Photovoltaics in action: Electron motion in a type-II InSe/GaAs semiconductor heterostructure has been recorded in a movie immediately after photoexcitation with high spatial and temporal resolution.
Electrons are the lifeblood of semiconductor devices, from transistors that power computers and smart phones, and semiconductor diodes that light up the night, to photovoltaic cells that harvest solar energy to power it all. Under the influence of applied voltages or light stimulations, electrons flow through nanoscale channels and plummet potential gradients at interfaces of disparate materials. In a semiconductor device this ebb and flow occurs several times every nanosecond within billions of transistors on a single microchip, unseen by human eye, but creating text, images and movies in strings of 0s and 1s. But the true time and spatial scales on which electrons are energized and transported span a range of hundreds of femtoseconds and tens of nanometres. Thus, to capture in a movie the physical phenomena of electrons in a device requires a truly extraordinary camera. Writing in Nature Nanotechnology, Man et al. report an experiment that performs just that. Specifically, they record a movie of electron flow in energy, space and time within a semiconductor heterojunction composed of GaAs in physical contact with InSe by imaging electrons emitted into vacuum through the joint action of femtosecond duration IR generation and UV electron emission laser pulses.