Function Of Growth Temperature Research Articles

Enhanced Phase Transition Properties of VO2 Thin Films on 6H-SiC (0001) Substrate Prepared by Pulsed Laser Deposition.

For growing high quality epitaxial VO2 thin films, the substrate with suitable lattice parameters is very important if considering the lattice matching. In addition, the thermal conductivity between the substrate and epitaxial film should be also considered. Interestingly, the c-plane of hexagonal 6H-SiC with high thermal conductivity has a similar lattice structure to the VO2 (010), which enables epitaxial growth of high quality VO2 films on 6H-SiC substrates. In the current study, we deposited VO2 thin films directly on 6H-SiC (0001) single-crystal substrates by pulsed laser deposition (PLD) and systematically investigated the crystal structures and surface morphologies of the films as the function of growth temperature and film thickness. With optimized conditions, the obtained epitaxial VO2 film showed pure monoclinic phase structure and excellent phase transition properties. Across the phase transition from monoclinic structure (M1) to tetragonal rutile structure (R), the VO2/6H-SiC (0001) film demonstrated a sharp resistance change up to five orders of magnitude and a narrow hysteresis width of only 3.3 °C.

Actively detached Pseudomonas aeruginosa biofilm cell susceptibility to benzalkonium chloride and associated resistance mechanism.

The present work aimed at studying physiological properties of Pseudomonas aeruginosa cells actively detached from biofilm formed on stainless steel and comparing them with their planktonic counterparts as a function of growth temperature (20°C and 37°C). The susceptibility of P. aeruginosa cells to benzalkonium chloride (BAC) was studied. Furthermore, the effect of BAC on the cell membrane integrity and the role of the cell membrane fluidity in the cell-scale-resistance mechanism were investigated. Our results showed that actively detached biofilm cells were more susceptible to BAC treatment than planktonic ones. A greater leakage of intracellular potassium after BAC addition was observed in actively detached biofilm cells, which reflects their membrane vulnerability. The rise of the growth temperature from 20 to 37°C increased the membrane rigidity of planktonic cells comparatively to their actively detached biofilm ones. Under experimental conditions developed in this work, our data highlighted that actively biofilm-detached and planktonic P. aeruginosa cells have distinguishable phenotypes.

Engineering V-shaped pits in InGaN layers grown by PA-MBE toward optimizing the active region of green LEDs

Surface morphology of thick InGaN with higher indium content, grown on GaN/sapphire templates using plasma-assisted molecular beam epitaxy (PA-MBE) under metal-rich growth condition, is investigated as a function of growth temperature and III–V ratio using atomic force microscopy. V-shaped pits on the InGaN surface are found to diminish with the decrease in growth temperature and remain unchanged with variation in III–V ratio, while exhibiting a significant impact on surface roughness, crystal quality, and optical properties. Fluctuation in density and depth of V-shaped pits is discussed, taking into account the low growth temperature of PA-MBE.

Multi-functional MnO2 nanomaterials for photo-activated applications by a plasma-assisted fabrication route.

Supported MnO2-based nanomaterials were fabricated on fluorine-doped tin oxide substrates using plasma enhanced-chemical vapor deposition (PE-CVD) between 100 °C and 400 °C, starting from a fluorinated Mn(ii) diamine diketonate precursor. Growth experiments yielded β-MnO2 with a hierarchical morphology tuneable from dendritic structures to quasi-1D nanosystems as a function of growth temperature, whose variation also enabled a concomitant tailoring of the system fluorine content, and of the optical absorption and band gap. Preliminary photocatalytic tests were aimed at the investigation of photoinduced hydrophilic (PH) and solid phase photocatalytic (PC) performances of the present nanomaterials, as well as at the photodegradation of Plasmocorinth B azo-dye aqueous solutions. The obtained findings highlighted an attractive system photoactivity even under visible light, finely tailored by fluorine content, morphological organization and optical properties of the prepared nanostructures. The results indicate that the synthesized MnO2 nanosystems have potential applications as advanced smart materials for anti-fogging/self-cleaning end uses and water purification.

Anisotropic optical properties of a-axis AlN films: a spectroscopic ellipsometry study

We report the uniaxial anisotropic optical constants of wurtzite-type a-axis oriented AlN films deposited on Si (100) using DC reactive magnetron sputtering as a function of growth temperature (Ts, 35 to 600 °C) using the spectroscopic ellipsometry (SE) technique. The thickness and roughness of these films are determined from the regression analysis of SE data, which are corroborated using TEM and AFM techniques. Highly oriented a-axis AlN film grown at 400 °C, exhibits high n and low k at 210 nm (deep-UV region) with small birefringence (−0.01) and dichroism (0.03) near the band edge. All these AlN films exhibit transparent nature from near-infrared (NIR) to 354 nm, where optical band gap energies vary between 5.7 to 6.1 eV.

Insights into the Plasma-Assisted Fabrication and Nanoscopic Investigation of Tailored MnO2 Nanomaterials.

Among transition metal oxides, MnO2 is of considerable importance for various technological end-uses, from heterogeneous catalysis to gas sensing, owing to its structural flexibility and unique properties at the nanoscale. In this work, we demonstrate the successful fabrication of supported MnO2 nanomaterials by a catalyst-free, plasma-assisted process starting from a fluorinated manganese(II) molecular source in Ar/O2 plasmas. A thorough multitechnique characterization aimed at the systematic investigation of material structure, chemical composition, and morphology revealed the formation of F-doped, oxygen-deficient, MnO2-based nanomaterials, with a fluorine content tunable as a function of growth temperature ( TG). Whereas phase-pure β-MnO2 was obtained for 100 °C ≤ TG ≤ 300 °C, the formation of mixed phase MnO2 + Mn2O3 nanosystems took place at 400 °C. In addition, the system nano-organization could be finely tailored, resulting in a controllable evolution from wheat-ear columnar arrays to high aspect ratio pointed-tip nanorod assemblies. Concomitantly, magnetic force microscopy analyses suggested the formation of spin domains with features dependent on material morphology. Preliminary tests in Vis-light activated photocatalytic degradation of rhodamine B aqueous solutions pave the way to possible applications of the target materials in wastewater purification.

Effect of temperature and carrier gas on the properties of thick [formula omitted]N layer

Thick (>150 nm) InxAl1-xN layers were grown on GaN/sapphire (0 0 0 1) by organometallic vapor phase epitaxy. Growth temperature of InxAl1-xN layers was reduced from 790 to 730 °C, to examine the effects of growth temperature in InxAl1-xN layers grown under H2 carrier gas. Indium incorporation, surface morphology, electrical, and optical properties of InxAl1-xN layers were examined as a function of growth temperature. Increase in In-molar fraction, as determined by high resolution X-ray diffraction, was observed with decreasing growth temperature of InxAl1-xN layers at the expense of surface roughness. Unstrained InxAl1-xN layer was achieved at 730 °C under H2 carrier gas with x = 0.18. However, InxAl1-xN layer grown under N2 carrier gas at 730 °C to study the effects of carrier gas, was observed with two times higher In-molar fraction (x = 0.37) and one order lower carrier concentration. This work shows the essential requirement of a multi-characterization approach to establish a connection between structural, electrical, and optical properties to improve our understanding towards InxAl1-xN. Edge threading dislocations density is found to be the most important parameter in deciding the characteristics of an InxAl1-xN layer.

Kinetics during endotaxial growth of CoSi2 nanowires and islands on Si(0 0 1)

We propose a phenomenological growth model describing the morphological evolution of two different (ridge and flat) types of endotaxial CoSi2 islands as a function of growth temperature from 500 °C to 800 °C and for Cobalt coverages from 0.1 monolayer (ML) to 0.5ML. The presence of low energy Type A and Type B CoSi2{1 1 1}-Si{1 1 1} interfaces drives the formation of flat-type islands and ridge-type nanowires respectively. The growth of these nanowires and islands are kinetically constrained, where the islands’ length, width and height are experimentally found to follow the Arrhenius relation with activation energies ranging between 0.4 eV and 1.6 eV. Our model suggests that the islands are kinetically constrained by thermally-activated processes such as island-edge diffusion anisotropy and corner energy-barriers, thereby restricting the adatom transfer between long and short sides of the islands. Consequently, at low growth temperatures, ridge-type nanowires are more dot-like while flat-type islands form small wire-like islands. But at high growth temperatures, the ridge islands form long nanowires while the flat islands become more dot-like instead.

Evolution of stratification in high-alloy content InGaN epilayers grown on (0001) AlN

ABSTRACTThe structural properties of InxGa1−xN epilayers, deposited on (0001) AlN templates by plasma-assisted molecular beam epitaxy, were studied by transmission electron microscopy and Raman spectroscopy, as a function of growth temperature. Single phase films with high indium content and well-ordered heteroepitaxial interfaces were attained at lower temperatures. Delayed plastic relaxation resulted in the structural stratification of high-temperature films due to the compositional pulling. Such films relaxed by a stacking fault mechanism, contrary to low temperature ones that exhibited relaxation by misfit dislocations. Despite the higher defect content of the former, their phonon mean free path was also higher, showing that alloying-induced fluctuations of the periodic potential constitute a more critical parameter. Cubic interfacial zones were suppressed at lower growth temperatures.This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.

In-plane x-ray diffraction for characterization of monolayer and few-layer transition metal dichalcogenide films

There is significant interest in the growth of single crystal monolayer and few-layer films of transition metal dichalcogenides (TMD) and other 2D materials for scientific exploration and potential applications in optics, electronics, sensing, catalysis and others. The characterization of these materials is crucial in determining the properties and hence the applications. The ultra-thin nature of 2D layers presents a challenge to the use of x-ray diffraction (XRD) analysis with conventional Bragg–Brentano geometry in analyzing the crystallinity and epitaxial orientation of 2D films. To circumvent this problem, we demonstrate the use of in-plane XRD employing lab scale equipment which uses a standard Cu x-ray tube for the analysis of the crystallinity of TMD monolayer and few-layer films. The applicability of this technique is demonstrated in several examples for WSe2 and WS2 films grown by chemical vapor deposition on single crystal substrates. In-plane XRD was used to determine the epitaxial relation of WSe2 grown on c-plane sapphire and on SiC with an epitaxial graphene interlayer. The evolution of the crystal structure orientation of WS2 films on sapphire as a function of growth temperature was also examined. Finally, the epitaxial relation of a WS2/WSe2 vertical heterostructure deposited on sapphire substrate was determined. We observed that WSe2 grows epitaxially on both substrates employed in this work under all conditions studied while WS2 exhibits various preferred orientations on sapphire substrate which are temperature dependent. In contrast to the sapphire substrate, WS2 deposited on WSe2 exhibits only one preferred orientation which may provide a route to better control the orientation and crystal quality of WS2. In the case of epitaxial graphene on SiC, no graphene-related peaks were observed in in-plane XRD while its presence was confirmed using Raman spectroscopy. This demonstrates the limitation of the in-plane XRD technique for characterizing low electron density materials.

Epitaxial structure and electronic property of β-Ga2O3 films grown on MgO (100) substrates by pulsed-laser deposition

We investigated heteroepitaxial growth of Si-doped Ga2O3 films on MgO (100) substrates by pulsed-laser deposition as a function of growth temperature (Tg) to find a strong correlation between the structural and electronic properties. The films were found to contain cubic γ-phase and monoclinic β-phase, the latter of which indicated rotational twin domains when grown at higher Tg. The formation of the metastable γ-phase and twin-domain structure in the stable β-phase are discussed in terms of the in-plane epitaxial relationships with a square MgO lattice, while crystallinity of the β-phase degraded monotonically with decreasing Tg. The room-temperature conductivity indicated a maximum at the middle of Tg, where the β-Ga2O3 layer was relatively highly crystalline and free from the twin-domain structure. Moreover, both crystallinity and conductivity of β-Ga2O3 films on the MgO substrates were found superior to those on α-Al2O3 (0001) substrates. A ratio of the conductivity, attained to the highest quantity on each substrate, was almost three orders of magnitude.

Temperature-Dependent Alkyl Glycerol Ether Lipid Composition of Mesophilic and Thermophilic Sulfate-Reducing Bacteria

The occurrence of non-isoprenoid alkyl glycerol ether lipids in Bacteria and natural environments is increasingly being reported and the specificity and diagenetic stability of these lipids make them powerful biomarkers for biogeochemical and environmental studies. Yet the environmental controls on the biosynthesis of these peculiar membrane lipids remain poorly documented. Here, the lipid content of two mesophilic (Desulfatibacillum aliphaticivorans and Desulfatibacillum alkenivorans) and one thermophilic (Thermodesulfobacterium commune) sulfate-reducing bacteria—whose membranes are mostly composed of ether lipids—was investigated as a function of growth temperature (20–40°C and 54–84°C, respectively). For all strains, the cellular lipid content was lower at sub- or supra-optimal growth temperature, but the relative proportions of dialkyl glycerols, monoalkyl glycerols and fatty acids remained remarkably stable whatever the growth temperature. Rather than changing the proportions of the different lipid classes, the three strains responded to temperature changes by modifying the average structural composition of the alkyl and acyl chains constitutive of their membrane lipids. Major adaptive mechanisms concerned modifications of the level of branching and of the proportions of the different methyl branched lipids. Specifically, an increase in temperature induced mesophilic strains to produce less dimethyl branched dialkyl glycerols and 10-methyl branched lipids relative to linear structures, and the thermophilic strain to decrease the proportion of anteiso relative to iso methyl branched compounds. These modifications were in agreement with a regulation of the membrane fluidity. In one mesophilic and the thermophilic strains, a modification of the growth temperature further induced changes in the relative proportions of sn-2 vs sn-1 monoalkyl glycerols, suggesting an unprecedented mechanism of homeoviscous adaptation in Bacteria. Strong linear correlations observed between different ratios of alkyl glycerols and temperature allow to hypothesize the use of these specific lipids as indicators of temperature changes in the environment.

Preparation and characterization of ZnS nanoparticles prepared by hydrothermal method

ZnS nanoparticles were prepared by hydrothermal method using zinc nitrate [Zn(NO[Formula: see text] ⋅ 6H2O] and thiourea [SC(NH[Formula: see text]] as sources of Zn[Formula: see text] and S[Formula: see text] ions and cetyltrimethyl ammonium bromide [CH3(CH2)[Formula: see text]N[Formula: see text](CH3)3 ⋅ Br[Formula: see text]; CTAB] as a surface active agent. The structural, surface morphology and optical properties of ZnS nanoparticles as a function of growth temperature were investigated. The studies show that ZnS nanoparticles have high transmittance over 70% in the visible light range of 400–800 nm, and it has a strong green emission band at about 520 nm which originates from the recombination of electrons from the energy level of sulfur vacancies with the holes from the energy level of zinc vacancies. With the increase of the growth temperature, the XRD peak intensity and the size of ZnS nanoparticles increase, while the green emission intensity firstly increases and then decreases.

Growth of few- and multilayer graphene on different substrates using pulsed nanosecond Q-switched Nd:YAG laser

In this report, few- and multilayer graphene was fabricated on different substrates by pulsed laser ablation of a highly ordered pyrolytic graphite target under optimized growth conditions, using a pulsed nanosecond Q-switched Nd:YAG laser at 355 nm (3.5 eV). The nondestructive micro-Raman spectroscopic study on our samples has revealed few- and multilayer graphene formation. The number of graphene layers was found to be reduced with the increase in growth temperature. At substrate temperature of 750 °C, the ratio of intensities (I 2D/I G) was calculated from the Raman spectra of the graphene samples to be 0.15 which confirms the multilayer graphene formation, while for graphene film grown at 800 °C, I 2D/I G ratio was 0.27 indicating formation of less than five layers of graphene or few-layer graphene. The thickness of few- and multilayer graphene was also confirmed using atomic force microscopy, whereas the microstructure of few- and multilayer graphene was investigated using scanning electron microscopy. The electrical properties in function of growth temperature were evaluated with two-point probe measurements. This work presents a simple, fast, and controllable alternative effective laser technique to synthesize few- or multilayer graphene.

Critical thickness of GaN on AlN: impact of growth temperature and dislocation density

Critical thickness and strain relaxation of c-plane GaN layers grown by molecular beam epitaxy on AlN were studied as a function of growth temperature and threading dislocation density (TDD). For this purpose we used AlN/sapphire templates and AlN single crystals with TDDs of ∼109 cm−2 and ∼103 cm−2, respectively. Whereas at high growth temperature (900 °C) the critical thickness for plastic relaxation is only 3 monolayers (MLs) for both substrates, this value drastically increases when decreasing the growth temperature. It reaches ∼30 MLs when GaN is deposited at 750 °C on AlN single crystals. We also observed that the strain relaxation rate strongly depends on TDD. These results exemplify the lack of efficient gliding planes in III-nitrides when grown along the c-axis, which, combined with low kinetics, allows for plastic relaxation to be frozen out. Achieving pseudomorphic GaN layers on AlN is of interest for two-dimensional electron gases based on AlN/GaN/AlN heterostructures lattice-matched to AlN single crystal substrates.

Growth of one-dimensional InSb nanostructures with controlled orientations on InSb substrates by MOCVD

We have synthesized InSb nanorods on InSb substrates by using metal-organic chemical vapor deposition (MOCVD). It is found that the crystal orientation of the substrate plays a remarkable role in controlling growth orientation of the InSb nanorods. On a [100]-oriented InSb substrate, the nanorods tend to lie on the substrate along its [1–10]-axis. In comparison, [111]-oriented InSb substrates tend to promote the vertical growth of InSb nanowires and free-standing InSb nanowires are consequently obtained. The evolutions of diameter and length of the nanowires on InSb (111) substrates as a function of growth temperatures, T, and input V/III source ratios, rV/III, provide the optimized growth condition, i.e., T = 400 °C and rV/III = 50. The orientated growth of one-dimensional InSb nanostructures is discussed and analized by employing a vapor-solid-solid (VSS) growth mechanism.

Morphologically tuned 3D/1D rutile TiO2 hierarchical hybrid microarchitectures engineered by one-step surfactant free hydrothermal method

Present investigation reports on the surfactant free hydrothermal synthesize of the morphologically tuned hierarchical hybrid rutile titanium oxide (TiO2) microarchitectures showing three dimensional microflower structures and cook pine tree like structures on the one dimensional nanorods formed over TiO2 seed layer coated glass substrates by tuning growth temperature. TiO2 seed layer of ∼100nm thick was coated on the glass substrates employing sol–gel spin coating method and then rutile TiO2 microarchitectures were synthesized on the TiO2 seed layer by one-step surfactant free hydrothermal method. Deposited samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, UV–vis spectroscopy and photoluminescence spectroscopy techniques. Influence of the growth temperature on the crystallinity, morphology and optical properties along with the growth mechanism to achieve hierarchical microarchitectures was investigated. Present work revealed that the structural, morphological and optical properties of the TiO2 hierarchical microarchitectures strongly depend on the growth temperature. Further we proposed a model for the cause to effect possible morphological changes of rutile TiO2 microarchitectures as a function of growth temperatures on the TiO2 seeded glass substrates.

Hydride CVD Hetero-epitaxy of B12P2 on 4H-SiC

Icosahedral boron phosphide (B12P2) is a wide bandgap semiconductor (3.35eV) that has been reported to “self-heal” from high-energy electron bombardment, making it attractive for potential use in radioisotope batteries, radiation detection, or in electronics in high radiation environments. This study focused on improving B12P2 hetero-epitaxial films by growing on 4H-SiC substrates over the temperature range of 1250–1450°C using B2H6 and PH3 precursors in a H2 carrier gas. XRD scans and Laue transmission photographs revealed that the epitaxial relationship was (0001)〈112¯0〉B12P2∥(0001)〈112¯0〉4H−SiC. The film morphology and crystallinity were investigated as a function of growth temperature and growth time. At 1250°C, films tended to form rough, polycrystalline layers, but at 1300 and 1350°C, films were continuous and comparatively smooth (RRMS≤7nm). At 1400 or 1450°C, the films grew in islands that coalesced as the films became thicker. Using XRD rocking curves to evaluate the crystal quality, 1300°C was the optimum growth temperature tested. At 1300°C, the rocking curve FWHM decreased with increasing film thickness from 1494 arcsec for a 1.1μm thick film to 954 arcsec for a 2.7µm thick film, suggesting a reduction in defects with thickness.

Spontaneous cracking of amorphous solid water films and the dependence on microporous structure

Vapor-deposited, porous, amorphous, water-ice films, also called amorphous solid water (ASW), crack spontaneously during growth when the film thickness exceeds a critical value (Lc). We measured the Lc during growth of ASW films as a function of growth temperature (Tg = 10 K, 30 K, and 50 K) and deposition angle (θ = 0°, 45°, and 55°) using a quartz crystal microbalance, an optical interferometer, and an infrared spectrometer. The critical thickness, 1–5 μm under our experimental conditions, increases with Tg and θ, an indication of film porosity. We suggest that ASW films undergo tensile stress due to the mismatch between substrate adhesion and contracting forces derived from the incompletely coordinated molecules on the surfaces of the pores. We provide a model to explain the observed dependences of Lc on the Tg and θ in the context of Griffith theory and estimate the tensile strength of low-temperature ASW to be ∼25–40 MPa. Our model can be applied more generally to describe fracture of other solids with microporous structures, such as metallic or ceramic materials with voids.

Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties

Atomic layer deposition (ALD) is based on self-limiting surface reactions. This and cyclic process enable the growth of conformal thin films with precise thickness control and sharp interfaces. A multilayered thin film, which is nanolaminate, can be grown using ALD with tuneable electrical and optical properties to be exploited, for example, in the microelectromechanical systems. In this work, the tunability of the residual stress, adhesion, and mechanical properties of the ALD nanolaminates composed of aluminum oxide (Al2O3) and titanium dioxide (TiO2) films on silicon were explored as a function of growth temperature (110–300 °C), film thickness (20–300 nm), bilayer thickness (0.1–100 nm), and TiO2 content (0%–100%). Al2O3 was grown from Me3Al and H2O, and TiO2 from TiCl4 and H2O. According to wafer curvature measurements, Al2O3/TiO2 nanolaminates were under tensile stress; bilayer thickness and growth temperature were the major parameters affecting the stress; the residual stress decreased with increasing bilayer thickness and ALD temperature. Hardness increased with increasing ALD temperature and decreased with increasing TiO2 fraction. Contact modulus remained approximately stable. The adhesion of the nanolaminate film was good on silicon.