Engineering and harnessing sub-bandgap states: From defect physics to quantum functionalities

Sub-bandgap states, originating from intrinsic defects, extrinsic dopants, or physical processes such as irradiation, interface engineering, and strain engineering, serve as a versatile mechanism for modulating properties across diverse semiconductor platforms. Existing reviews are typically confined to a single application or a single material system. This review introduces a unified framework that connects the origin of these states to the electronic structure via spectroscopic characterisation. Subsequently, it links the nature of these sub-bandgap states to optoelectronic and quantum functionalities. This framework has been applied across diverse material systems, including wide-bandgap oxides, transition-metal dichalcogenides, halide perovskites, hexagonal boron nitride, diamond, and SiC. A multi-modal defect-identification strategy is developed to emphasise the need for independent experimental and theoretical evidence before assigning a microscopic origin to a sub-bandgap feature. In addition, the mathematical frameworks used to quantify defect-related parameters are rigorously examined. Quantitative benchmarks are tabulated for sub-bandgap photodetectors, defect-based light emitters, quantum emitters and defect-engineered photocatalysts. The review closes by identifying the principal unresolved issues that separate laboratory demonstrations from technological deployment. This includes deterministic, atomically precise and scalable defect creation, charge-state and environmental instability, and the resulting limits on reproducibility and standardisation.