Temperature-driven Emergence of Negative Photoconductivity in Semimetal MoTe2 Film Probed with Terahertz Spectroscopy

TL;DR

This study uses terahertz spectroscopy to reveal temperature-dependent carrier dynamics in MoTe2, showing negative photoconductivity at low temperatures due to polaron formation, which impacts optoelectronic applications.

Contribution

It provides the first experimental evidence of small polaron formation in MoTe2, linking temperature-dependent electron-phonon interactions to its optical response.

Findings

High-temperature MoTe2 shows only positive photoconductivity.

Low-temperature MoTe2 exhibits negative photoconductivity after initial positive response.

Polaron formation time increases slightly with temperature, from 0.4 ps to 0.5 ps.

Abstract

Layered two-dimensional (2D) materials MoTe2 have been paid special attention due to the rich optoelectronic properties with various phases. The nonequilibrium carrier dynamics as well as its temperature dependence in MoTe2 are of prime importance, as it can shed light on understanding the anomalous optical response and potential applications in far infrared (IR) photodetection. Hereby, we employ time-resolved terahertz (THz) spectroscopy to study the temperature dependent nonequilibrium carrier dynamics in MoTe2 films. After photoexcitation of 1.59 eV, the 1T'-phase MoTe2 at high temperature behaves only THz positive photoconductivity (PPC) with relaxation time of less than 1 ps. In contrast, the Td-phase MoTe2 at low temperature shows ultrafast THz PPC initially followed by emerging THz negative photoconductivity (NPC), and the THz NPC signal relaxes to the equilibrium state in hundreds of ps time scale. Small polaron formation induced by hot carrier has been proposed to be ascribed to the THz NPC in the polar semimetal MoTe2 at low temperature. The polaron formation time after photoexcitation increases slightly with temperature, which is determined to be ~0.4 ps at 5 K and 0.5 ps at 100 K. Our experimental result demonstrates for the first time the dynamical formation of small poalron in MoTe2 Weyl semimetal, this is fundamental importance on the understanding the temperature dependent electron-phonon coupling and quantum phase transition, as well as the designing the MoTe2-based far IR photodetector.