Zinc oxide (ZnO) is a wide bandgap semiconductor with a direct bandgap of approximately 3.37 eV at room temperature, making it highly suitable for optoelectronic applications. Its properties are significantly influenced by intrinsic and extrinsic defects, which can alter luminescence and conductivity. Understanding these defects, their characterization, and mitigation strategies is critical for optimizing ZnO-based devices.

Intrinsic defects in ZnO arise from deviations in stoichiometry and lattice imperfections. The most common intrinsic defects include zinc interstitials (Zn_i), oxygen vacancies (V_O), zinc vacancies (V_Zn), and oxygen interstitials (O_i). Among these, oxygen vacancies and zinc interstitials are particularly significant due to their role in n-type conductivity and luminescence.

Oxygen vacancies (V_O) are one of the most studied defects in ZnO. They can exist in three charge states: neutral (V_O^0), singly ionized (V_O^+), and doubly ionized (V_O^2+). These vacancies introduce shallow donor levels within the bandgap, contributing to n-type conductivity. The presence of V_O is often associated with green luminescence, observed around 500–530 nm in photoluminescence spectra. This emission is attributed to the recombination of electrons in singly ionized oxygen vacancies with holes in the valence band or deep acceptor levels.

Zinc interstitials (Zn_i) are another intrinsic defect that acts as a shallow donor. They are highly mobile and can diffuse rapidly, even at moderate temperatures. Zn_i contributes to n-type conductivity and can also influence luminescence. However, their formation energy is higher than that of oxygen vacancies, making them less prevalent under equilibrium conditions. Zinc interstitials are often linked to blue or violet emission in the 400–450 nm range, resulting from transitions involving Zn_i-related energy levels.

Zinc vacancies (V_Zn) are acceptor-type defects that introduce deep levels in the bandgap. They are responsible for yellow-orange luminescence (~600 nm) due to transitions involving hole capture at V_Zn sites. Oxygen interstitials (O_i) are less common but can act as compensating acceptors, reducing n-type conductivity by trapping electrons.

Extrinsic defects in ZnO arise from impurities incorporated during growth or processing. Common extrinsic defects include hydrogen, which often bonds with oxygen (forming O-H complexes), and residual alkali or transition metals from precursors. Hydrogen is particularly notable because it can passivate acceptor defects, enhancing n-type conductivity. However, extrinsic defects are excluded from this discussion as the focus is solely on intrinsic defects and their impacts.

Characterization of defects in ZnO employs a variety of techniques. Photoluminescence spectroscopy (PL) is widely used to identify defect-related emissions. The green emission band (~520 nm) is commonly associated with oxygen vacancies, while blue/violet emissions (~400–450 nm) are linked to zinc interstitials. Deep-level transient spectroscopy (DLTS) can quantify defect energy levels and concentrations, providing insights into their electronic impact. X-ray photoelectron spectroscopy (XPS) helps determine the chemical state of oxygen and zinc, revealing the presence of oxygen vacancies through shifts in binding energy. Electron paramagnetic resonance (EPR) can detect paramagnetic defects such as singly ionized oxygen vacancies (V_O^+). Positron annihilation spectroscopy (PAS) is sensitive to open-volume defects like zinc vacancies.

The impact of defects on luminescence and conductivity is significant. Oxygen vacancies and zinc interstitials enhance n-type conductivity by introducing donor states near the conduction band. However, excessive defect concentrations can lead to non-radiative recombination, reducing luminescence efficiency. Zinc vacancies, acting as acceptors, can compensate donor defects but may also introduce unwanted deep-level recombination centers. Balancing defect concentrations is crucial for optimizing both electrical and optical properties.

Mitigation strategies for defects in ZnO focus on growth conditions and post-growth treatments. High-temperature annealing in oxygen-rich environments can reduce oxygen vacancies by promoting oxygen incorporation into the lattice. Conversely, annealing in zinc-rich conditions may increase zinc interstitials but must be carefully controlled to avoid excessive defect formation. Rapid thermal annealing (RTA) is effective in minimizing defect aggregation while improving crystallinity. Hydrothermal and solvothermal synthesis methods, conducted under controlled pH and temperature, can yield ZnO with lower intrinsic defect densities compared to vapor-phase methods.

Another approach involves defect passivation using non-doping elements. For example, hydrogen plasma treatment can passivate zinc vacancies and other acceptor-like defects, improving luminescence efficiency. However, excessive hydrogen can introduce its own defects, necessitating precise control. Surface passivation with organic or inorganic layers can also reduce surface-related defects that contribute to non-radiative recombination.

The role of stoichiometry cannot be overstated. Near-stoichiometric ZnO exhibits fewer intrinsic defects, leading to better optoelectronic performance. Techniques such as pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) allow fine control over stoichiometry by adjusting oxygen and zinc fluxes during growth. Solution-based methods, while cost-effective, often require post-synthesis treatments to achieve comparable defect control.

In summary, intrinsic defects like oxygen vacancies and zinc interstitials play a dominant role in determining the luminescence and conductivity of ZnO. Characterization techniques such as PL, DLTS, and XPS provide critical insights into defect types and concentrations. Mitigation strategies involving controlled growth conditions, annealing, and passivation are essential for optimizing ZnO properties. By understanding and managing these defects, the performance of ZnO-based devices can be significantly enhanced without relying on extrinsic doping.