Multiscale Investigation of the Structural, Electrical and Photoluminescence Properties of MoS2 Obtained by MoO3 Sulfurization

Abstract

In this paper, we report a multiscale investigation of the compositional, morphological, structural, electrical, and optical emission properties of 2H-MoS2 obtained by sulfurization at 800 °C of very thin MoO3 films (with thickness ranging from ~2.8 nm to ~4.2 nm) on a SiO2/Si substrate. XPS analyses confirmed that the sulfurization was very effective in the reduction of the oxide to MoS2, with only a small percentage of residual MoO3 present in the final film. High-resolution TEM/STEM analyses revealed the formation of few (i.e., 2–3 layers) of MoS2 nearly aligned with the SiO2 surface in the case of the thinnest (~2.8 nm) MoO3 film, whereas multilayers of MoS2 partially standing up with respect to the substrate were observed for the ~4.2 nm one. Such different configurations indicate the prevalence of different mechanisms (i.e., vapour-solid surface reaction or S diffusion within the film) as a function of the thickness. The uniform thickness distribution of the few-layer and multilayer MoS2 was confirmed by Raman mapping. Furthermore, the correlative plot of the characteristic A1g-E2g Raman modes revealed a compressive strain (ε ≈ −0.78 ± 0.18%) and the coexistence of n- and p-type doped areas in the few-layer MoS2 on SiO2, where the p-type doping is probably due to the presence of residual MoO3. Nanoscale resolution current mapping by C-AFM showed local inhomogeneities in the conductivity of the few-layer MoS2, which are well correlated to the lateral changes in the strain detected by Raman. Finally, characteristic spectroscopic signatures of the defects/disorder in MoS2 films produced by sulfurization were identified by a comparative analysis of Raman and photoluminescence (PL) spectra with CVD grown MoS2 flakes.

Esaki Diode Behavior in Highly Uniform MoS2/Silicon Carbide Heterojunctions

“…The main mechanism ruling the formation of MoS 2 during the sulfurization process is the heterogeneous vapor–solid reaction between S and MoO x , while the loss of MoO x by evaporation plays a not negligible role at the temperature of 700 °C. [ 34 ] According to other recent reports, [ 33,34 ] the sulfurization of ≈1 nm MoO x is expected to result in the formation of 1L MoS 2 . No reaction between Mo or S and 4H‐SiC occurs during the sulfurization process, while a slight oxidation of 4H‐SiC is observed, as indicated by Si 2p core level spectra of the as‐deposited and annealed samples (reported in Figure S1, Supporting Information).…”

“…In our work, highly uniform and controlled coverage of 4H‐SiC with ultrathin MoS 2 films (predominantly monolayer (1L)) was achieved by a facile synthesis method, [ 33 ] highly compatible with semiconductor fab processes, i.e., sulfurization of predeposited very‐thin (≈1.2 nm) Mo films at a temperature of 700 °C. Interestingly, current–voltage characteristics (at room temperature) of these MoS 2 heterojunctions with n + ‐doped 4H‐SiC showed a pronounced NDR, indicating the occurrence of band‐to‐band‐tunneling (BTBT) at the interface.…”

Multi-scale modeling of 2D GaSe FETs with strained channels

“…This induced-doping strain relation has been evidenced for other 2D materials, such as black-phosphorus (BP) [32] where strain-modulated bandgap significantly alters the density of thermally activated carriers, impacting the conductance of BP-based FETs; and in single-layer MoS 2 [33] where experimental evidence of the direct relation between strain and doping has been shown by Raman mapping and electrical measurements.…”

Stitching‐Induced Structural Corrugation of Twisted Grain Boundaries in CVD‐Grown MoS2 Domains

“…Given that the signature mode is only active on the thickness or strain, the possibility of thickness inconsistency could be ruled out. [ 16,28,29 ] It has also been demonstrated that local strain resulting from a corrugated structure can induce significant changes to Raman and PL properties. [ 30 ] Therefore, the observed shift of mode is undoubtedly attributed to the structural change induced strain.…”