THE DETERMINATION OF BARIUM STRONTIUM TITANATE THIN FILM BAND GAP ENERGY Ba0,15Sr0,85TiO3 USING ULTRAVIOLET-VISIBLE SPECTROSCOPY

A thin layer of Barium Strontium Titanate Ba0.15Sr0.85TiO3 (BST) was developed on a glass substrate using a sol-gel method with annealing temperatures and spin coating process at 3500 rpm for 30 seconds. The annealing temperature varied from 600C, 650C, and 700C. Characterization of optical properties was developed using UV-Vis spectroscopy to determine the energy bandgap. The values of the BST thin layer energy band at the annealing temperature were 3.55 eV, 3.32 eV, and 3.10 eV, respectively. The results indicate that the BST thin film was a semiconductor material.


INTRODUCTION
Science and technology as a necessity in human life influence various fields of life and become a criterion for the progress of a country. Technological advancements are indicated by an increase in the need for electronic devices [1]. The production of these electronic devices requires better quality materials [2]. It drives many researchers to study the materials that can be used as the raw materials for electronic devices. The various applications, such as the use of FRAM (Ferroelectric Random Access Memory), manufacturing thin-film capacitors, pyroelectric sensors due to temperature changes, pressure sensors (piezoelectric), a light sensor was utilizing the semiconductor connection properties, and optoelectronics [3,4].
Ferroelectric material, which is currently being developed, is Barium Strontium Titanate (BST). BST thin film is a ferroelectric material formed of Barium Titanate (BaTiO3), which is doped with strontium (Sr). Propagation is carried out to increase the dielectric constant and to reduce dielectric loss at low frequencies [5].
Barium Strontium Titanate (BST) is a ferroelectric material that is widely applied for storage media and has a perovskite crystal structure. Some researchers believe that BST has the potential to replace the thin layer of SiO2 in the Metal Oxide Semiconductor (MOS) circuit. Until now, the results of studies on BST films usually have dielectric constants, which are lower than the bulk form. The suitable grain microstructure, oxygen vacuum, interfacial layer formation, and oxidation of the bottom electrode or Si, are believed to be factors decreasing in electrical properties [6].
The preparation methods which are often used in the synthesis of layered and porous nanomaterials are the sol-gel, intercalation, and inclusion methods. The sol-gel method is a method of preparing solids with a low-temperature technique that involves the transition from a system with microscopic particles dispersed in a liquid (sol) to a macroscopic material (gel) containing liquid. When the liquid evaporates, it remains a hard material like glass. Sol-gel is an amorphous material and does not have uniform pore dimensions. Sol-gel synthesis generally goes through the stages of hydrolysis and condensation [7].
BST thin layer in this study was made by the sol-gel method or known as the Chemical Solution Deposition (CSD) method, by using a chemical solution on a substrate processed by the spin coating technique. The sol-gel method has the advantage of a simple process, and it can be done at room temperature, short fabrication cycles, high purity, the cost required is relatively inexpensive, and can control the stoichiometry layer well [8]. Where x is a percentage number for Ba and Sr in Ba1-xSrxTiO3, and Ti is for 100%. The ratio of Ba and Sr is 15% : 85%. The thin layer was characterized using Ultraviolet-Visible (UV-Vis) spectroscopy to determine the optical properties and the gap energy using the Tauc Plot method.
The energy band gap is a gap located between the valence band and the conduction band, where electrons will jump from one band to the other. This gap will show the nature of a solid, whether it is a conductor, an insulator, or a semiconductor. The gap energy is the minimum needed to excite electrons from the valence band to the conduction band. When a semiconductor is charged with energy corresponding to the bandgap energy, the electron will | 13 SPEKTRA: Jurnal Fisika dan Aplikasinya Volume 5 Issue 1, April 2020 be excited into the conduction band, leaving a positive charge called a hole. Energy band gap control is performed to obtain the energy needed to excite electrons or holes in the material or energy emitted by electrons or holes when returning to the ground state can be modified as needed [9].
The known value of the energy band is important to find out how much energy is needed to excite the electrons from the valence band to the conduction band so that a good application for this material can be determined. Energy bands that are too small will cause electron jumps from the valence band to the conduction band so that the electrons are less free, while energy bands that are too large will inhibit electron jumps so that the flow of electrons will be impeded [10].

METHOD
The manufacturing of a thin layer Ba0.15Sr0.85TiO3 uses the Sol-gel method.  The determination of the optical band gap in thin films can be done by processing the transmittance data, which was obtained from characterization using Ultraviolet-Visible spectroscopy. Wavelengths that used for measurement of transmittance are from ultraviolet light to visible light (300 nm to 800 nm). The value of the refractive index and thickness of the thin layer is determined using the following equation [11]: Thin layer thickness values can be determined from the results of the calculation of the refractive index value using the following equation:  The calculation result of thin-film thickness is then used to determine the coefficient of absorption of the thin layer for each wavelength using the following equation: The absorption coefficient of the thin layer has been obtained, then determine hν where hν= ℎ . Then determine the optical band gap using the Tauc plot method, which is the method of determining the optical band gap by extrapolating from the relationship graph (hν) as abscissa and (αhν) n as a coordinate to cut the energy axis and from the curve can be determined the gap energy value of each material Ba1-xSrxTiO3 which was examined. The value of (αhν) n , = 1 2 ⁄ for the direct transition process and n = 2 for the indirect transition process [12].

RESULT AND DISCUSSION
Absorbance values are obtained from the results of characterization using UV-Vis spectroscopy.   Transmittance at Ba0,15Sr0,85TiO3 thin film occurs in the increase of transmittance value at wavelengths of 300-450 nm. Based on the transmittance spectrum, results in the thin layer Ba0,15Sr0,85TiO3 shows that the higher of annealing temperature, the smaller the transmittance value. This is because the constituent atoms will become denser, resulting in collisions of light particles with the atomic constituent layers will be more frequent, so it is difficult for light to be able to pass through the layer. The higher the absorbance value, the lower the transmittance value will be. A high absorbance value means that many large particles are found in the thin film [9].
The refractive index value (n) is obtained from the calculation results by processing the maximum transmittance data (TM), minimum transmittance (Tm), and wavelength (λ) using equation (2). Furthermore, by using equation (     The energy gap indicates the movement of electrons across the valence band to the conduction band. From FIGURE 4, it is found that the width of the energy gap (Eg) decreases with increasing annealing temperature applied to the sample. This is because small crystalline crystals shrink and are swallowed up by larger crystalline grains. The growth of this grain occurs when primary crystallization stops [15]. Higher annealing temperature causes the crystal structure to be more dense and compact, so the energy gap is smaller. The energy gap range for each sample is not too far away because the annealing temperature range used is 50 o C. This indicates that if the gap width of the energy decreases, more electrons can undergo an electronic transition. Its transition from the valence band to the conduction band so that the thin layer is increasingly conductive [16].