Fabrication of large area two-dimensional PtX2 (X=S, Se) heterostructures for broadband photodetector studies

Abstract

Over the last few decades, two-dimensional (2D) materials, especially transitional metal dichalcogenides (TMDCs) have procured a huge attraction for their wide range of intriguing properties. The thickness-dependent band gap, high absorption coefficient, and high on-off ratio of TMDC based devices make them potential candidates in the optoelectronics field to develop photodetectors, phototransistors, and solar cells. Photodetectors are optoelectronic devices that convert light signals into electrical signals. In recent years, for photodetector applications, group-10 TMDCs, especially PtS2 and PtSe2 have become a spotlight for their narrow band gap (0-1.6 eV) and high carrier mobility. Despite these properties, the controlled growth of PtS2 on various substrates and high-performance PtX2 (X=S, Se) based broadband photodetectors with best response in near-infrared region (NIR) are less explored. In present thesis work, we try to solve these challenges. In the first work, a large area and controlled growth of PtS2 films is achieved via thermally assisted conversion (TAC) method. The main issue during the growth of PtS2 by TAC method is the formation of an intermediate PtS phase due to its more stability. The present study shows that by optimizing the growth parameters such as Pt film thickness, sulfur amount, and carrier gas flow rate, a controlled growth of PtS2 and PtS to PtS2 conversion can be obtained. A detailed Raman and X-ray photoelectron spectroscopy (XPS) further revealed that PtS2 is dominantly to form for thinner Pt film, sulfurized under sulfur-rich vapors at a moderate gas flow rate. Furthermore, the PtS2 growth is highly reproducible and can be grown on different substrates such as SiO2/Si, Sapphire, Si, and mica, and their photodetector performance is also checked. In the second part of the thesis, different PtS2 heterostructures (PtS2/MoS2, PtS2-x/Ga2O3) are fabricated and their broadband photodetector performance are discussed. A detailed interface study of heterojunction, including their band alignment, valence band offset (VBO), and conduction band offset (CBO) is calculated. The PtS2/MoS2 heterostructure is fabricated by directly growing PtS2 film on MoS2-coated substrate by TAC method. The fabricated PtS2/MoS2 photodetector gives a broad response in the range of 400-1200 nm (Visible to NIR) with a maximum responsivity of 30.2 AW-1 under 900 nm illumination. The PtS2-x/Ga2O3 heterostructure is fabricated by wet transfer of PtS2-x film over Ga2O3 film. The fabricated PtS2-x/Ga2O3 based photodetector gives a wide response covering ultraviolet (UV)-C to NIR (250-1100 nm) with a maximum responsivity of 38.7 AW-1 under 1100 nm illumination and exhibits a fast millisecond response. In the last part of the thesis, large PtSe2/MoS2 heterostructure fabrication and its photodetector study are explained. The problem of high dark current in bare PtSe2 owing to narrow and semimetal is resolved by fabricating its heterostructure with MoS2. Compared to bare PtSe2, the PtSe2/MoS2 heterostructure shows a 103 order of reduction in dark current and an increase in detectivity of the fabricated photodetector. Moreover, PtSe2/MoS2 heterostructure shows a broad detection response covering visible to NIR (400-1200 nm) with maximum detectivity of 9.8×1012 Jones under 900 nm irradiation. An ultrafast response with a rise/fall time of 103 μs /117 μs is also achieved in the PtSe2/MoS2 device which shows its high performance. In conclusion, we have grown a controlled and large area of PtS2 film on various substrates by optimizing the critical growth parameters. The different PtX2 heterostructure (PtS2/MoS2, PtS2-x/Ga2O3, PtSe2/MoS2) fabrication, band alignment study, and their broadband photodetector study have also been carried out. Thus, these results suggest that PtS2 and PtSe2 are suitable materials for developing future optoelectronic devices.