Halide-assisted atmospheric pressure growth of large WSe₂ and WS₂ monolayer crystals

Highlights

• We demonstrate halide-assisted chemical vapor deposition of WSe₂ and WS₂ monolayers.

• Halide-stimulated the formation of volatile tungsten oxyhalides from tungsten oxide results in growing WSe₂ and WS₂ monolayers, in atmospheric pressure at low temperatures (700–850 °C).

• The as-grown monolayers show high hole and electron mobility up to 102 and 26 cm² V⁻¹ s⁻¹ for WSe₂ devices and electron mobility of 14 for WS₂ devices.

Abstract

Chemical vapor deposition (CVD) of two-dimensional (2D) tungsten dichalcogenide crystals requires steady flow of tungsten source in the vapor phase. This often requires high temperature and low pressure due to the high sublimation point of tungsten oxide precursors. We demonstrate atmospheric pressure CVD of WSe₂ and WS₂ monolayers at moderate temperatures (700–850 °C) using alkali metal halides (MX where M = Na or K and X = Cl, Br or I) as the growth promoters. We attribute the facilitated growth to the formation of volatile tungsten oxyhalide species during growth, which leads to efficient delivery of the precursor to the growth substrates. The monolayer crystals were found to be free of unintentional doping with alkali metal and halogen atoms. Good field-effect transistor (FET) performances with high current on/off ratio ∼10⁷, hole and electron mobilities up to 102 and 26 cm² V⁻¹ s⁻¹ for WSe₂ and electron mobility of ∼14 cm² V⁻¹ s⁻¹ for WS₂ devices were achieved.

Introduction

Atomically thin crystals of transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional (2D) materials and triggered intense research efforts [1], [2]. Monolayer TMDs consist of a hexagonally packed layer of transition metal atoms sandwiched between two layers of chalcogen atoms. There are around 40 different types of TMDs with a wide variety of physical properties that make them attractive for fundamental studies as well as for practical applications [3], [4]. Depending on the selection of the metal and chalcogen species, TMDs can be semiconducting (e.g. MoS₂, WSe₂), metallic (e.g. NbS₂, TaSe₂), or semimetallic (e.g. WTe₂) [3]. 2D TMD crystals exhibit unique electronic properties that are absent in the bulk materials due to geometrical confinement and distinct crystal symmetry [5], [6], [7], [8]. Monolayers of group 6 TMDs have received significant attention for their direct bandgap in the visible range of wavelengths, high carrier mobility, excellent gate coupling, flexibility, and strong light-matter interaction [5], [9], [10], [11], [12], [13], [14]. These attributes make them appealing for ultrathin, lightweight, and flexible device applications [4], [15], [16].

Practical device applications of 2D TMDs rely on breakthroughs in the controlled and efficient growth of large-area films. While many groups have reported successful chemical vapor deposition (CVD) of various monolayer TMDs, the growth conditions and the quality of the film vary remarkably in the literature (Tables S1 and S2, see also Ref. [17]). One of the key challenges is the delivery of metal and chalcogen precursors to the growth substrates at a controlled flux. Typically, transition metal oxide and chalcogen in solid form are used as the precursors for the reaction. Since they have significantly different vapor pressures, the generation of steady vapor flux requires careful optimization of the temperature, pressure and carrier gas flow rate. Controlled growth of tungsten-based TMDs is particularly challenging due to the high sublimation temperature of tungsten oxide (e.g. WO₃ and WO_2.9). Recent reports on vapor transport and CVD growth of monolayer WSe₂ and WS₂ have shown that growth requires either high temperature (>925 °C), or low pressure (∼1 Torr) or both [13], [14], [18], [19], [20], [21], [22], [23], [24]. These conditions need to be met in order to achieve a sufficient flux of tungsten precursor in the vapor phase over a distance between the precursor to the growth substrates [14], [19], [24], [25]. The high temperature and low pressure requirements are partly relaxed when oxide precursor is placed in direct contact with growth substrates [26], [27]. However, this approach prevents controllable deposition of uniform large-area films. While the use of more volatile or gaseous tungsten-containing precursors such as WCl₆, W(CO)₆ and WOCl₄ has been demonstrated, the quality of the material or growth rate had to be compromised [28], [29], [30], [31].

Halogen molecules such as I₂ and Br₂ are commonly used as the transport agents in chemical vapor transport (CVT) growth of bulk WSe₂ and WS₂ crystals thanks to their reactivity with metal oxide to form oxyhalide species such as WO₂X₂ and WOX₄ (X = Cl, Br or I), which have significantly lower melting point compared to WO₃ and WO_2.9 [32]. These transport agents are, however, not suitable for CVD growth of TMD monolayers due to their high reactivity and high vapor pressure.

In this paper, we report atmospheric pressure growth of high quality WSe₂ and WS₂ monolayers using a variety of alkali metal halides as the growth promoters. We show that the presence of alkali metal halides allow the growth at temperatures as low as 700 °C under atmospheric pressure. The facilitated growth suggests chemical reaction between the tungsten oxide precursor and the alkali metal halides and formation of volatile tungsten-based halide species. We found that the flakes grown by this method are highly crystalline, chemically pure, and exhibit good field-effect transistors (FETs) performances with hole and electron mobilities of 102 and 26 cm² V⁻¹ s⁻¹ for WSe₂ and electron mobility of ∼14 cm² V⁻¹ s⁻¹ for WS₂ devices.