Mr Viesturs Spulis

Optoelectrical characterisation of individual non-radiative point defects in nitrides by time-resolved cathodoluminescence and drift-diffusion modelling

The most impactful technological triumphs in the modern era can be attributed to semiconductor research and the resulting device technologies. Transistors have enabled computational possibilities; light-emitting diode (LED) technology provide efficient and environmentally friendly lighting, while solar cells serve as one of the largest renewable energy sources. Many advancements in semiconductor technology can be attributed to the ability to control crystallographic point defects in crystalline materials. While some point defects — which can be as small as a single missing or misplaced atom — can give semiconductor materials desirable qualities, non-radiative point defects significantly reduce the efficiency of semiconductor devices by generating heat, or, as is in the case of LEDs, reducing light emission.

Despite the critical role non-radiative point defects play in limiting the performance of virtually every modern semiconductor device, our understanding of their behaviour remains largely confined to ensemble effects observable at the micron or device scale. This is, to a significant extent, due to an absence of measurement techniques capable of reliably probing these properties at the nanometre scale. Time-resolved cathodoluminescence (TR-CL) has emerged as a promising technique to address this knowledge gap, offering spatial resolution below the optical diffraction limit, the ability to excite wide-bandgap semiconductors, and a temporal resolution of approximately 10 picoseconds. While proof-of-concept studies have demonstrated TR-CL’s capability to estimate key parameters, such as relaxation times, of individual point defects, its full potential for characterizing large populations of individual point defects remains untapped.

In my PhD project, I am developing a systematic approach to probe individual non-radiative point defects in InGaN using ultrafast, nanoscale time-resolved cathodoluminescence combined with bespoke computational modelling. This work aims to enable the characterization of point defect types, densities, and their optoelectronic impacts. Additionally, correlating these findings with varied manufacturing parameters will, for the first time, allow the creation of a comprehensive database linking point defect properties to internal quantum efficiency, growth parameters, and relaxation dynamics. Developing such a database is essential for understanding and mitigating the adverse effects of point defects, ultimately leading to improved performance across a wide range of semiconductor devices.

If successful, this research would help fighting light poverty by improving efficiency of LEDs, improve renewable energy sources by raising efficiency of solar cells, reduce power consumption of microchips, and enable high efficiency ultraviolet LEDs as cheap and efficient replacements of mercury-based UV lights for the removal of biological pathogens from water in regions with limited water treatment infrastructure.