Optical and Luminescence Properties of Zinc Oxide

Abstract: We generalize and systematize basic experimental data on optical and luminescence properties of ZnO single crystals, thin films, powders, ceramics, and nanocrystals. We consider and study mechanisms by which two main emission bands occur, a short-wavelength band near the fundamental absorption edge and a broad long-wavelength band, the maximum of which usually lies in the green spectral range. We determine a relationship between the two luminescence bands and study in detail the possibility of controlling the characteristics of ZnO by varying the maximum position of the short-wavelength band. We show that the optical and luminescence characteristics of ZnO largely depend on the choice of the corresponding impurity and the parameters of the synthesis and subsequent treatment of the sample. Prospects for using zinc oxide as a scintillator material are discussed. Additionally, we consider experimental results that are of principal interest for practice.

• The paper reviews ZnO’s unique optical properties, revealing its direct bandgap of ~3.37 eV and high exciton binding energy of 60 meV that enable UV applications.
• The paper demonstrates that ZnO exhibits two key luminescence bands—a fast excitonic edge emission and a broad green emission linked to defect centers such as zinc or oxygen vacancies.
• The paper highlights that doping with elements like Ga, In, and Mg can tailor ZnO’s luminescent response, informing strategies to improve UV lasers and scintillators.

Optical and Luminescence Properties of Zinc Oxide: An Academic Review

The paper by P. A. Rodnyi and I. V. Khodyuk, published in "Optics and Spectroscopy", provides a comprehensive examination of the optical and luminescence properties of zinc oxide (ZnO), a wide bandgap semiconductor known for its potential in various optoelectronic applications. This review synthesizes experimental data across various forms of ZnO, including single crystals, thin films, powders, ceramics, and nanocrystals, focusing on the two primary emission bands observed in ZnO: the short-wavelength band near the absorption edge and the long-wavelength green luminescence band.

Key Findings and Mechanisms

1. Structure and Fundamental Properties: ZnO is characterized as a direct gap semiconductor with a hexagonal wurtzite structure prominent under ambient conditions. The bandgap of ZnO is approximately 3.37 eV at room temperature, suggesting its utility in ultraviolet (UV) applications. Its high exciton binding energy (60 meV) makes it a favorable candidate for UV laser diodes and light-emitting diodes (LEDs).
2. Luminescence Bands: The paper identifies two fundamental luminescence bands: the short-wavelength edge luminescence near 3.35 eV, associated with excitonic emissions, and the broad green luminescence band, typically peaking in the green spectral range. The edge luminescence, with a decay time of ~0.7 ns, is significant for high-speed optoelectronic devices, including lasers and scintillators.
3. Green Luminescence Models: Various defect centers, such as Cu²⁺ ions, zinc vacancies (VZn), and oxygen vacancies (VO), are explored as potential contributors to the green luminescence. Despite conflicting models, the consensus suggests that the nature of green luminescence is multifaceted, involving different centers depending on the sample conditions and preparation methods. Zinc vacancies are considered more plausible luminescence centers for n-type conductivity in oxygen-rich ZnO, whereas oxygen vacancies dominate in zinc-rich contexts.
4. Doping Effects: The introduction of impurities such as Ga, In, and Mg can modulate the luminescent properties by widening the bandgap and stabilizing donor centers. Conversely, p-type conductivity remains challenging due to the low solubility of typical p-type dopants.
5. Temperature Dependence: The study observes that the characteristics of the luminescence bands are sensitive to temperature changes, particularly affecting the green luminescence due to electronic transitions between different donor and acceptor levels.

Practical Implications and Future Directions

The exploration of ZnO's luminescence properties holds significant implications for developing advanced optoelectronic devices. ZnO's potential as a scintillator material is underscored by its high radiation resistance and considerable mechanical and chemical stability. Furthermore, ZnO-based UV lasers represent a promising area of research, with ongoing efforts to optimize p–n junctions and thin-film architectures to achieve stimulated emission.

Looking ahead, further elucidation of the exact mechanisms leading to green luminescence in ZnO could enhance material processing techniques, particularly in achieving high purity and controlled defect densities. Advances in doping strategies to achieve reliable p-type conductivity will be pivotal for the broader adoption of ZnO in commercial applications, including transparent electronics and photovoltaic technologies.

In conclusion, the optical and luminescent properties of ZnO present a rich field for continued research, particularly in enhancing the efficiency and stability of ZnO-based devices for use in novel optoelectronic applications.