Advanced Semiconductor Materials and Devices Group

Layer Transfer of 2D Materials Using Quasi-Dry Method

The quality of a synthesized film depends on various factors, including the choice of substrate for film deposition, what are the required precursors for growth, growth pressure and temperature, and so on. Most TMDCs are typically synthesized at temperatures above 500 °C, making it impractical to deposit on temperature-sensitive substrates. There are several layer transfer processes to overcome the temperature dependent issues such as the wet-etching method, water-assisted transfer method, double support layer-based transfer method, and so on that have been studied and optimized for two-dimensional (2D) materials in our lab group. Presently, the wet etching transfer is extensively employed for the transfer of MoS₂. The wet etching method involves applying a layer of polymethylmethacrylate (PMMA) onto the MoS₂ material through spin coating. The PMMA/MoS₂ stack is then separated from the substrate using chemical etching, typically employing hydrogen fluoride (HF) or other strong etchants. However, this chemical etching process can harm the MoS₂ film and degrade its quality, which is undesirable for the heterostructure formation. Therefore, there is a strong demand for a highly efficient, repeatable transfer approach that avoids damage and ensures the preservation of film integrity.

In our lab, we developed a novel transfer process named the quasi-dry layer transfer method, free from PMMA residues and chemical etchants. The schematic illustration for MoS₂ layer transfer by this process is shown in Figure 1. Since this layer transfer method utilizes only PDMS and water, it can be referred to as the quasi-dry layer transfer method. Furthermore, the quasi-dry layer transfer process has emerged as a highly versatile technique for creating 2D/2D (MoS₂/WS₂ and vice versa) and 2D/3D (MoS₂/β-Ga2O3) van der Waals (vdW) heterostructures as shown in Figure 2. We also fabricated MoS₂/mica and MoS₂/β-Ga₂O₃/mica flexible photodetectors and studied their performance with bending/tensile strain and temperature. The responsivity and detectivity of the MoS₂/mica flexible photodetector were found to be 1.10 mA/W and 3.86 × 1010 Jones, respectively. The PDCR, responsivity, and detectivity of the MoS₂/β-Ga₂O₃/mica flexible photodetector were calculated to be 103, 7.21 mA/W, and 2.4 × 1011 Jones, respectively. The photocurrent and responsivity were increased by 155% and 136% under 0.61% tensile strain.

Figure 1. Schematic of MoS₂ layer transfer by quasi-dry layer transfer process

Figure 2. (a) AFM image of the MoS₂/β-Ga2O3 heterostructure, (b) XPS spectra of the Ga 3d core level for pristine β-Ga₂O₃ and MoS₂/β-Ga₂O₃, (c) Schematic illustration of the MoS₂/β-Ga₂O₃ flexible photodiode, (d) Time-dependent photoresponse of the device under different bending states (strain states)