Low-defect-density WS2 by hydroxide vapor phase deposition

Abstract

Two-dimensional (2D) semiconducting monolayers such as transition metal dichalcogenides (TMDs) are promising channel materials to extend Moore's Law in advanced electronics. Synthetic TMD layers from chemical vapor deposition (CVD) are scalable for fabrication but notorious for their high defect densities. Therefore, innovative endeavors on growth reaction to enhance their quality are urgently needed. Here, we report that the hydroxide W species, an extremely pure vapor phase metal precursor form, is very efficient for sulfurization, leading to about one order of magnitude lower defect density compared to those from conventional CVD methods. The field-effect transistor (FET) devices based on the proposed growth reach a peak electron mobility ~200 cm²/Vs (~800 cm²/Vs) at room temperature (15 K), comparable to those from exfoliated flakes. The FET device with a channel length of 100 nm displays a high on-state current of ~400 µA/µm, encouraging the industrialization of 2D materials.

Fig. 1. Hydroxide Vapor Phase Deposition.

a Schematic of hydroxide vapor phase deposition (OHVPD) growth of WS₂ monolayers. b Nudged elastic band (NEB) simulation of kinetic energy barriers (ΔE) for bonded OH and O dissociating from the edge of WS₂. c, d Optical Image c and AFM image d of the OHVPD-WS₂ monolayer. e Photo of a 2-inch OHVPD-WS₂ monolayer film grown on a sapphire substrate.

Fig. 2. Optical characterizations of OHVPD- and CVD-WS₂ monolayers.

a Typical Raman spectra showing the characteristic modes of OHVPD- and CVD-WS₂ monolayers excited by 532 nm wavelengths. The hollow circles and coloured lines are the experimental and Lorentzian fit curves respectively. b, c Statistic distribution of out-of-plane mode A_1g Raman peak width and normalized intensity of longitudinal acoustic at M point in the Brillouin zone LA(M) Raman peak for OHVPD- and CVD-WS₂ monolayers. The dashed lines represent the normal distribution curves. d Low-temperature PL spectra of OHVPD- and CVD- WS₂ monolayers at 4 K. The solid lines and dashed ones are the experimental and fitted peaks respectively. The fitted peaks can be assigned to neutral exciton (X⁰), trion (X^T), and defect-bound exciton (X^D).

Fig. 3. Defect analysis by scanning tunneling microscopy (STM).

a, b STM images of a CVD-WS₂ (Bias Voltage (V) = 1.35 V, Current (I) = 40 pA) and b OHVPD-WS₂ monolayer (V = 1.15 V, I = 30 pA). c–f STM images (V = 1.1 V, I = 30 pA) of the commonly observed point defects in CVD- and OHVPD-WS₂: oxygen substituting sulfur (O_s) in the c top and d bottom sulfur plane; e Mo substitutional tungsten (Mo_W); f Positively charged defect (PCD) and g Negatively charged defect (NCD). h, Histograms table of observed point defect density in different OHVPD- and CVD- WS₂.

Fig. 4. Electrical performance of OHVPD-WS₂ monolayers.

a Four-probe conductivity as a function of Vg for OHVPD-WS₂ monolayer device on the 300 nm SiO_x substrate at different temperatures. Insect shows the device structure. (Scale bar: 5 μm) b Field-effect mobility as a function of temperature for OHVPD- and CVD-WS₂ monolayers. c Comparison of mobility distribution for our OHVPD-WS₂ results (orange), mechanical exfoliation WS₂ monolayers (ME, green), and conventional CVD-WS₂ (cyan) from literatures. d FET transfer curve of an OHVPD-WS₂ monolayer for the short channel device (L_CH = 100 nm).