When two-dimensional (2D) group-VI transition metal dichalcogenides such as tungsten disulfide (WS2) are grown by atomic layer deposition (ALD) for atomic growth control at low deposition temperatures (< 450 °C), they often suffer from a nanocrystalline grain structure limiting the carrier mobility. The crystallinity and monolayer thickness control during ALD of 2D materials is determined by the nucleation mechanism, which is currently not well understood. Here, we propose a qualitative model for the WS2 nucleation behavior on dielectric surfaces during plasma-enhanced (PE-) ALD using WF6, H2 plasma and H2S based on analyses of the morphology of the WS2 crystals. The WS2 crystal grain size increases from ~20 nm to 200 nm by lowering the nucleation density. This is achieved by lowering the precursor adsorption rate on the starting surface using an inherently less reactive starting surface, by decreasing the H2 plasma reactivity, and by enhancing the mobility of the adsorbed species at higher 2 deposition temperature. Since SiO2 is less reactive than Al2O3, and diffusion and crystal ripening is enhanced at higher deposition temperature, WS2 nucleates in an anisotropic island-like growth mode with preferential lateral growth from the WS2 crystal edges. This work emphasizes that increasing the crystal grain size while controlling the basal plane orientation is possible during ALD at low deposition temperatures, based on insight in the nucleation behavior, which is key to advance the field of ALD of 2D materials. Moreover, this work demonstrates the conformal deposition on 3D structures, with WS2 retaining the basal plane orientation along topographic structures.

AlO on Si is known to form an ultrathin interfacial SiO during deposition and subsequent annealing, which creates a negative fixed charge ( Q) that enables field-effect passivation and low surface recombination velocities in Si solar cells. Various concepts were suggested to explain the origin of this negative Q. In this study, we investigate Al-O monolayers (MLs) from atomic layer deposition (ALD) sandwiched between deliberately grown/deposited SiO films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10 cm/s at 1000 °C), are deposited at a constant rate of ∼5 × 10 Al atoms/(cm cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO imprints its tetrahedral near-order and bond length into the Al-O MLs. By variation in the tunnel-SiO thickness and the number of Al-O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Q but that a SiO/AlO interface within a tunneling distance from the substrate must be present. The Al-induced acceptor states at these interfaces have energy levels slightly below the Si valence band edge and require charging by electrons from either the Si substrate or from Si/SiO dangling bonds to create a negative Q. Hence, tunneling imposes limitations for the SiO and AlO layer thicknesses. In addition, Coulomb repulsion between the charged acceptor states results in an optimum number of Al-O MLs, i.e., separation of both interfaces. We achieve maximum negative Q of ∼5 × 10 cm (comparable to thick ALD-AlO on Si) with ∼1.7 nm tunnel-SiO and just seven ALD-AlO cycles (∼8 Å) after optimized annealing at 850 °C for 30 s. The findings are discussed in the context of a passivating, hole-selective tunnel contact for high-efficiency Si solar cells.

Multi-frequency electron spin resonance (ESR) study of p-type synthetic 2H MoS2 reveals a previously unreported signal of axial-symmetry [g// = 2.032(2); g⊥ = 2.270(2)] characteristic for a hole-type center in MoS2. It is identified as originating from N acceptor dopants, the N atoms substituting for S sites, with a density of ∼2.3 x 1017 cm-3, thus predominantly accounting for the p-type sample doping. For the applied magnetic field along the c-axis, the signal is mainly comprised of a 14N hyperfine 1:1:1 triplet of splitting A// = 14.7 ± 0.2 G with, on top, a center line accounting for ∼26% of the total signal intensity. The additional observation of a weak half-field signal (g = 3.92) correlating with the main full-field Zeeman response points to the presence of spin S ≥ 1 N agglomerates. The overall signal properties indicate that only ∼74% of the N acceptors occur as isolated decoupled dopants. Monitoring of the ESR signal intensity over a broad temperature range unveils the N dopant as a shallow acceptor of activation energy Ea = 45 ± 7 meV, thus well fit for stable substitutional p-type doping in MoS2-based novel nanoelectronic devices.

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Highlights  Investigation of process conditions for the degradation of sulfamethoxazole  Identification of degradation products for elucidation of the degradation mechanism  Elucidation of pH influence on formation of degradation products