Low-Temperature Deposition of Alumina Films by Ultrasonic Spray Pyrolysis with a Water-Based Precursor

Abstract

Alumina (Al₂O₃) is a key material in inorganic and hybrid electronics due to its excellent dielectric, chemical, and thermal stability properties. In this work, we present the first results of alumina films deposited by ultrasonic spray pyrolysis (USP) at low temperatures (40–100 °C) using water as the sole solvent, followed by an annealing step at 100 °C. The films were characterized by SEM, XRD, EDS, and UV-Vis spectroscopy to evaluate their morphology, structure, composition, and optical properties. Preliminary results show an average thickness of approximately 8 µm, with surface features consisting of agglomerates (average particle size ≈ 7.252 µm) distributed over the film. XRD patterns revealed the presence of tetragonal and orthorhombic phases, while EDS confirmed the presence of aluminum and oxygen with slight compositional variations depending on deposition and annealing conditions. UV-Vis absorption spectra exhibited characteristic bands between 259 nm and 263 nm. These results provide a comprehensive understanding of the optical, structural, and morphological behavior of Al₂O₃ films processed at low temperatures. The motivation for studying these films is to enable more eco-friendly gate oxides for organic MIS structures, as well as functional layers in photonic devices. This approach represents a sustainable and straightforward route for obtaining Al₂O₃ coatings compatible with temperature-sensitive substrates, paving the way for future applications in hybrid and organic electronics.

1. Introduction

Alumina (Al₂O₃) is a widely used material in the field of inorganic materials due to its excellent mechanical, thermal, and chemical properties, as well as its high stability and corrosion resistance [1,2]. Historically, its application in inorganic electronic devices has focused on its use as a dielectric in transistors, barrier layers in microelectronics, and protective coatings in sensors and optical devices [3,4]. In the field of organic materials for electronic devices, alumina has attracted interest as a barrier layer and dielectric in hybrid structures, particularly to improve the stability and performance of flexible optoelectronic devices [2,3,4,5,6].

Among the most widely used deposition techniques, ultrasonic spray pyrolysis stands out for its simplicity, low cost, and versatility, enabling control over the morphology and composition of the films. In the production of inorganic films, typical precursors include aluminum nitrate, aluminum isopropoxide, and aluminum chloride, dissolved in solvents such as water, alcohols, or hydroalcoholic mixtures [7]. For organic materials, the integration of alumina has been mainly explored through the deposition of thin layers onto polymers or hybrid structures, aiming for compatibility with low-temperature processes and environmentally friendly solvents [5,6,8,9,10,11,12,13,14,15,16,17].

In this work, we present the first results of alumina films deposited by ultrasonic spray pyrolysis at low temperatures using water as the sole solvent. The films were characterized in terms of morphology, structure, composition, and optical properties using Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), and Ultraviolet–visible spectroscopy (UV-Vis) techniques. The objective is to analyze their optical and structural properties in order to assess their application as gate oxides in organic MIS structures and their potential use in photonic devices. This approach offers a sustainable and straightforward route to obtain Al₂O₃ coatings compatible with temperature-sensitive substrates, highlighting their viability for future developments in hybrid and organic electronics.

2. Experimental Methods

The alumina was acquired from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany) with a purity of 80%. It was dispersed in 20% distilled water (TQE) and this was used as a precursor in the system to form the films on the substrate. The reagents were utilized in their original state, without any additional purification.

It is worth mentioning that one of the main objectives is to obtain films at low temperatures. For this reason, use of aluminum salt as a precursor to form Al₂O₃ would involve temperatures above 150 °C. Using the Al₂O₃ suspension allows very good quality films to be obtained at 100 °C. Another disadvantage of using the aluminum precursor is that the complete elimination of reaction by-products must be ensured in order to obtain a good quality film, but this requires high temperatures.

On the other hand, using water as a solvent has the advantage of being a non-toxic and environmentally friendly process. Another advantage of using an Al₂O₃ suspension is that, for film deposition, pyrolysis (thermal decomposition of the salt) is not necessary; only a suitable temperature to evaporate the solvent is required. This allows obtaining the Al₂O₃ films at low temperatures and on flexible substrates. Taking into account the above considerations, the films obtained in this work show uniformity although with agglomerates on the surface.

A Baku BK 2000 ultrasonic bath (Distrito Haizhu, Guangzhou, Guangdong, China) was used to achieve a homogeneous mixture of the solution, which was in the form of a suspension (heterogeneous dispersion). To deposit by using the USP technique it was used a Citizen Model CUN60 system (Stuttgart, Germany), with an ultrasonic frequency of 2.5 MHz and the atomization power is Approx. 10 W., the spray rate is 0.2–0.7 mL/min with 3-speed adjustable. These parameters were selected to promote a uniform dispersion of the obtained solution and the formation of homogeneous films at low temperatures. Corning glass slides were used as substrates for the deposition of the Al₂O₃ films, and a thermal hotplate (Chemat Technology, model TW-4H, Los Angeles, CA, USA) was used to carry out the thermal annealing of the Al₂O₃ films.

In order to obtain the alumina (Al₂O₃) films, a dispersion of alumina in water with a volume concentration of 4% was prepared. The dispersion was subjected to an ultrasonic system maintained at a temperature of 50 °C. The films were deposited on conventional Corning glass substrates on a hotplate for a period of five minutes at varying deposition temperatures (TD), specifically 40 °C (P6), 60 °C (P5), 80 °C (P4), and 100 °C (P1). Subsequently, the films were subjected to thermal annealing, conducted at an annealing temperature (Tr) of 100 °C for 30 min on a hotplate, with the objective of removing any residual impurities. The annealed samples were designated as follows: P1Tr, P4Tr, P5Tr, and P6Tr.

For the characterization of the films the X-ray diffraction patterns of the Al₂O₃ films were measured using a single-crystal X-ray diffractometer, Stoe-Stadivari model, Bruker brand (New York, NY, USA), equipped with a microfocus X-ray source, goniometer, and X-ray detector. The 2θ measurement range varies from 0 to 60°, and the sample measurement was performed in a horizontal orientation. SEM images of the alumina films were obtained with a JEOL JSM-7800F Schottky Field Emission Scanning Electron Microscope (Pleasanton, CA, USA). Cross-sectional and frontal view measurements were performed for the Al₂O₃ films, and images were obtained on a micrometer scale. The wide-range UV-VIS-NIR system, Varian (Agilent) Cary 5000 model (Santa Clara, CA, USA), with a PbSmart detector, was used with a measurement range from 175 nm to 3300 nm. Absorption spectra were measured with normal incidence over a spectral range from 200 to 700 nm.

3. Results and Discussion

Figure 1a shows the X-ray diffraction (XRD) patterns of the Al₂O₃ films P1, P5, and P6, and Figure 1b presents the patterns for P5Tr and P6Tr. Although all samples exhibit predominantly amorphous behavior, three characteristic peaks corresponding to alumina are observed. These peaks and their associated crystalline phases are identified using the ICDD reference cards shown in the figure. The presence of these peaks indicates the formation of crystal domains within the films during deposition and subsequent annealing. This process helps eliminate residual impurities and promotes the development of a chemically and thermally stable structure.

During this process, some of the surface-bound volatile species or residual moisture may be partially removed, improving the chemical stability of the material. However, it is acknowledged that low-temperature annealing is not sufficient to eliminate chemically bonded impurities such as hydrogen or oxygen within the agglomerates.

In Figure 1a, the intensity of the peaks remains relatively uniform across the different deposition and annealing temperatures. Notably, after annealing, the intensity of the peak at 57° increases for samples P5 and P6. The identification of the peaks is based on crystallographic databases that correlate specific planes with known alumina phases [18,19,20].

The low crystallinity observed in the films suggests a partially ordered structure compared to more defined crystalline phases such as α-alumina and γ-alumina [21,22]. This may result from the low deposition temperatures, which limit complete crystal formation. Nevertheless, the diffraction peaks suggest the presence of specific planes or reflections consistent with tetragonal and orthorhombic alumina phases [22,23,24]. In the tetragonal system, the a and b axes are perpendicular and equal in length, with all angles at 90°, while the orthorhombic system features unequal a, b, and c axes, also with 90° angles [25].

Bharthasaradhi and Nehru [12] reported that alumina films were deposited above 500 °C using a coprecipitation method, revealing well-defined diffraction planes. In contrast, our work demonstrates that alumina films can be obtained at much lower temperatures (around 100 °C) using an ultrasonic spray pyrolysis (USP) system. This method enables film deposition on a variety of substrates including flexible and cardboard types, expanding the range of potential applications, particularly in flexible electronic devices.

The XRD patterns presented in this work correspond to selected representative samples before and after annealing. Although measurements for the P1Tr sample and raw alumina powder were not performed, the analyzed patterns (including P4, P5, P6 and their annealed counterparts) provide sufficient information to observe trends in crystallinity and structural evolution under low-temperature processing. All XRD patterns were processed under identical conditions, but no normalization to the main peak was applied, since the primary aim was to evaluate the presence or absence of specific crystalline phases rather than quantify relative crystallinity. Future studies will consider including additional reference samples and applying peak normalization to further refine structural analysis [20,23,24,26,27,28].

The morphological characterization of the Al₂O₃ films was performed using scanning electron microscopy (SEM). Figure 2a presents a cross-sectional SEM image of sample P4, deposited at 80 °C for 5 min, showing a total thickness of approximately 8 µm. Agglomerated granules are visible in the cross-section, and the film appears non-uniform due to the low deposition temperature. Based on the image, the continuous film thickness is estimated to be 1.82 µm, while the agglomerated region accounts for approximately 6 µm, resulting in a total thickness of around 7.82~8 µm.

Figure 2b shows the surface morphology of film P4, where spherical structure is observed dispersed over the surface. These spheres have an average size of approximately 7.252 µm and may affect surface properties such as roughness and adhesion [26]. This analysis provides valuable insights into the morphology and structure of the Al₂O₃ films, both in cross-section and on the surface. The thickness was estimated by averaging 15 measurements in different surface regions of the samples by means of the micrographs, with an approximate error of ±0.3 µm. The particle size was obtained through image analysis using the ImageJ2 software. In addition, the agglomerates may affect the film uniformity and limit their reliability as an oxide gate, but in our case this Al₂O₃ was functional as an oxide gate. A detailed electrical study is on course and will be published elsewhere.

In Figure 2c, a large surface particle is observed, measuring approximately 2.617 µm. The presence of these particles may influence the film’s structural properties, particularly since the film was deposited at low temperature without annealing, which affects surface uniformity and growth mechanisms [11,29,30].

The formation of spherical particles may be attributed to various factors such as deposition rate, substrate temperature, precursor concentration, and interaction during the spray pyrolysis process. These spheres likely represent material agglomerates that condensed on the substrate due to limited mobility and crystallization, resulting from the low deposition temperature and the nature of the precursor system [29,30].

The non-uniform morphology observed in the films is consistent with the use of low deposition temperatures and the heterogeneous nature of the alumina suspension. These conditions limit the mobility of the precursor during the deposition process, leading to agglomeration and the formation of irregular surface features. Although the films are not homogeneous at the microscale, the deposition method still enables the formation of continuous layers with optical properties comparable to those of alumina films obtained at much higher temperatures. This supports the potential of this low-temperature approach for flexible electronics applications.

Figure 3 shows the EDS spectrum of Al₂O₃ films, corresponding to samples P4 and P5, both with and without annealing. The elemental weight percentages are summarized in

Table 1. Chemical composition of the Al₂O₃ films.

Samples	Al (Wt%)	O (Wt%)
P4	44.7	55.3
P5	47.9	52.1

.

The elemental composition reveals slight variations between films P4 and P5. For film P4 (deposited at 80 °C), the aluminum (Al) content is 44.7 wt% and oxygen (O) is 55.3 wt%. For P5, which underwent annealing at 100 °C, the Al content increases to 47.9 wt% and O decreases to 52.1 wt%. This change suggests that annealing may contribute to a more compact structure or induce partial desorption of surface adsorbed species, slightly modifying the film’s composition [10,21].

These results provide key insights into the chemical composition of the Al₂O₃ films and contribute to understanding their behavior before and after thermal treatment.

Figure 4 shows EDS elemental distribution maps of films P4 and P5. The spatial distributions of aluminum (blue), oxygen (red), and silicon (green) are illustrated on the film surfaces. The presence of aluminum and oxygen corresponds to the alumina composition, while silicon originates from the glass substrate.

In Figure 4a, a spherical particle is shown which contains both aluminum and oxygen, confirming its composition as part of the Al₂O₃ structure. Figure 4b displays the surface of film P4, revealing a non-uniform distribution of Al and O, with localized regions of higher concentration due likely to agglomeration and uneven deposition at 80 °C. In contrast, Figure 4c corresponds to film P5 (annealed at 100 °C), where the elemental maps still show aluminum and oxygen, but with increased dispersion of material on the surface, suggesting that annealing promotes the redistribution or partial sintering of the agglomerates.

These results confirm that the Al₂O₃ films are composed primarily of aluminum and oxygen, with varying degrees of homogeneity influenced by deposition and annealing conditions [10,22,23,24,25,26].

Once the structural, morphological, and compositional characterizations confirmed the formation of the Al₂O₃ films, their optical properties were evaluated by using UV-Vis spectroscopy.

Figure 5a shows the UV/Vis absorption spectra of the Al₂O₃ films P1, P4, P5, and P6, before (purple) and after (blue) annealing. A characteristic absorption band for alumina is observed in the range of 210 nm to 280 nm [10,27]. Films P1 and P5 exhibit a maximum absorption peak at 263 nm, while films P4 and P6 show a peak at 259 nm. After annealing at 100 °C, the absorption bands shift slightly, with a central wavelength near 259 nm.

Figure 5b displays the Tauc plots used to determine the optical band gaps of films P6 and P4Tr, calculated to be 4.44 eV and 4.49 eV, respectively. The band gaps were estimated by extrapolating the linear portion of the (αh$\nu$)² vs. photon energy (h$\nu$) plot, following the Tauc model for direct allowed transitions [2,10]. The Al₂O₃ films (P1, P4, P5, P6, and P1Tr, P4Tr, P5Tr, P6Tr) deposited at low temperatures (40–100 °C) and annealed at 100 °C, exhibit optical band gaps ranging from 4.44 to 4.49 eV. These values are consistent with those reported for α- and γ-alumina films deposited using high-temperature techniques, which typically fall within the 4–6 eV range [21,22]. The slight variations observed before and after annealing are minimal and fall in the expected margin of error for the measurement and calculation methods used. Nonetheless, the consistency across samples supports the conclusion that alumina films with comparable optical properties can be obtained at significantly lower processing temperatures. The optical band gap (Eg) of the samples Al₂O₃ was determined from the UV/Vis spectra using the Tauc [27] Equation (1):

$${\left(\alpha h\nu \right)}^{{\displaystyle \frac{1}{n}}}=C_1\left(h\nu -E_g\right)$$

(1)

where α = Absorption coefficient, h = Planck’s const., $\nu$ = incident radiation frequency, $C_1$ = proportionality constant, E_g = bandgap energy of the material, and n = power whose value depends on the electronic transition presented by the material (Al₂O₃ presents a direct allowed transition for which n = ½ [27]). The Tauc extrapolation diagram for the abscissa provides the value of the optical band gap (E_g) [27].

The lower value obtained can be attributed to several factors: (i) the presence of structural defects in films deposited at low temperatures, (ii) the amorphous nature of the material under these deposition conditions, and (iii) possible impurities from the commercial precursor (80% purity).

This result highlights the potential of the low-temperature ultrasonic spray pyrolysis (USP) method to fabricate the dielectric Al₂O₃ films with suitable optical properties for flexible electronics. The ability to achieve such band gaps at temperatures below 300 °C presents a significant advantage in terms of energy savings and compatibility with thermally sensitive substrates.

In previous studies, temperatures exceeding 300 °C were typically required to deposit alumina films, depending on the technique [1,14,28]. For instance, chemical vapor deposition (CVD) operates between 200 °C and 1000 °C [13,14,15], sputtering around 400 °C [14,16], and the sol–gel processes above 400 °C [17,18,19]. In contrast, the present work demonstrates the feasibility of using USP at temperatures below 200 °C to obtain alumina films with comparable optical features.

The observed absorption band in the 200–280 nm range aligns well with literature values for α- and γ-alumina [10,12,13,27], further confirming the optical behavior of the films obtained in this study.

The combined results from UV-Vis, XRD, SEM, and EDS analyses confirm that alumina films obtained via low-temperature ultrasonic spray pyrolysis exhibit structural, morphological, and optical features consistent with partially crystalline alumina. The presence of spherical particles on the surface is attributed to nucleation processes during deposition and thermal annealing. These findings demonstrate that even at temperatures as low as 100 °C, it is possible to obtain functional alumina films with properties suitable for use in dielectric layers and flexible electronics.

4. Conclusions

The Al₂O₃ films were successfully deposited using an economical and straightforward method ultrasonic spray pyrolysis at low temperatures of 40 °C, 60 °C, 80 °C, and 100 °C, using easily handled precursors. After annealing at 100 °C, modifications in their optical and compositional properties were observed. The films exhibited a thickness of approximately 8 µm, including agglomerated surface features.

X-ray diffraction (XRD) patterns indicated the presence of tetragonal and orthorhombic structural phases through the identification of specific crystallographic planes. SEM analysis showed the presence of surface agglomerates (average particle size ≈ 7.252 µm) distributed over a continuous underlying film. Energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of aluminum and oxygen, with slight compositional variations depending on deposition and annealing conditions. UV-Vis spectroscopy revealed characteristic alumina absorption bands between 259 nm and 263 nm.