75 research outputs found
Atomic resolution interface structure and vertical current injection in highly uniform heterojunctions with bulk GaN
The integration of two-dimensional with recently attracted
significant interest for future electronic/optoelectronic applications.
However, the reported studies have been mainly carried out using
heteroepitaxial templates on sapphire substrates, whereas the growth of
on low-dislocation-density bulk GaN can be strategic for the
realization of truly vertical devices. In this paper, we report the growth of
ultrathin films, mostly composed by single-layers (), onto
homoepitaxial on bulk substrates by sulfurization of a
pre-deposited film. Highly uniform and conformal coverage of the
surface was demonstrated by atomic force microscopy, while very low
tensile strain (0.05%) and a significant -type doping () of was evaluated by Raman mapping. Atomic
resolution structural and compositional analyses by aberration-corrected
electron microscopy revealed a nearly-ideal van der Waals interface between
and the -terminated crystal, where only the topmost
atoms are affected by oxidation. Furthermore, the relevant lattice parameters
of the heterojunction, such as the van der Waals gap, were
measured with high precision. Finally, the vertical current injection across
this 2D/3D heterojunction has been investigated by nanoscale current-voltage
analyses performed by conductive atomic force microscopy, showing a rectifying
behavior with an average turn-on voltage under forward bias,
consistent with the expected band alignment at the interface between
doped and .Comment: 21 pages, 6 figure
Multiscale Investigation of the Structural, Electrical and Photoluminescence Properties of MoS2 Obtained by MoO3 Sulfurization
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
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