Electron dynamics at semiconductor surfaces and interfaces on ultrashort length and ultrafast time scales

This project will be focused on studying the electrical properties of semiconductor surfaces and interfaces. The fields of scanning tunnelling microscopy (STM), scanning near-field optical microscopy (s-SNOM) and optical pump terahertz probe spectroscopy (OPTPS) will be combined in the genesis of a single instrument able to probe electrical properties of materials at unprecedented spatial and temporal resolution.

During the project the novel instrument will be exploited to understand nanoscale charge dynamics in two important classes of semiconductors which show great promise for application in energy conversion; metal halide perovskites and semiconductor nanowires. Elucidating the mechanisms of charge recombination, trapping and degradation at surfaces and grain boundaries will have a direct impact on the design and implementation of these materials in solar cells.

While the project is focussed on study of these semiconductors for energy conversion, the impact and potential of the work will extend well beyond that field. The research conducted during this project will develop, implement and promote new instrumentation for the development of novel high-efficiency photovoltaic technologies. The instrument will spatially and temporally resolve charge-carrier recombination and extraction across interfaces allowing a fundamental understanding of nascent photovoltaic technologies based on metal halide perovskite (MHP) and nanostructured III-V semiconductors.

Previously we have developed and applied optical pump terahertz probe spectroscopy (OPTPS) to measure femtosecond charge dynamics in semiconductor nanostructures and MHPs, but the technique is applied to ensembles and lacks microscopic detail. Recently there has been great interest in scattering-type Scanning Near-field Optical Microscopy (s-SNOM) which allows ~20nm spatial resolution imaging from the visible to THz region of the spectrum, based on scattering of light from an AFM tip.

A less common approach to achieving even higher spatial resolution is terahertz scanning tunnelling microscopy (THz-STM) in which a THz pulse creates rectified tunnel current. The combination of these techniques in a single instrument is extremely powerful and of high novelty. The objectives of the proposed project are:

1. To develop a novel instrument for studying charge dynamics at semiconductor surfaces interfaces with unprecedented temporal and spatial resolution and create a theoretical framework for processing and modelling data.

2. To answer the unresolved questions surrounding the function of grain boundaries in metal halide perovskite photovoltaics through a microscopic understanding of charge dynamics at grain boundaries and grain surfaces.

3. To observe charge dynamics at interfaces between different semiconductors on a femtosecond time scale in heterojunctions of III-V semiconductor nanowires and metal halide perovskite semiconductors, with the aim of designing more efficient multijunction solar cells.

The project underpins the EPSRC's objective of "delivering economic impact and social prosperity" via the Resilient Nation objective by addressing key societal challenges of energy security and global warming. The Productive Nation is also applicable given that PV is an important area with the global industry currently worth ~US$100 billion, and is likely to grow in coming years.

Beyond solar technologies the instrument developed in the project is ideally suited to the study of cutting-edge transistors and integrated circuits (semiconductor industry annual global revenue ~US$500 billion). Collaborators include:

University of Birmingham, University of Strathclyde, University of Cambridge, Australian National University (Australia), EPFL (Switzerland), IBM Zurich (Switzerland), RAL Space, Topica Photonics (Germany),