Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics

Abstract

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.

Recent Advances in 2D Metal Monochalcogenides

“…[206] In a similar manner, in-plane anisotropic layered 2D MMCs have recently been investigated as potentially promising materials in photovoltaics. [38,41,42,109,172,[207][208][209][210][211][212] Dai and co-workers [41] have designed and investigated a new photovoltaic system based on novel 2D MMCs. In particular, the photoresponse and the photovoltaic performance of GeS, GeSe, SnS, and SnSe monolayers were evaluated by means of theoretical quantum transport simulations.…”

“…[ 206 ] In a similar manner, in‐plane anisotropic layered 2D MMCs have recently been investigated as potentially promising materials in photovoltaics. [ 38,41,42,109,172,207–212 ]…”

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…Terahertz radiation is strongly attenuated by free charge-carriers, and thus optical pump-terahertz probe spectroscopy (OPTPS) became a powerful method for characterizing the charge carrier dynamics of semiconductors with sub-picosecond temporal resolution. 3 Over the years there have been countless insight into carrier dynamics using OPTPS, ranging from the determination of carrier lifetimes -and by-extension, mobilities [78][79][80] -to the quantification of electron-phonon coupling in perovskite crystals. 81,82 This method has shown growth and utility over the years, with new developments occurring to-date.…”

Injection current in ferroelectric group-IV monochalcogenide monolayers

“…Injection and shift current usually occur simultaneously and distinguishing one from another is challenging 9 . The shift current in MMs was recently studied theoretically using first-principles density functional theory (DFT) 16,17 and experimentally using Terahertz spectroscopy in the group of L. Titova 18,19 . It was found to be large, potentially able to overcome the limitation of bulk FEs.…”