Synthesis of two-dimensional hexagonal boron nitride and tin-derivative dichalcogenides for nanoelectronic applications
Keywords: heterostructures
transition metal dichalcogenides
doping
hexagonal boron nitride
tin disulfide
epitaxy
two-dimensional materials
photodetector
chemical vapour deposition

The discovery of graphene (Gr) has so far drawn significant attention and expanded into the development of other two-dimensional (2D) materials with complementary electronic properties over the past few years. Molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are examples of semiconducting transition metal dichalcogenides (TMDs), which have a direct band gap in the red spectral region (1.8−2.0 eV), making them promising candidates for the future flexible optoelectronic applications. However, exploiting other 2D materials with different bandgap values is still necessary for devices with comprehensive functionalities. This DPhil project aims to develop 2D insulating hexagonal boron nitride (hBN) and semiconducting tin-derivative dichalcogenides (e.g. tin sulfide SnS2, Sn doped WS2) using chemical vapour deposition (CVD) growth method, which possesses the advantages for mass production compared to the commonly-used exfoliation approach.
In the first part of growth of hBN, a new oxide-assisted CVD process was developed via the formation of thin oxide layer on Cu surface, which can passivate the nucleation sites of Cu surfaces before hBN growth. In the following hBN growth stage the oxide layer gradually decomposes in the reducing hydrogen atmosphere and only the limited fresh Cu surface is able to absorb the active species forming stable nuclei. This leads to increased domain sizes from 1 to 20 μm by reducing the nucleation density from 106 to 103 mm–2.
In the second part of growth of SnS2, a new precursor SnS was used for the growth and investigation for the photodetecting performance via combining them with Gr electrodes. The Gr-SnS2-Gr transistors exhibit prominent photodetector response with high detectivity and photoresponsivity. The response of all 2D photodetectors depends upon charge trapping and the Schottky barrier modulation at the interface of SnS2 and Gr. This indicates that SnS2 has photosensing performance that is comparable to other 2D TMDs, such as MoS2 and WS2, in the green spectral region when combined with Gr electrodes.
For the traditional growth of SnS2 on SiO2 substrates, one of the drawbacks is the high percentage of crystals with vertical growth, which is detrimental for the device applications. To improve this, the CVD-grown WS2 layers were utilised as the substrates for the growth of 2D SnS2. In addition to the formation of type II van der Waals (vdW) heterostructures, the lateral growth of SnS2 is stabilised with epitaxially aligned crystals on the basal planes whereas cluster growth appears on the edges of WS2 layers. This is attributed to the lower energy barrier of migration for adsorbed active species on the defect-free surface, resulting in facilitated surface diffusion compared to the defect-rich surface.
Lastly, a post-growth doping strategy in monolayer and bilayer WS2 crystals utilising a metal exchange mechanism was demonstrated, whereby Sn atoms become substitutional dopants in the W sites through energetically favourable replacement. This can be achieved by firstly growing WS2 single crystals and then reacting with a SnS precursor. The Sn dopants exhibit an n-type doping behaviour in the WS2 layers based on the decreased threshold voltage obtained from transistor device measurements. Annular dark field scanning transmission electron microscopy (ADF-STEM) shows that in bilayer WS2 the Sn doping occurs only in the top layer, creating vertical heterostructures with atomic layer doping precision.