Deep-level transient spectroscopy (DLTS) is a critical tool for investigating trap states in oxide semiconductors, particularly in materials like indium gallium zinc oxide (IGZO) and titanium dioxide (TiO2). These materials are widely used in applications such as thin-film transistors, photovoltaics, and sensors, where understanding defect states is essential for optimizing performance. Oxygen vacancies, interface states, and metastable defects significantly influence carrier transport, making DLTS an invaluable technique for characterizing these electronic traps.

Oxygen vacancies are among the most common defects in oxide semiconductors. In IGZO, oxygen vacancies act as shallow donors, contributing to n-type conductivity. However, they can also form deep-level traps that capture charge carriers, leading to threshold voltage shifts and reduced mobility in thin-film transistors. DLTS studies on IGZO reveal multiple trap levels within the bandgap, with activation energies ranging from 0.1 eV to 0.6 eV below the conduction band. These traps are associated with oxygen vacancy complexes and metal cation disorder. The high resistivity often observed in IGZO films is partly attributed to these defect states, which scatter carriers and reduce effective mobility.

In TiO2, oxygen vacancies play a dual role. While they enhance n-type conductivity in reduced TiO2, they also introduce deep traps that degrade charge carrier lifetimes. DLTS measurements on rutile and anatase TiO2 identify electron traps with activation energies between 0.3 eV and 0.8 eV below the conduction band. These traps are linked to singly and doubly ionized oxygen vacancies, which act as recombination centers in photocatalytic and photovoltaic applications. The presence of these defects can lead to Fermi-level pinning, limiting the efficiency of TiO2-based devices.

Interface states in oxide semiconductors are another critical area of study using DLTS. In IGZO thin-film transistors, the interface between the semiconductor and the gate dielectric (e.g., SiO2 or Al2O3) hosts trap states that influence device stability. DLTS spectra of IGZO-SiO2 interfaces reveal traps with activation energies of 0.2 eV to 0.5 eV, attributed to dangling bonds and disorder-induced defects. These interface states contribute to hysteresis in transfer characteristics and bias stress instability. Similar issues arise in TiO2-based heterostructures, where interface traps between TiO2 and metals or other oxides affect charge injection and recombination dynamics.

Metastable defects pose a unique challenge in oxide semiconductors. These defects exhibit time-dependent behavior, often activated by electrical bias, light exposure, or environmental conditions. DLTS is particularly effective in capturing their transient nature. For example, in IGZO, metastable oxygen vacancy complexes can switch between active and passive states under bias stress, leading to threshold voltage instabilities. DLTS measurements under varying bias conditions help quantify the density and emission kinetics of these defects. In TiO2, metastable traps associated with hydroxyl groups or adsorbed species can alter conductivity over time, complicating device reliability.

The high resistivity of oxide semiconductors further complicates DLTS measurements. Many oxide materials, especially in their stoichiometric or lightly doped forms, exhibit low free carrier concentrations. This results in weak DLTS signals, requiring high sensitivity instrumentation and careful sample preparation. Techniques like Laplace DLTS, which offers higher energy resolution, are often employed to resolve closely spaced trap levels in these materials. Additionally, the high resistivity can lead to non-uniform electric fields during DLTS measurements, necessitating corrections for accurate trap profiling.

Challenges also arise from the complex defect chemistry of oxide semiconductors. Unlike conventional semiconductors like silicon, oxides often have multiple charge states for a single defect type. Oxygen vacancies in TiO2, for instance, can exist in neutral, singly ionized, or doubly ionized states, each contributing differently to DLTS spectra. Distinguishing these states requires complementary techniques like electron paramagnetic resonance (EPR) or photoluminescence spectroscopy. Furthermore, the interaction between intrinsic defects and extrinsic impurities (e.g., hydrogen or transition metals) can create additional trap levels, complicating the interpretation of DLTS data.

Despite these challenges, DLTS provides critical insights into the performance limitations of oxide semiconductors. In IGZO-based thin-film transistors, identifying and quantifying oxygen vacancy-related traps helps optimize deposition conditions and post-processing treatments (e.g., annealing in oxygen-rich atmospheres) to reduce defect densities. For TiO2 photocatalysts, DLTS studies guide doping strategies to minimize recombination-active traps. The technique also aids in understanding the role of interface states in heterojunction devices, enabling better interface engineering for improved charge transport.

In summary, DLTS is a powerful method for probing trap states in oxide semiconductors, with applications spanning from fundamental defect studies to device optimization. Oxygen vacancies, interface states, and metastable defects are key factors influencing carrier transport and device performance. While challenges like high resistivity and complex defect chemistries exist, advancements in DLTS instrumentation and complementary characterization techniques continue to enhance our understanding of these materials. By addressing these challenges, researchers can further improve the performance and reliability of oxide semiconductor devices.