Structural and Electrical Characteristics of ZnO Thin Films on Polycrystalline AlN Substrates

Fostered by the increasing use of AlN in highly thermally conductive electronic packages, it is attempted to grow ZnO thin films on polycrystalline AlN in the present work. Effects of the polycrystalline substrate on the structural and electrical properties of the ZnO films are highlighted. The ZnO films are synthesized by low-cost spin-coating technique from a precursor solution containing zinc acetate, 2-methoxyethanol, and monoethanolamine. Structural analysis was performed using X-ray diffraction, which showed all the films to be polycrystalline of hexagonal phase with no preferred orientation, in sharp contrast to numerous reports of strong c -axis orientation on glass substrates. The films, which were heat-treated at 700°C followed by firing in nitrogen containing 5% hydrogen at 500°C, exhibited better crystallinity and dense microstructure among all the samples and had the lowest resistivity (1.36 × 10⁻¹Ω·cm). The minimum resistivity obtained in the present work is comparable to the values reported for the films grown using similar technique, thus indicating the negligible dependence of electrical properties on orientation of the films.     

Effects of deposition temperature on the crystallinity of Ba0.5Sr0.5TiO3 epitaxial thin films integrated on Si substrates by pulsed laser deposition method

Epitaxial Ba_0.5Sr_0.5TiO₃ (BSTO) thin films were grown on TiN buffered Si (0 0 1) substrates by PLD method and the effects of deposition temperature on their crystallinity and microstructure were studied. BSTO thin films were prepared with substrate temperature ranging from 350 to 650 °C. The BSTO films grown at below 400 °C showed amorphous phase and the film grown at 450 °C showed mixed phase of crystalline and amorphous, where crystalline phase was observed only at the top surface portion of the film. The BSTO films with fully crystalline phase were obtained in the samples deposited at above 500 °C. The (0 0 l) preferred orientation and the crystallinity of the BSTO films were improved with increasing the temperature. The dielectric constant, measured at 100 kHz and at room temperature, of the BSTO film grown at 650 °C was measured to be as high as 1129.     

Rapid Formation of Zinc Oxide Films by an Atmospheric-Pressure Chemical Vapor Deposition Method

Zinc oxide films were prepared by an atmospheric-pressure chemical vapor deposition method using (acetylacetonato)-zinc as a source material. Transparent and uniform ZnO films of considerable area (20 × 70 mm, ∼0.3 μm thick) could be obtained easily on a crown glass (CGW #200) with a high deposition rate. The deposition rate first remained constant with increasing substrate temperature ( T _s), then increased abruptly from 120 nm/min at T _s= 550°C to 220 nm/min at T _s= 600°C, and finally stopped increasing above T _s= 600°C. The maximum preferred orientation and best crystallinity of the films were obtained at T _s= 550°C.     

(100)-Orientation of Pseudocubic Perovskite-Type LaNiO3 Thin Films on Glass Substrates via the Sol–Gel Process

(100)-oriented, perovskite-type LaNiO₃ thin films were prepared on SiO₂ glass substrates by the sol–gel method. Effects of thermal processing on the orientation, surface morphology, and electrical properties of the films were investigated. The nearly complete (100)-orientation was achieved by drying the films at temperatures above 350°C before a final heat treatment at 700°C. It was the key to heat the substrate from the other side to obtain the oriented films. A possible mechanism of the orientation is proposed on the basis of surface energies of the films and the substrates and the interfacial energy between them.     

The Effect of Deposition Temperature and Postanneal on the Bi3.63Pr0.3Ti3O12 thin Films Prepared by RF-Magnetron Sputtering Method

Praseodymium doped Bi₄Ti₃O₁₂ (BTO) thin films with composition Bi_3.63Pr_0.3Ti₃O₁₂ (BPT) were successfully prepared on Pt/Ti/SiO₂/Si substrates by RF-magnetron sputtering method at substrate temperatures ranging between 500° and 750°C. The structural phase and orientation of the deposited films were investigated in order to understand the effect of the deposition temperature on the properties of the BPT films. As the substrate temperature was increased to 700°C, the films started showing a tendency of assuming a c -axis preferred orientation. At lower temperatures, however, polycrystalline films were formed. The Pt/BPT/Pt capacitor showed an interesting dependence of the remnant polarization (2 P _r) as well as dc leakage current values on the growth temperature. The film deposited at 650°C showed the largest 2 P _r of 29.6 μC/cm². With the increase of deposited temperature, the leakage current densities of films decreased at the same applied field and the film deposited at 750°C exhibited the best leakage current characteristics. In addition, the ferroelectric fatigue and Raman measurements were carried out on the as-prepared, postannealed in air and postannealed in oxygen BPT films. It was revealed that the BPT film postannealed in air exhibited the weakest fatigue-resistance characteristics and highest frequency shifted Raman vibration modes, indicating the highest oxygen vacancy concentration in this film.     

Sol-Gel Preparation of Transparent and Conductive Aluminum-Doped Zinc Oxide Films with Highly Preferential Crystal Orientation

Transparent aluminum-doped zinc oxide (ZnO) films were prepared via the sol-gel method on silica-glass substrates from 2-methoxyethanol solutions of zinc acetate and aluminum chloride that contained monoethanolamine. Dip coating was conducted at room temperature, with substrate withdrawal rates of 1.2-7.0 cm/min. After each deposition, the films were heat-treated in air at 200°-450°C for 10 min (pre-heat-treatment). After six to fourteen layers had been deposited, the films were then subjected to annealing in air at 500°-800°C for 1 h (the first post-heat-treatment), followed by annealing in nitrogen at 500°-700°C for 15 min to 4 h (the second post-heat-treatment). All the films obtained were transparent and showed only an extremely sharp ZnO (002) peak in the X-ray diffractometry (XRD) patterns. The effects of the aluminum content, the substrate withdrawal speed, and the heat-treatment conditions on the electrical resistivity of the films were studied. All these factors strongly affected the resistivity. The lowest resistivity value (6.5 10^-3 Omegacm) was achieved in a film that contained 0.5 at.% aluminum, prepared with a low substrate withdrawal speed (1.2 cm/min), and a pre-heat-treatment of individual layer at 400°C in air and a post-heat-treatment of the entire film at 600°C in air, followed by a post-heat-treatment at 600°C in nitrogen. These preparation parameters also affected the degree of crystal orientation, which was revealed by the intensity of the ZnO (002) XRD peak. Higher crystal orientation was effective in reducing the film resistivity, whereas the higher grain-packing density and possible aluminum segregation were thought to have positive and negative effects, respectively, in reducing the resistivity.     

Effect of Substrate Texture on Orientations of rf-Sputtered Ba2Si2TiO8 Thin Films

Radio-frequency-sputtered barium silicon titanate, Ba₂Si₂TiO₈ (BST), thin films were grown on various substrates at substrate temperatures ranging from 750° to 955°C. The asdeposited films were characterized using X-ray diffraction, optical microscopy, and scanning electron microscopy (SEM). The results of the morphology analysis and X-ray diffraction analysis show no crystalline structure for films deposited on the fused quartz and unetched Si (100) substrates at temperatures lower than 865°C. At a substrate temperature of 900°C, tiny tetragon-like grains were observed for the film grown on the fused quartz, and the grains grew at a rate of 0.18 μm/min in the initial deposition stage. Optical and SEM micrographs reveal (001)-oriented, tetragon-like grains grown on HF-etched Si (100) wafers in the initial deposition stage and in later stages at substrate temperatures from 822° to 865°C. The results of X-ray diffraction analysis show a high (001) orientation for the thicker films, which corresponds to the tetragon-like configuration of the grains in the initial stage. Triangle-like grains were observed on the films deposited on etched Si (111) substrates in a temperature range from 822° to 899°C, but the grain configuration changed from the initial triangle shape to a tetragon shape as the films grew thick. The X-ray diffraction spectra show a high (001) orientation for the thicker films, which indicates an orientation transformation during the deposition due to a very low interfacial energy between the (001)-oriented BST overgrowth and the BST substrates in the later stage of deposition. Both the tetragon-like and the triangle-like grains have quasi-two-dimensional polygon shapes and have nonliquid-like characteristics in coalescence, which suggests a strong interaction between the BST deposits and the Si substrates.     

Orientation Control Growth of Lanthanum Nickelate Thin Films Using Chemical Solution Deposition

Highly (100)- and (110)-oriented LaNiO₃ (LNO) thin films were successfully prepared on a Si (100) substrate using the chemical solution deposition method. It was somewhat surprising to find that the orientation of LNO films depended on the heating rates of the temperature range of 200°–400°C. The samples with heating rates beyond 10°C/s showed the preferential (100) orientation, while those with heating rates below 6.67°C/s showed the preferential (110) orientation. The orientation mechanism is controlled by the thermodynamics of nucleation and crystal growth. LNO films with controlled orientation having low resistivities of 2 mΩ·cm are a good basis for integrating ferroelectric applications.     

Preparation of Ba2YCu3O7-δ Thin Films with Preferred Orientation through an Organometallic Route

Superconducting Ba₂YCu₃O_7-δ thin films were prepared through an organometallic route. Single-phase Ba₂YCu₃O_7-δ thin films with preferred orientation were successfully prepared on SrTiO₃ (100) single-crystal substrates at 800°C by a dip coating method using partially hydrolyzed Ba-Y-Cu organometallic solutions. Preferentially oriented Ba₂YCu₃-O_7-δ thin films were also prepared on MgO (100) substrates. By controlling the partial hydrolysis conditions, a coating solution for precursor thin films was kept accurately at the stoichiometric composition. The use of ozone gas during the pyrolysis of the precursor thin films was found to suppress the formation of BaCO₃. Ba₂YCu₃O_7-δ thin films with c -axis orientation perpendicular to a SrTiO₃ (100) substrate, which were heat-treated at 900°C for 15 min, exhibited a superconductivity transition with an onset of 90 K and an end of 75 K.     

Sol-Gel Route to Ferroelectric Layer-Structured Perovskite SrBi2Ta2O9 and SrBi2Nb2O9 Thin Films

Precursors for layer-structured perovskite thin films of SrBi₂Ta₂O₉ (SBT) and SrBi₂Nb₂O₉ (SBN) were prepared by the reactions of a strontium-bismuth double methoxyethoxide and tantalum or niobium methoxyethoxide in methoxyethanol, followed by partial hydrolysis. Several spectroscopic techniques, such as ¹H-, ¹³C-, and ⁹³Nb-NMR (nuclear magnetic resonance), and Fourier-transform infrared spectroscopy were used to analyze the arrangement of the metals and oxygen in the precursor molecules. The precursors contained Sr-O-M (where M is Ta or Nb) bonds (i.e., a strontium is connected to two MO₆ octahedra) and Sr-O-Bi bonds with a bismuth atom bonded to the oxygens of the MO₆ octahedron. The arrangement of metals and oxygens was considered to be similar to the layer-structured perovskite crystal sublattice. As a result, the sol-gel-derived SBT thin films crystallized, by rapid thermal annealing in an oxygen atmosphere below 550°C, and they exhibited preferred (115) orientation. The crystallinity improved and the crystallite size increased with temperature up to 700°C. In the case of SBN thin films, a low heating rate (2°C/min) was necessary for the control of the crystallographic (115) orientation, whereas a rate of 200°C/s (rapid thermal annealing) produced films that exhibited c -axis orientation. The (115) SBT thin film, heated to 700°C, exhibited improved ferroelectric properties.     

Preparation of Highly Oriented Aluminum Nitride Thin Films on Polycrystalline Substrates by Helicon Plasma Sputtering and Annealing

Highly c -axis-oriented aluminum nitride (AlN) thin films were prepared on polycrystalline Si₃N₄ and SiC substrates by helicon plasma sputtering. The difference in the substrate materials scarcely influenced the crystal structures of the films. The full width at half-maximum (FWHM) of the X-ray rocking curves of the films was 3.2°, which is the smallest value for AlN thin films deposited on polycrystalline substrates to our knowledge. Annealing at 800°C in vacuum decreased the FWHM from 3.2° to 2.8°. The structure of the films was densely packed and consisted of many fibrous grains. The films developed c -axis orientation on the polycrystalline substrates, because the interaction between the films and substrate surfaces was small, and (100) and (110) AlN planes grew preferentially. The films were piezoelectric and generated electrical signals in response to mechanical stress.     

Preparation of β-BaB2O4 Powders and Thin Films by Sol–Gel Method

β-BaB₂O₄ (β-BBO) powders and films were successfully synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of barium metal with boron triethoxide in ethanol by addition of diethanolamine. The drip-coated precursor films began to crystallize to β-BBO on Pt substrates at 500°C and converted to β-BBO films with preferred orientation to the c -axis at 700°C.     

Deposition of Pb(Zr,Ti)O3 Thin Films by Metal-Organic Chemical Vapor Deposition Using β-diketonate Precursors at Low Temperatures

Lead zirconate titanate (PZT) thin films were deposited by metal-organic chemical vapor deposition (MOCVD) using β-diketonate precursors and 0₂ at temperatures below 500°C on variously passivated Si substrates. PZT thin films could not be deposited on bare Si substrates, owing to a serious diffusion of Pb into the Si substrate during deposition. Pt/SiO₂/Si substrates could partially block the diffusion of Pb, but a direct deposition of PZT thin films on the Pt/SiO₂/Si substrates resulted in a very inhomogeneous deposition. A TiO₂ buffer layer deposited on Pt/SiO₂/Si substrates could partially suppress the diffusion of Pb and produce homogeneous thin films. However, the crystallinity of PZT thin films deposited on the TiO₂-buffered Pt/SiO₂/Si substrate was not good enough, and the films showed random growth direction. PZT thin films deposited on the PbTiO₃-buffered Pt/SiO₂/Si substrates had good crystallinity and a- and c-axis oriented growth direction. However, the PZT thin film deposited at 350°C showed fine amorphous phases at the grain boundaries, owing to the low chemical reactivities of the constituent elements at that temperature, but they could be crystallized by rapid thermal anneaiing (RTA) at 700°C. PZT thin film deposited on a 1000-å PbTiO₃,-thin-film-buffered Pt/SiO₂/Si substrate at 350°C and rapid thermally annealed at 700°C for 6 min showed a single-phase perovskite structure with a composition near the morphotropic boundary composition.     

Sol–Gel Route to Porous Lanthanum Cobaltite (LaCoO3) Thin Films

Sol–gel-derived LaCoO₃ thin films were deposited on yttria-stabilized zirconia (YSZ) substrates from a lanthanum isopropoxide–cobalt acetate (with 2-methoxyethanol) precursor solution. A chelating agent (2-ethylacetoacetate) and polyethylene glycol (PEG) were used to modify the above-mentioned precursor solution. The La-Co precursor solution was sufficiently viscous, and transparent LaCoO₃ gel films were prepared successfully using a spin-coating technique. Crystallization behavior and microstructure evolution were investigated using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). A single-phase perovskite thin film with the grain size of ∼50 nm was obtained by heat-treating the spin-coated gel film at a temperature of 600°C. SEM observations revealed that the microstructure of LaCoO₃ thin films that were prepared from the precursor solution with PEG was porous, and the LaCoO₃ thin film maintained its porous microstructure to a temperature of 800°C.     

Oriented Lead Titanate Film Growth at Lower Temperatures by the Sol-Gel Method on Particle-Seeded Substrates

An approach to fabricate lead titanate (PbTiO₃) films with preferred orientation on arbitrary substrates by a sol-gel method was developed. To ensure a preferred crystallographic orientation on the substrates with different crystal structure, well-defined platelet PbTiO₃ particles were used as seeds. Because the basal plane of the platelet-shaped particles was the (001) plane, the particles aligned with the c-axis perpendicular to the substrates. During crystallization of the sol-gel PbTiO₃ films on the particle-seeded substrate, preferential nucleation occurred on seeded particles with the same crystallographic orientation as the particles at a much lower temperature. In the current study, (100) and (001) textured PbTiO₃ films have been produced on various substrate materials such as silicon, silicate glass, indium tin oxide (ITO) glass, and titanium metal at temperatures as low as 275°C. The microstructure of the films was examined by scanning electron microscopy and atomic force microscopy. Limited ferroelectric properties also were determined, to underscore the preferred orientation that was produced in these materials.     

Synthesis of Lead Barium Niobate Powders and Thin Films by the Sol-Gel Method

Crack-free, dense, and transparent Pb_0.6Ba_0.4Nb₂O₆ (PBN60) thin films have been prepared by a sol-gel method with metal alkoxides and metal acetate. A homogeneous and stable precursor solution was obtained from Ba metal, Pb(CH₃COO)₂, and Nb(OEt)₅ in 2-methoxyethanol. PBN60 powder crystallized to the hexagonal phase at 600°C and then completely transformed to the orthorhombic phase of the tungsten bronze structure at 1250°C. The hexagonal phase was formed on SiO₂ glass, MgO(lOO), and sapphire(R) substrate at 600°C, while the orthorhombic phase was only on a sapphire(C) substrate. Orthorhombic PBN60 films with c -axis preferred orientation were successfully synthesized on sapphire(C) substrates at 600°C.     

Formation of High-Quality, Epitaxial La2Zr2O7 Layers on Biaxially Textured Substrates by Slot-Die Coating of Chemical Solution Precursors

Crystallization studies were performed of epitaxial La₂Zr₂O₇ (LZO) films on biaxially textured Ni–3at.%W substrates having thin Y₂O₃ (10 nm) seed layers. LZO films were deposited under controlled humid atmosphere using reel-to-reel slot-die coating of chemical solution precursors. Controlled crystallization under various processing conditions has revealed a broad phase space for obtaining high-quality, epitaxial LZO films without microcracks, with no degradation of crystallographic texture and with high surface crystallinity. Crack-free and strong c -axis aligned LZO films with no random orientation were obtained even at relatively low annealing temperatures of 850°–950°C in flowing one atmosphere gas mixtures of Ar–4% H₂ with an effective oxygen partial pressure of P(O₂)∼10⁻²² atm. Texture and reflection high-energy electron diffraction analyses reveal that low-temperature-annealed samples have strong cube-on-cube epitaxy and high surface crystallinity, comparable to those of LZO film annealed at high temperature of 1100°C. In addition, these samples have a smoother surface morphology than films annealed at higher temperatures. Ni diffusion rate into the LZO buffer film is also expected to be significantly reduced at the lower annealing temperatures.     

IR Absorption and Raman Spectra of As-Deposited and Annealed Hydrogenated Amorphous Carbon Films by Plasma Decomposition of Toluene

Hydrogenated amorphous carbon (a-C:H) films, prepared by the rf plasma decomposition of toluene, have been deposited at substrate temperatures of 35° to 60°C. Sharp C─H stretch absorption bands occur around 3000 cm⁻¹, which suggest the microstructure of the films. A weak band at 3304 cm⁻¹, which gives evidence for C─C triple bonds in a-C:H films, diminishes in intensity at increasing annealing temperatures and disappears after annealing at 200°C.     

Alumina Thin Films as Optical Waveguides

Amorphous AI₂O₃, fabricated by reactive rf magnetron sputtering, has been evaluated as a planar waveguide material. The microstructure and optical properties of planar waveguides were examined as a function of deposition (substrate temperature, O₂ flow rate) and annealing conditions. X-ray and electron diffraction verified that as-deposited films were amorphous for substrate temperatures up to 500°C and for a wide range of O₂ flow rates. This amorphous phase was stable through anneals up to 800°C, but crystallized to " y -Al₂O₃ at 1000°C and to α-Al₂O₃ at 1200°C. The amorphous films had transmission windows that extended from 200 nm to 7 (Am with an average refractive index of 1.65 and reproducible losses as low as ∼1 dB/cm at 632.8 nm. The refractive index increased with substrate temperature, but was independent of O₂ flow rate. The losses decreased with substrate temperature and increased as a function of O₂ flow rate. As a final check on the amorphous structure, Cr-doped films were prepared by codeposition. Fluoresence was detected only in annealed crystalline films.     

Nanocrystalline ZnO Thin Film Synthesis Using Glycerol in Aqueous Polymeric Precursor Processing

Synthesis of high-quality ZnO thin films via simple and cost effective processing technique is a major challenge. In this work, the preparation of nanocrystalline ZnO thin films by a novel polymeric precursor processing using glycerol as chelating agent is presented. The process has advantages of being cost-effective and environment friendly. ZnO thin films were prepared by a single spin-coating deposition of aqueous polymeric precursor prepared with zinc nitrate [Zn(NO₃)₂] and glycerol as chelating agent. The thermal decomposition of polymeric precursors was studied by thermogravimetric analysis and Fourier transform infrared (FTIR) spectroscopy. Annealing of these films were performed over the range of 300°–600°C, and the effect of annealing on the degree of crystallization, surface morphology, crystallite size, and optical properties was investigated. X-ray diffraction analysis shows that the thin films are polycrystalline with wurtzite structure. The thin films are 80% dense, have crack free microstructure, and transparency of >85% in the visible region. These films exhibit absorption edge at 375 nm. On measuring at room temperature, the optical band gap energy of ZnO thin films, annealed at 450° and 600°C, was determined to be 3.295 and 3.267 eV. Room temperature photoluminescence spectra of these films show strong UV emission and a broad yellow-green emission in the range 525–600 nm. The intensity of UV emission peak increases with increase in annealing temperature that is attributed to an improvement in crystallinity.