Optical Pump–Terahertz Probe Measurements of Photocarrier Multiplication in 2D Bulk Single-Crystal 2H-MoTe₂

Overcoming the Shockley–Queisser limit in photovoltaic systems is an ongoing focus. Processes such as impact ionization that produce carrier multiplication in bulk materials are typically inefficient due to the requirements of energy and momentum conservation. Transition metal dichalcogenides, which are of interest for numerous reasons, have, additionally, recently been shown to exhibit very efficient carrier multiplication (CM) in thin films of 2H-MoTe2 and 2H-WTe2 formed by chemical vapor deposition (CVD). The photoconductivity in the thin films exhibited ultrafast (<1 ps) dynamics, atypical of most indirect semiconductors, that may be hard to translate to increased photovoltaic efficiency. To provide a window to the impact of material quality on this intriguing CM effect, we used optical pump–terahertz probe (OPTP) techniques to investigate carrier lifetimes and the efficiency for carrier multiplication for bulk single crystals of 2H-MoTe2 produced by chemical vapor transport (CVT). Efficient carrier multiplication is observed in this bulk single crystal, and photocarrier lifetimes are increased by orders of magnitude although there is still evidence of photocarrier localization in the THz frequency response.