Effect of interface on mid-infrared photothermal response of MoS₂ thin film grown by pulsed laser deposition

This study reports on the mid-infrared (mid-IR) photothermal response of multilayer MoS₂ thin films grown on crystalline (p-type silicon and c-axis-oriented single crystal sapphire) and amorphous (Si/SiO₂ and Si/SiN) substrates by pulsed laser deposition (PLD). The photothermal response of the MoS₂ films is measured as the changes in the resistance of the MoS₂ films when irradiated with a mid-IR (7 to 8.2 μm) source. We show that enhancing the temperature coefficient of resistance (TCR) of the MoS₂ thin films is possible by controlling the film-substrate interface through a proper choice of substrate and growth conditions. The thin films grown by PLD are characterized using X-ray diffraction, Raman, atomic force microscopy, X-ray photoelectron microscopy, and transmission electron microscopy. The high-resolution transmission electron microscopy (HRTEM) images show that the MoS₂ films grow on sapphire substrates in a layer-by-layer manner with misfit dislocations. The layer growth morphology is disrupted when the films are grown on substrates with a diamond cubic structure (e.g., silicon) because of twin growth formation. The growth morphology on amorphous substrates, such as Si/SiO₂ or Si/SiN, is very different. The PLD-grown MoS₂ films on silicon show higher TCR (-2.9% K^-1 at 296 K), higher mid-IR sensitivity (ΔR/R = 5.2%), and higher responsivity (8.7 V·W^–1) compared to both the PLD-grown films on other substrates and the mechanically exfoliated MoS₂ flakes transferred to different substrates.