Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals

Abstract

A variety of deposition methods for two-dimensional crystals have been demonstrated; however, their wafer-scale deposition remains a challenge. Here we introduce a technique for depositing and patterning of wafer-scale two-dimensional metal chalcogenide compounds by transforming the native interfacial metal oxide layer of low melting point metal precursors (group III and IV) in liquid form. In an oxygen-containing atmosphere, these metals establish an atomically thin oxide layer in a self-limiting reaction. The layer increases the wettability of the liquid metal placed on oxygen-terminated substrates, leaving the thin oxide layer behind. In the case of liquid gallium, the oxide skin attaches exclusively to a substrate and is then sulfurized via a relatively low temperature process. By controlling the surface chemistry of the substrate, we produce large area two-dimensional semiconducting GaS of unit cell thickness (∼1.5 nm). The presented deposition and patterning method offers great commercial potential for wafer-scale processes.

Figure 1. GaS representation and printing process of the 2D layers.

Stick-and-ball representation of GaS crystal. (a) Side view of bilayer GaS, showing a unit cell c=15.492 Å made of two GaS layers. (b) Top view of the GaS crystal. The GaS crystal lattice is composed of Ga–Ga and Ga–S covalent bonds that extend in two dimensions, forming a stratified crystal made of planes that are held together by van der Waals attractions. The Ga atoms are green and the S atoms are blue. (c,d) Functionalization of the substrate with FDTES changes the contact angle between a Ga drop and the substrate by 12.9°. The SiO₂ substrate becomes more hydrophobic and thus resists wetting by liquid Ga. (e,h) Schematics of the synthesis process for patterning 2D GaS via printing the skin oxide of liquid Ga. (e) Lithography process for establishing the negative pattern of the photoresist. (f) Covering the exposed area of the substrate with vaporized FDTES. (g) Placing Ga liquid metal and removing it with soft PDMS that leaves a cracked layer of Ga oxide. (h) Two concurrent steps of chemical vapour treatment: first, GaCl₃ layer is formed via exposure to HCl vapour and, second, sulfurization by exposure to S vapour forming GaS.

Figure 2. Material characterizations of printed GaS.

(a) XRD of printed GaS film (after multiple printing steps) with XRD of the printed oxide skin and SiO₂/Si substrate for comparison and GaS planes indicated (traces have been normalized and offset for clarity). The GaS peak corresponding to the 002 plane (11.6°) reveals a layer spacing of 7.62 Å that is in good agreement with previous studies. (b) Raman spectrum of the 2D GaS with GaS vibrational peak shifts indicated. (c,d) XPS of the 2D GaS for the regions of interest (c) Ga 3d and (d) S 2p. A small peak from elemental Ga is also observed in the Ga 3d region that has been associated with the slow decomposition of GaS in moist air. XPS analysis of the printed 2D GaO_x and GaCl₃ are shown in Supplementary Fig. 4.

Figure 3. Morphology and characteristics of the printed 2D films.

(a) AFM image with (b) height profile of the printed 2D GaS layer confirming bilayer deposition. The profile is offset to the substrate's surface. (c,d) Confocal PL map shows a patterned area made of 2D GaS (green) on a SiO₂/Si substrate (black), demonstrating that a near-uniform 2D layer is obtained in a large area along with the PL spectra from areas noted on map (c, patterns 1 and 2). Due to the excitation wavelength limitation of confocal PL, the deep trap emission is used for assessing the uniformity of the PL pattern. (e) PL emission spectra demonstrating contributions of the interband transitions (red) and deep trap recombinations (black). The deep trap emission spectrum is scaled by a factor of 10 for clarity. (f) High-resolution transmission electron microcsopy (HRTEM) of a flake mechanically scratched from the printed GaS film. Additional HRTEM images are presented in Supplementary Fig. 5. A full discussion on continuity of the 2D films, grain boundaries and the concentration and nature of defects are presented in the Supplementary Information (Supplementary Figs 1c,d, 5 and 6).

Figure 4. Statistical Raman analysis of the 2D GaS films.

Raman line scan analysis. (a) Optical image, the line scan was performed on the black line indicated. (b) Contour plot with phonon modes indicated. (c) Relative intensity of the E¹_2g phonon peak with standard error indicated. The measurements and analysis show a good uniformity for the deposited bilayer GaS. The relative intensity of the E¹_2g peak is shown to present a standard error of only 2.5% across an area spanning 100 μm.

Figure 5. Optical and electronic properties of 2D GaS.

Hybrid DFT computation of the band structures in bilayer (a) and bulk (b) GaS. For comparison, the DFT calculations for three- and four-layer GaS are presented in Supplementary Fig. 7. (c) Tauc plot used for determining the electronic band transition with simplified electronic band diagram (inset). Red lines are the valence and conduction band edges, black is the Fermi level and blue is the location of the deep trap band. (d) PESA demonstrating the valence band maximum (VBM) energy is −5.15 eV. (e) XPS valance analysis revealing an energy difference of 1.8 eV between the VBM and Fermi level. (f) PL decay with fitted exponential (red) and mean excitonic decay (blue) time indicated.

Figure 6. Characterization of 2D GaS FETs.

(a) Schematic representation of back gated GaS FET with tungsten/WS₂ electrodes featuring band energy diagram. (b) Raman spectrum of the W electrodes after sulfurization. Phonon modes of WS₂ are indicated demonstrating the growth of WS₂ on the skin of the electrodes. (c) Optical images of the fabricated devices and electrode gap (inset). The scale bars are 500 and 20 μm, respectively. The fabrication process is outlined in Supplementary Fig. 8. We tested five FETs at different locations and the mobility measurements have been consistent. (d) I/V characteristics of the FET at different gate voltages. (e) I/V phototransistor characteristics with no gate bias.

Figure 7. Statistical analysis of photodetector devices.

(a) Responsivity of each individual photodetector device and (b) distribution of responsivities. (c) On/off ratio of each device and (d) distribution of ratios. The median responsivity is ∼6.4 A W⁻¹ and the median on/off ratio is ∼170.