Zinc Oxide (ZnO)

High purity zinc (ZN) or zinc compounds (such as ZNO powder) are used as the main raw material, and the purity is usually required to be greater than 99.99%.

• Manufacturing process of zinc oxide material

1. Sol gel method (SOLGEL)

The process: Zinc salt (zinc nitrate) is used as the precursor, and the sol is formed by hydrolysis and condensation. Then the nano ZNO film or powder is generated after drying and annealing.

Advantages: low cost, controllable components, suitable for large area uniform film formation.

Key parameters: precursor concentration, annealing temperature (usually 400600℃) control grain size.

2 Chemical vapor deposition (CVD)

Steps: Single crystal ZNO films are deposited by reacting zinc sources (such as diethyl zinc) with oxygen on a high temperature substrate (such as sapphire).

Advantages: The film has excellent purity and crystallinity, which is suitable for photoelectric devices.

2 Magnetron sputtering

Steps: The film was sputtered in an oxygen atmosphere with ZN or ZNO target as the source.

4. Doping and defect engineering

N-type doping: commonly AL, GA, IN and other elements are doped to increase the carrier concentration (10¹⁸-10²⁰ CM⁻³).

Difficulties of P-type doping: P-type doping is difficult due to ZNO intrinsic defects (such as oxygen vacancy), and N and P doping or compound structure (such as ZNO/MGO heterojunction) are commonly used.

5. Post-treatment process

Annealing optimization: Zinc oxide films are annealed at 300500℃ to improve their crystal structure, reduce grain boundary defects, and improve mobility (up to 200 CM²/ (V S)).

Surface passivation: SIO₂ or AL₂O₃ coating is used to suppress surface recombination and enhance light response.

• Application technology

1. Transparent Conductive Film (TCO)

Low resistivity: After doping treatment, the resistivity of zinc oxide film can reach 10⁻⁴ Ω CM level, close to the level of ITO (indium tin oxide).

High carrier concentration: The carrier concentration can reach 10²⁰ CM⁻³, and the mobility is high, between 1050 CM²/ (V S).

Visible light transmittance>85% (wavelength 400800 NM), resistivity as low as 10⁻⁴ Ω CM (AL doping).

Compared with ITO (indium tin oxide), it has low cost and strong resistance to hydrogen plasma corrosion, which is suitable for flexible display devices.

Applications: touch screens, solar cell electrodes (such as perovskite cells), energy-saving window coatings.

2. Ultraviolet photoelectric devices

High transparency: In the visible light range, the transmittance of zinc oxide film is over 85%, which makes it very suitable for display devices.

Wide band gap: The band gap width of zinc oxide is about 3.37 EV, which can effectively block ultraviolet light, so it can be used to make UV detectors, the response speed of UV detectors is less than 10 NS, and the quantum efficiency is more than 80% (wavelength 365 NM)

The excitation binding energy is up to 60 MEV (higher than the 25 MEV of GAN), suitable for room temperature ultraviolet luminescence and detection.

Applications: UV LED, day-blind ultraviolet detector (military/environmental monitoring).

3. Piezoelectric and sensing applications

The piezoelectric coefficient D₃₃ is about 12.4 PM/V (higher than quartz), and the electromechanical coupling coefficient is high.

Gas sensor sensitivity: the detection limit of NO₂ and H₂ is as low as PPB (due to the rich active sites of oxygen vacancies on the surface).

Applications: piezoelectric nanogenerator (PENG), microelectromechanical system (MEMS), environmental monitoring sensor, piezoelectric sensor, acoustic wave device

4. Photocatalysis and antibacterial

Photocatalytic degradation efficiency (such as methylene blue)>90% (UV light for 2 hours), antibacterial rate>99.9% (for E. coli).

Visible light response can be extended by doping (such as AG and TIO₂ compound).

Applications: self-cleaning coatings, sewage treatment, medical antibacterial materials.

5. Wide temperature stability

Excellent thermal stability (melting point 1975℃) and can work in extreme environment of 200~1000℃.

It has strong radiation resistance and is suitable for aerospace electronic devices.

•  Compare the core advantages of other semiconductors

| characteristic                | ZnO                  | GaN                 | SiC

| Bandgap（eV）       | 3.37                 | 3.4                 | 3.2（4HSiC）

| Exciton binding energy（meV）    | 60                   | 25                  |

| Piezoelectricity              | powerful                 |weak                  | none

| cost                 | Low (abundant raw materials)       | high                  | Very high

| UV response efficiency         | Excellent                | good                | generally

• Challenges and future directions

1. P-type doping problem: a stable P-type doping process (such as LIN co-doping) needs to be developed.

2. Interface defect control: reduce the interface state density in heterojunction devices (such as ZNO/SI).

3. Flexible integration: Develop low temperature process compatible flexible substrates (such as PET).

Zinc oxide has become an ideal material to replace ITO due to its advantages of high transparency, low resistance, flexibility and abundant resources. It has a broad application prospect in flexible electronics, clean energy and sensors. In the future, with the continuous optimization of process, zinc oxide is expected to be applied in more high-end fields.