Towards large area and continuous MoS2 atomic layers via vapor-phase growth: thermal vapor sulfurization

We report on the effects of substrate, starting material, and temperature on the growth of MoS(2) atomic layers by thermal vapor sulfurization in a tube-furnace system. With Mo as the starting material, atomic layers of MoS(2) flakes are obtained on sapphire substrates while a bell-shaped MoS(2) layer, sandwiched by amorphous SiO(2), is obtained on native-SiO(2)/Si substrates under the same sulfurization conditions. An anomalous thickness-dependent Raman shift (A(1g)) of the MoS(2) atomic layers is observed in Mo-sulfurizations on sapphire substrates, which can be attributed to the competition between the effects of thickness and the surface/interface. Both effects vary with the sulfurizing temperatures for a certain initial Mo thickness. The anomalous frequency trend of A(1g) is missing when using MoO(3) instead of Mo as the starting material. In this case, the lateral growth of MoS(2) on sapphire is also largely improved. Furthermore, the area density of the resultant MoS(2) atomic layers is significantly increased by increasing the deposition temperature of the starting MoO(3) to 700 °C; the adjacent ultrathin MoS(2) grains coalesce in one or other direction, forming connected chains in wafer scale. The thickness of the so-obtained MoS(2) is generally controlled by the thickness of the starting material; however, the structural and morphological properties of MoS(2) grains, towards large area and continuous atomic layers, are strongly dependent on the temperature of the initial material deposition, and on the temperature of sulfurization, because of the competition between surface mobility and atom evaporation.

Synthesis and phase evolutions in layered structure of Ga2S3 semiconductor thin films on epiready GaAs (111) substrates

We report on synthesis and properties of p-type Ga2S3 semiconductor thin films that were prepared by sulfurizing epiready n-type GaAs (111) surface at elevated temperatures. Comparisons of structural and optical properties among the thin films, peeling-off resulted microtubes, and the remains after peeling-off give a clear clue to the crystal growth and phase evolutions of Ga2S3. Three layers of Ga2S3 are clearly identified in the thin films. They are layer i, cubic Ga2S3 epitaxially grown on the GaAs (111) substrate; layer ii, polycrystalline cubic Ga2S3 on top of layer-i; and layer iii, monoclinic and/or hexagonal Ga2S3 on top of layer ii. The onset of peeling-off occurred in layer i and/or at the interface between layer i and ii. Both the phase evolutions and the location of peeling-off are associated with a Ga out diffusion growth mechanism. Absorption spectroscopy revealed a direct bandgap of 3.0 eV, whereas photoluminescence spectra showed defects (excited Ga vacancies) related red (1.62 eV) and green (2.24 eV) emissions of the Ga2S3 films; both are qualitatively consistent with those reported values obtained at lower sample temperatures from Ga2S3 single crystals. These results, together with a large on/off current ratio (i.e., ∼14 at a bias of 4.0 V) of the resultant hetero p-Ga2S3/n-GaAs junction under a blue laser (405 nm, 3.0 mW) illumination, shed light on consequent integrations of Ga2S3- and GaAs-based optoelectronic devices, e.g., high-power laser radiation sensors.

Vapor-phase growth and characterization of Mo(1-x)W(x)S2 (0 ≤ x ≤ 1) atomic layers on 2-inch sapphire substrates

Atomically thin Mo(1-x)W(x)S2 (0 ≤ x ≤ 1) ternary compounds have been grown on 2-inch c-plane sapphire substrates with high uniformity by sulfurizing thin Mo(1-x)W(x) layers that were deposited at room temperature using a co-sputtering technique. Atomic force microscopy (AFM), Raman scattering, and optical absorbance spectroscopy (OAS) studies reveal that the Mo(1-x)W(x)S2 films consist of crystallites of two-to-four monolayers in thickness. X-ray photoelectron spectroscopy (XPS) shows that the core levels of Mo3d and W4f shift to lower binding energies while that of S2p shifts to higher ones with the increase in W compositions, which can be related to the larger electron affinity of W (0.8163 eV) than that of Mo (0.7473 eV). OAS has also shown that the direct bandgap of Mo(1-x)W(x)S2 is tuned from 1.85 to 1.99 eV by increasing x from 0 to 1. Both E(1/2)(g) and A(1g) phonon modes of the Mo(1-x)W(x)S2 films exhibit a two-mode behavior. The bandgap tuning and the two-mode phonon behaviors are typically the same as those recently observed in monolayer Mo(1-x)W(x)S2 obtained by mechanical exfoliation, thus shedding light on the bottom-up growth of large-scale two-dimensional Mo(1-x)W(x)S2 ternary alloys.