Light absorption in semiconductor crystals without inversion symmetry can produce electric currents. Recently, researchers at the Max Born Institute generated directed currents at THz frequencies, which is much faster than the clock rates of current electronics. They have shown that electronic charge transfer between adjacent atoms in the crystal lattice is the underlying mechanism.
Solar cells transform the energy from light into a dc which can augment an electric supply grid. Critical steps in this power conversion are the separation of charges after light absorption and their transport to the device’s contacts. Negatively charged electrons carry the electric currents, and positive charge carriers (holes) cause the so-called intraband motions in different electronic bands of the semiconductor.
Physicists want to answer the questions, “What is the smallest unit within a crystal that can provide a photo-induced direct current?”, “Up to which maximum frequency can one generate such currents?”, and, “At the atomic scale, which mechanisms are responsible for such charge transport?”
The smallest unit of a crystal is known as the unit cell, a distinct arrangement of atoms that chemical bonds determine (atom diagram below). The unit cell of the prototype semiconductor GaAs embodies a configuration of Ga and As atoms without a center of inversion. In the ground state of the crystal designated by the electronic valence band, the valence electrons are concentrate on the bonds between Ga and the As atoms.
After absorbing near-infrared or visible light, an electron is promoted from the valence band to the next higher band, the conduction band. In the new state, the electron charge shifts towards the Ga atoms. (See the position graph of electron density in both the valence band and the conduction band above). This charge movement corresponds to a local electric current, the interband or shift current, which is fundamentally distinct from the electron motions in intraband currents. Until recently, theoreticians have debated whether the experimentally observed photo-induced currents are due to intraband or interband motions.
For the first time, researchers at the Max Born Institute in Berlin, Germany, have studied optically-induced shift currents in the semiconductor gallium arsenide (GaAs) on ultrafast time scales as fast as 50 femtoseconds. They reported the results in the current issue of the journal Physical Review Letters 121, 266602 (2018).
Using intense and ultrashort light pulses ranging from the near infrared (λ = 900nm) to the visible (λ = 650nm, orange), they generated shift currents in GaAs which oscillated, thus, emitting terahertz radiation with a bandwidth up to 20THz. The underlying electron motions and the properties of these currents are fully reflected in the emitted THz waves.
These THz waves are detected in amplitude and phase. According to the researchers, the THz radiation demonstrates that the ultrashort current bursts of rectified light occur at frequencies which are about 5000 times higher than the highest clock rate of modern computer technology.
The properties of the observed shift currents exclude an intraband motion of electrons or holes. However, model calculations based on the interband transfer of electrons in pseudo-potential band structures reproduce the experimental results. These model calculation show that a real-space transfer of electrons over the distance on the order of just a bond length is a chief mechanism for this type of current generating.
In fact, this process operates within each unit cell of the crystal, which in this example is on a sub-nanometer length scale. This process causes the rectification of the optical field. According to the researchers, the effect can be exploited at even greater frequencies, offering novel applications in high-frequency electronics.