Partially Coherent Transport in IGZO Advances Understanding of Thin-Film Transistor Performance

Amorphous oxide semiconductors hold considerable promise for advancing computer technology, potentially enabling smaller, more efficient memory and three-dimensional chip designs, yet the precise way electrons move within these materials remains a significant puzzle. Ying Zhao, Michiel J. van, and Anastasiia Kruv, working at imec in Leuven, Belgium, alongside Pietro Rinaudo, Harold Dekkers, Jacopo Franco, and colleagues, now reveal that charge transport in indium gallium zinc oxide (IGZO) occurs via partially coherent electrons, challenging long-held assumptions about how these materials function. The team demonstrates that electrons traverse insulating gaps between small, locally coherent regions, rather than relying on previously proposed mechanisms like hopping between states or movement through a broad energy range. This discovery, underpinned by a new theoretical framework called field-effect-aware fluctuation-induced tunnelling, accurately predicts the behaviour of IGZO transistors and provides a pathway to optimise performance and unlock the full potential of this important semiconductor technology.

Promising candidates exist for enabling further DRAM scaling and 3D integration, which are critical for advanced computing. Despite extensive research, the charge transport mechanism in these disordered semiconductors remains poorly understood. This work investigates charge transport in the archetypical amorphous oxide semiconductor material, indium gallium zinc oxide (IGZO), across a range of compositions and temperatures using both thin-film transistors and Hall bar structures. The results show that the electrons involved in transport exhibit partially spatial coherence and non-degenerate conduction, revealing that electron transfer occurs primarily across insulating gaps separating locally coherent regions.

Simulating Charge Transport in Amorphous IGZO

This research details computational methods used to study the properties of amorphous Indium Gallium Zinc Oxide (a-IGZO), a material used in thin-film transistors (TFTs) for applications like displays and potentially DRAM. The team employed Density Functional Theory (DFT) with the cp2k software package, utilizing a mixed Gaussian and plane wave approach to calculate the electronic structure of the material. These simulations address material imperfections, such as defects and disorder, which significantly impact electrical properties, and consider the density of states and the impact of oxygen vacancies.

IGZO Charge Transport Reveals Partial Coherence

Scientists have achieved a breakthrough in understanding charge transport within amorphous oxide semiconductors, specifically IGZO. The research team investigated IGZO across a range of compositions and temperatures using both thin-film transistors and Hall bar structures, revealing that electrons exhibit partial spatial coherence and non-degenerate conduction. Experiments demonstrate that electron transfer occurs primarily across insulating gaps separating locally coherent regions, challenging previously held assumptions about transport mechanisms. Hall measurements, combined with first-principles calculations, confirm that electrons maintain partial coherence within finite regions, with the size of these regions directly influenced by the material’s composition.

The team developed a new framework, field-effect-aware fluctuation-induced tunnelling (FEAFIT), to accurately model this behavior. This model accurately predicts experimental data across all tested compositions, temperatures, and gate voltages, allowing for the extraction of fundamental transport parameters. These parameters were then correlated with electron coherence dimensions and the degree of energetic disorder, providing a deeper understanding of the relationship between material properties and electrical performance.

Coherent Transport in Amorphous Oxide Semiconductors

This research significantly advances understanding of charge transport within amorphous oxide semiconductors, materials crucial for next-generation electronic devices. Scientists demonstrated that electrons in indium gallium zinc oxide do not behave as previously assumed, exhibiting partial spatial coherence rather than the fully delocalized or completely localized states often proposed in existing models. The team established that conduction occurs primarily through electron transfer across insulating gaps separating small, locally coherent regions, a mechanism fundamentally different from conventional percolation or hopping theories. Researchers developed a new framework, fluctuation-induced tunnelling with field-effect awareness, which successfully predicts experimental results across varying material compositions, temperatures, and electrical conditions. By correlating tunnelling parameters with first-principles calculations, they revealed a clear link between gallium content, electron coherence dimensions, and the degree of energetic disorder within the material.