Van der Waals epitaxy of the layered semiconductors SnSe2 and SnS2: morphology and growth modes (original) (raw)
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Layer‐Dependent Optoelectronic Properties of 2D van der Waals SnS Grown by Pulsed Laser Deposition
Advanced Electronic Materials, 2019
research for electronic devices. The critical size of field effect transistors (FETs) is aggressively scaled down to the nano meter regime to acquire higher chip density, higher speed, and lower power consumption. [3,4] Among the low-dimensional materials, 2D layered materials [5] such as graphene, [6] black phosphorus (BP), [7-9] and transition metal dichalcogenides (TMDCs) [10] have been widely investigated for their potential development in future electronics and optoelectronics technology due to their unique physical, chemical, optical, and electronic properties. As carrier transport is confined in plane, 2D layered materials are considered as promising candidates for high-mobility "tunnel-electronic" devices. In addition, their flexibility and tunable band gaps promote the potential deployment in future electronics, photonic and optoelectronics technologies. [11,12] While n-type FETs represented by InGaZnO [13]-based thinfilm transistors (TFTs) gradually mature and step forward toward practical use, p-type FETs clearly need further development. Recently, researchers have paid particular attention to the intrinsic p-type tin monochalcogenides (SnX) (X = O, S, Se, and Te) [14,15] with the same crystal structure as black phosphorus. Tin monosulfide (SnS) has already attracted attention as a solar cell material owing to the good absorption across the electromagnetic spectrum with the proper optical properties exhibiting narrow band gaps (1.3-1.5 eV [direct], 1.0-1.1 eV [indirect] and a good absorption coefficient that is larger than 10 4 cm −1 in the visible region. [16] SnS has both low-temperature stable α-SnS (Pnma-D 2h 16 Layered metal monochalcogenides have attracted significant interest in the 2D family since they show different unique properties from their bulk counterparts. The comprehensive synthesis, characterization, and optoelectrical applications of 2D-layered tin monosulfide (SnS) grown by pulsed laser deposition are reported. Few-layer SnS-based field-effect transistors (FETs) and photodetectors are fabricated on Si/SiO 2 substrates. The premium 2D SnS FETs yield an on/off ratio of 3.41 × 10 6 , a subthreshold swing of 180 mV dec −1 , and a field effect mobility (µ FE) of 1.48 cm 2 V −1 s −1 in a 14-monolayer SnS device. The layered SnS photodetectors show a broad photoresponse from ultraviolet to near-infrared (365-820 nm). In addition, the SnS phototransistors present an improved detectivity of 9.78 × 10 10 cm 2 Hz 1/2 W −1 and rapid response constants of 60 ms for grow-time constant τ g and 10 ms for decay-time constant τ d under extremely weak 365 nm illumination. This study sheds light on layer-dependent optoelectronic properties of 2D SnS that promise to be important in next-generation 2D optoelectronic devices.
2D Materials, 2018
Tin disulfide (SnS2) is a n-type semiconductor with a hexagonally layered crystal structure and has promising applications in nanoelectronics, optoelectronics and sensors. Such applications require the deposition of SnS2 with controlled crystallinity and thickness control at monolayer level on large area substrate. Here, we investigate the nucleation and growth mechanism of two-dimensional (2D) SnS2 by chemical vapor deposition (CVD) using SnCl4 and H2S as precursors. We find that the growth mechanism of 2D SnS2 is different from the classical layerby-layer growth mode, by which monolayer-thin 2D transition metal dichalcogenides can be formed. In the initial nucleation stage, isolated 2D SnS2 domains of several monolayers high are formed. Next, 2D SnS2 crystals grow laterally while keeping a nearly constant height until layer closure is achieved, due to the higher reactivity of SnS2 crystal edges than basal planes. We infer that the thickness of the 2D SnS2 crystals is determined by the height of initial SnS2 islands. After layer closure, SnS2 grows on grain boundaries and results in 3D growth mode, accompanied by spiral growth. Our findings suggest an approach to prepare 2D SnS2 with a controlled thickness of several monolayers and add more knowledge on the nucleation and growth mechanism of 2D materials.
Layer-dependent properties ofSnS2andSnSe2two-dimensional materials
Physical Review B, 2016
The layer dependent structural, electronic and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, non-local 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex ∼ 0.9 eV, is substantially reduced to Ex = 0.14 eV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to identify number of layers in a 2D sample.
Growth of MoSe2 thin films with Van der Waals epitaxy
Journal of Crystal Growth, 1991
The concept of Van der Waals epitaxy that has been recently introduced removes severe lattice matching requirement by using materials which only have strong bonding in two dimensions. We demonstrate that an epilayer of MoSe2 deposited on various substrates can produce films of high crystalline quality despite of large mismatch. RHEED oscillation, observed in-situ for growing MoSe2 epilayers, shows a layer-by-layer growth with evidence for bilayer type growth, from which the 2Hb polytype is determined. STM provides real space images of the morphology of the epilayer, and shows novel structures resulting from the large lattice mismatch where the epilayer atoms are commensurated.
ACS Nano, 2014
Two-dimensional (2D) layered tungsten diselenides (WSe 2 ) material has recently drawn a lot of attention due to its unique optoelectronic properties and ambipolar transport behavior. However, direct chemical vapor deposition (CVD) synthesis of 2D WSe 2 is not as straightforward as other 2D materials due to the low reactivity between reactants in WSe 2 synthesis. In addition, the growth mechanism of WSe 2 in such CVD process remains unclear. Here we report the observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe 2 flakes and pyramid-like structures using a sulfur-assisted CVD method. Few-layer and pyramid-like WSe 2 flakes instead of monolayer were synthesized by introducing a small amount of sulfur as a reducer to help the selenization of WO 3 , which is the precursor of tungsten. Clear observations of steps, helical fringes, and herring-bone contours under atomic force microscope characterization reveal the existence of screw dislocations in the as-grown WSe 2 . The 2 generation and propagation mechanisms of screw dislocations during the growth of WSe 2 were discussed. Back-gated field-effect transistors were made on these 2D WSe 2 materials, which show on/off current ratios of 10 6 and mobility up to 44 cm 2 /V•s.
Epitaxial intergrowth of the layered crystals SnTaS2 and SnS2
1980
Layered crystals were obtained, with an overall composition of about Sn3Ta2S6, which gave unusual electron diffraction patterns. The patterns are explained in terms of an intergrowth of thln, alternating layers of SnTaS 2 and epitaxially deformed SnS 2. The SnS 2 has a monocIinic structure wlth a = 15.78 A, b = 5.67 A and B = 95 °. Similar crystals based on Nb rather than Ta have also been obtained.