# Synthesis and Dielectric Studies of Monoclinic Nanosized Zirconia

Authors: I. Flavia Princess Nesamani; V. Lakshmi Prabha; Aswathy Paul; D. Nirmal
Journal: Advances in Condensed Matter Physics
 (2014)

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## Abstract

Zirconium dioxide is a prospective high-κ material that can replace silicon dioxide. Zirconium dioxide nanoparticle has been synthesized using sol-gel process at room temperature. The structural and morphological characterization of the nanoscaled zirconium dioxide is done using FTIR, SEM, X-ray diffraction, and TEM. The particle size of the synthesized ZrO2 is observed in the range of 50–80 nm with an average crystallite size of 2–10 nm. The results are compared with commercial coarse zirconia which showed a particle size in the range of 900 nm–2.13 µm and crystallite size of 5.3 nm–20 nm. It is expected that both nanoscaling and the high dielectric constant of ZrO2 would be useful in replacing the low-κ SiO2 dielectric with high-κ ZrO2 for CMOS fabrication technology. The synthesized ZrO2 is subjected to impedance analysis and it exhibited a dielectric constant of 25 to find its application in short channel devices like multiple gate FinFETS and as a suitable alternative for the conventional gate oxide dielectric SiO2 with dielectric value of 3.9, which cannot survive the challenge of an end of oxide thickness ≤ 1 nm.


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## Body

## 1. Introduction

The ever growing semiconductor industry has turned towards nanoscaled devices with high-κ metal oxide gate dielectrics along with tuneable metal gates in transistor manufacturing processes to meet the need for high speed transistors, while maintaining low power consumption. The best metal oxide to replace Silicon dioxide is hafnia or zirconia (HfO2/ZrO2). The particles when synthesized in the magical dimension scale of nanometer [1] have wide applications with the improvement of the electrical, mechanical, and optical properties of any compound due to the increase in surface area. Zirconia, a metal oxide, is expected to be invariant with higher thermal expansion ratio in an FET configuration. A figure of merit to judge a high-κ gate dielectric layer is the equivalent oxide thickness (EOT)=(κSiO2/κhigh-κ)dκ. If a dielectric material with a higher dielectric constant (high-κ) can replace SiO2, the dielectric layer thickness can be increased proportionally while keeping the same gate capacitance [2, 3]. Since a thicker layer is used for insulation, the tunnelling current is drastically reduced in the tunnelling region. Meanwhile, before incorporating a new high-κ material into the present ULSI process flow, many requirements, like particle size, capacitance at different thickness of the material, and so forth, need to be optimized [4–8]. One of the most crucial elements that allow the successful scaling is certainly the material and electrical properties of ZrO2 [4, 5, 9–11] offsets compared to the reported band gap of 5.16 to 7.8 eV for ZrO2. Among the high-κ materials, ZrO2 is a promising candidate in multiple gate FinFETS and a lot of research works focus on this material [2]. Pure zirconia exhibits three stages of polymorphism. The monoclinic, tetragonal, and cubical symmetries are stable up to the temperature range of 1100°C, between 1100 and 2370°C, and above 2370°C, respectively. Zirconium dioxide, from the group of ceramic semiconductors, possesses special transition-metal oxide properties. The ceramic based zirconia is used in variety of applications such as technological and commercial applications, for example, catalyst, oxygen sensors, and high dielectric constant material for large scale integrated circuit. Zirconium dioxide has good thermal stability, high strength, low thermal conductivity, and high corrosion resistance. It is an excellent high-κ gate oxide dielectric widely accepted in high-κ/metal gate stacks for CMOS fabrication technology. Zirconium oxide can be prepared by sol-gel technique (Brinker and Scherer, 1990 [12]) using inorganic precursors like ZroCl2 or metal organic precursors like zirconium alkoxides [13]. Considering the development of new materials, the experimental conditions can be adjusted in order to get stable or metastable compounds. Most studies performed in the field focus on the synthesis of materials with mixed composition [14–17]. The physical and chemical properties of mixed oxides prepared through sol-gel process are of great interest due to their thermal and chemical stability. Besides they are highly acidic because of the presence of OH groups on the surface [18]. Stable dispersions, or sols, of small particles (less than 0.1 micrometre) are formed from precursor chemicals such as metal alkoxides or other metalorganics [18]. By partial evaporation of the liquid or addition of a suitable initiator, a polymer-like, three-dimensional bonding takes place within the sol to form a gelatinous network or gel. The gel can then be dehydrated and calcined to obtain a fine, intimately mixed ceramic powder. This paper presents the synthesis using solgel process and material characterization of the nanoscaled zirconia through FTIR, SEM, TEM, and XRD analysis along with the evaluation of dielectric constant through impedance spectroscopy test.


## 2. Experimental

### 2.1. Synthesis Method

In this work, one of the widely used cost effective sol-gel method [1, 5, 14, 16, 18] is employed for the preparation of ZrO2. The powders with different particle size, morphology, and phase composition can be prepared by varying parameters such as temperature, duration of the processing, and concentration of chemical species to solution [1]. 15 mmol of zirconium oxychloride (ZrOCl2 Sigma Aldrich) is dissolved in distilled water and aqueous ammonia is added drop by drop under constant stirring using magnetic stirrer to form gel-like precipitate until it reached a pH value of 10. The chemical equation is given as
(1)ZrOCl2+2H2O+2NH3→BenzylAlcoholSurfactantpH10ZrO2+2NH4Cl+H2OZrO2(microparticles)→SodiumdodecylSulphateZrO2(nanoparticles)30 mmol of benzyl alcohol was added as surfactant to reduce the surface tension. 30 mmol of sodium dodecyl sulphate is used as phase transition catalyst to form nanoparticle size. Then, the precipitate is filtered through high grade Whattman filter of nanopore size and thoroughly washed with water until elimination of chloride using deionized water. The precipitate is then dried in an oven for 24 hr at 120°C. The solid zirconium dioxide is calcined at 750°C for 2 hrs twice to obtain zirconium dioxide in the white powder form.


### 2.2. Characterization

The particle size distribution was analyzed by Fourier Transform Infrared Spectroscopy done using Schimadzu IR Prestige-21 using KBR pelletizer in the wavelength range of 400–4000 cm−1 with a resolution of 2 cm−1. X-ray diffraction was taken with Schimadzu, Japan Model XRD 6000 at room temperature using target of Cukα with a voltage of 40.0 (kV) and a current of 30.0 (mA) for 2θ between 10° and 90° with a sampling pitch of 1° in scan mode of continuous scan and scan speed of 10 (deg/min). SEM microstructural analysis was performed using a JEOL JSM-6390 microscope. The samples were ultrasonically dispersed in ethanol, deposited on a carbon film supported on a copper grid, and subjected for SEM analysis. TEM microstructural analysis was performed using JOEL JM 2010. Advanced analytical high resolution enables us to give morphological, chemical, and structure information. Electron diffraction crystallographic studies were done on synthesized zirconia powder and SAED (selected area electron diffraction) pattern and CBED (convergent beam electron diffraction) pattern were taken for the sample. The sample was mixed with ethanol and ultrasonically vibrated for 5 minutes and it is coated on carbon coated copper grid and the powder is allowed to dry for 5 minutes before taking the patterns. For the impedance analysis, the powder is sprayed in a die of diameter 10 mm and the thickness of 1.29 mm applying a pressure of 200 Psi. The pellet was sintered at 300°C for 1 hr. Then, the pellet is calcined at various temperatures upto a maximum of 750°C. Increase in processing temperatures (sintering and calcining) results in increase of grain size and reduction of pores or vacancies. Increase in temperature increases the grain size which in turn influences the dielectric and electrical properties of the sample. The growth of grain size is not seen after 750°C and, hence, the dielectric constant also remains constant after this temperature. The grain size growth was observed upto 750°C [19–23] and also it is confirmed that the grain did not grow bigger in size after 750°C [2]. This helps zirconia to have a constant dielectric. The impedance measurements were made with the Impedance Analyzer Hioki 3532-50 LCR Hi Tester with a frequency range of 42 Hz–5 MHz at room temperature as well at elevated temperatures from 50°C to 550°C at the intervals of 50°C. To avoid any diffusion problems, electrodes were not applied on the substrate but the sample was ensured to make complete contact with the electrodes by pressure contact.


## 2.1. Synthesis Method

In this work, one of the widely used cost effective sol-gel method [1, 5, 14, 16, 18] is employed for the preparation of ZrO2. The powders with different particle size, morphology, and phase composition can be prepared by varying parameters such as temperature, duration of the processing, and concentration of chemical species to solution [1]. 15 mmol of zirconium oxychloride (ZrOCl2 Sigma Aldrich) is dissolved in distilled water and aqueous ammonia is added drop by drop under constant stirring using magnetic stirrer to form gel-like precipitate until it reached a pH value of 10. The chemical equation is given as
(1)ZrOCl2+2H2O+2NH3→BenzylAlcoholSurfactantpH10ZrO2+2NH4Cl+H2OZrO2(microparticles)→SodiumdodecylSulphateZrO2(nanoparticles)30 mmol of benzyl alcohol was added as surfactant to reduce the surface tension. 30 mmol of sodium dodecyl sulphate is used as phase transition catalyst to form nanoparticle size. Then, the precipitate is filtered through high grade Whattman filter of nanopore size and thoroughly washed with water until elimination of chloride using deionized water. The precipitate is then dried in an oven for 24 hr at 120°C. The solid zirconium dioxide is calcined at 750°C for 2 hrs twice to obtain zirconium dioxide in the white powder form.


## 2.2. Characterization

The particle size distribution was analyzed by Fourier Transform Infrared Spectroscopy done using Schimadzu IR Prestige-21 using KBR pelletizer in the wavelength range of 400–4000 cm−1 with a resolution of 2 cm−1. X-ray diffraction was taken with Schimadzu, Japan Model XRD 6000 at room temperature using target of Cukα with a voltage of 40.0 (kV) and a current of 30.0 (mA) for 2θ between 10° and 90° with a sampling pitch of 1° in scan mode of continuous scan and scan speed of 10 (deg/min). SEM microstructural analysis was performed using a JEOL JSM-6390 microscope. The samples were ultrasonically dispersed in ethanol, deposited on a carbon film supported on a copper grid, and subjected for SEM analysis. TEM microstructural analysis was performed using JOEL JM 2010. Advanced analytical high resolution enables us to give morphological, chemical, and structure information. Electron diffraction crystallographic studies were done on synthesized zirconia powder and SAED (selected area electron diffraction) pattern and CBED (convergent beam electron diffraction) pattern were taken for the sample. The sample was mixed with ethanol and ultrasonically vibrated for 5 minutes and it is coated on carbon coated copper grid and the powder is allowed to dry for 5 minutes before taking the patterns. For the impedance analysis, the powder is sprayed in a die of diameter 10 mm and the thickness of 1.29 mm applying a pressure of 200 Psi. The pellet was sintered at 300°C for 1 hr. Then, the pellet is calcined at various temperatures upto a maximum of 750°C. Increase in processing temperatures (sintering and calcining) results in increase of grain size and reduction of pores or vacancies. Increase in temperature increases the grain size which in turn influences the dielectric and electrical properties of the sample. The growth of grain size is not seen after 750°C and, hence, the dielectric constant also remains constant after this temperature. The grain size growth was observed upto 750°C [19–23] and also it is confirmed that the grain did not grow bigger in size after 750°C [2]. This helps zirconia to have a constant dielectric. The impedance measurements were made with the Impedance Analyzer Hioki 3532-50 LCR Hi Tester with a frequency range of 42 Hz–5 MHz at room temperature as well at elevated temperatures from 50°C to 550°C at the intervals of 50°C. To avoid any diffusion problems, electrodes were not applied on the substrate but the sample was ensured to make complete contact with the electrodes by pressure contact.


## 3. Results and Discussion

The results of Fourier Transform Infrared Spectroscopy of coarse zirconia showed a strong peak of absorption at 1082.07 cm−1 and two weak peaks at 856.39 cm−1 and 877.61 cm−1 ensuring that the commercial sample is cubic zirconia; however, the synthesized zirconia after calcination at 750°C for two hours twice showed none of the major peaks except a small peak at 860.1 cm−1. Structural and morphological elucidations were checked using XRD (Figure 1), SEM (Figure 2), and TEM (Figure 3). The crystallite size of ZrO2 was estimated by XRD peaks. The reduction of size by sol-gel procedure was confirmed by different trials and all samples exhibit only peaks corresponding to typical ZrO2, [111], [200], and [220] planes. Using the diffraction peaks the crystallite size was calculated through Scherrer’s equation D = (0.9λ/βcosθ) and was found to fall in a range from 2 nm to 10 nm.(a) and (b): XRD pattern of coarse zirconia and synthesized zirconia.
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(b)[(a)-(b)]: SEM images of coarse zirconia; [(c)–(f)]: SEM images of synthesized zirconium dioxide.
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(f)(a) Corresponding selected area diffraction, (b) 100 nm, (c) 50 nm, and (d) 5 nm. TEM images of sol-gel synthesized zirconium dioxide.
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(d)The size of the particles of sol-gel synthesized zirconia, an average diameter of 50 to 80 nm particles sampled at random, was assessed from the corresponding SEM micrographs, whereas for coarse zirconia, the particle size was found to be in the range of 900 nm to 2.13μm (Figures 2(a) and 2(b)).Coarse zirconia has a cubical structure which is inferred from the lattice parameters as indicated from JCPDS file “a=b=c=5.129Å” with a crystallite size of 5.3 nm to 20 nm and particle size of 900 nm–2.13 μm and the synthesized zirconia after calcination at 750°C for 4 hours showed monoclinic structure with the lattice parameters indicated from JCPDS file as “a=5.25, b=5.204 and c=5.213Å” with particle size of 50 nm–80 nm and crystallite size of 2–10 nm determined by Debye-Scherrer method calculated using formula D=kλ/βcos⁡⁡θ. The XRD pattern (Figure 1) of resulting synthesized zirconia is of monoclinic structure that is stable up to 1100°C preferable for CMOS fabrication and it can replace SiO2 which has same stability. The SEM and TEM micrographs indicate that the ZrO2 nanoparticles after calcination were small, monoclinic crystallites in [111], [200], and [220] planes, and that they agglomerated to some extent because of nanocrystalline size and particles getting closer to each other. The obtained impedance spectra were fit to the theoretical equivalent circuit (Figure 4) using Z-Plot software [2, 24]. The values obtained from the fit and the actual data at 550°C are as follows. The equivalent circuit is obtained using the fit and consists of Rs = 1.5K Ω, R1 = 3.05 × 106 Ω, CPEIT = 1.174 × 10−11 F, and CPEIP = 0.98 with the peak frequency f = 3x104 Hz, where ωτ=1. The capacitance and dielectric loss were also measured at this frequency and is found to be CP = 1.7376 × 10−11 F. The same analysis has been made for all the temperatures from room temperature to 550°C. The dielectric constant of zirconia was found from the formula C=Є0ЄrA/d, where Є0 is permittivity of vacuum, Єr is permittivity of zirconia, A is Area of the pellet, and d is diameter of the pellet. The calculated value of nanosized zirconium oxide obtained from the results is 25. Also the conductivity values were verified using log F versus log σ plot (Figure 5) and cole-cole plot (Figures 6(a)–2(f)) and the conductivity was found to be equal at room temperature and at 450°C. The resistance was in the range of 106–107 proving that the nanosized synthesized zirconia has good dielectric constant of 25 [2, 25–28].Figure 4
Equivalent circuit of impedance fit.Figure 5
Comparison of conductivity graph versus logσ for the results ranging from room temperature to 400°C.(a)–(e) Cole-cole plot of synthesized zirconia at different temperatures. (f) Impedance spectra plot of zirconium oxide with fit results at 450°C.
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## 4. Conclusion

In this paper, nanocrystallite monoclinic zirconia powder with particle size of 50–100 nm and crystallite size of 2 nm–10 nm is obtained by sol-gel method, whereas coarse zirconia has a particle size of 1μm to 2 μm with a crystallite size of 50 nm–90 nm. The significant divergence of the phase composition, morphology, and the particle size has been found. The monoclinic phase is found to be predominant for the calcination temperatures up to 750°C [29]. The impedance profile of the nanosized zirconia gave a dielectric constant of 25 [2, 23, 26–28]. This work will be useful in the fabrication of nanosized zirconia as thin films to the required equivalent oxide thickness and analyzing the same for the high-κ gate dielectric materials. When compared with HfO2, a rare earth metal, ZrO2, is cheaper and exhibits the same electrical properties as that of HfO2, which made it viable to be used in designing a high-κ/metal gate, multigate MOSFET, with improved electrical characteristics.



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*Source: 828492-2014-02-16.xml*