Injection Metalorganic Chemical Vapor Deposition Research Articles

Dimensionality Effect on Surface States and Piezoelectric Behavior in ZnO Nanowires Grown by Pulsed-Liquid Injection Metal–Organic Chemical Vapor Deposition

Dimensionality Effect on Surface States and Piezoelectric Behavior in ZnO Nanowires Grown by Pulsed-Liquid Injection Metal–Organic Chemical Vapor Deposition

Correlation between the Dimensions and Piezoelectric Properties of ZnO Nanowires Grown by PLI-MOCVD with Different Flow Rates

Zinc oxide nanowires (ZnO NWs) have gained considerable attention in the field of piezoelectricity in the past two decades. However, the impact of growth-process conditions on their dimensions and polarity, as well as the piezoelectric properties, has not been fully explored, specifically when using pulsed-liquid injection metal–organic chemical vapor deposition (PLI-MOCVD). In this study, we investigate the influence of the O2 gas and DEZn solution flow rates on the formation process of ZnO NWs and their related piezoelectric properties. While the length and diameter of ZnO NWs were varied by adjusting the flow-rate conditions through different growth regimes limited either by the O2 gas or DEZn reactants, their polarity was consistently Zn-polar, as revealed by piezoresponse force microscopy. Moreover, the piezoelectric coefficient of ZnO NWs exhibits a strong correlation with their length and diameter. The highest mean piezoelectric coefficient of 3.7 pm/V was measured on the ZnO NW array with the length above 800 nm and the diameter below 65 nm. These results demonstrate the ability of the PLI-MOCVD system to modify the dimensions of ZnO NWs, as well as their piezoelectric properties.

Numerical Study of Growth Rate and Purge Time in the AlN Pulsed MOCVD Process

The relationship between the purge time and the overall growth rate in pulsed injection metal–organic chemical vapor deposition with different V/III ratios is studied by numerical analysis. The transport behavior of TMAl and TMAlNH3 during the process is studied to understand the effect of the adductive reaction. The results show that, as the V/III ratio increases, there is a significant reduction in the average growth rate per cycle, without the addition of a purging H2 pulse between the III and V pulses, due to stronger adductive reaction. The adductive reaction can be reduced by inserting a purging pulse of pure H2 between the III and V pulses, but there is a decrease in the overall growth rate due to the longer cycle time. At smaller V/III ratios, the growth rate decreases with increasing purge times, since the gain in reducing the adductive reaction is offset by the detrimental effect of extending the cycle time. The degree of reduction in the adductive reaction is higher for larger V/III ratios. When the benefit of reducing the adductive reaction overcomes the deficiency of the extending cycle time, a remarkable enhancement of the growth rate can be obtained at high V/III ratios by inserting a pure H2 purge pulse between the III and V pulses. A higher overall growth rate can be achieved at higher V/III ratios by choosing an appropriate purge time.

Tuneable polarity and enhanced piezoelectric response of ZnO thin films grown by metal–organic chemical vapour deposition through the flow rate adjustment

The formation process of ZnO thin films grown by pulsed-liquid injection metal–organic chemical vapour deposition (PLI-MOCVD) has a major impact on its morphological, structural, electrical and piezoelectric properties, but their correlation has not been elucidated yet nor decoupled from the thickness effects.

Low temperature Direct Liquid Injection MOCVD of amorphous CrCx coatings in large-scale reactors: An original route to nanostructured multilayer coatings

The effect of low temperature deposition on the quality and the microstructure of hard a -CrC x coatings grown by Direct Liquid Injection Metal-Organic Chemical Vapor Deposition (DLI-MOCVD) was investigated. At deposition temperatures higher than 525 °C, the coatings are bi-phased and polycrystalline , essentially composed of Cr 7 C 3 with Cr 3 C 2 as a minor phase. By decreasing the temperature in the 525–350 °C range, they are amorphous and exhibit a monolithic glassy-like microstructure without grain boundary. For temperatures lower than 350 °C, the uniform amorphous microstructure changes for a lamellar multilayer structure, while the composition of the gas phase was kept constant at the inlet of the reactor for all these CVD runs. The origin of this surprising lamellar structure has been investigated. After having discarded the idea of a self-organized multilayer growth, sometimes encountered in CVD processes, evidence was found for rapid changes in the deposition mechanism. They are induced by oscillations of the surface temperature around a critical value due to the regulation of the furnace used for such large-scale reactors. These almost periodic temperature changes lead to modulations in the surface composition of nutrient species, and subsequently, to the growth of multilayers. The individual composition of lamellae changes regularly, while the overall composition of the coatings remains identical to that of uniform monolithic coatings. For growth conditions leading to local temperature close to the critical value, a regular and controlled nanostructuring can be obtained with a period close to a hundred nanometers. Similar to monolithic a -CrC x coatings, these a -CrC x multilayer coatings exhibit a good barrier behavior against high temperature oxidation, according to the test performed. As future prospects, the main interest for these lamellar coatings could be their mechanical properties thanks to the numerous interfaces capable of deflecting and dissipating the propagation of cracks. • Chromium carbide coatings are deposited by DLI-MOCVD at very low temperatures. • A lamellar microstructure forms at temperatures as low as 325 °C. • Oxidation barrier behavior is fully conserved with the multilayer structure. • Changes in temperature and reaction mechanisms explain the lamellar growth.

Effects of thermal annealing on the structural and electrical properties of ZnO thin films for boosting their piezoelectric response

The growth process and post-deposition treatment have strong effects on the structure of ZnO thin films, which can also affect significantly their electrical and piezoelectric properties. In this report, the effect of high temperature annealing under oxygen atmosphere on ZnO stacked thin films grown on silicon by pulsed-liquid injection metal-organic chemical vapor deposition is investigated in detail. By varying the annealing temperature from 600 to 1000 °C, the structural analysis reveals the occurrence of c-axis oriented grain coarsening and growth processes associated with the improvement of crystallinity and removal of carbon impurities. The electrical resistivity also varies strongly with thermal annealing. Those variations are accompanied by an enhancement of the piezoelectric amplitude measured by piezo-response force microscopy (PFM). Interestingly, the PFM measurements reveal the predominant development of O- and Zn-polar domains at low and high temperature annealing, respectively, originating from the anisotropic grain boundary velocity. The present findings show that the high temperature annealing under oxygen atmosphere in ZnO thin films can enhance not only its piezoelectric amplitude but also its Zn-polarity uniformity, which unambiguously leads to the improvement of its piezoelectric performance.

Engineering Copper Adhesion on Poly-Epoxy Surfaces Allows One-Pot Metallization of Polymer Composite Telecommunication Waveguides

Mass gain in the aerospace sector is highly demandable for energy savings and operational efficiency. Replacement of metal parts by polymer composites meets this prerequisite, provided the targeted functional properties are recovered. In the present contribution, we propose two innovative and scalable processes for the metallization of the internal faces of carbon fiber reinforced polymer radiofrequency waveguides foreseen for implementation in telecommunications satellites. They involve sequential direct liquid injection metalorganic chemical vapor deposition of copper and cobalt. The use of ozone pretreatment of the polymer surface prior deposition, or of cost effective anhydrous dimethoxyethane as solvent for the injection of the copper precursor, yield strongly adherent, 5 µm Cu films on the polymer composite. Their electrical resistivity is in the 4.1–5.0 μΩ·cm range, and they sustain thermal cycling between −175 °C and +170 °C. Such homogeneous and conformal films can be obtained at temperatures as low as 115 °C. Demonstration is achieved on a polymer composite waveguide, composed of metallized 60-mm long straight sections and of E-plane and H-plane elbows, that paves the way towards the metallization of scale one devices.

Interface structure and strain relaxation in Nd0.96MnO3 epilayers grown on (001) SrTiO3 substrates

In this work we focus on the growth of highly oriented Nd0.96MnO3 (NMO) perovskite epilayers of different thickness on single-crystalline (001)SrTiO3 (STO) template, using an injection metal-organic chemical vapor deposition process. X-ray diffraction revealed that the epilayers have an orthorhombic Pnma structure and were purely ((1¯01) oriented parallel to the (001) plane of the substrates. The orientation relationships between the film and substrate are rather well defined in the vicinity of the interface as 1¯01NMO//[001]STO (out-of-plane), 101NMO//[100]STO and [010]NMO//[010]STO (in plane). It can be concluded that the film thickness significantly influences the strain state of the NMO epilayers deposited on STO. There was a contraction of out-of-plane layer network spacing leading to a progressive relaxation in the growth direction. The out-of-plane lattice parameter is lower than the bulk value. As the film thickness increases, the NMO epilayer strain reduces so that out-of-plane lattice parameters tend towards their bulk values. The calculated strain goes from −0.4%(thickness of 150 nm) to 0% (thickness of 600 nm). These epilayers are therefore strained at the interface and relax with the thickness. The out-of-plane lattice parameter observed for the 600 nm thick epilayer relaxed toward the bulk NMO. No traces of extra phases are detected. An atomic model of interfaces has been built using cross-sectional transmission electron microscopy image, as well as a crystallographic simulation software CrystalMaker.

Morphology Transition of ZnO from Thin Film to Nanowires on Silicon and its Correlated Enhanced Zinc Polarity Uniformity and Piezoelectric Responses.

ZnO thin films and nanostructures have received increasing interest in the field of piezoelectricity over the last decade, but their formation mechanisms on silicon when using pulsed-liquid injection metal-organic chemical vapor deposition (PLI-MOCVD) are still open to a large extent. Also, the effects of their morphology, dimensions, polarity, and electrical properties on their piezoelectric properties have not been completely decoupled yet. By only tuning the growth temperature from 400 to 750 °C while fixing the other growth conditions, the morphology transition of ZnO deposits on silicon from stacked thin films to nanowires through columnar thin films is shown. A detailed analysis of their formation mechanisms is further provided. The present transition is associated with strong enhancement of their crystallinity and growth texture along the c-axis together with a massive relaxation of the strain in nanowires. It is also related to a prevailed zinc polarity, for which its uniformity is strongly improved in nanowires. The nucleation of basal-plane stacking faults of I1-type in nanowires is also revealed and related to an emission line at about 3.326 eV in cathodoluminescence spectra, further exhibiting fairly low phonon coupling. Interestingly, the transition is additionally associated with a significant improvement of the piezoelectric amplitude, as determined by piezoresponse force microscopy measurements. The Zn-polar domains exhibit a larger piezoelectric amplitude than the O-polar domains, showing the importance of controlling the polarity in these deposits as a prerequisite to enhance the performances of piezoelectric devices. The present findings demonstrate the high potential in using the PLI-MOCVD system to form ZnO with different morphologies and polarity uniformity on silicon. They further reveal unambiguously the superiority of nanowires over thin films for piezoelectric devices.

Metalorganic chemical vapor deposition and investigation of nonstoichiometry of undoped BaSnO3 and La-doped BaSnO3 thin films

Wide bandgap (~3 eV) perovskite transparent conductive oxide material exhibiting superior electric properties is highly desired in today's optoelectronics. Just recently La-doped barium stannate (LBSO) bulk n-type semiconductor with outstanding carrier mobility (~320 cm2 V−1 s−1) at high carrier density (1020 cm−3) has been discovered. Highly conductive thin LBSO film synthesis has been challenging due to large lattice mismatch and nonstoichiometry as being the least investigated. Therefore, in this work, the Pulsed injection metalorganic chemical vapor deposition method was used for the growth of undoped BaSnO3 (BSO) and La-doped BaSnO3 thin films. Film depositions were carried out on monocrystalline LaAlO3, SrTiO3, Al2O3 substrates in wide stoichiometric range. Deviation from stoichiometric composition in BSO (Sn/Ba) and LBSO (Sn/(Ba + La)) had a significant impact on microstructure, optical, and electric properties while maintaining cubic symmetry. Near-stoichiometric films even grown at a high rate of ~10 nm/min showed sufficient electric properties.

Morphological, structural, optical, and electrical study of nanostructured thin films: Charge transport mechanism of p-type Co3O4

The normal cubic spinel Co3O4 () is the most stable crystalline cobalt oxide and is a promising material for many applications due to its high abundance, low cost, and low toxicity. Co3O4 is successfully used as magnetic material [1], as supercapacitor [2,3], as electrochromic windows [4], as well as corrosion protective coatings [5]. It is noteworthy that Co3O4 is a direct p-type semiconductor and is extensively studied in photovoltaics cells [6], in gas sensors [7], in anode materials in lithium-ion batteries [8] and in photocatalysis [9]. Low band gap, large surface exchange and high conductivity are essential properties for the achievement of efficient photocatalytic materials. A large variety of physical and chemical deposition methods are available to tailor the films microstructures and their inherent physical and electrochemical properties. In particular, various Co3O4 deposition methods have been reported as epitaxy [10,11], magnetron sputtering [12,13], pulsed laser deposition [6], thermal oxidative decomposition method [4], chemical spray pyrolysis [3,14], sol–gel processes like dip coating [15], metal organic chemical vapor deposition (MOCVD) [16], plasma enhanced MOCVD [9], pulsed liquid injection MOCVD [5], and atomic layer deposition (ALD) [17,18]. Moreover, the electrical properties are strongly related to the deposition method. MOCVD [16] and ALD [18,19] dense thin films show the lowest resistivity ranging from 0.5 to 100 Ω cm, while sol-gel [20], chemical precipitation [21], and spray pyrolysis [3,[22], [23], [24]] deposition methods show lower conductive films with resistivity of about two orders of magnitude higher. The main hypothesis to explain this significant difference is the morphology and the grain size. The conduction paths in the highest conductive Co3O4 thin films is discussed as grain boundaries [18] or through the grains by variable range hopping (VRH) conduction of holes as proposed by Cheng et al. [16]. However, to the best of our knowledge, the latter hypothesis is not demonstrated experimentally using nano-scale characterization. In this context, the main objective of this paper is to investigate the influence of the Co3O4 morphology on the electrical properties at macro and nanoscale.In the following, we study the effect of two substrate temperatures (Ts) of 400 °C and 500 °C on the morphology, on the optical and the electrical properties of ~140-nm-thick Co3O4 films deposited by thermally activated pulsed liquid-gas injection MOCVD.

Black Co oxides coatings for thermosensitive polymer surfaces by low-temperature DLI-MOCVD

Black coatings are deposited at low temperature in order to enable the functionalization of thermosensitive substrates, such as epoxy-based carbon fiber reinforced polymers (CFRP). The direct liquid injection metalorganic chemical vapor deposition of Co oxide films is performed with the dicobalt octacarbonyl precursor, Co2(CO)8, in the temperature range 50°C–160°C, on Si substrates, first. Films morphology can be described by a dense sublayer on which the typical “cauliflower” microstructure grows, with a large amount of voids and open porosity. We obtain nanocrystalline CoO in the deposition temperature range 50°C–125°C, and nanocrystalline (CoO+Co3O4) above 125°C. The bulk composition of the films is Co(45)O(45)C(10). Over the deposition temperatures tested, films processed at 125°C repetitively show the lowest reflectivity in the visible range. An important role in the optical reflectivity is attributed to the carbon content, although it is not possible to decorrelate microstructural changes from the carbon elimination in calcination experiments. Finally, we reproduce the above-mentioned results with success on CFRP substrates, and demonstrate the applicability of the process on thermosensitive composite parts with results comparable to the state-of-the-art in the visible range.

Magnetoresistance Relaxation Anisotropy of Nanostructured La-Sr(Ca)-Mn-O Films Induced by High-Pulsed Magnetic Fields

The results of investigation of colossal magnetoresistance relaxation in nanostructured La-Sr(Ca)-Mn-O films upon removal of magnetic field pulse are presented. Thin films having thicknesses of 75-350 nm grown by pulsed injection metal-organic chemical vapor deposition technique were studied in pulsed magnetic fields having duration of 0.9 ms and amplitudes of 2-12 T. It was obtained that “fast” relaxation process occurring in hundred microseconds time scale exhibits anisotropy: the magnitude of remnant resistivity is approximately three times smaller and the process is faster when magnetic field is applied perpendicular to the film plane in comparison with in-plane direction. The dynamics of this relaxation process was analyzed using Kolmogorov-Avrami-Fatuzzo model, taking into account nucleation and reorientation of magnetic domains into equilibrium state. The “fast” remnant relaxation was not observed after the application of longer pulses (20 ms) having amplitude of 60 T. Influence of remnant relaxation on operation of B-scalar magnetic field sensors based on nanostructured manganite films is discussed.

Up-conversion luminescence in Er/Yb-doped YF3 thin films deposited by PLI-MOCVD

We have studied the NIR (near infrared) to visible up-conversion of Er/Yb co-doped YF3 thin films prepared by PLI-MOCVD (pulsed liquid injection metal-organic chemical vapor deposition). Violet, strong green, as well as red emission bands corresponding to intra-4f transitions in Er3+ were observed under 972nm excitation. The up-converted emission was studied as a function of the Yb concentration and in doing so, a change in the red/green emission intensity ratio was observed. We attributed this, on one hand, to an Er-to-Yb back transfer corresponding to 4S3/2 → 4I13/2:2F7/2 → 2F5/2, which starts to occur at increasing Yb concentrations. As a consequence, the red emission is shown to be enhanced through an upward energy transfer 4I13/2 → 4F9/2. On the other hand the depopulation of 4S3/2 is also related to an upward transfer from Yb to Er corresponding to transition 2F5/2 → 2F7/2 :4S3/2 → 4G9/2 which is enhanced by an increasing Yb concentration. We performed power-dependent up-conversion luminescence measurements to confirm the proposed excitation mechanisms. Finally, lifetimes of the intermediate states 4S3/2 and 4F9/2 were determined, to show the influence of the Yb concentration on the relaxation processes in Er3+.

Particular structural defects in Ta2O5 from crystallisation of amorphous thin films in O2–H2O atmosphere

Structural defects within Ta2O5 monoclinic structures were obtained by crystallisation at 850°C in atmosphere O2–H2O of amorphous thin films deposited by injection metal-organic chemical vapour deposition (i-MOCVD) on Si substrates. These defects, observed by high resolution transmission electron microscopy (HRTEM), contain alignments of Ta5+ vacancies along a crystallographic direction [001]. H2O gas of different partial pressures was obtained by bubbling O2 gas through deionised water or Na2CO3–K2CO3 aqueous solution maintained at different temperatures in a range of 25–45°C. The amount of structural defects increases dramatically with H2O partial pressure and annealing time from HRTEM observations and X-ray diffraction results. Such defects are thermally stable, at least up to 850°C. Using Na2CO3–K2CO3 aqueous solution, similar structural defects including mainly K+ species were formed. Na+ species contributes mostly to the formation of another phase of composition close to (Na0.8K0.2)2Ta4O11 of natrotantite type structure. Assuming that Ta5+ vacancies are occupied by H+ in a form of bridging hydroxyl groups Ta–OH–Ta in order to satisfy electrical neutrality, the presence of significant levels of hydrogen atoms within Ta2O5 concordant with H enrichment at defects was identified by time-of-flight secondary ion mass spectrometry (ToF-SIMS).

Modeling a MOCVD process to apply alumina films on the inner surface of bottles

Deposition inside a cavity with a narrow neck, i.e. a hollow body, is a challenge, in particular for the supply and the evacuation of the reactive phase and of the effluents through the same entrance. High gradients (pressure, velocity, species concentrations) and important convective mechanisms related to the impacting jet of the gaseous phase on the bottom inner wall of the cavity are created in that region. Given the increasing number of experimental parameters, modeling helps efficient process optimization with better understanding of physical and chemical phenomena occurring inside the hollow body. This study presents a numerical modeling using the CFD code Fluent to simulate a direct liquid injection metalorganic chemical vapor deposition of amorphous alumina coatings from aluminum tri-isopropoxide (ATI) on the inner walls of a glass bottle. The model is used to predict local profiles of gas velocity, temperature and species concentrations into the reactor as well as local growth rate based on an apparent heterogeneous kinetic law of ATI decomposition. Comparison with experimental thickness profiles allows validating the model and leads to a better insight in the phenomena which impact film thickness inhomogeneity along the inner surface. The model contributes to the improvement of the reactor configuration by increasing the distance between nozzle and bottle and the outlet section of the nozzle.

Growth and structural characterization of YMnO3 thin films grown by pulsed liquid injection MOCVD on Si and SrTiO3 substrates

Optimum parameters for the growing of YMnO3 films by pulsed liquid injection metalorganic chemical vapor deposition have been studied. Si substrates were used for the optimization of the deposition process. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that polycrystalline single phase YMnO3 films can be obtained for an optimal ratio of Y and Mn on the injected solution and either amorphous, metastable orthorhombic, and/or hexagonal YMnO3 phases can be obtained depending on the deposition temperature and precursors ratio. In a second stage, YMnO3 films were grown on SrTiO3 substrates. Pure epitaxial orthorhombic YMnO3 phase was confirmed by XRD. The films microstructure, characterized by scanning electron microscopy and TEM, shows a columnar growth. Each columnar grain grows epitaxially with three possible orientations.

Metal-organic chemical vapor deposition of high-k dielectric Ce–Al–O layers from various metal-organic precursors for metal–insulator–metal capacitor applications

The possibilities to grow thin layers of high-k dielectric CeAlO3 by pulsed injection metal-organic chemical vapor deposition using different metal-organic (MO) precursors have been investigated. Three pairs of MO precursors were studied for the growth of the films: Ce (IV) and Al(III) 2,2,6,6-tetramethylheptane-3,5-dionates, Ce tetrakis(1-methoxy-2-methyl-2-propoxide)-diethylaluminumethoxide and tris(isopropylcyclopentadienyl)cerium-tris(diethylamino)aluminum. Under optimized conditions, all three pairs of investigated precursors enabled the growth of close to stoichiometric Ce–Al–O films at reasonably low temperatures, 400–450°C, however, crystalline CeAlO3 phase was not present in as-deposited layers. Films were grown on Si(100) and Si(100)/TiN substrates. Two kinds of TiN electrodes were used — amorphous TiN (15–30nm thick) and crystalline TiN (70–100nm thick) layers, grown by chemical vapor deposition and physical vapor deposition techniques, respectively. The pure tetragonal CeAlO3 phase was crystallized in films by a short annealing in Ar or N2 at 800–850°C. Required annealing conditions (temperature and annealing duration) depended on the selected precursors and substrates. Thermomechanical degradation of Si/TiN/Ce–Al–O structures was observed by Scaning Electron Microscopy after the annealing of the samples. Lower degradation degree was observed for structures with a thin amorphous TiN layer.

Structural and magnetic properties of thin films of BaFeO3-δ deposited by pulsed injection metal-organic chemical vapor deposition

Thin films of BaFeO3-δ have been deposited by pulsed injection metal-organic chemical vapor deposition. The films adopt a pseudocubic structure as a result of the match with the cubic structure of the SrTiO3 substrate and the accommodation of a high density of oxygen vacancies. They display antiferromagnetic behavior at room temperature and high electrical resistance. Furthermore, we observed an ageing effect that may be recovered to some extent by annealing in oxygen atmosphere.

Thermal Stability of Neodymium Aluminates High‐κ Dielectric Deposited by Liquid Injection MOCVD Using Single‐Source Heterometallic Alkoxide Precursors

Thin films of neodymium aluminate (NdAlOx) have been deposited by liquid injection metalorganic chemical vapor deposition (MOCVD) using the bimetallic alkoxide precursor [NdAl(OPri)6(PriOH)]2. The effects of high‐temperature postdeposition annealing on NdAlOx thin films are reported. The as‐deposited thin films are amorphous in nature. X‐ray diffraction (XRD) and medium energy ion scattering (MEIS) show, respectively, no crystallization or interdiffusion of metal ions into the substrate after annealing at 950°C. The capacitance‐voltage (C-V) and current‐voltage (I-V) characteristics of the thin films exhibited good electrical integrity following annealing. The dielectric permittivity (κ) of the annealed NdAlOx was 12, and a density of interface states at flatband (Dit) of 4.01 × 1011 cm−2 eV−1 was measured. The deposited NdAlOx thin films are shown to be able to endure high‐temperature stress and capable of maintaining excellent dielectric properties.