Synthesis of SnO2 Thin Coatings by Indium and Aluminum Mixed Doping using the Sol-Gel Spin-Coating Technique

Article Info Received : February 4th, 2020 Revised : May 21th, 2020 Accepted: June 9th, 2020 Abstract: This research succeeded in creating a thin layer of SnO2 and SnO2: In+Al through a synthesis process with the sol-gel spin-coating technique on a glass substrate. The manufacture of this thin layer uses the basic material SnCl2.2H2O, and the doping material InCl3.4H2O and AlCl3. This thin layer is made with variations in doping concentration, number of layers, and heating temperature. The results of the synthesis of SnO2: In+Al films show that the thin film formed is more transparent when the doping concentration and the number of layers is increased. Meanwhile, the thin layer that forms is increasingly not transparent when heated at higher temperatures.


Introduction
Current technology is developing very rapidly and produces a very small technology, namely nanotechnology. Nanotechnology was introduced by Michael Faraday since 1850 and continues to develop (Battal et al, 2014. One form of nanotechnology is a thin film. The thin layer continues to experience growth both in terms of the way it is made, the materials used, and its application in people's lives (Mak & Shan, 2016). Equipment used by many people uses thin layers, such as LCD TVs, plasma TVs, touch screen handphones (cellphones) and automatic teller machines (ATMs), handheld game consoles, and car navigation systems. In addition, thin films are also used in semiconductor devices such as diodes, capacitors, sensors, and transistors (Anaraki et al, 2016). Thin films can be made from inorganic, organic, metal, metal-organic mixtures, and metalinorganic mixtures. Thin films have sizes on the nanometer scale (Mihailescu, 2019). Thin films can be as an insulator, conductor, semiconductor, or superconductor (Mak & Shan, 2016).
Making thin films usually uses materials such as In2O3, ITO, SnO2, TiO2, WO3, ZnO (DeAngelis, 2018). SnO2 is often used as a base for making thin films. This is related to its superiority in nature like its reflectivity, transparency, and low electrical resistance. In addition, SnO2 thin films have high chemical stability and good transparency properties to light with an energy gap of ~ 3.6 eV (Carvalho et al, 2012). SnO2 thin film is widely applied in the manufacturing industry of transparent electrodes, solar cells, optical-electrical equipment, and gas sensors. This is based on the cheap price compared to other semiconductors, can to respond to some gases properly, has a long service life, and only requires simple electronic devices in the implementation of sensing. In addition, the properties of SnO2 are very dependent on the crystallographic quality and surface morphology of deposited layers (Morales et al, 2015). Experimental results show that materials such as TiO2, ZnO, and SnO2 are proven capable of detecting O2, CO, and several types of hydrocarbon gases (Mishra at al, 2017).
Pure or doped SnO2 thin films have been produced through various techniques, such as sol-gel spin coating to synthesize SnO2 thin films with Aluminum doping , pyrolysis spray to synthesize SnO2 thin films with Fluorine doping (Bakr et al, 2016), synthesis of SnO2 by doping Fluorine using the sol-gel spin-coating technique , and sol-gel dip coating to synthesize the SnO2 thin layer (Carvalho et al, 2012). The synthesis of SnO2 doped Indium thin films tends to be more transparent when the doping concentration is increased and will become less transparent when the heating temperature is increased . The effect of the addition of aluminum doping concentrations on the quality of transparency of SnO2 thin-film was also proven to increase the level of transparency .
This study will use the sol-gel spin coating technique because it has several advantages, namely having a short crystallization process, the use of low temperatures, nanoparticles, pure yields, economic and simple in nature (Yati et al, 2017;. Some of the variables involved in the sol-gel spin coating technique are solution concentration, doping concentration, rotational speed, turnaround time, solution aging time, layer repetition, and heat treatment .

Method
The basic material used as a coating in this study was 0.902 grams of Tin (II) chloride dihydrate (SnCl2.2H2O with a molar mass of 225.63 grams/mol, 98% purity, Merck). The solvent uses 40 ml of ethanol ((C2H5OH) with a molar mass of 46.07gram/mol, 98% purity, Merck) at room temperature. The ingredients for doping are 0.4266; 0.8532; 1.2797; 1.7063; and 2.1329 grams In+Al. The substrate used is glass with a size of 10 mm x 10 mm x 3 mm. Other supporting materials are aquades water, detergent soap, and alcohol which are used to clean substrates. SnO2: In+Al thin film synthesis includes substrate preparation, sol-gel manufacturing, thin-film manufacturing, and the heating process.
Substrate preparation is done by washing the glass substrate using detergent soap, then stirring using a shaker for 30 minutes. After that, rinse it using distilled water, and soak it using alcohol for 30 minutes. Then heat it using the furnace for one hour at 100 o C (Doyan and Humaini, 2017).
Making sol-gel is done by mixing SnCl2.2H2O with InCl3.4H2O and AlCl3 with a mixture concentration of 1 M, so as to produce SnO2 sol-gel doped with Indium and Aluminum with different concentrations.
Making thin films is done by the sol-gel spincoating technique using a spin coater. SnO2: In+Al solution is dropped on a glass substrate, then rotated using a spin coater at 2000 rpm for 2 minutes. Samples were made as many as 120 samples, with variations in doping concentration, number of layers, and heating temperature. After all, samples have been rotated, they are stored at different temperatures, namely room temperature, 50 o C, 100 o C, 150 o C, and 200 o C. Each sample at this temperature amounted to 24 samples.

Result and Discussion
The process of making SnO2: In+Al thin films consists of the stages of substrate preparation, sol-gel manufacturing, thin-film manufacturing, and the heating process. The substrate used is glass. The glass is washed first to ensure the glass is clean of other substances. After that, the glass is heated to vaporize the remnants of other substances attached to the glass. The next step is the process of making sol-gel by mixing SnCl2.2H2O with InCl3.4H2O and AlCl3 with a mixture concentration of 1 M using ethanol solvent. The amount of ethanol used as a solvent is 20 ml . The amount of SnCl2.2H2O, InCl3.4H2O, and AlCl3 dissolved in 20 ml of ethanol in order to obtain the desired concentration ratio can be seen in Table 1. The results of the SnO2: In+Al solution formed by the sol-gel technique can be seen in Figure 1.  The next process is the manufacture of thin films using spin-coating techniques. The glass substrate is placed on a spin coater, then drops of SnO2: In+Al solution. Glass substrate that has been dropped by SnO2: In+Al solution is rotated at 2000 rpm for 2 minutes. This screening is intended so that the SnO2: In+Al solution is spread evenly on the glass substrate.
After all, samples have been rotated, the process of adding layers and heating the samples at different temperatures is carried out. It aims to determine the effect of many layers and heating temperature on the quality of the thin film formed. Previous research has proven that the concentration of doping Indium  and Aluminum doping  and different heating temperatures have an effect on the quality of the thin film formed. In other research, it has been proven that the more concentration of doping Aluminium and Zinc which is added to SnO2, the more transparency of the thin films which is produced (Ikraman et al, 2017.  Figure 2 shows that the SnO2: In+Al thin films from the picture (a) to picture (f) experience a decreasing level of transparency. This shows that SnO2: In+Al thin films with higher doping concentrations have higher transparent levels.
The number of layers in a thin layer also affects the transparency level of the thin film. The results of the synthesis of SnO2: In+Al thin films with different amounts of layers can be seen in Figure 3.  Figure 3 shows that the SnO2: In+Al thin film formed is more transparent when the number of layers is added. This proves that the number of layers in the thin layer has an effect on the level of transparency.
In addition to doping concentration and the number of layers, the heating temperature also affects the transparency quality of the formed SnO2: In+Al films. The results of the synthesis of thin films at different temperatures can be seen in Figure 4.  Figure 4 shows that from the picture (a) to picture (e) the SnO2: In+Al thin layer is becoming less transparent. This shows that the addition of the heating temperature makes the SnO2: In+Al thin layer more transparent.

Conclusion
The process of making sol-gel can take place normally at room temperature. The sol-gel spin-coating technique is good and efficient for making thin-film samples. The thin film produced by the sol-gel spin-coating technique has good surface properties. The surface quality of SnO2: In+Al films produced is influenced by doping concentration, the number of layers, and heating temperature. The more concentration and the number of layers makes the level of transparency of the thin layer higher. Meanwhile, the thin layer becomes less transparent when the heating temperature is increased.