Molecular layer deposition of ZrO2-based organic-inorganic nanohybrid thin films for organic thin film transistors

Molecular layer deposition (MLD) technique can be used for preparation of various organic-inorganic nanohybrid superlattices at a gas-phase. The MLD method is a self-controlled layer-by-layer growth process under vacuum conditions, and is perfectly compatible with the atomic layer deposition (ALD) method. In this paper, we fabricated a new type organic-inorganic nanohybrid thin film using MLD method combined with ALD. A self-assembled organic layer (SAOL) was formed at 170 °C using MLD with repeated sequential adsorptions of CC terminated alkylsilane and zirconium hydroxyl with ozone activation. A ZrO₂ inorganic nanolayer was deposited at the same temperature using ALD with alternating surface-saturating reactions of Zr(OC(CH₃)₃)₄ and H₂O. The prepared SAOL-ZrO₂ organic-inorganic nanohybrid films exhibited good mechanical stability, excellent insulating properties, and relatively high dielectric constant k (~ 16). They were then used as a 23 nm-thick dielectric for low voltage pentacene-based thin film transistors, which showed a maximum field effect mobility of 0.63 cm²/V s, operating at − 1 V with an on/off current ratio of ~ 10³.     

原子层淀积制备金属氧化物薄膜研究进展

原子层淀积(ALD)技术作为一种先进的薄膜制备方法近年来越来越得到重视,它能精确地控制薄膜的厚度和组分,实现原子层级的生长,生长的薄膜具有很好的均匀性和保形性,因而在微电子和光电子等领域有广泛的应用前景。本文综述了ALD技术的基本原理,及其在金属氧化物薄膜制备上的研究进展。     

Atomic layer deposition and characterization of biocompatible hydroxyapatite thin films

Atomic layer deposition (ALD) was used to produce hydroxyapatite from Ca(thd)₂ (thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) and (CH₃O)₃PO onto Si(100) and Corning (0211). Film crystallinity, stoichiometry, possible impurities and surface morphology were determined. The as-deposited films contained significant amounts of carbonate impurities however, annealing at moist N₂ flow reduced the carbonate content even at 400 °C. The as-deposited Ca-P-O films were amorphous but rapid thermal annealing promoted the formation of the hydroxyapatite phase. Mouse MC 3T3-E1 cells were used for the cell culture experiments. According to the bioactivity studies cell proliferation was enhanced on as-deposited ALD-grown Ca-P-O films and greatly enhanced on films annealed at 500 °C in comparison with reference cells on borosilicate glass or cell culture polystyrene.     

Characterizations of NbAlO thin films grown by atomic layer deposition

Niobium-aluminate (NbAlO) thin films have been prepared on silicon (100) with different Nb₂O₅:Al₂O₃ growth cycle ratio by atomic layer deposition (ALD) technology. The structural, chemical and optical properties of NbAlO thin films are investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The results show that all the obtained NbAlO films are amorphous and fully oxidized. It is also found that the proportion of components in the NbAlO film can be well-controlled by varying the ALD growth cycles of the independent oxides. Furthermore, the refraction index of the prepared films is observed to increase with an increase of the concentration of Nb in the mixtures.     

Atomic layer deposition of thin films of ZnSe—structural and optical characterization

Thin films of sphalerite-type ZnSe were grown by atomic layer deposition (ALD) from elemental Zn and Se precursors. These films, grown on various substrates, show bright blue ‘edge’ emission accompanied by donor-acceptor pair emissions in the blue, green and red spectral regions. Red, green and blue emissions mixed together give a white color, with a color temperature between 2400 and 4500 K depending on a layer thickness and temperature. ZnSe grown by ALD is in consequence a promising material for the fabrication of semiconductor-based white light emitting thin film electroluminescence displays.     

Atomic layer deposition of Ru films from bis(2,5-dimethylpyrrolyl)ruthenium and oxygen

Ru thin films were grown on hydrogen terminated Si, SiO₂, Al₂O₃, HfO₂, and TiO₂ surfaces by atomic layer deposition from bis(2,5-dimethylpyrrolyl)ruthenium precursor and oxygen. The 4-20 nm thick films on these surfaces consisted of nanocrystalline hexagonal metallic ruthenium, regardless of the deposition temperature. At the lowest temperatures examined, 250-255 °C, the growth of the Ru films was favored on silicon, compared to the growth on Al₂O₃, TiO₂ and HfO₂. At higher temperatures the nucleation and growth of Ru became enhanced in particular on HfO₂, compared to the process on silicon. At 320-325 °C, no growth occurred on Si-H and SiO₂-covered silicon. Resistivity values down to 18 μΩ·cm were obtained for ca. 10 nm thick Ru films.     

Electrical resistivity of Ti-Zn mixed oxide thin films deposited by atomic layer deposition

Ti-Zn mixed oxide thin films, with thickness less than 50 nm, were grown with atomic layer deposition (ALD) technique at low temperature (90 °C) varying the composition. ALD is a powerful chemical technique to deposit thin films with thickness of few atomic layers. ALD oxide material growth is achieved by dosing sequentially the metal precursor and the oxidizing agent. Thanks to ALD nature of layer by layer growth it was possible to realize mixed metal, Ti and Zn, oxide thin films with controlled composition, simply by changing the number of cycles of each metal oxide layer. Structural and electrical properties of the prepared thin films were studied as a function of their composition. Synchrotron radiation X-ray diffraction technique was used to follow thin film crystallization during sample annealing, performed in situ. It was observed that the onset temperature of crystallization raises with Ti content, and sample structure was Zn₂TiO4 phase. Electrical resistivity measurements were performed on crystalline samples, annealed at 600 °C, revealing an increase in resistivity with Ti content.     

Low temperature atomic layer deposition of titania thin films

This paper presents a comprehensive study of atomic layer deposition of TiO₂ films on silicon and polycarbonate substrates using TiCl₄ and H₂O as precursors at temperatures in the range 80-120 °C. An in-situ quartz crystal microbalance was used to monitor different processing conditions and the resultant films were characterised ex-situ using a suite of surface analytical tools. In addition, the contact angle and wettability of as-deposited and UV irradiated films were assessed. The latter was found to reduce the contact angle from ≥ 80° to < 10°. Finally, the effect of surface pre-treatment on film toughness and adhesion was investigated and the results show a significant improvement for the pre-treated films.     

The uniformity of Al distribution in aluminum-doped zinc oxide films grown by atomic layer deposition

We investigated the aluminum distribution in aluminum-doped zinc oxide films grown by atomic layer deposition. Surface morphology, structure, composition and electrical properties of obtained films were studied. For the aluminum content less than 2 at.%, a periodicity of Al distribution along the layer depth was observed. This periodicity diminished significantly after annealing the samples in nitrogen atmosphere at 300 °C. For the Al content higher than 2 at.%, its distribution in ZnO:Al films was uniform within the depth measurement accuracy of ∼5-10 nm.     

Microstructure and magnetic properties of zinc ferrite thin films produced by pulsed laser deposition

Zinc ferrite thin films were deposited from a target of zinc ferrite onto a MgO substrate using XeCl excimer laser operating at 308 nm and frequency of 30 Hz. The crystallographic characterizations of the films were performed using X-ray diffraction (XRD). Microstructure, surface morphology, chemical composition and grain size, as well as surface roughness were obtained from scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM). The magnetic properties of the thin films were studied in the temperature range 5–300 K and in fields of up to 5 T using SQUID magnetometry. Data on temperature and field dependence of magnetization provide a strong evidence for superparamagnetism. Paper presented at 8 AGM of MRSI, BARC, Mumbai, 1997.     

Atomic layer deposition of tantalum nitride based thin films from cyclopentadienyl type precursor

Tantalum nitride based thin films have been deposited on p-Si (100) and SiO₂/Si by thermal Atomic Layer Deposition (ALD) using either the Ta(= N^tBu)(NEt₂)₃ or a derivative, in which one dialkylamido ligand is substituted by a η⁵-cyclopentadienyl (η⁵-Cp), as metal organic precursors with ammonia as reducing agent. TaN_xC_y self-limiting temperature dependent ALD growth was achieved for the TaCp(= N^tBu)(NEt₂)₂/NH₃ process with a growth rate of 0.51-0.91 Å cycle⁻¹ in the 400-425 °C temperature range while between 240 and 280 °C, the growth of TaN based films from the Ta(= N^tBu)(NEt₂)₃ was accompanied by a partial decomposition of the precursor. The η⁵-cyclopentadienyl type compound allows lower nitrogen content in the precursor and thereafter in the deposited film. Although N/Ta ratio is close to one at temperatures of 390 and 400 °C, as analyzed by Rutherford Back Scattering and Nuclear Reaction Analysis, films were amorphous independently of the deposition temperature. Since Ta-C bonds are present in the Cp derivative, the TaCp(= N^tBu)(NEt₂)₂ tends more likely to form tantalum carbide compared to Ta(= N^tBu)(NEt₂)₃, which leads to lower thin film resistivity. For both precursors, employed in their respective ALD window, films were smooth with a root-mean-square roughness close to 1 nm.     

Goniocolorimetric study of aluminum oxide films deposited by atomic layer deposition

The interference colors resulting from thin films of Al₂O₃ deposited by atomic layer deposition (ALD) on silicon have been rigorously analyzed using a recently developed robotic gonioreflectometer. A series of eleven increasingly thick films was deposited, up to 1613 Å, and their reflectance values obtained for the visible spectrum. A comparison of these values with the predictions of computer simulations employing Fresnel equations has revealed that while there was generally good agreement between predicted and measured spectra, there are some spectral regions that exhibit large deviations from predicted reflectances, typically at near-normal measurement angles and shorter wavelengths. The effect of these discrepancies on color appearance was investigated in the CIE L*a*b* color space for the daylight illuminant D65. Large iridescence is both predicted and measured for most film thicknesses. Chroma and hue differences as large as 20 CIELAB units between the predicted and the measured color centers were obtained. Simulation also predicts larger iridescence than what is actually measured. A likely cause for the observed discrepancies is that the dielectric constants of the ALD films deviate from the literature values for the bulk material.     

Conformity and structure of titanium oxide films grown by atomic layer deposition on silicon substrates

Conformity and phase structure of atomic layer deposited TiO₂ thin films grown on silicon substrates were studied. The films were grown using TiCl₄ and Ti(OC₂H₅)₄ as titanium precursors in the temperature range from 125 to 500 °C. In all cases perfect conformal growth was achieved on patterned substrates with elliptical holes of 7.5 μm depth and aspect ratio of about 1:40. Conformal growth was achieved with process parameters similar to those optimized for the growth on planar wafers. The dominant crystalline phase in the as-grown films was anatase, with some contribution from rutile at relatively higher temperatures. Annealing in the oxygen ambient resulted in (re)crystallization whereas the effect of annealing depended markedly on the precursors used in the deposition process. Compared to films grown from TiCl₄, the films grown from Ti(OC₂H₅)₄ were transformed into rutile in somewhat greater extent, whereas in terms of step coverage the films grown from Ti(OC₂H₅)₄ remained somewhat inferior compared to the films grown from TiCl₄.     

Monocrystalline zinc oxide films grown by atomic layer deposition

In the present work we report on the monocrystalline growth of (00.1) ZnO films on GaN template by the Atomic Layer Deposition technique. The ZnO films were obtained at temperature of 300 °C using dietylzinc (DEZn) as a zinc precursor and deionized water as an oxygen precursor. High resolution X-ray diffraction analysis proves that ZnO layers are monocrystalline with rocking curve FWHM of the 00.2 peak equals to 0.07°. Low temperature photoluminescence shows a sharp and bright excitonic line with FWHM of 13 meV.     

Electrical properties of quaternary HfAlTiO thin films grown by atomic layer deposition

Quaternary alloyed HfAlTiO thin (~ 4-5 nm) films in the wide range of Ti content have been grown on Si substrates by Atomic Layer Deposition technique, and the effect of both the film composition and the interfacial reactions on the electrical properties of HfAlTiO films is investigated. It is shown that depending on the Ti content, the permittivity and the leakage current density I_leak in HfAlTiO films vary in the range k = 18 ÷ 28 and 0.01-2.4 A cm^− 2, respectively. The incorporation of ultra thin SiN interlayer in Al/HfAlTiO/SiN/Si stack gives rise to the sharp (× 10³) decrease of the I_leak ~ 6 · 10^− 5 A/cm² at the expense of the rather low capacitance equivalent thickness ~ 0.9 nm.     

Growth of aluminum nitride thin films prepared by plasma-enhanced atomic layer deposition

Aluminum nitride (AlN) thin films were deposited by atomic layer deposition from aluminum chloride (AlCl₃) and an ammonia/hydrogen plasma. The most important role of the ammonia/hydrogen plasma was to act as a reducing agent to extract Cl from AlCl₃, and to form AlN subsequently. The growth rate was saturated at ∼0.042 nm/cycle, and the thickness was proportional to the number of reaction cycles. Repeating this reaction cycle led to precisely controlled growth. The film properties were analyzed using Auger electron spectroscopy, X-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy and time-of-flight elastic recoil detection analysis. The concentration of chlorine and hydrogen impurities was 0.23 and 2.01 at.%, respectively. AlN films showed good anti-oxidation properties when O₂ was annealed at 650 °C for 30 min.     

An overview of highly porous oxide films with tunable thickness prepared by molecular layer deposition

This paper is a short review about the principle, preparation, and applications of ultra-thin oxide films prepared by molecular layer deposition (MLD). Porous oxide films, with well-defined porous structures and precisely controlled thicknesses down to several angstroms, can be prepared from dense organic/inorganic hybrid polymer films grown by MLD. The organic constituents in the film can be removed either by calcination at elevated temperatures or mild water etching at room temperature. Because of the layer-by-layer growth process for MLD, the deposited polymer films have regular structures and the removal of organic components from MLD polymer films produces uniform interconnected highly porous structures with a high surface area. For example, porous aluminum oxide films prepared by such a method have both micropores and mesopores with a BET surface area as high as 1250 m²/g. Examples of the versatility of the technique for fabrication of novel functional materials for various applications are discussed, including thermally stable, highly selective metal nanoparticle catalysts, defect-free inorganic membranes for gas separation, and photocatalytic layers prepared from titanium alkoxide MLD films.     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Atomic layer deposition and characterization of zirconium oxide-erbium oxide nanolaminates

ZrO₂ and Er₂O₃ thin films and nanolaminates were grown by atomic layer deposition from tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium, bis(methylcyclopentadienyl)methoxymethylzirconium and ozone as precursors at 350 °C. Nanolaminates consisted of 3-8 nm thick ZrO₂ and Er₂O₃ layers alternately deposited on planar substrates and on three-dimensional substrates with aspect ratio 1:20. The erbium content was 5-15 at.%. ZrO₂-Er₂O₃ films crystallized already in as-deposited states. Upon annealing at 650 °C, the films were stabilized in the form of cubic or tetragonal ZrO₂ polymorph and cubic Er₂O₃. Dielectric properties of the nanolaminates were comparable to those of the constituent oxides.     

Deposition of Al doped ZnO layers with various electrical types by atomic layer deposition

AlZnO thin films with various Al/Zn composition ratios were deposited by atomic layer deposition (ALD) at 200 °C. The effect of the composition of the AlZnO films on their electrical and optical characteristics was investigated. The AlZnO films with an Al content of up to 10 at.% showed high conductivity, while further increasing in the Al content resulted in the abrupt formation of an insulating oxide film. The lowest electrical resistivity of the ALD-deposited AlZnO film was 6.5 × 10^− 4 [Ω cm] at 5 at.% Al. The AlZnO films with up to 5 at.% Al exhibited crystalline phases and a near-band-edge emission. With increasing Al content, the optical band edge showed a blue shift, and a sudden shift associated with an insulating bandgap was observed in the AlZnO films containing 20 at.% Al.