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.     

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.     

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.     

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.     

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.     

Influence of thickness and growth temperature on the properties of zirconium oxide films grown by atomic layer deposition on silicon

ZrO₂ films of thicknesses varied in the range of 3–30 nm were atomic layer deposited from ZrI₄ and H₂O–H₂O₂ on p-Si(100) substrates. The effects of film thickness and deposition temperature on the structure and dielectric properties of ZrO₂ were investigated. At 272 and 325 °C, the growth of ZrO₂ started with the formation of the cubic polymorph and continued with the formation of the tetragonal polymorph. The ratio between the lattice parameters increased with the film thickness and growth temperature. The effective permittivity, determined from the accumulation capacitance of Hg/ZrO₂/Si capacitors, increased with the film thickness, reaching 15–17 in 25-nm-thick films. The permittivity decreased with the increasing growth temperature. The hysteresis of the capacitance–voltage curves was the narrowest for the films deposited at 325 °C, and increased towards both lower and higher deposition temperatures.     

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.     

X-ray mirrors on flexible polymer substrates fabricated by atomic layer deposition

Atomic layer deposition (ALD) techniques were used to fabricate W/Al₂O₃ superlattices with high X-ray reflectivity on flexible Kapton® polyimide and polyethylene naphthalate (PEN) polymer substrates. Reflectivities of 78% and 74% at λ = 1.54 Å were measured for 6-bilayer W/Al₂O₃ superlattices on Kapton® polyimide and PEN, respectively. These excellent X-ray reflectivities are attributed to precise bilayer thicknesses and ultrasmooth interfaces obtained by ALD and smoothing of the initial polymer surface by an Al₂O₃ ALD layer. The conformal ALD film growth also produces correlated roughness that enhances the reflectivity. These W/Al₂O₃ superlattices on flexible polymers should be useful for ultralight and adjustable radius of curvature X-ray mirrors.     

Low-temperature atomic layer deposition of ZnO thin films: Control of crystallinity and orientation

Low-temperature atomic layer deposition (ALD) processes are intensely looked for to extend the usability of the technique to applications where sensitive substrates such as polymers or biological materials need to be coated by high-quality thin films. A preferred film orientation, on the other hand, is often required to enhance the desired film properties. Here we demonstrate that smooth, crystalline ZnO thin films can be deposited from diethylzinc and water by ALD even at room temperature. The depositions were carried out on Si(100) substrates in the temperature range from 23 to 140 °C. Highly c-axis-oriented films were realized at temperatures below ~ 80 °C. The film crystallinity could be further enhanced by post-deposition annealing under O₂ or N₂ atmosphere at 400-600 °C while keeping the original film orientation intact.     

Structure and morphology of aluminium doped Zinc-oxide layers prepared by atomic layer deposition

The aim of this work is to study the effects of deposition temperature and aluminium incorporation on the crystalline properties, orientation and grain size of atomic layer deposited ZnO layers. X-ray diffraction analysis revealed a change in the dominant crystallite orientation with increasing substrate temperature. The most perfect crystal structure and largest grain size was found at 2 at.% aluminium content. Accumulation of compressive strain developed a monotonous increase with the growth temperature. Electric resistivity showed no anisotropy despite the change in the orientation, therefore the dominant conduction mechanism is not grain boundary related.     

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.     

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.     

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.     

Native oxide consumption during the atomic layer deposition of TiO2 films on GaAs (100) surfaces

The consumption of the surface native oxides is studied during the atomic layer deposition of TiO₂ films on GaAs (100) surfaces. Films are deposited at 200 °C from tetrakis dimethyl amido titanium and H₂O. Transmission electron microscopy data show that the starting surface consists of ~2.6 nm of native oxide and X-ray photoelectron spectroscopy indicates a gradual reduction in the thickness of the oxide layer as the thickness of the TiO₂ film increases. Approximately 0.1-0.2 nm of arsenic and gallium suboxide is detected at the interface after 250 process cycles. For depositions on etched GaAs surfaces no interfacial oxidation is observed.     

Self-limiting low-temperature growth of crystalline AlN thin films by plasma-enhanced atomic layer deposition

We report on the self-limiting growth and characterization of aluminum nitride (AlN) thin films. AlN films were deposited by plasma-enhanced atomic layer deposition on various substrates using trimethylaluminum (TMA) and ammonia (NH₃). At 185 °C, deposition rate saturated for TMA and NH₃ doses starting from 0.05 and 40 s, respectively. Saturative surface reactions between TMA and NH₃ resulted in a constant growth rate of ~ 0.86 Å/cycle from 100 to 200 °C. Within this temperature range, film thickness increased linearly with the number of deposition cycles. At higher temperatures (≥ 225 °C) deposition rate increased with temperature. Chemical composition and bonding states of the films deposited at 185 °C were investigated by X-ray photoelectron spectroscopy. High resolution Al 2p and N 1s spectra confirmed the presence of AlN with peaks located at 73.02 and 396.07 eV, respectively. Films deposited at 185 °C were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction. High-resolution transmission electron microscopy images of the AlN thin films deposited on Si (100) and glass substrates revealed a microstructure consisting of nanometer sized crystallites. Films exhibited an optical band edge at ~ 5.8 eV and an optical transmittance of > 95% in the visible region of the spectrum.     

Effect of deposition temperature on the properties of CeO2 films grown by atomic layer deposition

Cerium oxide dielectric thin films have been grown on n-type silicon by atomic layer deposition using a monomeric homoleptic Ce^IV alkoxide precursor with water vapour. Herein we report the dielectric properties of CeO₂ films deposited from tetrakis(1-methoxy-2-methyl-2-propanolate)cerium. The resulting films exhibit permittivities in the range 25-42 at 1 MHz with a strong dependency on the deposition temperature. The microstructural origin of this behaviour has been investigated. The as-deposited films were found to be crystalline and they exhibited the cubic fluorite structure for deposition temperatures in the range 150 °C to 350 °C. Variations in the crystallite sizes are governed by the deposition temperature and have been estimated using a Debye-Scherrer analysis of the X-ray diffraction patterns. The changing crystallite size correlates with changes seen in the triply-degenerate F_2g first-order Raman line half-width at 465 cm^− 1. It is concluded that the frequency dependency of the film dielectric properties is strongly influenced by the crystallite size which in turn is governed by the growth temperature.     

Tris(dialkylamino)aluminums: Syntheses, characterization, volatility comparison and atomic layer deposition of alumina thin films

The syntheses and characterization of both tris(diethylamino)aluminum and tris(diisopropylamino)aluminum are presented in this letter. Characterization includes vapor pressure measurements and comparison of the two non-pyrophoric precursors showing them to be viable alternatives to trimethylaluminum. Ultimately, tris(diisopropyl)aluminum was successful in the atomic layer deposition of alumina thin films.     

Thermal properties of TiO2 films grown by atomic layer deposition

We study the thermal properties of amorphous TiO₂ thin films of various thicknesses t, grown by atomic layer deposition. The thermo-optic coefficient dn/dT and the temperature coefficient dρ/dT of film density ρ are determined from ellipsometric data in wavelength range 380 < λ < 1800 nm with the Cauchy model and the Lorentz-Lorenz relation. It is found that dn/dT exhibits negative values for films with t < 150 nm and positive values for thicker films, while no significant changes in the two coefficients take place if t < 200 nm. A qualitative physical explanation based on porosity of the thin films is suggested. Films with t = 60 nm are illustrated in detail at λ = 640 nm: the room-temperature values of the coefficients are found to be dn/dT = − 3.1 × 10^− 5 °C^− 1 and dρ/dT = − 4.8 × 10^− 5g cm^− 3° C^− 1.     

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.     

Properties of atomic layer deposited HfO2 thin films

The growth, composition and morphology of HfO₂ films that have been deposited by atomic layer deposition (ALD) are examined in this article. The films are deposited using two different ALD chemistries: i) tetrakis ethylmethyl amino hafnium and H₂O at 250° and ii) tetrakis dimethyl amino hafnium and H₂O at 275 °C. The growth rates are 1.2 Å/cycle and 1.0 Å/cycle respectively. The main impurities detected both by X-ray Photoelectron Spectroscopy and Fourier transform infrared spectroscopy (FTIR) are bonded carbon (~ 3 at.%) and both bulk and terminal OH species that are partially desorbed after high temperature inert anneals up to 900 °C. Atomic Force Microscopy reveals increasing surface roughness as a function of increasing film thickness. X-ray diffraction shows that the morphology of the as-deposited films is thickness dependent; films with thickness around 30 nm for both processes are amorphous while ~ 70 nm films show the existence of crystallites. These results are correlated with FTIR measurements in the far IR region where the HfO₂ peaks are found to provide an easy and reliable technique for the determination of the crystallinity of relatively thick HfO₂ films. The index of refraction for all films is very close to that for bulk crystalline HfO₂.