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 $g^{(2)}(\tau)$ 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 $g^{(2)}(\tau)$ 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 ($T_2/2T_1 \sim 1$) between spectral jumps

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

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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

13-carbon isotopically depleted n-type phosphorus doped diamond homoepilayer

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