Thermally Assisted Conversion

All of the synthesis methods so far discussed suffer from a drawback - either in scalability, quality or control of growth. This is the focus of much current research. In order to address some of these issues, a variant of CVD known as TAC was explored. The development of TAC synthesis methodologies, and characterising the resultant materials constitutes the major body of work in this thesis.

The TAC process involves depositing metal97–99on a suitable substrate, fol- lowed by exposure of the metal layer to chalcogen vapour at an elevated tempera- ture. The chalcogen diffuses into the metal layer forming an alloy, and thus the transition metal is converted into a TMD. This method produces polycrystalline films with good morphological control, due to well-regulated metal deposition techniques. Initial investigations into the TAC synthesis route focused on MoS2.

The first example of this method was by Zhanet al.,100who sulfurised Mo films in a hot-wall furnace, forming polycrystalline MoS2. TMDs can be reliably synthe-

sised at around 700 °C,101,102however with plasma enhancement the temperature can be lower than 500 °C.53Any MX3formed during the growth decomposes to

the thermodynamically more stable MX2, at temperatures well below those used

for typical growths. For example, MoS3decomposes to MoS2+ S at 310 °C.103

With TAC, selective large-area growth is easily realised; because the metal doesn’t enter the vapour phase, standard lithographic techniques can be used to

define metal patterns which are then converted into TMD patterns.101This enables simple fabrication schemes for a wide range of devices, as has been demonstrated for gas sensors, TFTs and Hall bars.102,104Another advantage is that the thickness can be controlled in the metallisation step, and therefore produce TMDs with good thickness control, down toward the monolayer.101

A number of groups have demonstrated that the alignment of the TMD layers formed during TAC depends on the conversion conditions used. Cross-sectional transmission electron microscopy (XTEM) studies from several groups have shown the 2D layers to be well aligned with the substrate for thin films grown at low partial pressure, and high temperatures.105–108Laskaret al.105have demon- strated well ordered, well aligned MoS2on sapphire, by TAC of 5 nm Mo, this

is shown in fig. 2.6(a). Tarasov et al. described the synthesis of large-scale, high-quality MoS2 tri-layer films by TAC (at temperatures above 1000 °C) of

1 nm of Mo metal.108 In these cases, the material produced is continuous, and TMD layers are parallel to the substrate.

This type of behaviour is not always seen for TAC films with a thicker initial metal layer, (≥10nm). Liuet al.have shown that under normal growth conditions (i.e. high temperature, low pressure) thick films grown on SiO2prefer to form with

protruding bell-type morphology. This is imaged in fig. 2.6(b) and schematically visualised in fig. 2.6(c).109,110This quality is due to the random alloying nature for the TAC process and the reduced surface energy causing the Mo to form clusters prior to sulfurisation. The clusters then convert from the outside inwards, resulting in the growth of bell-shaped TMD features.

It has also been reported that the TAC process at elevated pressure and/or lower temperature leads to the 2D layers forming perpendicular to the substrate which has allowed for novel electrical and electrochemical device structures. This

(a) (b)

Before TAC

After TAC

(c) (d)

Fig. 2.6 XTEM of TAC MoS2 (a) aligned 5 nm Mo on sapphire, (b) bell-like

thicker films on SiO2, (c) schematic of bell-like growth, (d) vertical growth.

Figures adapted from: (a) Laskaret al.,105(b) Liuet al.,110 (c) Liu et al.,111(d) Konget al.112

was first investigated by Ganut et al. for WS2.113 They determined that low

temperature (650-850 °C) and excess chalcogen availability led to vertical growth. Higher temperature (1000 °C) produced horizontal films, and the crystallinity improved with longer growth times. Recently, Konget al.112have demonstrated vertical TAC growth of MoS2and MoSe2at 550 °C, (>100 mTorr) with a large

excess of chalcogen, as can be seen in fig. 2.6(d). They attribute the perpendicular growth to a kinetically-driven process, with the chalcogen preferentially diffusing

into the film through the van der Waals gaps. Thickness dependence of the metal layers on vertical growth was also investigated by Junget al.,114 who showed that preferential vertical growth of MoS2and WS2resulted only when the initial

metal layer was thicker than 10 nm.

A recent study by Feiet al.115has allowed for direct TEM imaging of MoS2

growth mechanisms. While they do not investigate TAC growth, (their work involves the thermolysis of a single source precursor, ((NH4)2MoS4) their system

is very similar. They show that growth around 400 °C leads to layer by layer vertical growth, while increasing the temperature to 780 °C leads to the MoS2

structures adopting a horizontal configuration. At 820 °C precipitation of horizon- tal MoS2particles on the surface occurs, which become enlarged (with increasing

temperature up to 850 °C) a variety of ways including an orientated attachment mechanism and conventional Ostwald ripening. Since TAC involves gaseous sulfur source, it is likely that growth also proceeds through a conventional crystal growth mechanism, whereby high energy facets grow faster by sulfur adsorption.

TAC is a method of synthesising 2D TMDs in a potentially industrially relevant manner. Advantages include scalable, large area growth that is well suited to lithographic processes and good thickness control. Further, the material produced is dependent on synthesis conditions and growth parameters can be easily adjusted to produce different configurations tailored to specific applications. Drawbacks include polycrystalline films and the difficulty of controlling the introduction of dopants.