In the past year, our collaboration partners at Eindhoven University of Technology have churned out many papers related to atomic layer deposition (ALD) and 2D transition metal dichalcogenides (TMDs). For many, the question is 'why there is such an interest in this research field?' and 'what is the potential?'. In this blog, we will tackle some basic and a few advanced questions to give some context and highlight these recent works.
2D materials are at the very limit of thin-film dimensions with thicknesses down to a single atom. These materials exhibit superlative electronic and optoelectronic properties which researchers today are trying to harness for next-generation devices for electronics, optoelectronics and energy applications.
While graphene kick-started exploration and application of these ultra-thin materials, it has created a vast field of exploration and application of several other 2D materials like nitrides (hBN), transition metal dichalcogenides (MoS2, WSe2 etc.) and even 2D oxides. Especially the transition metal dichalcogenides (TMDs), specifically the sulfides such as MoS2 and WS2, have gathered a lot of attention due to their properties and possible synthesis routes.
While these materials can be found in nature and can be exfoliated from the bulk crystals, chemical vapor-based techniques are employed to allow easy scale-up for future devices. For instance, high temperature (up to 1200°C) chemical vapor deposition (CVD) such as possible in our PlasmaPro 100 Nano (formerly Nanofab) can be utilized to grow high-quality 2D materials. Also, lower temperature techniques (up to 600 °C) such as ALD can be used in case of the TMDs. Here, our FlexAL2D configuration of the FlexAL ALD system which was first installed at Eindhoven University of Technology can be used.
Chemistry and process window for plasma ALD of 2D MoS2. The surface control of ALD and the reactivity of the plasma allow growth at relatively low temperatures.
For CVD processes, typically temperatures of over 800 °C are needed. That is often fatal for applications in semiconductors because the high temperature increases the diffusion of the atoms, which makes it harder to have them at the right spot. Researchers would like to have a process that yields materials of high quality at lower temperatures. This is especially important for stacks of 2D materials since at lower temperatures less diffusion of atoms between the layers will occur.
Due to its high reactivity, plasma is known to often allow film deposition at a lower temperature, and this is exemplified by the deposition of MoS2 at CMOS compatible temperatures where crystalline material was obtained already at 300 °C by plasma ALD. Furthermore, the self-limiting nature of ALD gives the promise of precise digital thickness control and uniform growth over a large area such as 200 mm wafers. When sulfurization of seed materials is used (such as MoO3) then the ability to deposit seed oxides at really low temperatures is an advantage. The Eindhoven group has shown patterned MoS2 by low-temperature deposition of the seed MoO3 by plasma ALD on patterned resist and a subsequent lift-off step to realize the pattern.
Conformal MoS2 on the corner of a trench. Plasma ALD of MoO3 was performed at 150 °C with subsequent sulfurization at 850 °C in an oven to obtain the MoS2 phase as shown by the planes.
Credit: Sharma et al., PhD Thesis (2018).
Current research focusses mostly on the sulfide TMDs where MoS2, WS2, TiS2 and TiS3 have been demonstrated on the FlexAL2D in Eindhoven. Graphene would be an interesting one but developing an ALD process for graphene is extremely challenging and high-temperature CVD routes are probably better there. Note that ALD is used extensively for a wide range of non-2D materials and, therefore, can be used for instance to grow dielectric, conductive and protective layers on top of 2D materials as well.
Obtaining a large crystal grain size is beneficial for many applications and a lot of the extraordinary properties of 2D materials often require large grains. For instance, electron mobility can be limited by scattering at the boundaries between the crystal grains. Using high-temperature CVD or high-temperature sulfurization of ALD seed material such as MoO3 are known methods do get relatively large grain size and good material quality.
Getting large grain size at lower temperatures is still a challenge, but the expectation is that the stepwise control of ALD should give options to allow some control and increase of the currently obtained relatively small grain sizes.
One example has been demonstrated where adding an additional plasma step in the ALD cycle resulted in an increase in the grain size. This method is often referred to as a three-step ABC cycle, as opposed to the normal AB cycle in ALD.
Adding an additional plasma step in the recipe can reduce the number of vertical oriented grains and can increase the grain size. Here the effect is shown for plasma ALD of WS2.
Credit: Balasubramanyam et al. 2019
The first related question is how to determine the properties of 2D materials. Many general thin-film diagnostics are of use, but Raman spectroscopy is particularly powerful for 2D materials. Raman can relatively easily show you whether you have crystalline material, and which phase it is. With further analysis, Raman can also give insight into the orientation of the 2D crystal planes. You might think that flat 2D materials are the only structure of interest, but for water-splitting applications, for instance, out-of-plane crystals can be quite beneficial. The ratio between planar and out-of-plane crystallites is dependent on the growth parameters and quite some study is going into controlling this.
Controlling the phase and composition can also be a challenge, many 2D TMDs can exist in multiple phases and with different composition (for instance with 2 or 3 sulfur atoms per metal atom). Recently it was demonstrated that by using plasma ALD, both TiS2 and TiS3 could be grown, both of which have their own target applications. The high reactivity of the plasma step and the reactive sulfur plasma species seem to allow for growth of S-rich compounds difficult to make by thermal ALD. Note that some of these phases can be superconductors so this could also be of interest for quantum devices.
Two different phases of 2D TiSx each with its own applications and properties. By changing the growth temperature and using subsequent anneals, both can be obtained by plasma ALD. Note that TiS3 is in fact a “quasi-1D” material since there are no bonds between the triangles.
Credit: Basuvalingam et al. 2019
The recent stream of publications and the general interest in the field suggest there are still many interesting things to come. There is more than enough parameter space to explore and it is still early days in this research field so please keep an eye out on the developments.
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Dr Harm Knoops
Atomic Scale Segment Specialist, Oxford Instruments Plasma Technology