Fabricating two-dimensional nanostructured tellurium thin films via pyrolyzing a single-source molecular precursor

A simple route to fabricate tellurium (Te) thin film with a well-defined two-dimensional nanostructure is presented in this study. The method involves dip-coating and subsequently pyrolyzing a single-source molecular precursor diethyldithiocarbamato tellurium (TDEC) onto a glass substrate. The pyrolysis temperature and the initial thickness (?) of TDEC film exhibited strong influence on the morphology of Te film, where films composed of uniform Te nanoflakes have been obtained at 440 °C and ? ≈ 490 nm. Investigations on the thermodynamic properties of TDEC through thermogravimetry, differential thermogravimetry, and differential scanning calorimetry techniques suggest that the production of Te from TDEC features random and continuous nucleation, shedding light on the possible growth mechanism of the two-dimensional Te film.     

Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

Bias enhanced nucleation of diamond thin films in a modified HFCVD reactor

In this study we investigated the nucleation of synthetic diamond thin films on Si substrates by double bias enhanced Hot Filament Chemical Vapour Deposition (HFCVD) method. First, we investigated the influence of the bias voltage and secondly the influence of the nucleation time under different bias voltages. The bias voltage was varied from −120 V up to −220 V as well as the nucleation time was changed from 30 up to 120 min in order to obtain the optimized nucleation conditions for following growth of continuous diamond layer. Samples were analyzed by Scanning Electron Microscopy (SEM) and Raman Spectroscopy. SEM was used for determination of cluster sizes and their distribution on the Si surface, while Raman Spectroscopy for determination and analysis of carbon phases.     

Germanium on insulator near-infrared photodetectors fabricated by layer transfer

We report on novel pn Ge photodetectors fabricated on glass. The fabrication consists of wafer bonding and layer splitting, followed by a low-temperature epitaxial growth of Ge. The photodiodes are characterized in terms of dark current and responsivity, and their performance compared with devices realized on either Ge or Si. The minimum current density is 50 μA/cm² at 1 V reverse bias, the responsivity is 0.2 A/W in the photovoltaic mode, with a maximum of 0.28 A/W at 1.55 μm at a reverse voltage of 5 V.     

Effects of substrate bias on the structure and mechanical properties of (Al1.5CrNb0.5Si0.5Ti)Nx coatings

(Al_1.5CrNb_0.5Si_0.5Ti)N_x high-entropy nitride coatings were designed and investigated in this study. Nitride coatings are deposited under a sufficient amount of nitrogen at 415 °C on Si by direct current magnetron reactive sputtering from a non-equimolar Al_1.5CrNb_0.5Si_0.5Ti high-entropy alloy target. The effects of substrate bias (V_s) on film structure and mechanical properties are studied. All these coatings have a single NaCl-type face-centered cubic structure and nearly stoichiometric ratio of (Al_1.5CrNb_0.5Si_0.5Ti)₅₀ N₅₀. A distinct refinement of microstructure of the films is observed when V_s varies from 0 V to − 100 V. Typical columnar structure transits into a dense and featureless structure and grain size decreases from 70 nm to 5 nm. Similar refinement remains at larger bias(− 150 or − 200 V). At the same time, the residual compressive stress increases from near zero to − 3.9 GPa at − 150 V and then decreases to − 3.2 GPa at − 200 V. The hardness increases from 12 GPa at 0 V, peaks at 36 GPa at − 100 V, and then decreases to 26 GPa at − 200 V. The structural evolution strengthening mechanism are discussed and compared with equimolar high-entropy nitrides.     

Influence of electrochemical treatment of ITO surface on nucleation and growth of OLED hole transport layer

Indium-tin-oxide (ITO) surfaces were electrochemically treated with voltages from 0 to + 2.8 V in 0.1 M K₄P₂O₇ electrolyte. The initial growth mode of hole transport layer (HTL) was investigated by atomic force microscope (AFM) observation of thermally deposited 2 nm N,N-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) on the electrochemically treated ITO surfaces. The results showed that the morphology of NPB thin film was significantly influenced by the treating voltage via the change in surface energy, especially the polar component. The treatments with + 2.0 and + 2.4 V were found to be most effective for more uniform and denser nucleation of NPB. The influence of the electrochemical treatments on the nucleation and growth mode of HTL and therefore the device performance were discussed.     

Electrochemical characterization of electrodeposited carbon nanotubes

Carbon nanotubes were electrodeposited in acetonitrile solution at room temperature using Cu, and Fe-Ni nanoparticles as nucleation sites on HF-etched Si(100) wafer substrate. The electrochemical behavior of the deposition was investigated by voltammetry and chronoamperometry techniques. In order to obtain the optimum growth condition, the deposition critical parameters including current density range, potential and time were studied and calculated. Carbon nanotubes with approximate external diameter of 40-100 nm were fabricated under potentiostatic condition and diffusion control at − 20 V in 4-6 h. The film crystallinity was investigated by means of X-ray diffraction and the tubes structure was revealed using scanning electron microscope and transmission electron microscope images. Raman spectroscopy was also employed to characterize the nanostructural features and single wall carbon nanotubes were detected.     

The structure of Ta nanopillars grown by glancing angle deposition

Regular arrays of Ta nanopillars, 200 nm wide and 500 nm tall, were grown on SiO₂ nanosphere patterns by glancing angle sputter deposition (GLAD). Plan-view and cross-sectional scanning electron microscopy analyses show dramatic changes in the structure and morphology of individual nanopillars as a function of growth temperature T_s ranging from 200 to 700 °C. At low temperatures, T_s ≤ 300 °C, single nanopillars develop on each sphere and branch into subpillars near the pillar top. In contrast, T_s ≥ 500 °C leads to branching during the nucleation stage at the pillar bottom. The top branching at low T_s is associated with surface mounds on a growing pillar that, due to atomic shadowing, develop into separated subpillars. At high T_s, the branching occurs during the nucleation stage where multiple nuclei on a single SiO₂ sphere develop into subpillars during a competitive growth mode which, in turn, leads to intercolumnar competition and the extinction of some nanopillars.     

In-situ reflection high-energy electron diffraction study of epitaxial growth of Cu on NaCl (100) under oblique angle vapor deposition

Epitaxial growth of copper on annealed NaCl(100) surface was carried out using thermal evaporation at an oblique angle of incidence (75 ± 5)° with respect to the substrate normal. The substrate was kept at a temperature of (150 ± 5)°C. The crystalline structure of the Cu film was studied in situ by reflection high energy electron diffraction at various deposition times. We observed that the film grows through nucleation of epitaxial islands followed by coalescence and then flattening of the film. The chevron shaped diffraction patterns formed by the refraction effect of electrons were used to identify the crystal facets. With longer deposition times, instead of columnar structures, a continuous epitaxial film was formed despite the oblique angle incidence of the vapor. The morphology of the final film was characterized ex situ by atomic force microscopy and shows L-shaped pores asymmetric with respect to the vapor incident direction.     

Optimization of GaN nucleation layer deposition conditions on sapphire substrates in HVPE system

The influence of deposition conditions of nucleation GaN layer on the properties of high-temperature GaN layer, grown on sapphire substrates, was investigated. The hydride vapor phase epitaxy (HVPE) three-section horizontal hot-wall furnace technique was applied. Various temperatures, HCl flows and time intervals of nucleation layer growth were utilized. Based on previous studies the following experimental conditions were selected: temperature was kept at 450 or 570 °C, and HCl flows were 8 or 10 sccm/min. The duration of nucleation layer deposition was 5, 7 and 9 min. The scanning electron microscopy technique was applied for the investigation of nucleation layer morphology after migration. Thick GaN layers were deposited during the three-step growth process at 1060 °C. Samples with various surface morphologies were obtained. Photoluminescence spectra and X-ray measurements were performed, which permitted clarifications of the influence of growth conditions of the nucleation layer on the properties of high-temperature layers.     

Repeated nucleation in an undercooled tin droplet by fast scanning calorimetry

The heterogeneous nucleation kinetics of a single undercooled tin droplet was measured at a cooling rate of 1 × 10⁴ K/s by fast scanning non-adiabatic calorimetry. The nucleation rates were obtained by a statistical analysis of nearly 1000 identical events. Applying classical heterogeneous theory in conjunction with the available thermodynamic data, it was found that a single mechanism spherical cap model was capable of describing the experimental data. And the present study shows that fast scanning calorimetry allows studying nucleation in micron sized droplets and may open a way for detailed investigations of the influence of droplet size to discriminate surface and bulk nucleation.     

The phase transformation and crystallization kinetics of (1 − x)Li2O–xNa2O–Al2O3–4SiO2 glasses

The phase transformation and crystallization kinetics of (1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses have been studied by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) analysis. The crystallization temperature at the exothermic peak increases from 1171 to 1212 K when the Na₂O content increases from 0 to 0.6 mol. The crystalline phase is composed of spodumene crystallization when the Na₂O content increases from 0 to 0.6 mol. The activation energy of spodumene crystallization decreases from 444.0 ± 22.2 to 284.0 ± 10.8 kJ mol⁻¹ when the Na₂O content increases from 0 to 0.4 mol. Moreover, the activation energy increases from 284.0 ± 10.8 to 446.0 ± 23.2 kJ mol⁻¹ when the Na₂O content increases from 0.4 to 0.6 mol. The crystallization parameters m and n approach 2, indicating that the surface nucleation and two-dimensional growth are dominant in (1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses.     

Effect of eutectic particles on the grain size control and the superplasticity of aluminium alloys

Aluminium alloys containing eutectic particles of the Al-Ni, Al-Mg-Si, Al-Ni-Ce and Al-Cu-Ce systems are investigated. The particles which control grain growth and stimulate grain nucleation are studied. The Zener-Smith law about dependence between grain size and particle parameters is confirmed and experimental coefficients are found. Experimental coefficients of the Zener-Smith equation obtained in this study depend on the particle size and differ from theoretical coefficients proposed by Zener and Smith. Some alloys with grain size about 3 μm demonstrate very good superplasticity indicators, namely: the strain rate sensitivity index m = 0.5-0.6 and the elongation over 400% at constant strain rate 5 × 10⁻³ s⁻¹.     

The microstructure evolution and nucleation mechanisms of dynamic recrystallization in hot-deformed Inconel 625 superalloy

Hot compressions tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator at different strains between 900 °C and 1200 °C with a strain rate of 0.1 s⁻¹. Optical microscope, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the microstructure evolution and nucleation mechanisms of dynamic recrystallization. It was found that both the size and fraction of dynamically recrystallized grains increase with increasing deformation temperature. However, the size of dynamically recrystallized grains almost remains constant with increasing deformation strain. The dominant nucleation mechanism of dynamic recrystallization in Inconel 625 superalloy deformed at 1150 °C is the discontinuous dynamic recrystallization, which is characterized by the bulging of the original grain boundaries accompanied with twining. The continuous dynamic recrystallization characterized by progressive subgrain rotation occurs simultaneously in dynamic recrystallization process, although it can only be considered as an assistant nucleation mechanism at the early stage of hot deformation.     

Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001)

Ultra-thin gallium nitride (GaN) films were deposited using the ion-beam assisted molecular-beam epitaxy technique. The influence of the nitrogen ion to gallium atom flux ratio (I/A ratio) during the early stages of GaN nucleation and thin film growth directly, without a buffer layer on super-polished 6H-SiC(0001) substrates was studied. The deposition process was performed at a constant substrate temperature of 700 °C by evaporation of Ga and irradiation with hyperthermal nitrogen ions from a constricted glow-discharge ion source. The hyperthermal nitrogen ion flux was kept constant and the kinetic energy of the ions did not exceed 25 eV. The selection of different I/A ratios in the range from 0.8 to 3.2 was done by varying the Ga deposition rate between 5 × 10¹³ and 2 × 10¹⁴ at. cm^− 2 s^− 1. The crystalline surface structure during the GaN growth was monitored in situ by reflection high-energy electron diffraction. The surface topography of the films as well as the morphology of separated GaN islands on the substrate surface was examined after film growth using a scanning tunneling microscope without interruption of ultra-high vacuum. The results show, that the I/A ratio has a major impact on the properties of the resulting ultra-thin GaN films. The growth mode, the surface roughness, the degree of GaN coverage of the substrate and the polytype mixture depend notably on the I/A ratio.     

Optimization of silicon nanocrystals growth process by low pressure chemical vapor deposition for non-volatile memory application

The processes of silicon nanocrystals (Si-NCs) growth on both SiO₂ and Si₃N₄ substrates by low pressure chemical vapor deposition have been systematically investigated. A two-step process was adopted for Si-NCs growth: nucleation at a high temperature (580-600 °C) and growth at a low temperature (550 °C). By adjusting the pre-deposition waiting time and deposition time, the density, size and uniformity can be effectively controlled. Compared to the growth of Si-NCs on SiO₂, the coalescence speed of Si-NCs on Si₃N₄ is faster. Uniform Si-NCs with a high density of 1.02 × 10¹² cm^− 2 and 1.14 × 10¹² cm^− 2 have been obtained on SiO₂ and Si₃N₄, respectively. Finally, a Si-NCs-based memory structure with a 2.1 V memory window was demonstrated.     

Investigation on the crystal growth and characterisation of an organometallic non linear optical crystal – Tetrathiourea mercury tetrathiocyanato manganate

An organometallic complex tetrathiourea mercury tetrathiocyanato manganate (TMTM) was synthesised by two step chemical route. The influence of pH on the solubility, stability of the growth solution and growth of TMTM single crystal was studied. The bulk crystal of dimension 2.7 cm × 0.5 cm × 0.4 cm was grown at an optimized pH value of 3.0 by solvent evaporation method using water as a solvent. The structure and the crystallinity of the grown crystal were confirmed from single crystal XRD and powder XRD studies. The presence of thiourea and thiocyanate ligand and their coordination with the metal ions are confirmed from the IR studies. The grown single crystal of TMTM shows 69% transparency over the entire range of visible region with a cut-off wavelength of 362 nm. The SHG efficiency of TMTM was found to be 0.08 time that of MMTC. The complex exhibits good thermal stability up to 205 °C and it is non-hygroscopic in nature. The surface analysis was carried out by etching studies which shows a layer growth pattern revealing the 2D nucleation growth mechanism. From the Vickers hardness studies, the work hardening coefficient and the elastic stiffness constant were calculated.     

Nucleation of CdTe thin films deposited by close-space sublimation under a nitrogen ambient

The early stages of the close-space sublimation growth of CdTe/ITO (indium tin oxide) at 500°C under 26.7 kPa (200 Torr) of N₂ were investigated, the relatively high pressure being used to slow the growth. Film development was monitored over 60 mins by ex-situ AFM (atomic force microscopy), the growth being controlled by a shutter. The films formed by the ‘island’ or Volmer-Weber growth mechanism. Developments in the areal island growth rate, the island density and spatial distribution type were explained using growth mechanisms. Significant changes in these phenomena at t ≥ 10 mins were attributed to a change in growth mechanism from surface migration limited, to a regime in which island coalescence and direct impingement of vapour species on the islands became important. Since the islands are characterised by distinct crystalline facets, this indicates the growth mechanism to be uninterrupted step-flow like addition of material to each island. Arguments are given to relate the final grain size in the films to the density of nuclei that are stable to re-evaporation after t = 10 mins.     

A novel, substrate independent three-step process for the growth of uniform ZnO nanorod arrays

We report a three-step deposition process for uniform arrays of ZnO nanorods, involving chemical bath deposition of aligned seed layers followed by nanorod nucleation sites and subsequent vapour phase transport growth of nanorods. This combines chemical bath deposition techniques, which enable substrate independent seeding and nucleation site generation with vapour phase transport growth of high crystalline and optical quality ZnO nanorod arrays. Our data indicate that the three-step process produces uniform nanorod arrays with narrow and rather monodisperse rod diameters (∼ 70 nm) across substrates of centimetre dimensions. X-ray photoelectron spectroscopy, scanning electron microscopy and X-ray diffraction were used to study the growth mechanism and characterise the nanostructures.     

Evaluation of incipient plasticity from interfaces between ultra-thin gold films and compliant substrates

Nanoindentation experiments were conducted for 30 nm-thick Au films on two types of substrates, polyimide (compliant) and glass (stiff), to clarify the dominant mechanics of incipient plasticity from the interface. A high resolved shear stress τ_r could be effectively applied to the Au/polyimide interface due to the compliant substrate, and plastic deformation was initiated at the interface. The critical resolved shear stress τ_crss at the interface was determined to have a value of 0.4 ∼ 0.5 GPa. On the other hand, in Au/glass, τ_r peaked within the Au film, and the maximum values were 1.1 ∼ 2.2 GPa depending on the tip radius, whereas the values of τ_r at the Au/glass interface were almost identical at 0.5 ∼ 0.7 GPa. Therefore, plastic deformation might be initiated from the Au/glass interface. The values of τ_crss for heterogeneous nucleation at the interfaces were smaller than that for homogeneous nucleation in the Au films.