ALD2018 Session NS+ALE-TuA: Nanostructures II + ALE

To meet the milestones outlined in the semiconductor technology roadmap, precisely controlled layer-by-layer etching processes for metal oxides are required to enable the fabrication of 3D-semiconductor microelectronic devices. In addition, 2D-layered transition metal dichalcogenides (TMDs) with tunable electronic properties exhibit attractive material properties and are under intense investigation as alternatives to silicon. Therefore, integration of 2D-materials with compatible dielectric metal oxide growth as well as etching can pave the way to successful device fabrication. Moreover, both selective material growth and selective etching are required for the cost effective manufacturing of advanced microelectronics.

Here we have developed etching processes using alternating exposures to molybdenum hexafluoride (MoF₆) and H₂O vapor for ALD grown metal oxides and 2D-TMDs. The MoF₆-H₂O precursors offer several advantages including low cost, low processing temperature, and rapid and effective etching spanning the range from continuous etching to atomic layer etching (ALEt).

We have used in-situ quartz crystal microbalance (QCM) and Fourier-transform infrared spectroscopy (FTIR) measurements to monitor the deposition and etching of the metal oxides and 2D-TMDs layers. Next, the etched metal oxides and 2D-TMDs thin films were analyzed by spectroscopic ellipsometry to determine the thickness and refractive index, and the composition was determined by X-ray photoelectron spectroscopy (XPS). These ex-situ measurements confirmed the etching behavior findings from our in-situ studies. Here we will discuss the details of the MoF₆-H₂O based etching chemistry.

Atomic layer deposition (ALD) is a thin film technology that in the past two decades rapidly developed from a niche technology to an established method. It proved to be a key technology for the synthesis of ultrathin film, the surface modification and the fabrication of complex nanostructured materials as well as the membrane tuning. [1]

In this work, we will give an overview about our activities on ALD, from the design of nanomaterials to membrane applications. After a short introduction to ALD, the versatility of the technique for the fabrication of novel nanolaminates thin films [2] and functional nanomaterials [3, 4] will be showed. Selected examples, focused on its use for the engineering of nanostructured functional materials and membranes targeting applications in energy (osmotic energy, biofuel cells and gas separation), environmental ((bio)-sensor and water purification) and health (DNA and proteins sensing) fields [5-9] will be discussed.

[1] Advanced Materials, 2012, 24, 1017

[2] J. Phys. Chem. C, 2016, 120, 5124–5132

[3] Nano Energy 1 (2012) 696

[4] Nanoscale 7 (2015) 5794

[5] Journal of Materials Chemistry A, 2016, 4, 6487-6494

[6] Journal of Materials Chemistry A, 2016,4, 17686-17693

[7] Biosensors and Bioelectronics, 2017, 92, 763–769

[8] Journal of Materials Chemistry A, 2014, 2 (48), 20650 – 20658

[9] ACS Appl. Mater. Interfaces, 2017, 9, 16669–16678

Recently, two-dimensional electron gas (2DEG) has attracted great attention due to the observation of 2DEG at various heteroepitaxial perovskite oxide interfaces. Typically, epitaxial LaAlO₃/single-crystal SrTiO₃ (LAO/STO) heterostructure shows high density of electrons (~10¹³-10¹⁴ cm^-2) confined at the oxide interface, where the density is about ~100 times higher than those of a typical semiconductor interface (~10¹¹-10¹² cm^-2). Despite its high electrical performance, process schemes are not suitable for practical device applications because of their high cost in the production of the oxide single crystal and epitaxial layer. Although the formation of 2DEG using γ-Al₂O₃ and amorphous LAO with Al₂O₃ over-layers on STO substrate was demonstrated recently, single crystalline STO substrates are still required. Therefore, 2DEG at an oxide interface has not been realized via a mass-production compatible thin film deposition process, in contrast to 2DEG at the semiconductor interfaces.

In this work, we demonstrate a realization of 2DEG at the interface in an extremely thin film (< ~10 nm) heterostructure comprised of two binary oxide stacks of Al₂O₃/TiO₂ via atomic layer deposition (ALD) on a SiO₂/Si substrate without using an oxide single crystal or epitaxial substrate. The Al₂O₃/TiO₂ heterostructure was composed of a few nm-thick amorphous Al₂O₃ over-layer and poly-crystalline anatase-TiO₂ bottom layer. We achieved high electron density (~10¹³-10¹⁴ cm^-2) and mobility (~4 cm²/V∙s) in the 2DEG at the interface of ultrathin Al₂O₃/TIO₂ heterostructures, comparable to those obtained from epitaxial oxide heterostructures. More importantly, we demonstrate a new type of field-effect transistors (FETs) using Al₂O₃/TIO₂ heterostructures with a high on-current (I_on, > 12 A/m), high on/off current ratio (I_on/I_off > ~10⁸), low off current (I_off, ~10^-8 A/m), and low sub-threshold swing (SS, ~100 mV/dec.), which outperforms the oxide heterostructure-based FETs reported so far. Ultrathin (< ~7 nm) TiO₂ bottom layer in the proposed Al₂O₃/TiO₂ heterostructure prone to be fully depleted, allows an extremely low I_off, high I_on/I_off ratio and low SS with maintaining high I_on via 2DEG channel with a high carrier density at the interface. The proposed new-type devices would provide a great opportunity for practical applications and mass-production of 2DEG devices, allowing a multi-level three-dimensional (3D) integration scheme. The detailed experimental results including new materials systems will be presented.

Two-dimensional (2D) transition metal dichalcogenides such as molybdenum and tungsten disulfide (MoS₂, WS₂) emerge as semi-conducting three-atom-thick layers that are widely applicable, for example as the complement of Si in ultra-scaled nanoelectronic devices at the back-end-of-line (BEOL) [1]. To exploit the potential of 2D layers in BEOL structures, they need to be grown using manufacturable deposition techniques in a highly crystalline structure with control over the orientation of the basal plane at low deposition temperatures (< 450°C).

When grown by atomic layer deposition (ALD) for atomistic growth control and compatibility with temperature sensitive structures, the crystallinity and structure of 2D materials is determined by the nucleation mechanisms, which are currently not yet understood. In our earlier work, polycrystalline WS₂ with well-controlled composition (S/W ratio ~2) and 2D structure was grown by PEALD from WF₆, H₂ plasma and H₂S on Al₂O₃ at 300°C [2]. The WS₂ layers suffered from a nanocrystalline grain structure (< 20 nm).

In this work, we show that the WS₂ crystal grain size can be increased from ~20 nm to beyond 200 nm by lowering the nucleation density. The latter is achieved by using a starting surface with a lower reactivity towards the PEALD precursors (i.e., thermally grown SiO₂), and by enhancing the mobility of the adsorbed species at higher deposition temperature (< 450°C) and reactor pressure (Figure 1).

By analyzing the morphology of the WS₂ crystals, we derive a qualitative model for the nucleation behavior of WS₂ during the PEALD process. The SiO₂ starting surface is less reactive to the PEALD precursors compared to Al₂O₃ starting surfaces, which results in growth inhibition on SiO₂. The PEALD precursors preferentially adsorb on the existing WS₂ nuclei. In combination with enhanced surface diffusion of the adsorbed species across both the SiO₂ starting surface as well as the growing WS₂ nuclei, lateral growth from WS₂ crystal edges is promoted which maximizes the WS₂ crystal grain size and develops a strong (0002) texture.

This work demonstrates how insight in the nucleation behavior of 2D materials can be used to increase the crystal grain size and control the basal plane orientation during ALD. A better understanding of these nucleation mechanisms is crucial to advance the field of ALD of 2D materials.

[1] 47^th IEEE European Solid-State Device Research Conference (ESSDERC) 2017, pp. 212–215. T. Schram, et al.

[2] Chem. Mater.2017, 29 (7), 2927–2938. B. Groven, et al.

Transition metal dichalcogenides like WS₂ are promising candidates for sustainable production of H₂ through electrochemical hydrogen evolution reaction (HER), when engineered into nanostructures with exposed reactive edge sites. Controlling the various parameters during the plasma exposure step of plasma-enhanced ALD (PEALD) allows tailoring of material properties which can influence the HER performance. In this work, we demonstrate a novel approach to maximize the density of reactive edge sites in nanostructured WS₂ by tuning the composition of co-reactant gas mixture during the plasma exposure step of PEALD.

H₂S+Ar and H₂ diluted H₂S+Ar were used as co-reactant gas mixtures for tuning the plasma composition during PEALD of WS₂. The addition of the strong reducing agent H₂ to the H₂S+Ar plasma gas mixture can significantly influence the WS₂ growth behaviour and resulting material properties. The H₂S+Ar plasma yielded a GPC of 0.8Å while the H₂S+Ar+H₂ plasma increased GPC to 1.4Å for the same metalorganic precursor bis(tertbutylimido)-bis(dimethylamido)-tungsten, at a low temperature of 300°C. Transmission electron microscopy (TEM) studies revealed the growth of WS₂ ‘nanoflakes’ when using H₂S+Ar plasma, while H₂S+Ar+H₂ plasma resulted in the growth of WS₂ ‘fins’. The nanoflakes comprised of closely packed basal planes with their edges predominantly terminating on the top surface, while the fins appeared to have higher surface area and were observed to taper out on the top surface. Rutherford backscattering spectroscopy studies indicated an excess S content for nanoflakes (S:W=2.2), while a S deficiency was observed for fins (S:W=1.9). In line with TEM studies, X-ray diffraction measurements showed differences in preferential orientation of crystals for nanoflakes and fins.

The HER performance of WS₂ nanoflakes was significantly better than of WS₂ fins. To reach a current density of 10mA/cm², a relatively lower overpotential of ~390mV was sufficient for the nanoflakes while a significantly higher overpotential of ~460mV was required for the fins. In line with HER results, copper underpotential depositions on the respective nanostructures revealed a three-fold increase in the amount of reactive edge sites for nanoflakes when compared with fins. In order t o further enhance the HER performance, catalytically superior WS₂ nanoflakes were grown on top of high surface-area WS₂ fins. This WS₂ stack yielded the best HER performance in our work (overpotential of ~365mV). To summarize, we demonstrate how PEALD can be used as a new approach to nanoengineer and enhance the HER performance of WS₂ by maximizing the density of reactive edge sites at low temperature.