Chemical vapor deposition of monolayer-thin WS2 crystals from the WF6 and H2S precursors at low deposition temperature.

Monolayer-thin WS2 with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF6 and H2S at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous Al2O3 starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS2 crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the Al2O3 starting surface forms predominantly monolayer-thin WS2 crystals and AlF3 crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS2 layer proceeds with the next WS2 layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the Al2O3 surface as soon as the WS2 layer completely covers the Al2O3 surface and that the WS2 crystal basal planes and AlF3 crystals have a low reactivity for WF6 adsorption at 450 °C. Nonetheless, a second layer of WS2 starts to form before the first WS2 layer completely covers the starting surface, albeit the surface coverage of the second layer is low (<20%, after 25 min of CVD reaction). During the steady-state growth regime, predominantly the WS2 crystals in the second monolayer continue to grow in lateral dimensions up to ∼40 nm. These crystals reach larger lateral dimensions compared to the crystals in the bottom, first layer due to low reactivity for WF6 adsorption on the WS2 basal plane compared to Al2O3. Presumably, they grow laterally by precursor species that adsorb on and diffuse across the WS2 surface, before being incorporated at the more reactive edges of the WS2 crystals in the second layer. Such a process proceeds slowly with only up to 40% surface coverage of the second WS2 layer after 150 min of CVD reaction. The CVD reaction is mediated by the starting surface: WF6 precursor preferentially adsorbs on Al2O3, whereas adsorption is not observed on SiO2. Nevertheless, WS2 grows on SiO2 in close proximity to Al2O3 in 90 nm pitch Al2O3/SiO2 line patterns. Hence, functionalization of the starting surface (e.g., SiO2 with Al2O3) can provide opportunities to grow monolayer-thin WS2 crystals at predetermined locations by selective, lateral growth with tunable crystal size, even at low deposition temperatures.

Structural characterization of SnS crystals formed by chemical vapour deposition.

The crystal and defect structure of SnS crystals grown using chemical vapour deposition for application in electronic devices are investigated. The structural analysis shows the presence of two distinct crystal morphologies, that is thin flakes with lateral sizes up to 50 µm and nanometer scale thickness, and much thicker but smaller crystallites. Both show similar Raman response associated with SnS. The structural analysis with transmission electron microscopy shows that the flakes are single crystals of α-SnS with [010] normal to the substrate. Parallel with the surface of the flakes, lamellae with varying thickness of a new SnS phase are observed. High-resolution transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), first-principles simulations (DFT) and nanobeam diffraction (NBD) techniques are employed to characterise this phase in detail. DFT results suggest that the phase is a strain stabilised ß' one grown epitaxially on the α-SnS crystals. TEM analysis shows that the crystallites are also α-SnS with generally the [010] direction orthogonal to the substrate. Contrary to the flakes the crystallites consist of two to four grains which are tilted up to 15° relative to the substrate. The various grain boundary structures and twin relations are discussed. Under high-dose electron irradiation, the SnS structure is reduced and ß-Sn formed. It is shown that this damage only occurs for SnS in direct contact with SiO2 .

Observation and understanding of anisotropic strain relaxation in selectively grown SiGe fin structures.

The performance of heterogeneous 3D transistor structures critically depends on the composition and strain state of the buffer, channel and source/drain regions. In this paper we used an in-line high resolution x-ray diffraction (HRXRD) tool to study in detail the composition and strain in selectively grown SiGe/Ge fin structures with widths down to 20 nm. For this purpose we arranged fins of identical dimensions into larger arrays which were then analyzed using an x-ray beam several tens of micrometers in size. Asymmetric reciprocal space maps measured both parallel and perpendicular to the fins allowed us to extract the lattice parameters in all three spatial directions. Our results demonstrate an anisotropic in-plane strain state of the selectively grown SiGe buffer in case of narrower fins with significantly reduced relaxation in the direction along the fin. This observation was verified using nano-beam electron diffraction, and is explained based on the reduced probability for dislocation half-loops to evolve in trenches narrower than a few times the critical radius. Moreover, we introduce and discuss in detail a methodology for the determination of the composition in case of an anisotropic in-plane strain state which differs from the procedure commonly used for blanket layers. Our findings verify the importance of in-line HRXRD measurements for process development and monitoring as well as the fundamental study of relaxation and defect formation in confined volumes.

Single-electron capacitance spectroscopy of individual dopants in silicon.

Motivated by recent transport experiments and proposed atomic-scale semiconductor devices, we present measurements that extend the reach of scanned-probe methods to discern the properties of individual dopants tens of nanometers below the surface of a silicon sample. Using a capacitance-based approach, we have both spatially resolved individual subsurface boron acceptors and detected spectroscopically single holes entering and leaving these minute systems of atoms. A resonance identified as the B+ state is shown to shift in energy from acceptor to acceptor. We examine this behavior with respect to nearest-neighbor distances. By directly measuring the quantum levels and testing the effect of dopant-dopant interactions, this method represents a valuable tool for the development of future atomic-scale semiconductor devices.

Low temperature deposition of 2D WS2 layers from WF6 and H2S precursors: impact of reducing agents.

We demonstrate the impact of reducing agents for Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) of WS2 from WF6 and H2S precursors. Nanocrystalline WS2 layers with a two-dimensional structure can be obtained at low deposition temperatures (300-450 °C) without using a template or anneal.