Review

, Volume: 15( 2)

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Preparation of Nanocrystalline Aluminum Oxide Thin Films: A Review

*Correspondence:

Ho SM , Faculty of Information Technology, Maths and Science, INTI International University, Putra Nilai, Negeri Sembilan, Malaysia, Tel: +6067982000; E-mail: soonmin.ho@newinti.edu.my

Abstract

Nowadays, binary compound such as aluminium oxide thin films are well known for applications in a wide range of microelectronics devices and optoelectronics. This work demonstrates that it is possible to prepare aluminium oxide thin films by using various deposition methods including chemical deposition method and physical deposition technique. Characterization of thin films is a broad science. Here, the characterization of the films was done with x-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared absorption and UV-visible spectrophotometer.

Keywords

Thin films; Aluminium oxide; Semiconductor; Solar cells

Introduction

Recently, there has been growing interest to prepare various types of metal chalcogenide thin films [1-30] and metal oxide films [31-45] by using different deposition methods. Aluminium oxide thin films have been extensively studied due to their promising applications in optoelectronics devices, microelectronics devices, water repellent coating, protective coating, dielectric and sensing layer. These materials have many properties include chemical inertness, good adhesion to glass substrate, transparency over wide wavelength range, mechanical strength, high thermal conductivity, and high dielectric constant.

There are several different deposition techniques either physical deposition method or chemical deposition technique have been applied for the preparation of aluminium oxide thin films such as reactive magnetron sputtering method, thermal evaporation method, pulsed laser deposition, plasma enhanced chemical vapor deposition method, sol gel technique, atomic layer epitaxy and chemical vapor deposition. Here, a critical analysis was carried out in order to prepare aluminium oxide films. Then the obtained films were characterized using X-ray photoelectron spectroscopy, scanning electron microscopy, x-ray diffraction and Fourier transform infrared.

Literature survey

Zhao et al. [46] have reported the synthesis of aluminum oxide thin films by using off plane filtered cathodic vacuum arc system. They figure out that aluminum oxide films could be deposited with high deposition rate (1.5 nm/s to 0.5 nm/s) on quartz and Si (100) substrates at room temperature under various oxygen pressures (0.01 Pa to 0.0373 Pa). The obtained results show that the as-grown films are amorphous with low stress (lower than 0.5 GPa) and good uniformity could be seen. They conclude that the film properties display the potential applications in optical coatings industry. On the other hand, the influence of annealing process on the properties of aluminium oxide thin films has been discussed by Zhao and Tay [47]. It is noticeable that the surface of films remained smooth up to 600°C. The experiment results indicated that crystallization is induced for the films when heated at 900°C. Also, they claim that the refractive index increased with increasing the annealing temperature from 200°C to 900°C.

Sol gel technique has been used to prepare aluminum oxide thin films by Nursen et al. [48]. In the experiment described, acetyl acetone, aluminium sec-butoxide and alcohol solution (ethanol, isopropanol and n-butyl alcohol) was used as a chelating agent, raw materials, and solvent, respectively. The data obtained show that the most stable one was synthesized by using n-butyl alcohol if compared to other solvents. On the other hand, highly transparent, homogeneous and amorphous aluminium oxide films were observed on silicon substrate after annealing the films at 500°C. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared absorption (FTIR) spectroscopy tests verify that the deposited films were hydroxide free.

Atomic layer epitaxy deposition and chemical vapor deposition were used to prepare aluminium oxide film as proposed by Hiltunen et al. [49]. Atomic layer deposition technique has many benefits include low defect density could be achieved, can produce pin hole free films and desired specified thickness. Meanwhile, chemical vapor deposition method has many advantages such as can deposit films which are hard to evaporate, high growth rates and can grow epitaxial films. In their study, the AlCl3, water, oxygen and aliphatic alcohol were used as aluminium and oxygen source respectively. Finally, the electrical constants and environmental stability analysis show that the obtained films are suitable for applications such as insulating and protecting layers. In other case, Pradeep et al. [50] have synthesized Al2O3 films with thicknesses of 400, 300 and 200 nm on silicon and soda lime glass substrates using atomic layer deposition. The optical behavior was studied in the 400 to 1800 nm wavelength range. The obtained outputs reflect that the films grown on glass substrate have higher refractive indices as compared to the films deposited on silicon.

Plasma enhanced chemical vapor deposited method has been used to prepare aluminum oxide thin films as suggested by Wen and Suhr [51]. The choice of this method due to it possesses a number of advantages such as low operation temperature, lower chances of cracking deposited layer and good step coverage. In their experiments, the aluminium oxide films were deposited onto various substrates includes glass, quartz, steel, silicon, and nickel using aluminium acetyl acetonate as precursor. It is interesting to note that the deposits are hard (up to 2370 HK) and indicate good adherence to the substrates.

Aluminum oxide thin films were prepared by using pulsed laser deposition method as described by Arrieta et al. [52]. This deposition method has several advantages such as cost effective, fast and versatile method. They point out that the obtained films are amorphous and slightly deficient in oxygen. Also, these films indicate a smooth surface with dispersed splashed particles with diameters ranging from 0.2 to 0.5 micrometers. In their study, the thermo luminescent properties of films were investigated. They conclude that thermo luminescent curve indicate two peaks at 110°C and 176°C. Furthermore, a linear relationship between absorbed dose and the thermoluminescent response for doses span from 150 Gy to 100 Gy was detected. In another case, Al₂O₃ films of 800 nm thicknesses were prepared by Ion et al. [53] using pulsed laser deposition method. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis show that diameter of grains about 50 nm. Energy dispersive X-ray spectroscopy (EDX) investigation points out that a stoichiometric transfer from target to film. The atomic percentage of oxygen and aluminium is 63.25% and 36.75%, respectively.

Electro spraying method has been applied as a means for the deposition of Al₂O₃ films on a metal substrate. Jaworek et al. [54] have described that electrospray system contained stainless steel capillary nozzle and a heated table of diameter of 120 mm. The SEM micrographs reflect that the particles produce a tight layer of particles without visible agglomerates. They further explain that the bigger grains (estimated about 1 μm to 2 μm) are due to an effect of coagulation of the smaller ones.

Aluminium oxide thin films were prepared using thermal evaporation method on aluminium substrate in open air environment as reported by Khan et al. [55]. The surface morphology has been studied by scanning electron microscopy. The formation of rounded grains (1 μm to 2.5 μm) and irregular patch of particles was detected for 1 and 2 hours. However, the surface morphology displayed the formation of irregular patches distributing uniformly for the 5 hours. They conclude that the formation of microstructure mainly depended on treatment time (1 to 5 hours). On the other hand, the XRD data show that crystallinity of various planes varied with increasing treatment time. Also, XRD patterns display the formation of Al₂O₃, Al(OH)₃, and Al₂O₃.H₂O phases.

Solvothermal approach was employed to prepare aluminium oxide thin films as suggested by Duan et al. [56]. This method allows for the precise control over the size, shape distribution and crystallinity of films. The SEM analysis shows that the obtained films are dense and grainy surface morphology. The thickness of the films was estimate to be 150 nm to 300 nm.
Chemical liquid phase deposition was employed for the first time to prepare Al₂O₃ films as described by Jie and Sun [57]. They observe that the growth rate is 12 nm/h with the deposition carried out at 15°C by using 0.0837 M of Al₂(SO4)₃ and 0.214 M of NaHCO₃. They also found that post growth annealing not only densifies and purifies the films, but lead to film crystallization as well.

The preparation of Al₂O₃ thin films using electrodeposition method has been proposed by Farzana et al. [58]. The films are electrodeposited at 5°C, at 3 V in deposition time from 30 to 120 minutes. The thickness increases from 50 nm to 250 nm as the deposition time was increased from 30 to 120 minutes. They claim that these materials could be used as barrier coatings due to the surface roughness is in the range of 5.9 nm to 15.4 nm. Lastly, they suggest that the band gap reduces from 4.38 eV to 4 eV with increase in deposition time.

X-ray diffraction (XRD) analysis was conducted in order to learn more about the film structure. Effect of bath temperature on the properties of electrodeposited Al₂O₃ films was investigated and reported by Imran et al. [59]. XRD results confirmed the formation of aluminium oxide at the bath temperature of 5°C to room temperature. Also, XRD analysis indicates that the crystallite size reduces as the bath temperature is increased.

Reactive magnetron sputtering method was used to prepare aluminium oxide thin films on soda glass substrate as described by Koushki et al. [60]. The XRD profile suggests that the formation of cubic aluminium oxide phase. SEM analysis indicates that the average grain size of 25 nm was detected. On the other hand, the influence of introduced oxygen flow on optical properties of films was studied. The optical spectra highlight that the highest transmission value belongs to highest amount of introduced oxygen. Researchers have claim that the deposition process was controlled by target voltage. For example, the maximum deposition rate obtained was 215 nm/minutes, 77% of the rate of metallic aluminium as reported by Juliet et al. Meanwhile, Kari et al. [61] have observed that the maximum deposition rate was 150 nm/minutes from the value of metallic minimum. In the other case, the influence of annealing temperature on the properties of aluminium oxide films was investigated by Lim et al. [62]. They conclude that the crystallite size was detected to increase, whereas the micro strain and dislocation density were reduced by increasing the annealing temperature. They explain that due to the reduction in the lattice strain.

Conclusions

Aluminium oxide thin films have been prepared by using different deposition methods. The properties of obtained thin films were influenced by various experimental conditions such as annealing temperature, target voltage, oxygen flow and deposition time.

Acknowledgement

Inti International University is gratefully acknowledged for the financial support of this work.

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