ 200 nm, indicating respectively the dominance of SPP excited at the film-substrate interface and at the metal surface. For 30 nm < h < 200 nm, decreasing of the film thickness results in transition from the pure surface mode to its coupling with and finally domination of the electromagnetic wave excited at the film-substrate interface, providing evidence of the SPP mechanism of LIPSS formation under laser irradiation of thin metallic films.

Competing growth of titanium nitrides and silicides in Ti thin films processed in expanding microwave plasma: Morphology and microstructural properties

The diffusion of nitrogen into Ti silicide films allows the performance of complementary metal oxide semiconductor (CMOS) components to be improved. In this work, the thermochemical treatment is carried out in an expanding microwave plasma reactor using (Ar-33%N2-1%H2) gas mixtures. This process promotes the chemical reactions on the surface of metals. The diffusion of nitrogen into the film is improved by the reducing effect of NHx and/or H species towards passive layers such as oxides which form a barrier of diffusion in the surface layers during the process. The simultaneous formation of Ti nitrides and silicides at the surface and at the film-substrate interface, respectively gives rise to two competing processes which result in the growth of the Ti nitride phase at the expense of the Ti silicide phase at a critical temperature of 800°C. This paper reports on a comprehensive analysis of the evolution of TiSi2 and TiN phases and microstructural properties of films by means of X-ray diffraction, Raman spectroscopy, transmission electron microscopy and selected area electron diffraction investigations. Square shaped crystals of TiN are identified on the top of round shaped crystals of TiSi2. The growth of the TiN phase at the expense of TiSi2 induces a catastrophic decrease of the intensity of the (040) diffraction line of TiSi2 and a huge increase of the (220) reflection line of TiN. The microstructural properties changes during the process such as the formation of TiN crystals of nanometric size in the bulk of the TiSi2 phase as well as the migration of free Si which epitaxially grows at the film-substrate interface have been evidenced by very detailed investigations for the first time. The results are related to the mechanism of formation of TiN from the reaction between TiSi2 and nitrogen.

Structure of Diamond-Like Silicon–Carbon Films Alloyed by Vanadium

The structure of diamond-like silicon–carbon films formed on silicon substrates by magnetron and plasmatron codeposition using a closed-field magnetron and a plasmatron activated by tungsten cathode has been studied by transmission electron microscopy. The main feature of the films alloyed by vanadium to concentrations of 12–31 at % was found to be a layered structure of the film cross section. It was established that vanadium alloying leads to the formation of vanadium carbide (VC) nanocrystals; the nanocrystal size increases from 1–2 to 10 nm. At the maximum vanadium content, VC nanocrystals have an anisotropic shape: they are extended in the direction perpendicular to the film–substrate interface.

Atomic-level insights through spectroscopic and transport measurements into the large-area synthesis of MoS2 thin films

Abstract

Synchrotron x-ray microdiffraction study of residual stresses in BaTiO3 films deposited at room temperature by aerosol deposition

The residual stress through the thickness of a BaTiO3 ceramic film deposited on steel substrate at room temperature with aerosol deposition has been analyzed using synchrotron x-ray microdiffraction. A gradient in stress distribution was evident through the film and the maximum biaxial compressive stress of −800 MPa was observed at the film-substrate interface. Heat-treatment was found to relax the internal compressive stress, due to thermal expansion mismatch between the film and substrate. Variation in ferroelectric response was correlated to the change in stress state by thermal treatment. This analysis is crucial for development of micro-and nanoelectronic devices with AD films.

Consequences of elastic anisotropy in patterned substrate heteroepitaxy

The role of elastic anisotropy on quantum dot formation and evolution on a pre-patterned substrate is evaluated within the framework of a continuum model. We first extend the formulation for surface evolution to take elastic anisotropy into account. Using a small slope approximation, we derive the evolution equation and show how it can be numerically implemented up to linear and second order for stripe and egg-carton patterned substrates using an accurate and efficient procedure. The semi-infinite nature of the substrate is used to solve the elasticity problem subject to other boundary conditions at the free surface and at the film-substrate interface. The positioning of the quantum dots with respect to the peaks and valleys of the pattern is explained by a competition between the length scale of the pattern and the wavelength of the Asaro–Tiller–Grinfeld instability, which is also affected by the elastic anisotropy. The alignment of dots is affected by a competition between the elastic anisotropy of the film and the pattern orientation. A domain of pattern inversion, wherein the quantum dots form exclusively in the valleys of the patterns is identified as a function of the average film thickness and the elastic anisotropy, and the time-scale for this inversion as function of height is analyzed.

Structural Features of PLZT Films

Lead zirconate titanate (PZT) films doped with lanthanum, Pb(1–х)Laх(Zr0.48Ti0.52) (х = 0, 0.02, 0.05, 0.08, or 0.01), have been investigated by electron microscopy and X-ray diffraction. Films were formed on Si–SiO2–TiO2–Pt substrates by chemical vapor deposition from a solution and annealed at temperatures T = 650 and 750°C. The main structural features of the films, differing them from undoped PZT films fabricated by the same method, have been established. It is found that doping with lanthanum delays the pyrochlore–perovskite transformation in the film bulk, i.e., in the regions distant from the film–substrate interface. The fraction of metastable pyrochlore phase increases with an increase in the La molar content in the films. The main reason for the delay is the deficit of lead in the intergranular perovskite space, especially in the upper part of the film. Annealing at T = 750°C reduces the content of pyrochlore phase but does not completely remove it, which was never observed for undoped PZT films. Doping with lanthanum leads to a change in the lattice period c and a tetragonal distortion of the perovskite lattice (c/a ratio). Hence, the [100] texture of the films obtained, in contrast to the typical [111] texture of PZT films, is due to the increase in the lattice mismatch between the film and platinum layer when lead atoms are replaced with lanthanum. Lattice distortions of “transrotational” character, whose value exceeds 160 deg/μm, are found to arise in growing crystals.