Introduction
The study of new materials with tailored properties plays a significant role in advancing fields such as electronics, photonics, and sensor technology. Among promising semiconductor materials, zinc oxide (ZnO) has garnered attention due to its wide bandgap (3.3-3.4 eV), optical transparency, and potential applications in optoelectronic devices. This article discusses research conducted on the fabrication of ZnO thin films doped with aluminum (Al), cobalt (Co), and nickel (Ni) using the electro-spraying deposition method, along with their structural, electrical, and optical properties.
Electro-Spraying Deposition Method
Electro-spraying, also known as electrohydrodynamic atomization, is a process where a liquid precursor is dispersed into fine droplets using an electric field. This technique allows precise control over film thickness, uniformity, and morphology. In this study, a novel electro-spraying system was designed and optimized to improve deposition efficiency and safety. The system features temperature-controlled substrates (up to 450°C), multi-axis movement, and automated precursor delivery.
Fabrication of ZnO Thin Films
Precursor Preparation
ZnO thin films were synthesized using zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) as the Zn source. Ethanol and distilled water were used as solvents, with acetic acid as a stabilizer. To enhance the functional properties, ZnO films were doped with Al, Co, and Ni at varying concentrations.
Deposition Parameters
- Voltage: 16-25 kV
- Emitter-to-Collector Distance: 5-6 cm
- Precursor Flow Rate: 0.01-0.03 mL/min
- Substrate Temperature: 150°C to 300°C
- Substrate Types: Silicon (Si) and Glass
Structural and Morphological Characterization
Surface Morphology
Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed that film morphology is strongly influenced by deposition temperature and doping. Higher temperatures (200-250°C) resulted in more uniform and smoother films, while lower temperatures (150°C) led to porous structures.
Crystallinity
X-ray diffraction (XRD) analysis confirmed that all films exhibited a hexagonal wurtzite ZnO structure. The preferred crystal orientation shifted from (002) to (100) with increasing deposition temperature, indicating temperature-dependent crystallographic growth.
Optical and Electrical Properties
Optical Transparency and Bandgap
Spectrophotometric measurements showed that ZnO thin films had an optical transmittance of approximately 85% in the visible range. The optical bandgap (Eg) was estimated using Tauc plots:
- Pure ZnO: 3.31 eV
- Al-Doped ZnO: Slight increase in Eg
- Co/Ni-Doped ZnO: Minor bandgap narrowing due to impurity states
Conductivity and Magnetic Properties
Doping with Al improved electrical conductivity, making Al-doped ZnO (AZO) a candidate for transparent conductive electrodes. Co and Ni doping introduced magnetic properties, suggesting potential applications in spintronics and diluted magnetic semiconductors.
Applications and Future Prospects
The ZnO thin films developed in this study have applications in:
- Transparent Conductive Electrodes: AZO can replace indium tin oxide (ITO) in display and solar cell technologies.
- Photocatalysis: ZnO and doped variants show potential for environmental purification and self-cleaning surfaces.
- Sensors: Doped ZnO thin films enhance sensitivity in gas and biosensors.
- Spintronic Devices: Magnetic properties introduced by Co and Ni doping enable applications in memory and logic devices.
Conclusion
By adjusting deposition parameters and doping concentrations, ZnO films with enhanced conductivity, transparency, and magnetic functionality can be achieved. Future work could explore alternative dopants and further optimization of the deposition process for large-scale applications.