Tuning the Optoelectronic Properties of Pulsed Laser Deposited “3D”-MoS2 Films via the Degree of Vertical Alignment of Their Constituting Layers

Abstract

Pulsed-laser-deposition (PLD) is used to deposit MoS2 thin films at substrate temperatures (Td) ranging from 25 to 700 °C. A Td = 500 °C is identified as the optimal temperature that yields MoS2 films consisting of highly-crystallized 2H-MoS2 phase with a strong (002) preferential orientation, a direct optical bandgap (Eg) of ∼1.4 eV and a strong photoresponse of ∼1500%. Raman spectroscopy revealed that the degree of vertical alignment of MoS2 layers in the films also reaches its maximum at Td = 500 °C. High-resolution-transmission-electron-microscopy has provided a clear-cut evidence that the PLD-MoS2 films predominantly consist of vertically aligned MoS2 layers over all the film thickness of ∼90 nm, enabling those “3D” films to behave as a direct-bandgap “2D-MoS2” with exceptional optoelectronic properties. Indeed, at Td = 500 °C, the PLD-MoS2 based photodetectors (PDs) devices are shown to exhibit the highest responsivity (R) and detectivity (D*) values (125 mA W−1 and 9.2 × 109 Jones, respectively) ever reported for large area (≥ 1 cm2) MoS2-based PDs operating at a voltage as low as 1 V. For the first time, a constant-plus-linear relationship between Eg, R, and D* of the PDs and the degree of vertical alignment of the MoS2 layers is established. Such a correlation is fundamental for the controlled growth of PLD-MoS2 films and the tuning of their properties in view of their integration with standard large-scale-integration processing.