Initial Stage Of Film Growth Research Articles

Preparation and electrical properties of ultra-thin Bi2212 films by sol-gel method

In order to achieve flexible applications of superconducting thin film materials in devices, the surface quality of the film is critical. The thickness of the film is one of the important factors affecting its surface quality. In this paper, Bi2212 superconducting thin films of different thicknesses were prepared on LaAlO3(100) and SrTiO3(100) single crystal substrates by sol-gel method and the effects of film thickness and different heterogeneous structures on their crystallographic, morphological and superconducting properties were investigated. The results showed that the roughness gradually increased with increasing Bi2212 thickness at the initial stage of film growth and stabilized after reaching a certain thickness, while the superconducting properties were destroyed at very small sample thickness, which was consistent in both single-crystal substrate films. In the Bi2212/LaAlO3 structure, the surface quality of the sample was excellent with a roughness of 5.4 nm at a thickness of about 18 nm for Bi2212, and the growth of the precipitated phase was inhibited to a greater extent. The film also had good superconductivity. This work provided a further research basis for the preparation of high quality Bi2212 films and offers further possibilities for their practical application.

Time-resolved in-situ investigation of Co-nitride thin film growth by grazing incidence X-ray absorption spectroscopy

The growth of cobalt nitride thin films prepared by reactive sputtering on float glass substrates was in-situ investigated using surface sensitive, grazing incidence X-ray absorption fine structure spectroscopy. Time-resolved measurements employing a quick-scanning monochromator allow to follow the growth on time-scales of a few seconds per spectrum, so that the film growth could be studied in detail. A quantitative modeling of the obtained data clearly show that a zinc blende type CoN is formed already during the initial stages of film growth, and the formation of small crystalline clusters finally lead to a 3D-growth of polycrystalline CoN-films.

Evolving surface morphology during epitaxy of NiO on Ag(001)

Reactive molecular beam epitaxy of NiO on Ag(001) has been characterized by high-resolution low energy electron diffraction performed during deposition of Ni on Ag(001) in molecular oxygen atmosphere for deposition temperatures of 300–420 K. The initial stage of film growth at high deposition temperature is determined by competing processes of Ni oxidation and Ni interdiffusion into Ag(001) with Ni-Ag alloying. Ni oxidation is favored at low deposition temperatures. Thus, NiO bilayers are immediately formed. At high deposition temperatures, however, alloying with Ag is enhanced and the formation of the initial NiO bilayer is delayed. After formation of a closed NiO bilayer, alloying is prevented and all deposited Ni is oxidized and NiO growth proceeds in the layer-by-layer growth mode. Later stages of epitaxy are governed by relaxation processes induced by the formation of mosaics due to interface dislocations. The tilt angle of the mosaic decreases with increasing film thickness due to elastic deformation of the NiO lattice. Dislocation related mosaics are paired and form grooves. Postdeposition characterization shows periodic arrangement of grooves related to the formation of a regular dislocation network.

Growth Evolution of AZO thin Films Deposited by Magnetron Sputtering at Room Temperature

Thin AZO films were grown by RF magnetron sputtering for different deposition times in argon plasmas. Optical, structural, and morphological properties, together with elemental composition, were studied and correlated with the observed effects on the electrical properties and compared with two models of mobility scattering (ionized impurities and grain boundaries). The results suggest that the carrier density in the studied case is limited to below 15% owing to the low ionization efficiency caused by the formation of neutral impurities as homologous phases. While the spread in the mobility during the initial stages of film growth is strongly influenced by grain boundaries, in thicker films the limitation on ion efficiency becomes more significant.

Growth of InN ultrathin films on AlN for the application to field-effect transistors

Herein, we report the growth method of ultrathin indium nitride (InN) films on aluminum nitride (AlN) templates by sputtering and its application to field-effect transistors (FETs). Although island-like InN surfaces were formed at the initial film growth stage, the height of the islands on the surface could be controlled by changing the growth temperature. The height of the InN islands grown at 500 °C was lower than those grown at 440 °C and 480 °C. To demonstrate an application of the InN/AlN heterostructure to FETs, we fabricated FETs using a 2-nm-thick InN film grown on an AlN template. The FET with the InN channel grown at 500 °C exhibited a drain current density of 0.19 mA/mm and an on/off ratio of approximately 102 although the drain current of the FET with the InN grown at 450 °C was not adequately controlled by the gate bias. These results indicate that the growth condition of InN films is responsible for the characteristics of InN/AlN FETs.

Atomistic Simulation of Stresses in Growing Silicon Dioxide Films

Dependence of stress values in silicon dioxide films on its thickness in the initial stage of film growth was investigated using atomistic molecular dynamics simulation. It was shown that the stress in normally deposited films was compressive and varied slightly with growth of film thickness. The stress in the glancing angle deposited films was several times lower than the stress in the normally deposited films, and varied from compressive stress to tensile stress with increasing film thickness. An essential anisotropy of stress tensor components was revealed for the case of glancing angle deposition. The calculated stress values were in the interval of experimental data.

Effect of ultrasonic field on grain refinement and stress relaxation of Be films by thermal evaporation

The beryllium (Be) films were prepared by vacuum evaporation with ultrasound on the glass slide substrates. The Be coating mainly consists of α-Be phase independent of ultrasonic vibration, while the texture and crystallinity are affected by ultrasonic vibration. With the increasing of ultrasonic power, the surface displays polygonal grains, but the grain size decreases from 130 nm to 60 nm. The cross section views show a transition from regular columnar structure to broken columnar structure owing to the ultrasonic mechanical agitation. When ultrasonic vibration is applied, the microstructure transforms from isolated islands to bicontinuous islands in the initial stage of film growth. In addition, the residual stress of Be films changes from a tensile stress to compressive stress under the effect of ultrasonic vibration, which is closely related to the transformation of microstructure.

In-situ growth of iron mononitride thin films studied using x-ray absorption spectroscopy and nuclear resonant scattering

We studied the structural and magnetic properties of in-situ grown iron mononitride (FeN) thin films. Initial stages of film growth were trapped utilizing synchrotron based soft x-ray absorption near edge spectroscopy (XANES) at the N K-edge and nuclear resonant scattering (NRS). Films were grown using dc-magnetron sputtering, separately at the experimental stations of SXAS beamline (BL01, Indus 2) and NRS beamline (P01, Petra III). It was found that the initial stages of film growth differs from the bulk of it. Ultrathin FeN films, exhibited larger energy separation between the t2g and eg features and an intense eg feature in the N K-edge pattern. This indicates that a structural transition is taking place from the rock-salt (RS)-type FeN to zinc-blende (ZB)-type FeN when the thickness of films increases beyond 5nm. The behavior of such N K-edge features correlates very well with the emergence of a magnetic component appearing in the NRS pattern at 100K in ultrathin FeN films. Combining the in-situ XANES and NRS measurements, it appears that initial FeN layers grow in a RS-type structure having a magnetic ground state. Subsequently, the structure changes to ZB-type which is known to be non-magnetic. Observed results help in resolving the long standing debate about the structure and the magnetic ground state of FeN.

Observations of the initial stages on reactive gas-timing sputtered TaO thin films by dynamic in situ spectroscopic ellipsometery

Dynamic in-situ spectroscopic ellipsometry (iSE) was employed to investigate the growth mechanism of the tantalum oxide (TaO) film on silicon wafer substrate deposited by reactive gas-timing RF magnetron sputtering compared with conventional reactive sputtering. The effect of reactive gas timing parameter on physical structure and optical property were analyzed by both the continuous and island film growth model with Tauc-Lorentz optical model. The variation of refractive index, film thickness and void volume fraction were obtained from dynamic fits of iSE data indicated that the growth process of the amorphous TaO thin films was divided into two regimes: the nucleation stage and continuous-layer stage. These stages were observed during deposition although the initial film growth stage corresponds to the island film growth model. The study demonstrates that sputtered TaO by the RGT technique at the O2 on-off gas timing has played a crucial factor in improving the formation of nucleation stage and that the deposited TaO thin films were high deposition rate with high refractive index. The complementary field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) showed a good agreement with the film thickness and morphology obtained from dynamic iSE measurement. The real-time monitoring of iSE offers important evidences to understand the growth mechanism of reactive gas-timing technique.

Fractal simulation of thin film nucleation growth process using a diffusion-limited aggregation model

In order to study the initial nucleation and growth process of the films, a two-dimensional (2D) DLA model was established, and fractal dimension was calculated by sandbox method. Compared with the experimental results, the model can well characterize the morphology of nucleation and growth in the initial stage of film growth. In addition to the number of particles and deposition probability involved in other studies, the size of substrate and the location of central particles are also considered in this work. The growth morphology, fractal dimension and the number of simulated steps are presented in this paper.

Observation of the growth of MAPbBr3 single-crystalline thin film based on space-limited method

Organic-inorganic hybrid perovskite single-crystalline thin films (SCTFs) are promising for enhancing photoelectric device performance. One key aspect is to be able to synthesize SCTFs with large surface areas. Here, we investigated the formation of MAPbBr3 SCTF by space-limited inverse temperature crystallization method. It was found that the weak wetting substrate could reduce friction force and promote precursor ion diffusion, which enabled a continuous growth of the thin perovskite single crystal along in-plane direction. SCTFs could be obtained at the initial stage of film growth and we also observed some signs of island growth model. Pyramid-shaped islands formed first and then converged to achieve top surface growth of SCTFs. After further growth, the precursor ion concentration near the SCTF gradually decreased. The lateral crystal growth was limited by the inefficient and anisotropic long range transportation of precursor ions along the micrometer-size or nanometer-size gap, which resulted in transformation of SCTFs into polycrystalline films. Low ion diffusion in the narrow gap is still the main constraint for the preparation of the larger and thinner SCTFs.

Excitation wavelength dependence of Raman scattering and substrate dependence on hydrogenated silicon-based thin film

Thin films of hydrogenated silicon-based thin film were deposited on Corning 7059 glass and single-crystalline silicon substrates in a capacitive-coupled radio-frequency (13.56 MHz) plasma-enhanced chemical vapor-deposition system in the presence of direct-current bias stimulation. The thickness of the films was measured using a surface profile–measuring system. X-ray diffraction was employed to characterize the structure of the silicon films and determine the grain size. Raman spectroscopy was carried out to verify the crystalline structure and to illustrate the structural composition of the silicon films. Raman scattering is exerted on those films by three different excitation wavelengths, from red to near ultraviolet. It is validated that the Raman spectrometry is different by varying the incident wavelength. The experiment result is well explained and it is inferred that suitable laser should be chosen to get further information of material microstructure. The topography and the roughness of the films were investigated using atomic force microscopy. Small differences were found to exist in the film thickness and the deposition rate was unaffected by the use of different substrates, whereas the microstructure of the thin films varied with the different substrates. Under the same deposition conditions, the films on single-crystalline silicon showed better crystallinity than those on glass substrates, while they had much smaller root-mean-square roughness than those deposited on single-crystalline silicon. It implies that the silicon atoms on the surface of glass have no significant migration and essentially condense wherever they land on the substrate surface. It is also suggested that the chemical nature of the substrate surface influences the initial stages of film growth. These results should have important impacts on the engineering of silicon-based thin film and related devices.

Elucidating doping driven microstructure evolution and optical properties of lead sulfide thin films grown from a chemical bath

Doping driven remarkable microstructural evolution of PbS thin films grown by a single-step chemical bath deposition process at 60 °C is reported. The undoped films were discontinuous with octahedral-shaped crystallites after 30 min of deposition, whereas Cu doping led to a distinctly different surface microstructure characterized by densely packed elongated crystallites. A mechanism, based on the time sequence study of microstructural evolution of the films, and detailed XRD and Raman measurements, has been proposed to explain the contrasting microstructure of the doped films. The incorporation of Cu forms an interface layer, which is devoid of Pb. The excess Cu ions in this interface layer at the initial stages of film growth strongly interact and selectively stabilize the charged {111} faces containing either Pb or S compared to the uncharged {100} faces that contain both Pb and S. This interaction interferes with the natural growth habit resulting in the observed surface features of the doped films. Concurrently, the Cu-doping potentially changed the optical properties of the films: A significant widening of the bandgap from 1.52 eV to 1.74 eV for increase in Cu concentration from 0 to 20% was observed, making it a highly potential absorber layer in thin film solar cells.

Effects of the surface modification of polyurethane substrates on genotoxicity and blood activation processes

The aim of this study was to determine the mutagenic and thrombogenic potential of a material composed of a thin coating deposited on a polymeric substrate. In this work, a surface was modified in a manner that would mimic the function of cellular niches. Finally, the surfaces should actively capture and differentiate progenitor cells from the blood stream. Thin films with 10 to 500nm thicknesses were deposited by unbalanced, pulsed DC magnetron sputtering on smooth polyurethane. Such high energy conditions led to a stiffening of the polymer surface layers by pseudodiffusion during the initial stages of film growth. Both the high intrinsic film stress due to high energy film growth and the huge difference in the elastic properties of the films and polymer substrates resulted in hierarchical and self-adapting nanowrinkling. Surface modifications of synthetic materials for future use in regeneration of the circulatory system must be tested in terms of their thrombogenicity and mutagenicity. Point mutations in many cases can lead to many serious haematologic complications. Genotoxicity was determined by testing for reverse histidine mutations in selected strains of Salmonella typhimurium. The analysis was performed in the presence and absence of metabolic activation system S9 containing liver microsomal fraction of rats. Based on these results, no mutagenicity of the tested material was observed. The interaction of blood and the material under dynamic conditions was described. Blood from above the analysed surface was collected after the test, and the quality of the blood was assessed along with the type of cellular response to the surface. In the obtained results of the coagulation processes, it was found that the tested material reduced the process of platelet activation under hydrodynamic conditions in comparison to the control material, polyurethane.

Probing the Interfacial Chemistry of Ultrathin ALD-Grown TiO2 Films: An In-Line XPS Study

Ultrathin TiO2 films received renewed attention in the field of photoelectrochemical water splitting as corrosion protection layers for unstable, small-bandgap semiconductors. Because nucleation on the substrate can differ from steady-state growth of the film itself, it is important to understand the nucleation behavior on a specific surface. In this work, we studied the nucleation mechanism of atomic layer deposition-grown TiO2 from TiCl4 and H2O on as-received silicon by means of in-line X-ray photoelectron spectroscopy. Within a region of ∼0.4 nm of the SiO2/TiO2 interface, the presence of Ti3+ states are detected. In this region, the Ti, O, and Cl species are found to be more strongly bonded. At the initial stages of film growth, prolonged TiCl4 exposure is necessary to reach a saturated surface chemistry, which is in contrast to the outcome of growth per cycle saturation curve analysis. A prolonged water exposure experiment suggests that residual chlorine impurities can be prevented by using a suffic...

Growth of Ni and Ni-Cr alloy thin films on MgO(001): Effect of alloy composition on surface morphology

The effects of substrate treatment, growth temperature, and composition on the surface morphology of Ni-Cr thin films grown on MgO(001) are studied by scanning tunneling microscopy and atomic force microscopy. We demonstrate that a combination of acid-etched substrates and high temperature deposition (400 °C) will result in smooth films with well-defined terraces (up to 30 nm wide) that are suitable for the study of progression of chemical reactions on the surface. Two different treatments are used to prepare the MgO substrates for deposition and they introduce characteristic differences in film surface morphology. Thin films that are grown on the phosphoric acid-treated substrates present reduced nucleation density during the initial stages of film growth which results in long and wide terraces. Due to the ≈16% lattice mismatch in the Ni(001)/MgO(001) system, film growth at 400 °C yields discontinuous films and a two-step growth process is necessary to obtain a continuous layer. Ni films are deposited at 100 °C and subjected to a post-growth annealing at 300 °C for 2 h to obtain a smoother surface. The addition of just 5 wt. % Cr drastically changes the film growth processes and yields continuous films at 400 °C without de-wetting in contrast to pure Ni films. With increasing Cr content, the films become progressively smoother with wider terraces. Ni5Cr alloy thin films have an rms surface roughness of 3.63 ± 0.75 nm, while Ni33Cr thin film is smoother with an rms roughness of only 0.29 ± 0.13 nm. The changes in film growth initiated by alloying with Cr are due to changes in the interfacial chemistry which favorably alters the initial adsorption of the metal atoms on MgO surface and suggests a reduction of the Ehrlich-Schwoebel barrier. The growth of smooth Ni-Cr thin films with a well-defined surface structure opens up a new pathway for a wide range of surface science studies related to alloy performance.

Microstructure and intrinsic stress evolution during epitaxial film growth of an Ag0.93Al0.07 solid solution on Si(111); excessive planar faulting due to quantum confinement

The correlation of microstructural development and the kinetics of film growth has been investigated during the epitaxial film growth of an ultrathin binary Ag0.93Al0.07 solid solution on a Si(111)-7×7 surface at 300 K by the combination of high-resolution transmission electron microscopy, X-ray diffraction, scanning tunneling microscopy, low energy electron diffraction, and real-time in-situ stress measurements. Up to a film thickness of 6 ± 2 nm, epitaxial Ag0.93Al0.07 film growth is characterized by the strikingly extensive formation of planar faults parallel to the film/substrate interface, while at larger thickness the film grows practically defect-free. As revealed by real-time in-situ stress measurements, the extensive formation of planar faults at the very initial stage of growth is not driven by the reduction of the system's elastic strain energy but is rather caused by a striking thickness-dependence of the stacking-fault energy owing to a quantum size effect of the ultrathin metal alloy film, resulting in a frequent succession of fcc and hcp stackings of close-packed layers during the initial stage of film growth. The extensive development of planar faults at the initial stage of film growth (<6 ± 2 nm) is associated with the occurrence of a high density of kinks and corners at thereby atomically rough surface ledges, which strongly enhances the downward transport of adatoms from higher to lower terraces (interlayer mass transport) by a reduction of the effective diffusion barrier at the edge of surface steps and by increasing the driving force for adatoms to attach to the surface ledges. As a result, the epitaxial Ag0.93Al0.07 film initially grows in a 2D layer-by-layer type of growth and thus establishes atomically smooth film surfaces. For the practically planar-fault-free growth at thicknesses beyond 6 ± 2 nm, interlayer mass transport becomes distinctively limited, thereby inducing a transition from 2D to 3D type of film growth.

Nanocrystal growth and morphology of PbTeSe-ZnSe composite thin films prepared by one-step synthesis method

The microstructure of polycrystalline PbTe1−xSex-ZnSe composite thin films has been studied by scanning transmission electron microscopy and electron diffraction. The films were prepared by the one-step synthesis method using simultaneous evaporation of PbTe and ZnSe. The nanocrystals of PbTe1−xSex are formed in a ZnSe matrix. Tellurium concentration can be tuned by controlling the PbTe evaporation source temperatures between 753 K and 793 K. Binary PbSe nanocrystals were formed at 753 K, while ternary PbTe1−xSex nanocrystals were formed at 793 K. The nanocrystals grow in a granular shape at the initial stage of film growth, and the morphology changes to nanowire-shape as the film grows, irrespective of the Te concentration. The ternary PbTe1−xSex nanocrystals were composed of two phases with different Te concentration; Te-rich (Se-poor) granular crystals were formed near the bottom half parts of the film and Te-poor (Se-rich) nanowires were formed at the upper half parts of the film. Columnar ZnSe crystals contain high-density {111} stacking faults due to the low stacking fault energy of ZnSe. A balance of deposition and re-evaporation on the substrate during the film growth will be responsible for the resultant nanocrystal morphology.

TiOx thin films grown on Pd(100) and Pd(111) by chemical vapor deposition

The growth of ultrathin TiOx (0≤x≤2) films on Pd(100) and Pd(111) surfaces by chemical vapor deposition (CVD), using Titanium(IV)isopropoxide (TTIP) as precursor, has been investigated by high resolution photoelectron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. Three different TiOx phases and one Pd–Ti alloy phase have been identified for both surfaces. The Pd–Ti alloy phase is observed at the initial stages of film growth. Density functional theory (DFT) calculations for Pd(100) and Pd(111) suggest that Ti is alloyed into the second layer of the substrate. Increasing the TTIP dose yields a wetting layer comprising Ti2+ species (TiOx, x ∼0.75). On Pd(100), this phase exhibits a mixture of structures with (3×5) and (4×5) periodicity with respect to the Pd(100) substrate, while an incommensurate structure is formed on Pd(111). Most importantly, on both surfaces this phase consists of a zigzag pattern similar to observations on other reactive metal surfaces. Further increase in coverage results in growth of a fully oxidized (TiO2) phase on top of the partially oxidized layer. Preliminary investigations indicate that the fully oxidized phase on both Pd(100) and Pd(111) may be the TiO2(B) phase.

Effect of the initial stage of film growth on device performance of organic transistors based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)

The initial stage of organic film growth is considered to be vital for the carrier transport in organic thin-film transistors with bottom gate configuration. The same topographies of 40nm dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) films on para-sexiphenyl (p-6P) monolayer and bare SiO2 exhibited quite different field-effect mobilities, 1.9 and 0.1cm2/Vs, respectively. The further investigation indicated there were different growth behaviors at their initial stages of film growth. Column islands with high density were observed on SiO2, while lamina islands on p-6P monolayer due to the good diffusion ability and their good epitaxial relationship. The latter is beneficial to obtain high quality film with less boundaries and defects. The work demonstrated that the initial stage of film growth is an important factor to determine the device performance of organic transistors, which is significant to improve the device fabrication and optimize the device performance.