Microstructure, Optical, and Electrical Properties of Barium Titanate (BaTiO₃) and Ba_1−xNd_xTiO₃ Thin Films Deposited by Chemical Solution Deposition (CSD) Method

Abstract

Barium titanate (BaTiO₃) and Ba_1−xNd_xTiO₃ thin films used by chemical solution deposition method and prepared with spin-coating technique. The thin films of BaTiO₃ and Ba_1−xNd_xTiO₃ were grown on a quartz and silicon (Si) substrate. The research aim is to get the best of mole variation of Nd doping to ferroelectric material properties. Doping Nd was varied at 5, 10% in BaTiO₃. The thin films were annealed at 850 °C with one hour holding time. The crystal structure, optical and electrical properties of BaTiO₃ and Ba_1−xNd_xTiO₃ have been investigated. The microstructure of BaTiO₃ and Ba_1−xNd_xTiO₃ were analyzed using XRD equipment, which showed that BaTiO₃ and Ba_1−xNd_xTiO₃ crystal had a perovskite shape. The addition mole dopant (x) neodymium (Nd³⁺), the diffraction angle of the XRD results were seen shifting to the right. As the mole dopant (x) Nd increased, so crystallinity from the Ba_1−xNd_xTiO₃ thin films decreased. The crystallite size of BaTiO₃, Ba_0.95Nd_0.05TiO₃, and Ba_0.9Nd_0.1TiO₃ were 70; 21; 28 nm. The addition mole dopant (x) Nd³⁺ resulted in smaller grain sizes but it did not significantly affect thickness. The greater the addition mole dopant (x) Nd, the smaller the band-gap produced. The results of the IV characterization showed that the Ba_1−xNd_xTiO₃ thin films had a response to light and photovoltaic effects.

1 Introduction

The development of science and technology is increasingly advanced, particularly in Physics in the area of electronics. The growth of thin films is mostly done and applied in the electronics field such as solar cells. Solar cells are the use of solar energy as an alternative energy that is made by converting solar energy into electrical energy. Ferroelectric is a dielectric material that is able to change the electric polarization spontaneously due to the application of an external electric field [1]. The development of ferroelectric thin film research is increasingly becoming a great potential issue since it can be applied to solar cells [2]. The electric field in the ferroelectric solar cells can control and regulate the response of solar cells [3]. When ferroelectric material is applied to solar cells under ultraviolet lighting, this ferroelectric material will have a high output voltage i.e., above 1 kV [2].

BaTiO₃ is one of the materials that attract the attention of many researchers because its ferroelectric nature has a high dielectric constant and a large electro-optical coefficient [4]. The BaTiO₃ thin film has an ABO₃ perovskite structure, where A is located at the corner of the cube, B is located on the diagonal of the cube space, and O is located at the diagonal plane of the cube [5]. Perovskite BaTiO₃ has five types of crystal structure including hexagonal, cubic, tetragonal, orthorhombic, and rhombohedral. Each of which depends on the temperature of the phase [6]. BaTiO₃ can be made in the form of thin films using several deposition methods such as vacuum and non-vacuum methods. The vacuum method consists of Laser Ablation, Chemical Vapor Deposition (CVD), PVD, Ion Plating [7]. The non-vacuum method consists of Screen Printing, Dip Coating, Electrophoresis, Electrodeposited, Spray Pyrolysis, and Spin Coating [5], sol-gel or Chemical Solution Deposition (CSD) [6]. CSD is a thin-film manufacturing method that combines physical and chemical methods. The steps of making thin films by using CSD method include making solutions, conducting spin coating process with a certain rotational speed, then conducting annealing process [8].

BaTiO₃ has a wide gap energy (Eg > 3 eV) [3]. To use the BaTiO₃ as solar cell material, it is necessary to reduce the energy gap by giving doping. Materials that can be used for doping administration such as Ni²⁺, Er³⁺, Mn⁴⁺, Zn²⁺, Hf⁴⁺, Fe²⁺, Ce⁴⁺, W⁶⁺, Gd³⁺, Nd³⁺, Sr²⁺ and Co²⁺ [3]. Atoms in perovskite materials can be doped using other atoms provided in which the radius of the atom is almost the same with the radius of the atom replaced [8]. The addition of neodymium (Nd³⁺) can replace Ba²⁺ in the position of perovskite A. This happens since the atomic radius Nd³⁺ (0.995 Å) is smaller than Ba²⁺ (1.35 Å) and the radius of Ti⁴⁺ (0.68 Å) [8]. The addition mol dopan neodymium (Nd³⁺) in the making process of BaTiO₃ is ion donor (donor doping) that donates excess valence electrons to ion Ba²⁺ [8]. The doping material given can affect the crystal structure, microstructure, and optical properties of a material [3]. In this paper presented BaTiO₃ substituted ionic Nd³⁺ [8].

In this research, BaTiO₃ thin film is doped with (Nd³⁺) use the chemical solution deposition (CSD) method with the spin coating technique. The solution BaTiO₃ and Ba_1−xNd_xTiO₃ was grown on quartz and silicon (Si) substrates deposited using spin coating with a rotational speed of 5000 rpm. The samples annealing process used 850°C with a holding time of 1 h. Samples were characterized using X-Ray Diffraction (XRD) equipment used to determine the microstructure, Scanning Electron Microscopy (SEM) used to determine the grain size and the thickness of the film, UV-Vis used to determine the amount of absorbance and transmittance of thin films, and a Keithley I–V Meter used to determine the amount of current and voltage of thin films.

2 Experiment

The thin films of BaTiO₃ and Ba_1−xNd_xTiO₃ used by chemical solution deposition method (CSD). Doping Nd was varied at 5, 10 in BaTiO₃. The material used in this research was Barium Acetate [Ba(CH₃COO)₂] 99.00%, Neodymium Acetate [Nd(CH₃COO)₃] 99.9%, Acetic Acid [(CH₃COOH)] 100%, Titanium Isopropoxide [Ti(OC₃H₇)₄] 97%, Ethylene Glycol [(HOCH₂CH₂OH)] 100%. There were several steps of making BaTiO₃ and Ba_1−xNd_xTiO₃ solutions including preparing tools and materials and weighing ingredients to mixing materials. The solution was deposited on the Quartz and silicon (Si) substrate, prepared with spin-coating technique at a rotational speed of 5000 rpm. The samples were annealed at 850 °C with a holding time of 1 h. The material properties of the samples were identified using XRD equipment for microstructure testing. The XRD equipment had a Lamda Cu (1.5406 Å). Samples were characterized using SEM to determine the morphology and the thickness of the samples. In addition, samples were characterized using UV-Vis spectrophotometer to determine the amount of absorbance and transmittance of thin films. Ultimately, samples were characterized using a Keithley I–V Meter to determine the amount of current and voltage of thin films.

3 Result and Discussion

3.1 Characterization of Microstructure

The XRD characterization results was show on Fig. 1. Based on Fig. 1, it can be seen that there are almost no other phases that affect the crystal structure of BaTiO₃ thin film and Ba_1−xNd_xTiO₃. In this experiment, increasing mole dopant Nd succeeded in replacing Ba, therefore the diffraction pattern produced by Ba_1−xNd_xTiO₃ was similar to BaTiO₃ pattern.

Fig. 1

Diffraction pattern of BaTiO₃ and Ba_1−xNd_xTiO₃

Based on the XRD results, with the addition mole dopant Nd, the diffraction angle is seen shifting to the right. These results are consistent with what has been done by Sandi et al. [9]. The ionic Nd³⁺ can replace Ba²⁺ in the perovskite structure. Along with the addition mole dopant Nd, it was also seen that the intensity value produced from XRD was getting higher. In accordance with the research of Iriani et al. the additional mole dopant Nd³⁺ on the BaTiO₃ thin films can increase the intensity of value formed [8].

The calculation of the crystallinity samples can be seen in Table 1. The addition mole dopant Nd causes the crystallinity of the thin film decrease. The crystallinity of the samples is calculated from the magnitude of the intensity of the diffraction peaks formed. The crystallinity of the samples is a picture of the level of regularity of the crystals that are formed in each film.

Table 1 Crystallinity and crystallite size of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films

Table 2 Lattice parameters of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films from the calculation result

The crystallite size of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films can be calculated using the following Scherrer equation [3]:

$$D = \frac{k\lambda }{\beta \cos \theta }$$

(1)

where the value of $\beta$ is the value of FWHM, $\theta$ is the diffraction angle, $k$ is the constant of Scherrer which is at 0.9, and $\lambda$ is the wavelength of X-ray. The calculation of the crystallite size samples can be seen in Table 1.

Based on the additional mole dopant Nd, the result of crystallite size dwindled. In accordance with studies that have been carried out by Gaur et al. that the crystallite size decreases with the additional mole dopant Nd [8]. The result of the crystallite size ranges from 70 to 28 nm.

The refined XRD results using the GSAS software can be seen in Fig. 2. This software processes with a refinement process so that the crystal structure and lattice parameters a, b, and c of the thin film can be identified.

Fig. 2

The refinement of BaTiO₃ and Ba_1−xNd_xTiO₃ diffraction patterns

The lattice parameter values from the GSAS software refinement results can be seen in Table 3. The result shows that thin film BaTiO₃ and Ba_1−xNd_xTiO₃ has a tetragonal structure since it has a lattice parameter value a = b ≠ c and the angle α = β = γ = 90°. The results of refinement Ba_1−xNd_xTiO₃ thin film diffraction pattern with a variation mole dopant Nd, show the alteration of lattice parameter because of dopant Nd³⁺ into BaTiO_3. Decrease in the Ba_1−xNd_xTiO₃ lattice parameter with respect to the initial lattice parameter BaTiO₃ caused the diffraction angle of the Ba_1−xNd_xTiO₃ thin film with a variation mole dopant Nd has shifted from the BaTiO₃ diffraction angle on ICDD which has a certain field orientation (hkl). It can affect the distance between atoms in the crystal and cause changes in parameters including grid a, b, and c. These results are in accordance with research conducted by Iriani et al. [8]. The changing in lattice parameters in the results of the refinement, thin film diffraction patterns in this study were not very significant (Table 2).

Table 3 Lattice parameters of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films from the result of the refinement software GSAS

3.2 Characterization of Morphology

The morphology of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films with magnitude 80,000× are shown in Fig. 3. Based on the addition mole dopant Nd in the BaTiO₃, the porosity of the thin film decreases. This happens since the granules in the thin film have experienced perfect diffusion.

Fig. 3

SEM image of a BaTiO₃, b Ba_0.95Nd_0.05TiO₃, c Ba_0.90Nd_0.10TiO₃

Based on the result of SEM characterization, it is obtained the grain size and the thickness of BaTiO₃ and Ba_1−xNd_xTiO₃ that can be seen in Table 4. The grain size of BaTiO₃ thin films cannot be calculated since the granules are completely diffused. When there is an increase in the concentration of mole dopant Nd in the thin film of BaTiO₃, the grain size decreases. The results of this study are in accordance with the research conducted by Zhang et al. [10]. However, in the sample of Ba_0.90Nd_0.10TiO_3, the grain size of the sample increased. The reason for the morphological change was not determined. It can only be assumed that the grain boundaries in the SEM results are clearly visible so that the number of grain size that can be calculated and the average is increasing.

Table 4 Grain size and thickness of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films

The thickness was got from the results of cross-section measurements on the BaTiO₃ and Ba_1−xNd_xTiO₃ thin film, as presented in Table 4. The thickness of BaTiO₃ and Ba_1−xNd_xTiO₃ films affects the addition mole dopant Nd. When BaTiO₃ was given an additional mole dopant Nd, the thickness of the film got thinner. However, the more mole concentrations dopant Nd was added, the thickness of the film got thicker. As the results of research from Iriani et al. [8]. The thickness of Ba_0.95Nd_0.05TiO₃ is 300 nm, Ba_0.90Nd_0.10TiO₃ is 335 nm, and the thickness of BaTiO₃ pure is 633 nm.

3.3 Characterization of Optical Properties

The calculation of band-gap is using the method of tauch-plot with the following equation:

$$\left( {\alpha h\upsilon } \right)^{2} = A\left( {h\upsilon - E_{g} } \right)$$

(2)

where $h\upsilon$ is the photon energy, $A$ is a constant, $\alpha$ is the coefficient of absorption, and $E_{g}$ is the optical band-gap [3].

The transmittance spectrum of quartz substrate grown by BaTiO₃ and Ba_1−xNd_xTiO₃ is presented in Fig. 4. It can be seen that the higher the concentration mole dopant Nd is given, the lower the peak transmittance value. Based on Fig. 4, it is found that the thin film of BaTiO₃ reaches the highest transmittance value. The amount of this transmittance value is inversely proportional to the value of the absorbance value.

Fig. 4

Transmittance of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films

The transmittance value of BaTiO₃ thin film in the visible light area is greater if mole dopant Nd is increased. The addition mole dopant Nd can increase the absorption of visible light in the thin film of BaTiO₃. The transmittance of the Ba_1−xNd_xTiO₃ film decreases with the decreasing wavelengths to a certain extent. Waves of absorption from thin films Ba_1−xNd_xTiO₃ are bigger if it is compared to BaTiO₃. This result is in accordance with the research by Fu et al. [11].

Based on UV-Vis and cross-section test results, the band-gap value is obtained. The band-gap values of the BaTiO₃ and Ba_1−xNd_xTiO₃ thin films are presented in Table 5. The results show that the band-gap of BaTiO₃ thin films is greater than Ba_1−xNd_xTiO₃. This happens since the addition mole dopant Nd can reduce the band-gap of the thin film. In this study, the Nd³⁺ ionic replaces the Ba²⁺ ion on the A-side so that it can cause the narrowing of the conduction band-gap. This is in accordance with the research of Jiang et al. [3]. In this study, the value of the band-gap of the thin film of BaTiO₃ and Ba_1−xNd_xTiO₃ is around 3.00–3.64 eV for the overall concentration of mole.

Table 5 Energy gap of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films

3.4 Characterization of Electrical Properties

The characterization of electrical properties (current–voltage) from BaTiO₃ and (Ba_1−xNd_xTiO₃) thin films was got by varying mole dopant Nd³⁺ using Keithley I–V meters. Figure 5 shows the current and voltage relationship curves of BaTiO₃ and Ba_1−xNd_xTiO₃ films in dark and light conditions which indicate that the thin film has diode characteristics. The curve shows that in the Ba_1−xNd_xTiO₃ film with Si substrate (p-type semiconductor), a connection has occurred. This curve shows that there is an induced current flow (photocurrent) so that the Ba_1−xNd_xTiO₃ film shows the response to the light. The Ba_1−xNd_xTiO₃ film when it is irradiated (in light conditions) has more conductive properties if it is compared to that when it is in dark conditions due to the absorption of photon energy. The result of the photon energy charge can be separated and collected by the cathode and anode. The results of this study are in accordance with the research conducted by Wang et al. [12]. The amount of I_sc and V_oc value from BaTiO₃ and Ba_1−xNd_xTiO₃ film are shown in Table 6. Based on Table 6, it can be seen that the additional mole dopant Nd³⁺ can increase the value of I_sc and V_oc. The results are in accordance with the research conducted by Jiang et al. [3].

Fig. 5

Characterization curve I–V of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films

Table 6 I_sc and V_oc value of BaTiO₃ and Ba_1−xNd_xTiO₃ thin films

4 Conclusions

The addition of mole dopant Nd³⁺ on BaTiO₃ caused the change of angel diffraction. The diffraction angle were seen shifting to the right. As the mole dopant Nd increased, so crystallinity from the Ba_1−xNd_xTiO₃ thin films decreased. The crystallite size of BaTiO₃, Ba_0.95Nd_0.05TiO₃, and Ba_0.9Nd_0.1TiO₃ were 70; 21; 28 nm. Result of refine with GSAS software show the dopan Nd have been included in the structure BaTiO_3. The addition mole dopant Nd³⁺ resulted in smaller grain sizes but it did not significantly affect the thickness. The greater the addition mole dopant Nd, the smaller the band-gap produced. Moreover, the results of the IV characterization show that the Ba_1−xNd_xTiO₃ thin films have a response to the light and the photovoltaic effects.