Influence of annealing temperature on the performance of TiO2/SiO2 nanolaminated films

Yun Cui; Yuanan Zhao; Ge Zhang; Meiping Zhu; Chen Song; Chunxian Tao; Tan Shu; Jianda Shao

doi:10.3788/COL202119.121406

Abstract

Different laminated structures of

{TiO}_{2} / {SiO}_{2}

composite film were prepared via atomic layer deposition (ALD) on alumina substrates. The effect of the annealing temperature in the air on the surface morphologies, crystal structures, binding energies, and ingredient content of these films was investigated using X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. Results showed that the binding energy of Ti and Si increased with decrease of the Ti content, and the

{TiO}_{2} / {SiO}_{2}

nanolaminated films exhibited a complex bonding structure. As the annealing temperature increased, the thickness of the nanolaminated films decreased, and the density and surface roughness increased. An increase in the crystallization temperature was proportional to the

{SiO}_{2}

content in

{TiO}_{2} / {SiO}_{2}

composite film. The annealing temperature and thin thickness strongly affected the phase structure of the ALD

{TiO}_{2}

thin film. To be specific, the

{TiO}_{2}

thin film transformed into an anatase phase from an amorphous phase after an increase in the annealing temperature from 400°C to 550°C, and the

{TiO}_{2}

film exhibited an anatase phase until the annealing temperature reached 850°C, owing to its extremely small thickness. The annealing process caused the Al ions in the substrate to diffuse into the films and bond with O.

Keywords

annealing atomic layer deposition nanolaminated film thin films

1. Introduction

Titania ( ${TiO}_{2}$ ) has been extensively used in solar energy applications, aerospace, and medicine fields^[1–4] because of its nontoxicity and good light transmission characteristics. ${TiO}_{2}$ has been used as a photocatalysis material because it can generate hydroxyl radicals with strong oxidizing properties^[5–8]. It has been widely used in optoelectronic systems^[9,10] because of its high refractive index, high dielectric constant^[11], and semiconductor characteristics^[12].

${TiO}_{2}$ is often doped with ${SiO}_{2}$ to expand its application range and enhance its performance^[13]. For example, after adding ${SiO}_{2}$ to the ${TiO}_{2}$ aerogel, the density of the resulting film increased^[14], and the mechanical performance and thermal stability^[15] were enhanced. There are two common doping methods. One involves doping ${SiO}_{2}$ in ${TiO}_{2}$ aerogel before the film production. When the molar ratio of ${SiO}_{2}$ to ${TiO}_{2}$ was between 1 and 5^[16–18], the photocatalytic performance of the ${TiO}_{2} / {SiO}_{2}$ film increased with an increase in the ${SiO}_{2}$ content. The other method involves vapor deposition of both ${TiO}_{2}$ and ${SiO}_{2}$ materials simultaneously to obtain different refractive indices and prepare thin film elements with specific properties. Studies have shown^[19] that at low ${TiO}_{2}$ concentrations, Ti ions could be inserted into the ${SiO}_{2}$ network in a tetrahedral structure; at high ${TiO}_{2}$ concentrations, Ti evolved from a tetrahedral structure to a Ti octahedral structure. When the concentration of ${SiO}_{2}$ in ${TiO}_{2}$ did not exceed 30%, the photoelectric current could be increased by changing the thickness of the film in the range of 55–90 nm^[20], and this film was used as an anti-reflection coating for solar cells.

In summary, ${TiO}_{2} / {SiO}_{2}$ composite films have been prepared mainly through doping ${SiO}_{2}$ in the original solution or the simultaneous vaporization of two materials. There are few reports on the preparation of ${TiO}_{2} / {SiO}_{2}$ composite films via atomic layer deposition (ALD) although ALD^[21] is considered as one of the most promising thin film technologies.

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Herein, ALD was used to prepare the ${TiO}_{2} / {SiO}_{2}$ nanolaminated films, and the effects of the annealing temperature on surface morphologies, crystal structures, binding energy (BE), and ingredient content of the ${TiO}_{2} / {SiO}_{2}$ nanolaminated films were studied. This study aims to provide a better understanding of ${TiO}_{2} / {SiO}_{2}$ nanolaminated films through ALD, which will eventually be able to contribute to the application of nanolaminated films in solar energy, medicine fields, and optoelectronic systems.

2. Experiments

2.1. Sample preparation

${TiO}_{2}$ and ${SiO}_{2}$ nanolaminated films were deposited alternately using R200 ALD equipment from Picosun, Finland.

One cycle of ${TiO}_{2}$ included the following: $C_{8} H_{24} N_{4} Ti$ feeding into the ALD chamber for 1.6 s at room temperature, $N_{2}$ purging (5 s), $O_{2}$ feeding (11 s) with 2500 W RF plasma power and at a reaction temperature of 150°C, and Ar purging (8 s). The saturated growth rate of the ${TiO}_{2}$ film was 0.057 nm/cycle, which was marked as T. One cycle of ${SiO}_{2}$ included the following: $C_{6} H_{18} N_{3} Si$ feeding into the ALD chamber for 0.4 s at 90°C, $N_{2}$ purging (19 s), $O_{2}$ feeding (11 s) with 2500 W RF plasma power and at a reaction temperature of 150°C, and Ar purging (8 s). The saturated growth rate of ${SiO}_{2}$ film was 0.103 nm/cycle, which was marked as S, as shown in Table 1. When the film thickness reached 30 nm, the X-ray diffractometer could be used to obtain high-quality data about the crystal phase, thickness, and density of the film; in order to reduce the deposition time, 30 nm was selected as the analysis thickness due to the slow deposition rate of the ALD film. The samples with different atomic ratios of Si and Ti were designed by changing the cycle number; the samples with a total thickness of 30 nm were designed by changing the stack times, as shown in Table 2. The repeated stack times of ${SiO}_{2} / {TiO}_{2}$ nanolaminated films were 10, 18, and 24, as shown in Fig. 1.

Figure 1.Schematic diagrams of the structures of the nanolaminated films.

Deposited Film	Precursor	Purity (N)	Precursor Temperature	Reaction Temperature (°C)	Thickness of One Circle (nm)
${SiO}_{2}$	3DMAS ( $C_{6} H_{18} N_{3} Si$ )	$\geq 5$	Room	150	0.103 (S)
${TiO}_{2}$	TDMAT ( $C_{8} H_{24} N_{4} Ti$ )	$\geq 5$	90°C	150	0.057 (T)

Table 1. One Cycle of the ${SiO}_{2}$ and ${TiO}_{2}$ Process Conditions Using ALD

View all Tables

Sample	Total Thickness (nm)	Nano-Laminated Structure	Substrate
A-S0T4	30	Sub/526T × 1 stack time	${Al}_{2} O_{3}$
A-S1T3	30	Sub/(10S37T) × 10 stack times	${Al}_{2} O_{3}$
A-S2T2	30	Sub/(10S12T) × 18 stack times	${Al}_{2} O_{3}$
A-S3T1	30	Sub/(10S4T) × 24 stack times	${Al}_{2} O_{3}$

Table 2. Design Structures of the Nanolaminated Films

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2.2. Experimental conditions and sample characterization

The prepared samples were annealed at different temperatures. The samples were transferred to a quartz glass tube furnace for annealing treatment. The temperature of the furnace was raised from room temperature to 400°C, 550°C, 700°C, and 850°C sequentially; the heating rate was 10°C/min, and the set temperature was maintained for 1 h, as shown in Fig. 2. Finally, the sample was naturally cooled to room temperature.

Figure 2.Annealing programs of the samples at 400°C, 550°C, 700°C, and 850°C.

Surface morphologies were analyzed via field emission scanning electron microscopy (FESEM; Auriga, Zeiss). The resolution was 1 nm for the electron beam at 15 kV. The acceleration voltage varied continuously from 0.1 to 30 kV at 10 V intervals. The magnification range was 12– $1, 000, 000$ .

Phase identification and film thickness analyses were conducted via X-ray diffraction (XRD; Empyream, PANalytical) in the continuous scanning mode, using Cu Kα1 radiation ( $λ = 0.15406 nm$ ) at 40 kV and 40 mA. The working pattern was generated using grazing-incidence XRD for the crystal phase analysis and X-ray reflectivity (XRR) for the thickness and density analyses. The minimum step size was 0.0001°, and the repeatability of the entire machine was 0.001°.

Component analysis was conducted through X-ray photoelectron spectroscopy (XPS; KAlpha, Thermo Scientific) using a monochromatic Al Kα source, with a characteristic emission line of 1486.6 eV. The depth profile was cut using ${Ar}^{+}$ ions with ion energy of 1 keV. The reference etching rate of ${Ta}_{2} O_{5}$ was 0.13 nm/s, and the raster area was $4 mm \times 2 mm$ . The energy resolution of Kα was greater than 0.5 eV. The BE of C for 1 s at 248.8 eV was used as the calibration peak position.

3. Results

3.1. Crystal phase and surface morphologies

The crystalline phase and surface morphologies of the four samples before and after annealing were analyzed. As shown in Fig. 3, sample A-S0T4 did not crystallize after annealing at 400°C, and the anatase phase (reference ICDD code: 00-002-0406) appeared after annealing at 550°C, indicating that its phase transition temperature was between 400°C and 550°C; the anatase phase of the film remained until after annealing at 850°C. The inset of Fig. 3 shows that as the annealing temperature increased, the diffraction angle increased slightly, indicating that the interplanar spacing decreased as the annealing temperature increased. Figure 4 presents the surface morphologies of sample A-S0T4 before and after baking at different temperatures. No change in the surface morphologies was observed before and after annealing at 400°C, as shown in Figs. 4(a) and 4(b), and large grains appeared on the surface of the film after annealing at 550°C, as shown in Fig. 4(c). With an increase in the annealing temperature, the sizes of the particles of ${TiO}_{2}$ films gradually increased, and noticeable grain boundaries appeared on the surface of the film after annealing at 850°C, as shown in Fig. 4(d). The ${TiO}_{2}$ film retained the anatase phase and did not transform into the rutile phase (reference ICDD code: 00-001-1292) after annealing at 850°C because of its nanoscale thickness; the formation of large grains after annealing reduced interplanar spacing.

$Diffraction spectra of sample A-S0T4 before and after annealing; the inset shows the change in the diffraction angle.$

Figure 3.Diffraction spectra of sample A-S0T4 before and after annealing; the inset shows the change in the diffraction angle.

Figure 4.Surface morphologies of sample A-S0T4 before and after annealing: (a) before annealing, (b) after annealing at 400°C, (c) after annealing at 700°C, and (d) after annealing at 850°C.

The films on A-S1T3 and A-S2T2 were amorphous before annealing, as shown in Figs. 5(a) and 5(b). The crystallization temperature increased with the increase in the ${SiO}_{2}$ content of the film, although the crystallinity was low and the diffraction peak was weak. Before annealing, the surface of the samples was smooth, as shown in Figs. 6(a), 6(c), and 6(e). After annealing at 850°C, the surface roughness decreased with decrease in ${TiO}_{2}$ content, as shown in Figs. 6(b) and 6(d). The film on A-S3T1 did not crystallize after annealing at 850°C, as shown in Fig. 5(c), and the surface morphology did not change significantly before and after annealing, as shown in Figs. 6(e) and 6(f).

$Diffraction spectra of the samples before and after annealing: (a) A-S1T3, (b) A-S2T2, and (c) A-S3T1.$

Figure 5.Diffraction spectra of the samples before and after annealing: (a) A-S1T3, (b) A-S2T2, and (c) A-S3T1.

Figure 6.Surface morphologies of the samples before and after annealing: (a) before annealing of A-S1T3, (b) after annealing of A-S1T3 at 850°C, (c) before annealing of A-S2T2, (d) after annealing of A-S2T2 at 850°C, (e) before annealing of A-S3T1, and (f) after annealing of A-S3T1 at 850°C.

3.2. Thickness and density

The thickness and density changes of four types of films were analyzed by measuring the XRR curves. As shown in Fig. 7, according to the critical angle of the curve, distance of oscillations, and shape of the curve, the density and thickness of the films were fitted and analyzed. Figure 8 shows the fitting results; the thickness was normalized. Results showed that as the annealing temperature increased, the film thickness decreased, and the density increased.

Figure 7.XRR curves of sample A-S0T4 before and after annealing.

Figure 8.Changes in the thickness and density of the four types of films with the annealing temperature.

3.3. Changes in the ingredient content and BE

The elemental composition of the four samples before and after annealing at 850°C was analyzed via XPS, and Fig. 9 shows the results. The content of Si gradually increased from sample A-S0T4 to sample A-S3T1, which was consistent with the design trend; however, the Si content was significantly less than the design result, which was attributed to the ${SiO}_{2}$ growth rate being lower than the saturated growth rate, owing to the different growth surface of the multilayer film and monolayer film. The O content did not change significantly in the four samples before annealing, which was about 66.6%, indicating that the ratio of Ti or Si to O was 1:2. Al ions were detected on the surface of four types of films after annealing at 850°C, indicating that the annealing process caused the Al ions to diffuse into the films.

Figure 9.Changes in the elemental composition of the four types of films before and after annealing at 850°C.

The changes in the BEs of Si, Ti, and O before annealing and the changes in the BEs of Al after annealing at 850°C in the four types of samples were analyzed, and Fig. 10 shows the results. The BEs of Ti, Si, and Al in films were between those of the pure elements and those of the pure oxides; these indicated that the nanolaminated films exhibited a complex bonding structure. The BEs of Ti, Si, and Al increased with the decrease of Ti content in the films, as shown in Figs. 10(a), 10(b), and 10(d), and the BEs shifts were caused by the substitution of Si or Al for Ti atoms, which was much less electronegative^[22]. As Si or Al replaced Ti atoms, and the average electronegativity of the Si/Al/Ti site increased, the electron density became more tightly bound. Consequently, electron density from the chemical environment surrounding the absorbing atom could stress to a greater extent around the core-hole produced during an XPS experiment, increasing the final-state energy and increasing the BE.

Figure 10.BE spectra for the four elements in different samples. (a) Ti 2p XPS core-line spectra: the BEs of Ti increased with decrease in the Ti content in the film before annealing. (b) Si 2p XPS core-line spectra: the BEs of Si increased with decrease in the Ti content in the film before annealing. (c) O 1s XPS core-line spectra: the BEs of O increased with decrease in the Ti content in the film before annealing. (d) Al 2p XPS core-line spectra: the BEs of Al increased with decrease in the Ti content in the film after annealing at 850°C.

The BE spectra of O showed a multipeak state, indicating that O comprised a variety of chemical bonds, as shown in Fig. 10(c).

As shown in Figs. 11(a) and 11(b), the O 1s XPS core-line spectra of sample A-S2T2 were subjected to peak splitting before and after annealing. Table 3 shows the atomic percentage and BEs. Al ions diffused into the film, and the O 1s XPS core-line spectrum also changed accordingly after annealing at 850°C. This indicated that a chemical bond was formed between the Al ions and O ions. The structure of the mixed oxide film was extremely complicated^[23], and it was often written in the form of each oxide: ${TiO}_{2} {\cdot SiO}_{2}$ before annealing and ${TiO}_{2} {\cdot Al}_{2} O_{3} {\cdot SiO}_{2}$ after annealing.

State	Type	Ti 2p	Si 2p	Al 2p	O 1s A (Ti–O)	O 1s B (Si–O)	O 1s C (Al–O)
Before baking	Atomic percentage (%)	26.0	9.0	–	46.2	18.8	–
Before baking	Binding energy (eV)	457.8	101.1	–	529.5	530.6	–
After baking	Atomic percentage (%)	18.9	11.1	5.7	29.8	22.7	11.8
After baking	Binding energy (eV)	458.0	102.1	73.1	529.6	531.6	530.7

Table 3. Atomic Percentage and BE of Each Constituent Element of Sample A-S2T2 before and after Annealing at 850°C

View all Tables

Figure 11.O 1s XPS core-line spectrum and the peak-fitting situation in sample A-S2T2 (a) before baking and (b) after annealing at 850°C.

4. Discussion

Herein, four types of ${TiO}_{2} / {SiO}_{2}$ nanolaminated films were prepared via ALD, and the influence of the annealing temperature on the surface morphologies, crystal structures, thickness, density, BEs, and ingredient content of the nanolaminated films was analyzed.

Results showed that the ${TiO}_{2} / {SiO}_{2}$ nanolaminated films exhibited a complex bonding structure. The BEs of Ti and Si were between those of the pure elements and pure oxides, and they increased with a decrease in the Ti content; the BEs shifts were caused by the substitution of Si for Ti atoms, which was much less electronegative. As the annealing temperature increased, the thickness of the nanolaminated film decreased, and the density increased, indicating that the annealing process would make the film denser.

The crystallization temperature of the films was proportional to the ${SiO}_{2}$ content of the film mainly because the crystallization temperature of ${SiO}_{2}$ was considerably higher than that of ${TiO}_{2}$ . As the annealing temperature increased, the pure ${TiO}_{2}$ film transformed from the amorphous phase into the anatase phase, the crystal grain size increased, and the lattice spacing decreased. When the annealing temperature increased to 850°C, the crystalline state of the ${TiO}_{2}$ film did not change from the anatase phase to the rutile phase; this phenomenon was attributed to the low thickness of the film.

The Al ions in the substrates diffused into the four types of nanolaminated films and bonded with O after 850°C annealing. In the subsequent tests, the elemental percentages of the four types of thin films on the ${SiO}_{2}$ substrate were measured. It was found that the Si content in the film also increased significantly after annealing at 850°C in the follow-up research results, indicating that the Si ions in substrates also diffused into the film. The experimental results showed that positive ions in the substrate, such as Al and Si, could diffuse into the films with a thickness of tens of nanometers at a certain baking temperature. The diffusion mechanism of Si and Al ions needs to be investigated further.

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