41 research outputs found
Effects of processing on the stability of molybdenum oxide ultra-thin films
The effects of wet chemical processing conventionally employed in device
fabrication standards are systematically studied on molybdenum oxide (MoOx)
ultra-thin films. We have combined x-ray photoelectron spectroscopy (XPS),
angle resolved XPS and x-ray reflectivity techniques to provide deep insights
into the changes in composition, structure and electronic states upon treatment
of films with different initial stoichiometry prepared by reactive sputtering.
Our results show significant reduction effects associated with the development
of gap states in MoOx, as well as changes in the composition, density and
structure of the films, systematically correlated with the initial oxidation
state of Mo.Comment: 16 pages, 5 figures, Appendix include
Functional Mn--Mg cation complexes in GaN featured by Raman spectroscopy
The evolution of the optical branch in the Raman spectra of (Ga,Mn)N:Mg
epitaxial layers as a function of the Mn and Mg concentrations, reveals the
interplay between the two dopants. We demonstrate that the various
Mn-Mg-induced vibrational modes can be understood in the picture of functional
Mn--Mg complexes formed when substitutional Mn cations are bound to
substitutional Mg through nitrogen atoms, the number of ligands being
driven by the ratio between the Mg and the Mn concentrations.Comment: Accepted for publication in Applied Physics Lette
Effects of Dielectric Stoichiometry on the Photoluminescence Properties of Encapsulated WSe2 Monolayers
Two-dimensional transition-metal-dichalcogenide semiconductors have emerged
as promising candidates for optoelectronic devices with unprecedented
properties and ultra-compact performances. However atomically thin materials
are highly sensitive to surrounding dielectric media, which imposes severe
limitations to their practical applicability. Hence for their suitable
integration into devices, the development of reliable encapsulation procedures
that preserve their physical properties are required. Here, the excitonic
photoluminescence of WSe2 monolayer flakes is assessed, at room temperature and
10 K, on mechanically exfoliated flakes encapsulated with SiOx and AlxOy layers
employing chemical and physical deposition techniques. Conformal flakes coating
on untreated - non-functionalized - flakes is successfully demonstrated by all
the techniques except for atomic layer deposition, where a cluster-like oxide
coating is observed. No significant compositional or strain state changes in
the flakes are detected upon encapsulation by any of the techniques.
Remarkably, our results evidence that the flakes' optical emission is strongly
influenced by the quality of the encapsulating oxide - stoichiometry -. When
the encapsulation is carried out with slightly sub-stoichiometric oxides two
remarkable phenomena are observed. First, there is a clear electrical doping of
the monolayers that is revealed through a dominant trion - charged exciton -
room-temperature photoluminescence. Second, a strong decrease of the monolayers
optical emission is measured attributed to non-radiative recombination
processes and/or carriers transfer from the flake to the oxide. Power- and
temperature-dependent photoluminescence measurements further confirm that
stoichiometric oxides obtained by physical deposition lead to a successful
encapsulation.Comment: 30 pages, 6 figure