Effect of Annealing Temperature on the Structure and Properties of La₂O₃ High-K Gate Dielectric Films Prepared by the Sol-Gel Method

Abstract

This article presents the sol-gel method for depositing La₂O₃ thin films on n-type Si substrates and quartz substrates, and investigates the impact of annealing temperature on the microcomposition, surface morphology, optical properties, and band characteristics of the films. X-ray diffraction (XRD) analysis indicates that the films are amorphous below 500 °C, with annealing resulting in a hexagonal-phase La₂O₃ (h-a₂O₃) and new non-hydrated impurities. Fourier-transform infrared (FTIR) analysis reveals that the prepared La₂O₃ film is unaffected by moisture. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) provide evidence that the La₂O₃ film has a smooth, uniform surface without cracks. The roughness increases from 0.426 nm to 1.200 nm, and the film thins from 54.85 nm to 49.80 nm as the annealing temperature rises. The film’s transmittance is above 75%, as measured by UV-Vis, and the calculated optical bandgap increases from 5.11 eV to 5.75 eV. The calculated band offset of the La₂O₃ film is greater than 1 eV, which meets the minimum requirements for MOS devices, thus providing promising prospects for La₂O₃ films in MOS applications.

1. Introduction

The microelectronics industry is still predominantly using the “scaling down” technique of silicon-based complementary metal-oxide-semiconductor (CMOS) devices to achieve miniaturization, low power consumption, and high reliability [1,2,3,4,5]. However, as the devices become smaller, the leakage-current- and power-consumption-related issues caused by the physical limit of the conventional gate oxide SiO₂ result in the failure of semiconductor devices [6]. Therefore, researchers have chosen high-k materials to replace SiO₂ as the gate material of CMOS devices to address the issues of the charge tunneling effect brought about by the development of the technology [7,8,9]. In addition to needing high k values, high-k materials used as gate dielectrics in CMOS devices need to meet the following requirements: (1) moderate bandgap width (E_g), conduction band offset (ΔE_c), and valence band offset (ΔE_v) [10]; (2) the high-k gate dielectric thin film should have a single crystal or amorphous morphology [11,12]; (3) good thermal stability between the substrate and the high-k dielectric material [13]. Among them, silicon oxynitride (SiO_xN_y) is one of the earliest high-k gate dielectric materials that have been studied. However, in the 2007 ITRS report, it could no longer meet the requirements of gate leakage for continued miniaturization [14]. Subsequently, materials such as HfO₂, TiO₂, ZrO₂, and rare earth oxides have been widely used as high-k gate dielectric materials [15,16].

Rare earth elements, represented by La₂O₃, have high dielectric constant (~27), wide bandgap (~5.3 eV), and significant conduction band offset with Si substrate, as well as good thermal stability, making it one of the most promising alternative materials in CMOS devices [17,18]. The high-k value and large bandgap of La₂O₃ result in relatively low leakage current in its application in CMOS devices. Wu et al. [19] applied a variety of high-k materials to ideal MOS devices and conducted quantum mechanical simulation experiments based on the WKB approximation. The results showed that the direct tunneling current decreases with the increase in dielectric constant or barrier height, with La₂O₃ having the lowest leakage current. La₂O₃ also possesses excellent properties in terms of electrical characteristics. Kakushima et al. [20] prepared La₂O₃ thin films and tested the electrical characteristics of sample MOSFETs. It was shown that the peak mobility increased from 60 cm²/Vs to 300 cm²/Vs and the subthreshold swing (SS) decreased from 120 mV/dec to 66 mV/dec. La₂O₃ exists in the cubic phase (c-La₂O₃) at low temperatures, while at high temperatures it exists mainly in the hexagonal phase (h-La₂O₃) [21,22]. h-La₂O₃ is commonly used as a gate dielectric in metal oxide semiconductor field effect transistors (MOSFETs) [23,24,25].

There are several methods for preparing films, including ion beam sputtering [26], chemical vapor deposition [27], and atomic layer deposition [28]. Although these technologies can produce films with excellent structures and properties, most of them require high-vacuum conditions and complex operations, resulting in high production costs. In comparison, the sol-gel method [29,30], which is inexpensive and easy to operate, is our preferred technology for preparing large-area, high-purity, and uniform thin films. Using the sol-gel method, sub-nanometer smooth La_xZr_yO_z (LZO) films have been successfully prepared and have exhibited excellent electrical properties [31]. However, in previous reports, the dielectric constant of La₂O₃ was found to be significantly lower than its theoretical value of 27, and even lower than 10 [32,33]. Researchers have found that one important reason for this is the strong hygroscopicity of La₂O₃, which is unstable and forms La(OH)₃ or LaOOH in atmospheric environment, leading to a decrease in its dielectric constant. The hygroscopicity of La₂O₃ is due to its high ionization, which causes a direct reaction with H₂O:

$${\mathrm{La}}_2{\mathrm{O}}_3\to 2{\mathrm{La}}^{3+}+3{\mathrm{O}}^{2-}$$

(1)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}^{2-}\to 2{\mathrm{OH}}^{-}$$

(2)

Experimental results showed that after being exposed to air for 12 h, the root mean square (RMS) surface roughness of La₂O₃ increased from 0.5 nm to 2.4 nm, which may be due to uneven volume expansion caused by moisture absorption of the La₂O₃ film [34]. Therefore, in this study, La₂O₃ thin films with excellent surface morphology were obtained by annealing in a nitrogen atmosphere and storing them in a vacuum environment. The sol-gel method, which is currently not extensively studied, was used to prepare La₂O₃ thin films to systematically investigate the microstructure, surface morphology, optical properties, and band characteristics of La₂O₃ thin films at different annealing temperatures. The feasibility of La₂O₃ thin films prepared by the sol-gel method as gate dielectric thin films in CMOS devices was evaluated.

2. Materials and Methods

2.1. Precursor Solution Preparation

The preparation of the precursor solution involves two processes: hydrolysis of lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O, Aladdin, Shanghai, China 99.5%) and aging of La(NO₃)₃·6H₂O. Firstly, La(NO₃)₃·6H₂O was dissolved in ethylene glycol monomethyl ether (C₃H₈O₂, Aladdin, Shanghai, China) to prepare a precursor solution with a concentration of 0.3 mol/L. Secondly, the solution was sonicated for 10 min and stirred for an hour in a stirrer. A solution of citric acid (C₆H₈O₇, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), with a molar ratio of 1:1 to La(NO₃)₃·6H₂O metal, was dissolved into 10 mL of C₃H₈O₂ and stirred for 1 h on a magnetic stirrer. The citric acid solution was slowly added dropwise to the stirring La(NO₃)₃·6H₂O solution and stirred for 2 h. Finally, 250 μL of deionized water was added and stirred for an additional 3 h to ensure complete hydrolysis reaction. The solution was then vacuum aged at 30 °C in a light-avoiding area for 48 h to obtain the La₂O₃ precursor solution.

2.2. Preparation of La₂O₃ Films

In this work, La₂O₃ films were prepared on n-type silicon (001) and quartz substrates with a resistivity of 0–20 Ω·cm using the sol-gel method. First, the substrates were sonicated in deionized water for 30 min, followed by sonication in acetonefor 20 min to remove organic substances and impurity ions on the surface. Next, the substrates were cleaned with ethanol to remove any acetone residue from the previous step. Then, 1% diluted hydrofluoric acid (HF, Tianjin Xinbote Chemical Co., Ltd., 40%, Tianjin, China) was used to sonicate the substrates and eliminate the naturally formed oxide layer on the surface. Finally, the substrates were sonicated in deionized water, vacuum dried, and stored for later use.

After filtering the precursor solution through a 0.2 μm polytetrafluoroethylene (PTFE) membrane filter, the solution was dropped onto a substrate fixed on a spin coater and spun at 3000 rpm for 30 s. The coated substrate was then covered with a cover glass on a heating plate (HP-1515, Wenzhou Hanbang Electronics, Zhejiang, China) and heated for 5 min at 150 °C to cure the film and remove excess organic solvent while isolating the air.

Finally, the samples were annealed in a nitrogen atmosphere at 400, 500, 600, and 700 °C for 2 h using a vacuum tube furnace (OTF-1200X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China). The annealing rate was set at 10 °C/min, and after annealing, the furnace was cooled to room temperature.

2.3. Film Characterization

The microstructure of La₂O₃ thin film was analyzed by X-ray diffraction (XRD) using Cu Kα radiation with a scanning rate of 3°/min and a scanning range of 20° ≤ 2θ ≤ 60°, performed with a Bruker instrument located in Karlsruhe, Germany; Fourier transform infrared spectroscopy (FTIR, Bruker, Karlsruhe, Germany); and steady-state and time-resolved fluorescence spectrometer (FLS1000/FS5, Edinburgh Instruments, Edinburgh, UK). Atomic force microscopy (AFM, Bruker Dimension Fastscan) and scanning electron microscopy (SEM, HELIOS Nano Lab 600i) were used to describe the morphology of the film. X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi) was used to determine the chemical bonding and valence band spectra of the film. Ultraviolet-visible spectrophotometry (UV-Vis, Lambda 650S, PerkinElmer, Norwalk, CT, USA) was used to measure the transmission spectra of the film deposited on a quartz substrate and to calculate the band gap.

3. Results

3.1. Microstructure Analysis

Figure 1 shows X-ray diffraction (XRD) patterns of La₂O₃ films deposited on the Si substrate at different annealing temperatures. The standard XRD pattern from the Powder Diffraction File (PDF-05-0602) of the Joint Committee on Powder Diffraction Standards (JCPDS) was added to compare with the sample XRD patterns and confirm the crystalline phase of the La₂O₃ film as h-La₂O₃. La₂O₃ has a hygroscopic property and tends to hydrate with water from the air. Therefore, thermal annealing for La₂O₃ film preparation was conducted in a vacuum environment and immediately followed by vacuum storage to avoid exposure to moisture. As a result, no diffraction peaks of h-La(OH)₃ or LaOOH were observed in the La₂O₃ film, indicating that the film was not affected by water vapor in the air during preparation and storage.

Thin films at 400 °C and 500 °C did not show any diffraction peaks, indicating that the films were in an amorphous state below 500 °C. As the temperature increased, weak diffraction peaks appeared in the film at 600 °C, located at 2θ = 29.96° and 39.53°, corresponding to the (101) and (102) crystal planes of standard h-La₂O₃ [35,36]. Similarly, in the diffraction peaks observed at 700 °C, characteristic diffraction peaks were observed at the same positions, and the crystallinity significantly increased. The results indicate that the crystalline state of the film did not undergo obvious changes with the increase in annealing temperature and no phase transition occurred, demonstrating good thermal stability. The inset in Figure 1 shows the photoluminescence (PL) spectrum of a crystallized thin film under 365 nm ultraviolet light irradiation. The curve in the figure is smooth, and no peak representing defects is observed. Moreover, with the increase of annealing temperature, no significant change can be seen in the PL spectrum, indicating that the structural changes are not caused by defects.

The FTIR spectroscopy is employed to detect the levels of hydroxides, water, and nitrates in La₂O₃ thin film after annealing at different temperatures. According to research, the remaining hydroxides and water are monitored by the broad O-H stretching mode peak near 3500 cm⁻¹ and the sharp H-O-H bending mode peak near 1650 cm⁻¹, respectively [31,37,38]. Figure 2 displays the FTIR spectra of La₂O₃ thin films annealed at different temperatures. It can be observed that there are no peaks appearing near 3500 cm⁻¹ and 1650 cm⁻¹ as mentioned above, and the curves are smooth, indicating that the vacuum environment was well maintained during the preparation process, and the thin film samples had either no adsorbed water or only a small amount present, and had undergone complete condensation. The content of NO_x (monitored via the N-O stretching mode with a center around 1280 cm⁻¹ and 1460 cm⁻¹) [31] decreased sharply at 600 °C and 700 °C. The NOx mainly originated from La(NO₃)₃·6H₂O. Similarly, the absorption band at 1465 cm⁻¹ corresponded to the carbonates produced by the reaction of CO₂ and water vapor in the atmosphere with the La₂O₃ surface. Due to the alkaline properties of La₂O₃, the carbonates were formed and chemically adsorbed on its surface [39,40]. As the temperature increased, there was a significant decrease in peak intensity within this wavenumber range. These losses are crucial for the stability of the film, but at temperatures as high as 700 °C, residual NO_x seems to still be retained in the film. The peak appearing at 750 cm⁻¹ corresponds to the vibrational mode of O-La-O [41,42].

3.2. Micromorphology Surface Morphology

Scanning electron microscopy (SEM) was performed to analyze the surface and cross-section of the thin film samples for morphological analysis. Figure 3 shows the SEM images of the surface and cross-section of La₂O₃ films annealed at different temperatures. It can be observed from the images that all La₂O₃ films are uniform, dense, and continuously intact. However, at an annealing temperature of 700 °C (Figure 3d), irregular bright grains appeared on the surface of the film, with clear particle boundaries, leading to an increase in surface roughness. The inserted figure shows the cross-sectional SEM image of the sample, and it can be clearly seen that the thickness of the film decreased from the initial 54.85 nm to 49.80 nm. This may be due to the transformation of hydroxy groups in the film prepared by sol-gel method into oxides, as well as the volatilization of residual solvents in the film. In addition, there is no gap between the film and the substrate bonding, which is tight and seamless.

Figure 4a–d gives the surface microstructure maps of the tested thin film samples at different annealing temperatures. AFM tests were performed for the samples at annealing temperatures of 400 °C, 500 °C, 600 °C, and 700 °C with a scan range of 1 µm × 1 µm. Three AFM tests were conducted on the same sample, and the corresponding RMS roughness standard deviations were found to be 0.403 ± 0.064 nm, 1.059 ± 0.096 nm, 1.201 ± 0.255 nm, and 1.240 ± 0.115 nm. The La₂O₃ films have a relatively rough surface and the surface roughness varies with large amplitude and high span, exhibiting a surface nearly twice as rough as 400 °C, at 500 °C, presumably due to the change in crystal shape. Through the XRD analysis, we found that the film was in the process of amorphous to crystalline transition at 500 °C. It was clearly observed that the film started to crystallize at 600 °C and the crystallinity increased with increasing temperature; therefore, it was speculated that the crystallization characteristics of the film led to the change in roughness. The increase in temperature also makes the molecules and atoms of the object more active, and after obtaining enough energy, the atomic clusters merge further, prompting the clustering of small molecules into large molecular clusters. The crystallization also produces the result of large grains [43].

3.3. Chemical Bonding Analysis

XPS analysis was employed to analyze the composition of La₂O₃ thin films deposited on silicon substrate at different annealing temperatures. Figure 5a shows the XPS full spectra of the La₂O₃ films at 400 °C–700 °C. It is evident from the full spectra that the samples contain elements such as lanthanum, oxygen, and silicon.

In Figure 5b, the La 3d spectrum of the La₂O₃ films at different temperatures can be seen. The La 3d spectrum has a main peak (La 3d_5/2) and a spin-splitting peak (La 3d_3/2), with their differences being 16.70 eV, 16.85 eV, 16.90 eV, and 17.00 eV at 400–700 °C, respectively. According to literature, the splitting energy of La 3d_3/2 and La 3d_5/2 is about 16.80 eV, representing the La+ state, and the binding energy range of La 3d_5/2 (La³⁺) in La₂O₃ is 834.0–835.1 eV [21]. The shift towards a lower binding energy indicates the presence of O–O bonding in the middle of the La₂O₃ film layer. As the temperature increases to 700 °C, the binding energy shifts towards a higher energy range, which can be explained by the second-nearest-neighbor effect, due to the La-silicate layer being closer to the interface of the Si substrate [44].

Figure 6 shows the O 1s XPS spectra of La₂O₃ thin films annealed at temperatures ranging from 400 °C to 700 °C. By fitting the O 1s spectra of the thin film samples, it was found that the film could be divided into lattice oxygen (La-O), oxygen vacancies (Vo), and hydroxyl groups (La-OH) covalently bonded with La ions at the annealing temperatures of 400 °C to 700 °C. The existence of La-OH was due to the incomplete evaporation of the sol-gel precursor [22]. It was observed that the contents of La-OH and Vo were high at 400 °C, and gradually decreased with increasing temperature, while the content of La-O increased significantly. This can be explained by the thermal-driven condensation of La-OH bonds into La-O bonds at higher temperatures, and the suppression of oxygen vacancies (Vo) generation by the annealing temperature, which increased the coordination number of metal ions and promoted the bonding between metal and oxygen ions [21].

3.4. Optical Properties

La₂O₃ thin films were prepared on quartz substrates at different annealing temperatures, and their optical properties were characterized using UV-Vis spectrophotometry in the wavelength range of 200–800 nm. Figure 7 shows that La₂O₃ thin films on quartz substrates exhibit high transmittance within the range of 200–800 nm, and the average transmittance of all the thin film samples is above 75%. The change in transmittance is caused by the alteration of the thin film crystal structure and thickness due to annealing temperature. Analysis of the XRD spectra shows that the thin film sample at 500 °C is in a mixed state of crystalline and amorphous phases with many defects. Therefore, the scattering of the thin film is strong, resulting in a lower transmittance at the temperature than that at 400 °C [21]. When the annealing temperature was set at 600 °C and 700 °C, the film thickness decreased to 49.80 nm, resulting in a higher transmittance of the thin film sample than that of the sample annealed below 500 °C. However, with the increase of crystallinity, the sample annealed at 700 °C had more interfacial scattering due to the increase of crystal planes, resulting in a lower transmittance compared to the sample annealed at 600 °C.

3.5. Energy Level Characterization

In this experiment, the transmittance of the La₂O₃ thin film deposited on a quartz substrate was measured in the UV range. Using the Tauc formula, the relationship between (αhυ)² and hυ was obtained, and the bandgap width was calculated [45,46]:

(αhυ)² = A(hυ − E_g)

(3)

where α, hυ, A, and Eg are the absorption coefficient, photon energy, constant, and optical band gap of the La₂O₃ film, respectively. Figure 8a–d show the curves of (αhυ)² versus hυ for the films deposited on the quartz substrate. The bandgap values are obtained by tangenting the curves related to hυ and (αhυ)² for the horizontal and vertical axes, respectively. Figure 8 shows the relationship between the optical bandgap and annealing temperature. The results indicate that the bandgap of La₂O₃ film increases from 5.31 eV to 5.74 eV as the annealing temperature increases up to 600 °C [46]. However, as the temperature is further increased from 600 °C to 700 °C, the bandgap width of the La₂O₃ film starts to decrease, dropping from 5.74 eV to 5.73 eV. This may be due to the transition of the film from a short-range ordered, long-range disordered state to a long-range ordered state with increasing temperature.

In this experiment, the determination of valence band discontinuity (ΔEv) and conduction band discontinuity (ΔEc) between the La₂O₃ thin film and the Si substrate was conducted using a core-level method based on the photoelectric effect. The values of ΔEv and ΔEc determined using this method can be obtained by calculating Equations (4) and (5) [47]:

$$\Delta {\mathrm{E}}_{\mathrm{v}}({\mathrm{La}}_2{\mathrm{O}}_3/\mathrm{Si})={\mathrm{E}}_{\mathrm{v}}\left({\mathrm{La}}_2{\mathrm{O}}_3\right)-{\mathrm{E}}_{\mathrm{v}}\left(\mathrm{Si}\right)$$

(4)

In this study, Ev(La₂O₃) refers to the highest valence band maximum (VBM) of the La₂O₃ thin film with respect to the silicon substrate, which is determined using linear extrapolation of the valence band maximum position of the La₂O₃ thin film. Figure 9a provides the valence band spectra of the La₂O₃ thin films annealed at different temperatures, with VBM values of 2.20 eV, 1.93 eV, 2.50 eV, and 2.82 eV for the thin films annealed at 400 °C, 500 °C, 600 °C, and 700 °C, respectively. Ev(Si) represents the valence band maximum of the silicon substrate, with a value of 0.50 eV. Based on the aforementioned equation, the VBO values at annealing temperatures of 400 °C, 500 °C, 600 °C, and 700 °C are determined to be 1.70 eV, 1.43 eV, 2.00 eV, and 2.32 eV.

The conduction band shift ΔEc of the La₂O₃ film can be calculated from (5) as:

$${\Delta \mathrm{E}}_{\mathrm{c}}({\mathrm{La}}_2{\mathrm{O}}_3/\mathrm{Si})={\mathrm{E}}_{\mathrm{g}}({\mathrm{La}}_2{\mathrm{O}}_3)-{\Delta \mathrm{E}}_{\mathrm{v}}({\mathrm{La}}_2{\mathrm{O}}_3/\mathrm{Si})-{\mathrm{E}}_{\mathrm{g}}\left(\mathrm{Si}\right)$$

(5)

In this study, Eg (La₂O₃) denotes the bandgap of the La₂O₃ thin film, which was measured using UV-Vis analysis of the film deposited on a quartz substrate, as described earlier. The bandgap value of the silicon substrate is known to be 1.12 eV. Using the relevant equation, the maximum conduction band values at annealing temperatures of 400 °C, 500 °C, 600 °C, and 700 °C are determined to be 2.49 eV, 3.10 eV, 2.62 eV, and 2.29 eV, respectively. Figure 9b illustrates the schematic energy band alignment of the La₂O₃ films on the silicon substrate at different annealing temperatures, based on the calculated and measured data. As previously reported, the band offset for gate dielectric layers should be greater than 1 eV. In this work, all La₂O₃ thin films fulfill this requirement, with a significant VBO and CBO exceeding 2 eV observed in the samples annealed above 600 °C, indicating that La₂O₃ is a promising high-k material for meeting the demand for band offset in CMOS devices, and can effectively suppress carrier tunneling phenomena.

4. Conclusions

In this study, La₂O₃ thin films were successfully prepared via the sol-gel method, and their microstructure, interfacial chemistry, optical properties, and bandgap width were systematically analyzed. The results showed that the La₂O₃ thin film started to crystallize at 600 °C and was confirmed to be h-La₂O₃ by comparison with the standard PDF card, with no impurity peaks generated. FTIR further confirmed that there was only a small amount of adsorbed water on the film surface. The film was tightly bound to the substrate, and the thickness decreased from 54.85 nm to 49.80 nm with the increase of annealing temperature, while the surface roughness showed an increasing trend. This indicates that although the increasing annealing temperature is conducive to improving the density of the film, too high a temperature may cause further atomic aggregation of the film. The La₂O₃ thin film has a high transmittance in the range of 200–800 nm, all above 70%, and the optical bandgap was calculated. The highest bandgap width of 5.74 eV was achieved at 600 °C, and both the conduction band offset and the valence band offset met the lowest requirements for CMOS barrier height.