Influence of Material Composition on Structural and Optical Properties of HfO₂-TiO₂ Mixed Oxide Coatings

Abstract

In this paper the influence of material composition on the structural, surface and optical properties of HfO₂-TiO₂ mixed oxide coatings was investigated and discussed. Five sets of thin films were deposited using reactive magnetron sputtering: HfO₂, TiO₂ and three sets of mixed HfO₂-TiO₂ coatings with various titanium content. The change in the material composition had a significant influence on the structural, surface and optical properties. All of the deposited coatings, except for (Hf_0.55Ti_0.45)O_x, were nanocrystalline with crystallites ranging from 6.7 nm to 10.8 nm in size. Scanning electron microscopy measurements revealed that surface of nanocrystalline thin films consisted of grains with different shapes and sizes. Based on optical transmission measurements, it was shown that thin films with higher titanium content were characterized by a higher cut-off wavelength, refractive index and lower optical band gap energy. The porosity and packing density were also determined.

1. Introduction

Thin oxide films with precisely defined properties are a strong requirement for the development of modern technologies. Transparent thin films based on titanium dioxide (TiO₂) and hafnium dioxide (HfO₂) are widely used in industrial applications such as optical and protective coatings or optoelectronic devices. HfO₂ and TiO₂ are characterized by many advantages, e.g., very good thermal, chemical, and mechanical stability and high transparency [1,2,3,4,5]. Both are also known as hard oxides with high wear and scratch resistance to mechanical damage.

Due to a high transmittance over a wide spectral range from the ultraviolet (200 nm) to near-infrared (1.2 µm), low optical absorption and dispersion [6,7,8], high reactive index (about 1.85–2.15) [2,9], wide band gap energy (E_g = 5.8 eV) [1,10] and hydrophobic properties [2,9], hafnium dioxide is one of the most commonly used materials in optical applications [11,12]. Depending on the conditions, hafnium oxide can exist in one of the three polymorphous forms. Monoclinic crystal structure is the most thermodynamically stable form at ambient temperature and pressure; however, in temperatures above 1700 °C it transforms into a tetragonal structure, and into a cubic one above 2200 °C [3,4,6,13,14]. Thin films based on HfO₂ are frequently used as innovative materials in numerous optical devices such as optical filters, ultraviolet heat mirrors, antireflection coatings and in cameras used for space applications [2,15]. According to the literature [11,12], amorphous HfO₂ films can be also used in flexible thin film capacitors, fiber-optic waveguides, computer memory elements and optical coatings deposited on polymer substrates.

The physical properties of titanium dioxide are also very good compared to HfO₂. Titanium dioxide is one of the basic “high index” materials, which can be used in the construction of optical coatings, for example [16]. Due to its high refractive index (2.2–2.6), high dielectric constant, optical transmittance in the visible range and photocatalytic activity, TiO₂ is used in various fields of industry [1]. TiO₂ thin films can be used in antireflective and protective coatings, self-cleaning and gas sensing films. It is also applied in solar cells and as antibacterial coatings [17,18,19]. This metal oxide can exist in one of the three crystal structures: anatase (E_g = 3.2–3.3 eV), rutile (E_g = 3.0–3.1 eV) or rarely applied brookite [20]. One can also distinguish mixtures of anatase and rutile or amorphous form. Anatase is metastable and transforms into the rutile at high temperature (ca. 700 °C) [17,18,19,20,21,22]. Rutile is the most stable phase of TiO₂. Titanium dioxide thin films with a rutile phase exhibit high refractive index and good thermal stability [21,22]. According to Lin et al. [21], TiO₂ thin films with a dominant rutile phase showed much greater hardness and Young’s modulus values than the anatase thin films. On the other hand, an anatase phase has higher photocatalytic activity [20,21].

Hafnium and titanium thin films can be prepared using various deposition methods, for example, the sol-gel method, electron beam evaporation, ion beam assisted deposition, chemical vapor deposition, direct current (DC) or radio frequency (RF) magnetron sputtering, atomic layer deposition, etc. [5,9,14,15,19,20,21].

In the literature, the HfO₂-TiO₂ pseudobinary systems have been investigated previously by others [17,23,24,25], especially in view of their electrical properties [26,27,28]. As was shown by Li et al. [26], mixed oxides of HfO₂-TiO₂ had, for example, higher permittivity compared to undoped HfO₂. In the case studies performed by Triyoso et al., the results revealed that mixed HfO₂-TiO₂ oxide and nanolaminate structures had improved charge trapping behavior as compared to undoped HfO₂ or TiO₂. On the other hand, Deen et al. [27] showed results that demonstrated enhanced electrical properties of high-k gate dielectrics based on a HfO₂/TiO₂ multilayer stack. Sequentially deposited HfO₂/TiO₂ thin layers lead to minimizing the gate oxide’s physical thickness, while the gate leakage current suppression characteristics for the 2 + 5 nm thick films were shown to be equivalent to the 10 nm thick TiO₂ films [28]. Therefore, such mixtures can combine the advantages of HfO₂ and TiO₂. These thin films can also exhibit high hardness and very good optical parameters, e.g., a low extinction coefficient [1]. Due to these qualities, mixed HfO₂-TiO₂ thin films can be used as functional optical coatings.

In this paper, the influence of composition of mixed hafnium-titanium thin films deposited by reactive magnetron sputtering on their structural, surface and optical properties have been described.

2. Experimental Section

Magnetron sputtering is one of the most efficient industrial methods for manufacturing thin-film optical coatings. Manufacturing of mixed oxide thin films usually requires sputtering of alloy or powder targets with defined chemical composition. In this work, HfO₂-TiO₂ mixed oxide coatings were prepared with the aid of a multitarget magnetron sputtering apparatus as a result of the simultaneous co-sputtering of Hf (99.5%) and Ti (99.99%) targets mounted on individually supplied magnetrons [29,30,31]. Applied system allows for the deposition of composite coatings from up to four targets. The distribution of the power supplied to each magnetron and their sputtering time were precisely controlled, which was possible due to application of a special control system that is managing the work of MSS2 (DORA Power System) power supply units. The power released to each magnetron is controlled by the pulse width modulation method. The distance between the targets and substrates was equal to 160 mm. The base pressure in the deposition chamber was ca. 5 × 10⁻⁵ mbar, while during the sputtering process it was equal to 2 × 10⁻² mbar. Thin films were sputtered in pure oxygen, without argon as a working gas. Five sets of thin films were prepared: HfO₂, TiO₂ and three HfO₂-TiO₂ mixed oxides with various material composition. The thin films were deposited on several silicon and fused silica substrates with a size of 20 × 20 mm². The silicon substrates were used to assess the material composition of deposited coatings, while the glass substrates were to determine the structural and optical properties. Substrates were not heated during the deposition processes.

The surface morphology of the thin films and their chemical composition were investigated using a FESEM FEI Nova NanoSEM 230 scanning electron microscope equipped with an EDS spectrometer (EDAX Genesis, EDAX Inc., Mahwah, NJ, USA). EDS measurements were performed five times for samples from each deposition process. The EDS used for measurements was calibrated for quantitative analysis and was accurate for qualitative analysis from approximately 0.1 at.%, while for quantitative analysis from ca. 1 at.% of the element content. The differences in material composition of the samples from various sputtering processes were negligible and beyond the error of measurement apparatus. Additionally, SEM images of the surface of deposited thin films were obtained.

The structural properties of TiO₂, HfO₂ and their mixtures were determined based on the results of X-ray diffraction (XRD). For the measurements, a PANalytical Empyrean PIXel3D powder diffractometer with Cu Kα X-ray (1.5406 Å) was used. The correction for the broadening of the XRD instrument was accounted for and the crystallite sizes were calculated using Scherrer’s equation [32].

Optical properties were evaluated on the basis of the transmission measurements. The experimental system was based on an Ocean Optics QE 65000 spectrophotometer and a coupled deuterium-halogen light source. Each transmission spectrum was averaged from five measurements performed for sample with various material compositions. Based on the obtained results, the cut-off wavelength, fundamental absorption edge and optical band gap energy (E_g) were determined. Changes of the cut-off wavelength were very small and in each case equal to ca. 2 nm, which was a value of inaccuracy of the measurement apparatus. A standard deviation of optical band gap energy was also calculated. Additionally, with the aid of reverse engineering method and SCOUT software, the refractive index (n) and extinction coefficient (k) were estimated.

3. Results and Discussion

The X-ray microanalysis was performed to investigate the titanium content in the deposited mixed HfO₂-TiO₂ coatings and revealed that it was equal to 17, 28 and 45 at.%, without taking into consideration signals from oxygen. In Figure 1, an exemplary distribution map of Hf, Ti and O elements in (Hf_0.83Ti_0.17)O_x thin film is shown, and it could be concluded that each element was homogenously distributed in the prepared coating. The area of investigation was ca. 16 μm × 12 μm. The EDS spectra, which show lines from Ti and Hf elements for each thin film, are also presented in Figure 1.

The XRD patterns for as-deposited HfO₂, mixed HfO₂-TiO₂ and TiO₂ thin films are shown in Figure 2. The hafnium dioxide, due to strong and wide diffraction lines, exhibited the nanocrystalline structure of a monoclinic phase with an average crystallites size of ca. 10.7 nm. Thin films with 17 and 28 at.% of titanium exhibited smaller crystallites, of 6.7 and 7.4 nm, respectively. However, further increase of the titanium concentration to 45 at.% in the prepared thin films resulted in a broad, amorphous-like pattern without visible peaks, which could be associated with hafnium dioxide or titanium dioxide phases. Therefore, it can be assumed that such increase of the titanium concentration hinders the crystal growth of prepared mixed oxide thin films. The amorphization of this coating might be caused by the introduction of the local lattice imperfections or very large mismatch of the HfO₂ and TiO₂ unit cell volume. For hafnium its unit cell volume is equal to ca. 140.3 ų, while for titanium it is only ca. 62.4 ų. This, in turn, can lead to the strong growth of the amorphous phase, which began to predominate over the crystalline structure. Similar behavior has been already observed for even small addition of ca. 10 at.% of Nd₂O₃ to TiO₂ thin films [33]. In the case of TiO₂, XRD measurements revealed a trace amount of fine crystallites related to the rutile phase. However, the peak at ca. 27.4 degrees (2θ) corresponding to the (110) rutile plane was broad and had very low intensity. Therefore, determined crystallites size of ca. 10.6 nm might be encumbered with small error. The broadening and low intensity of this peak can also indicate the appearance of a large amount of amorphous phase.

In the case of HfO₂ thin film, XRD measurements revealed a considerable shift of the diffraction peaks towards lower angle (2θ), which indicates presence of tensile stress. The addition of 17 and 28 at.% of titanium resulted in a shift towards a higher angle. However, tensile stress still occurred in the thin film that contain 17 at.% of Ti, while for a coating with 28 at.% of Ti, compressed stress was observed. For TiO₂, tensile stress was again observed. The type of stress occurring in measured coatings was determined on the basis of the Δd parameter from the following equation [34]:

$$\Delta d=\frac{d-d_{PDF}}{d_{PDF}}\times 100\%$$

(1)

where d—interplanar distance, d_PDF—the standard interplanar distance from [35,36].

Results of XRD measurements and analysis are presented in

Table 1. XRD measurements results for as-deposited HfO₂, mixed HfO₂-TiO₂ and TiO₂ thin films.

Thin Film	Crystal Plane	D (nm)	d (Å)	Δd (%)	Type of Stress
PDF No. 65-1142 HfO2–monoclinic [35]	(−111)	–	3.145	–	–
PDF No. 21-1276 TiO2–rutile [36]	(110)	–	3.247	–	–
HfO2	(−111)	10.7	3.171	+0.81	tension
(Hf0.83Ti0.17)Ox		6.7	3.155	+0.32	tension
(Hf0.72Ti0.28)Ox		7.4	3.129	−0.53	compression
(Hf0.55Ti0.45)Ox	amorphous	–	–	–	–
TiO2	(110)	10.8	3.261	+0.43	tension

D—average crystallite size; d—interplanar distance; Δd—percentage change of interplanar distance as-compared to standard (d_PDF) one.

. The positive sign of the Δd parameter speaks to tensile stress and the negative to the compressed one.

SEM images of the surface of as-deposited thin films are shown in Figure 3. All sputtered coatings were crack-free and continuous. HfO₂ thin films consisted of small grains with dimensions of ca. 20–30 nm. In the case of (Hf_0.83Ti_0.17O)_x thin film its surface was built from grains with round shapes, which had various dimensions in the range from ca. 25 nm to 95 nm. For (Hf_0.72Ti_0.28)O_x, the coating of its surface consisted of grains with round shapes of mostly small sizes of ca. 15–25 nm, however also few grains with larger size of ca. 50 nm were visible. A larger amount of Ti in the film, i.e., 45 at.%, resulted in a significant change to the surface morphology. The surface of this thin film was homogenous, very smooth and no grains were observed. SEM image confirmed the XRD results, which showed amorphous behavior of the investigated coating. In the case of the TiO₂ thin film, its surface was covered with particles of different shapes with various dimensions in the range from 30 nm to as much as 150 nm.

Optical properties such as cut-off wavelength (λ_cut-off) and optical band gap energy (E_g) were determined based on transmission spectra measurements in the range from ca. 200 nm to 1000 nm. The results of these measurements and analysis are presented in Figure 4. All as-deposited thin films were transparent in the visible wavelength range with the transmittance level of approximately 80%–90%, depending on the content of titanium. The highest average transmission was exhibited by pure hafnium, while the lowest (with the largest amplitude of optical interferences) was obtained for undoped titanium.

The results of cut-off wavelength measurements for all thin films are compared in Figure 4b. For hafnium, it is lower than 200 nm within the measured spectral range. The addition of 17 at.% of titanium to the thin film resulted in a significant shift of λ_cut-off towards longer wavelength, i.e., 277 nm. Further increases in titanium content to 28 at.% and 45 at.% caused a shift in λ_cut-off to 298 nm and 310 nm, respectively. In the case of undoped titanium, λ_cut-off is equal to 344 nm. It seems that increasing the titanium content to HfO₂ caused a redshift of the cut-off wavelength. The estimation of the absorption edge (Figure 4c) showed that it has decreased with the increase of titanium content in the thin film from 6.84 eV for undoped HfO₂ to 3.27 eV for pure TiO₂.

The optical band gap energy is shown in Figure 4c and was determined from Tauc plots (αhν)^1/2 in the function of photon energy (eV). It is important to estimate the optical band gap energy since it shows the energy needed for the transfer of the electron from the valence to the conduction band. For pure hafnium, E_g is equal to 6.08 eV for indirect transitions. The addition of 17 at.% of titanium caused a decrease in the value of the optical band gap to 3.41 eV. Further increases of the titanium amounts in deposited thin films to 28 at.% and 45 at.% caused a small, but gradual decrease of the optical band to 3.39 eV and 3.36 eV. The lowest value of E_g was obtained for undoped TiO₂, equal to 3.11 eV. Similarly to the cut-off wavelength, the change of the value of optical band gap is caused by the increase of TiO₂ content in the mixed oxide thin films.

In Figure 5, the spectral characteristics of the refractive index (n) and extinction coefficient (k) are presented. For the calculation of n and k dispersion curves, the reverse engineering method was used with the aid of SCOUT software [37]. Additionally, the thickness of each of the samples was also estimated. TiO₂ and HfO₂ thin films were sputtered from single Ti or Hf metallic targets. Their thickness was similar and equal to 359 nm and 371 nm, respectively. Thin films of mixed HfO₂-TiO₂ coatings were sputtered from two independently powered targets, and the thickness of Hf_0.83Ti_0.17O_x, Hf_0.72Ti_0.28O_x and Hf_0.55Ti_0.45O_x was equal to 453 nm, 472 nm and 526 nm, respectively.

The values of the refractive index and extinction coefficient are dependent on the TiO₂ concentration in the thin films. It was observed that with the increase of titanium content in the coatings, the refractive index gradually increased, while the extinction coefficient decreased. Changes in the refractive index and extinction coefficient determined for λ = 550 nm are presented in Figure 6a. The refractive index for as-deposited coatings increased from 1.81 for HfO₂ to 2.41 for TiO₂. The mechanism that might be related to the increase of n is directly related to the increase of the content of titanium in the thin films, which have significantly higher value than hafnium. Values of n for bulk HfO₂ with a monoclinic phase, and for TiO₂ with a rutile phase, are equal to 2.12 [38] and 2.65 [39], respectively. Therefore, the obtained values for thin films were slightly lower and stood out from the bulk ones.

Based on the obtained values of n, porosity (P) and packing density (PD) were calculated for different compositions. Porosity was estimated using following equation [40,41]:

$$P(\%)=\left[1-\left(\frac{n_f^2-1}{n_b^2-1}\right)\right]\times 100\%$$

(2)

where n_f—measured refractive index of deposited thin film, n_b—refractive index of the correspondent bulk material.

In turn, the packing density (PD) of a film is defined as the ratio of the average film density (ρ_f) and the bulk density (ρ_b) according to the equation [33,40,41]:

$$PD=\frac{{\rho }_f}{{\rho }_b}$$

(3)

The correlation between the film refractive index and its packing density can be expressed by [40,42,43]:

$$PD=\frac{\left(n_f^2-1\right)\times \left(n_b^2+2\right)}{\left(n_f^2+2\right)\times \left(n_b^2-1\right)}$$

(4)

It is also necessary to determine the refractive index of the correspondent bulk material for each HfO₂-TiO₂ composition according to the Lorentz-Lorentz relationship [40,44]:

$$\frac{(n_b^2-1)}{(n_b^2+2)}=f_1\cdot \frac{\left(n_1^2-1\right)}{\left(n_1^2+2\right)}+f_2\cdot \frac{\left(n_2^2-1\right)}{\left(n_2^2+2\right)}$$

(5)

where n₁, n₂ and f₁, f₂ are the refractive indices of the bulk components and their molar ratios in the HfO₂-TiO₂ composite material, respectively.

Using a determined refractive index of deposited thin films, it can be seen that the highest porosity and simultaneously the lowest packing density was obtained for (Hf_0.72Ti_0.28)O_x thin films. In the case of HfO₂, (Hf_0.0.83Ti_0.17)O_x and (Hf_0.55Ti_0.45)O_x porosity was very similar. The lowest value of porosity was obtained for undoped TiO₂. Therefore, it can be assumed that titanium exhibits the most closely packed structure, which was also confirmed by the calculation of the packing density. The packing density for all HfO₂ and mixed HfO₂-TiO₂ coatings revealed only negligible changes. The results of the dependence of porosity and packing density on TiO₂ content is shown in Figure 6b. Results of optical properties measurements are summarized in

Table 2. Optical properties measurements result of deposited thin films

Thin Film	λcut-off (nm)	Eg (eV)	n (at 550 nm)	k (at 550 nm)	P (%)	PD
HfO2	<215 ± 2	6.08 ± 0.07	1.81	3.7× 10−4	34.7	0.80
(Hf0.83Ti0.17)Ox	277 ± 2	3.41 ± 0.03	1.86	4 × 10−5	35.4	0.81
(Hf0.72Ti0.28)Ox	298 ± 2	3.39 ± 0.03	1.86	3 × 10−5	39.1	0.79
(Hf0.55Ti0.45)Ox	310 ± 2	3.36 ± 0.02	1.97	3 × 10−5	34.8	0.82
TiO2	344 ± 2	3.11 ± 0.02	2.41	2 × 10−5	20.0	0.92

.

4. Conclusions

In this paper HfO₂, mixed HfO₂-TiO₂ and TiO₂ coatings were deposited using magnetron sputtering. Through the change in the power released by each magnetron, it was possible to obtain mixed HfO₂-TiO₂ coatings with various amount of titanium content.

The change of the titanium content in mixed oxide thin films resulted in different structural, surface and optical properties. In the case of structural properties, XRD studies revealed that except for amorphous (Hf_0.55Ti_0.45)O_x thin film, all of the deposited coatings were nanocrystalline. The crystallite sizes of undoped hafnium and titanium were larger than those of mixed oxides by ca. 50%. The increase in the amount of titanium also resulted in the change of surface morphology. SEM studies in the case of (Hf_0.55Ti_0.45)O_x thin films seem to confirm the XRD measurements and speak to the amorphous behavior of this coating.

Optical properties changed significantly with the material composition of each thin film. The cut-off wavelength and refractive index increased with the increase in titanium, while the optical band gap and the extinction coefficient simultaneously decreased. It was determined that the most porous thin film, which also exhibited the lowest packing density, was (Hf_0.72Ti_0.28)O_x. On the other hand, undoped TiO₂ had the lowest porosity and the highest packing density.