title: Imaging the motion of electrons across semiconductor heterojunctions.
abstract: Technological progress since the late twentieth century has centred on semiconductor devices, such as transistors, diodes and solar cells. At the heart of these devices is the internal motion of electrons through semiconductor materials due to applied electric fields or by the excitation of photocarriers. Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resolution. Current studies of electron dynamics in semiconductors are generally limited by the spatial resolution of optical probes, or by the temporal resolution of electronic probes. Here, by combining femtosecond pump-probe techniques with spectroscopic photoemission electron microscopy, we imaged the motion of photoexcited electrons from high-energy to low-energy states in a type-II 2D InSe/GaAs heterostructure. At the instant of photoexcitation, energy-resolved photoelectron images revealed a highly non-equilibrium distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equilibrium photocarriers, we observed the spatial redistribution of charges, thus forming internal electric fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we produced a movie lasting a few trillionths of a second of the electron-transfer process in the photoexcited type-II heterostructure-a fundamental phenomenon in semiconductor devices such as solar cells. Quantitative analysis and theoretical modelling of spatial variations in the movie provide insight into future solar cells, 2D materials and other semiconductor devices.
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: Michael K L
authors.identifier: 0000-0001-6043-3631
authors.initials: MK
authors.lastname: Man
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: Athanasios
authors.initials: A
authors.lastname: Margiolakis
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: Skylar
authors.initials: S
authors.lastname: Deckoff-Jones
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: Takaaki
authors.initials: T
authors.lastname: Harada
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: E Laine
authors.initials: EL
authors.lastname: Wong
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: M Bala Murali
authors.initials: MB
authors.lastname: Krishna
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: Julien
authors.initials: J
authors.lastname: Madéo
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: Andrew
authors.initials: A
authors.lastname: Winchester
authors.affiliation: Department of Materials Science and Nanoengineering, Rice University, Texas 77005, USA.
authors.forename: Sidong
authors.initials: S
authors.lastname: Lei
authors.affiliation: Department of Materials Science and Nanoengineering, Rice University, Texas 77005, USA.
authors.forename: Robert
authors.identifier: 0000-0002-3942-8827
authors.initials: R
authors.lastname: Vajtai
authors.affiliation: Department of Materials Science and Nanoengineering, Rice University, Texas 77005, USA.
authors.forename: Pulickel M
authors.initials: PM
authors.lastname: Ajayan
authors.affiliation: Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan.
authors.forename: Keshav M
authors.identifier: 0000-0003-3917-6305
authors.initials: KM
authors.lastname: Dani