Luminescent Defects in a Few-Layer $h$-BN Film Grown by Molecular Beam Epitaxy

Luminescent Defects in a Few-Layer $h$ -BN Film Grown by Molecular Beam Epitaxy

A. Hernández-Mínguez, J. Lähnemann, S. Nakhaie, J. M. J. Lopes, and P. V. Santos

Phys. Rev. Applied 10, 044031 – Published 11 October 2018

Abstract

We report on luminescent centers contained in a few-layer-thick hexagonal boron nitride ( $h$ -BN) film grown on $Ni$ by molecular beam epitaxy. After transfer to a ${Si O}_{2}$ / $Si$ substrate, sharp lines are observed in photo- and cathodoluminescence spectra in both the ultraviolet and the visible range. Spatially resolved measurements reveal that the luminescent centers responsible for these lines are localized within microscopic multilayer islands that form at the nucleation centers of the $h$ -BN film. The comparison of their energy, polarization, and phonon-replica emission with previous theoretical predictions suggest that the $N_{B} V_{N}$ antisite could be one of the light emitters present in our sample. Moreover, we also observe evidence of other kinds of centers that could be associated to defects containing carbon or oxygen. The characterized luminescent defects could have potential applications as quantum light sources.

Received 26 June 2018
Revised 15 August 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.044031

Physics Subject Headings (PhySH)

Color centers Optoelectronics Point defects Single photon sources

Multilayer thin films Nitrides Semiconductor compounds

Molecular beam epitaxy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. Hernández-Mínguez^*, J. Lähnemann, S. Nakhaie, J. M. J. Lopes, and P. V. Santos

Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5-7, 10117 Berlin, Germany

^*alberto.h.minguez@pdi-berlin.de

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Vol. 10, Iss. 4 — October 2018

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Images

Figure 1
(a) Optical micrograph of the $h$ -BN film transferred to the ${Si O}_{2}$ / $Si$ substrate. The area covered by the $h$ -BN film is marked by the red dashed square. (b) Room-temperature photoluminescence spectra measured at two different points on the $h$ -BN film (green and blue curves), as well as a control spectrum recorded on a region without $h$ -BN (red curve). The curves are normalized to the amplitude of the $Si$ Raman peak at $\approx 2.56$ eV (marked in the graph) and vertically shifted for clarity. The inset displays the Raman line of the $h$ -BN after background subtraction. The solid line is a fit to the data using a Lorentzian function.
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Figure 2
(a) Scanning electron micrograph of a region of the $h$ -BN film after transfer to the ${Si O}_{2}$ / $Si$ substrate. (b),(c) Energy-filtered CL intensity maps centered at 320 nm (3.88 eV) and 600 nm (2.07 eV), respectively, recorded with a 36-nm spectral band pass. The CL images are displayed in a false color scale.
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Figure 3
Spatially resolved CL intensity maps of the island marked with a blue square in Fig. 2, for (a) the UV and (b) the visible emission range. Below the maps, exemplary spectra from two points of the map are given.
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Figure 4
(a) Characteristic PL spectrum of the luminescent center denoted as D1. It shows the ZPL at 2.16 eV (measured at 6 K), together with phonon side bands at lower energies associated to the vibration modes $A$ , $B$ , and $C$ described in the main text. The peaks marked as $A^{'}$ and $A^{″}$ correspond to phonon replicas involving the emission of two and three phonons, respectively. (b) Intensity of the ZPL as a function of the excitation power density, $I_{laser}$ . The solid curve is a fit to the data according to Eq. (1). (c) Spectrally resolved PL (measured at 20 K) of a center with ZPL at 2.166 eV. The color plot of the inset displays the intensity of the ZPL as a function of the polarization angle of the excitation laser. (d) Excitation (red squares) and emission (green circles) polarization curves for the ZPL of panel (c). The red and green curves are fits using a $\sin^{2} θ$ function.
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Figure 5
(a) Spectrally resolved PL of the center denoted as D2. It shows the ZPL at 2.003 eV (measured at 6 K) and no traces of a phonon side band. Its second-order autocorrelation function, $g^{(2)} (τ)$ , is displayed in the inset. The solid curve is a fit to the data according to Eq. (2). (b) Luminescence as a function of the polarization angle of the excitation laser. It consists of two emission lines, indicated as peak 1 and 2 in the figure. (c) Excitation polarization curves for peak 1 (red squares) and peak 2 (green circles). The red and green lines are fits using a $\sin^{2} (θ)$ function.
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