88 research outputs found
Characterization methods dedicated to nanometer-thick hBN layers
Hexagonal boron nitride (hBN) regains interest as a strategic component in
graphene engineering and in van der Waals heterostructures built with two
dimensional materials. It is crucial then, to handle reliable characterization
techniques capable to assess the quality of structural and electronic
properties of the hBN material used. We present here characterization
procedures based on optical spectroscopies, namely cathodoluminescence and
Raman, with the additional support of structural analysis conducted by
transmission electron microscopy. We show the capability of optical
spectroscopies to investigate and benchmark the optical and structural
properties of various hBN thin layers sources
Excitonic recombinations in hBN: from bulk to exfoliated layers
Hexagonal boron nitride (h-BN) and graphite are structurally similar but with
very different properties. Their combination in graphene-based devices meets
now a huge research focus, and it becomes particularly important to evaluate
the role played by crystalline defects in them. In this work, the
cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are
reported and compared to those of nanosheets mechanically exfoliated from them.
First the link between the presence of structural defects and the recombination
intensity of bound-excitons, the so-called D series, is confirmed. Low
defective h-BN regions are further evidenced by CL spectral mapping
(hyperspectral imaging), allowing us to observe new features in the
near-band-edge region, tentatively attributed to phonon replica of exciton
recombinations. Second the h-BN thickness was reduced down to six atomic
layers, using mechanical exfoliation, as evidenced by atomic force microscopy.
Even at these low thicknesses, the luminescence remains intense and exciton
recombination energies are not strongly modified with respect to the bulk, as
expected from theoretical calculations indicating extremely compact excitons in
h-BN
Exciton and interband optical transitions in hBN single crystal
Near band gap photoluminescence (PL) of hBN single crystal has been studied
at cryogenic temperatures with synchrotron radiation excitation. The PL signal
is dominated by the D-series previously assigned to excitons trapped on
structural defects. A much weaker S-series of self-trapped excitons at 5.778 eV
and 5.804 eV has been observed using time-window PL technique. The S-series
excitation spectrum shows a strong peak at 6.02 eV, assigned to free exciton
absorption. Complementary photoconductivity and PL measurements set the band
gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger
than 380 meV
Investigating the fast spectral diffusion of a quantum emitter in hBN using resonant excitation and photon correlations
The ability to identify and characterize homogeneous and inhomogeneous
dephasing processes is crucial in solid-state quantum optics. In particular,
spectral diffusion leading to line broadening is difficult to evidence when the
associated timescale is shorter than the inverse of the photon detection rate.
Here, we show that a combination of resonant laser excitation and second-order
photon correlations allows to access such fast dynamics. The resonant laser
drive converts spectral diffusion into intensity fluctuations, leaving a
signature in the second-order coherence function of the
scattered light that can be characterized using two-photon coincidences --
which simultaneously provides the homogeneous dephasing time. We experimentally
implement this method to investigate the fast spectral diffusion of a color
center generated by an electron beam in the two-dimensional material hexagonal
boron nitride. The function of the quantum emitter measured
over more than ten orders of magnitude of delay times, at various laser powers,
establishes that the color center experiences spectral diffusion at a
characteristic timescale of a few tens of microseconds, while emitting
Fourier-limited single photons () between spectral jumps
Quantum well confinement and competitive radiative pathways in the luminescence of black phosphorus layers
Black phosphorus (BP) stands out from other 2D materials by the wide
amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from
Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness.
This singularity made the optical and excitonic properties of BP difficult to
map. Specifically, the literature lacks in presenting experimental and
theoretical data on the optical properties of BP on an extended thickness
range. Here we report the study of an ensemble of photoluminescence spectra
from 79 passivated BP flakes recorded at 4 K with thicknesses ranging from 4 nm
to 700 nm, obtained by mechanical exfoliation. We observe that the exfoliation
steps induce additional defects states that compete the radiative recombination
from bound excitons observed in the crystal. We also show that the evolution of
the photoluminescence energy versus thickness follows a quantum well
confinement model appreciable from a thickness predicted and probed at 25 nm.
The BP slabs placed in different 2D heterostructures show that the emission
energy is not significantly modulated by the dielectric environment.
Introduction Confinement effectsComment: 11 pages, 3 figures - Main text 12 pages, 5 figures - Supporting
informatio
Distinguishing different stackings in layered materials via luminescence spectroscopy
Despite its simple crystal structure, layered boron nitride features a
surprisingly complex variety of phonon-assisted luminescence peaks. We present
a combined experimental and theoretical study on ultraviolet-light emission in
hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of
high-quality samples are measured via cathodoluminescence spectroscopy,
displaying characteristic differences between the two polytypes. These
differences are explained using a fully first-principles computational
technique that takes into account radiative emission from ``indirect'',
finite-momentum, excitons via coupling to finite-momentum phonons. We show that
the differences in peak positions, number of peaks and relative intensities can
be qualitatively and quantitatively explained, once a full integration over all
relevant momenta of excitons and phonons is performed.Comment: Main: 6 pages and 4 figures, Supplementary: 6 pages and 7 figure
Distinguishing Different Stackings in Layered Materials via Luminescence Spectroscopy
peer reviewedDespite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic
differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from “indirect,” finite-momentum excitons via coupling to finite-momentum phonons.We show that the differences in peak positions, number of peaks, and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed
Luminescence Spectroscopy of Bound Excitons in Diamond
International audienc
Etude d'un laser UV compact à semiconducteurs (Al, Ga)N pompé par micropointes
Ce mémoire présente les premiers résultats d'une étude visant la réalisation d'un laser UV continu, compact, de faible puissance (10mW) dans la gamme de longueur d'onde 250-350nm. Dans ce dispositif, la structure laser à émission latérale est à base de nanostructures (puits ou boîtes quantiques) de semiconducteurs nitrures (Al,Ga)N. Le pompage est assuré par des électrons énergétiques (10keV) émis par des cathodes à micropointes. L'étude séparée des éléments du laser a permis de déterminer les capacités et les limites actuelles des technologies utilisées. Le développement d'un canon à électrons miniature a bénéficié de l'étude d'un procédé de focalisation magnétique simple à base d'aimants permanents, et de matrices de micropointes adaptées à l'émission de forts courants (Ã.cm-2) dans un cône d'émission réduit. L'ensemble du dispositif de pompage a permis d'atteindre une densité de puissance de 12kW.cm-2 à 10kV sur la puce laser. Des hétérostructures laser à confinements séparés pour les porteurs et la lumière ont été réalisées en épitaxie par jets moléculaires à source plasma radiofréquence sur des substrats de SiC. L'effet laser a été obtenu à la température ambiante à 331nm à partir de boîtes quantiques GaN/AlxGa1-xN. Une étude expérimentale a permis d'attribuer l'origine des seuils encore trop élevés pour le pompage par micropointes, aux fortes pertes optiques internes des guides ternaires AlxGa1-xN/AlyGa1-yN (200cm-1). Un ensemble de procédés technologiques (gravure, clivage, polissage, miroirs diélectriques Hf02/SiO2) a été développé pour permettre la réalisation de cavités optiques à faibles pertes, adaptées au laser à boîtes quantiques.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF
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