Spin-Orbit Effects on Exciton Complexes in Diamond.

Ultrafine splittings are found in the optical absorption spectra of boron-doped diamond measured with high resolution. An analytical model of an exciton complex is developed, which permits assigning all absorption lines and sizing the interactions among the constituent charges and crystal field. We conclude that the entry of split-off holes in the acceptor-bound exciton fine structure yields two triplets separated by a spin-orbit splitting of 14.3 meV. Our findings thereby resolve a long-standing controversy [R. Sauer et al., Revised fine splitting of excitons in diamond, Phys. Rev. Lett. 84, 4172 (2000).PRLTAO0031-900710.1103/PhysRevLett.84.4172; M. Cardona et al., Comment on "Revised fine splitting of excitons in diamond,", Phys. Rev. Lett. 86, 3923 (2001).PRLTAO0031-900710.1103/PhysRevLett.86.3923; R. Sauer and K. Thonke, Sauer and Thonke reply, Phys. Rev. Lett. 86, 3924 (2001).PRLTAO0031-900710.1103/PhysRevLett.86.3924], revealing the underlying physics common in diverse semiconductors, including diamond.

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.

Molecular beam epitaxial growth of multilayer 2D-boron nitride on Ni substrates from borazine and plasma-activated nitrogen.

2D boron nitride (2D-BN) was synthesized by gas-source molecular beam epitaxy on polycrystalline and monocrystalline Ni substrates using gaseous borazine and active nitrogen generated by a remote plasma source. The excess of nitrogen atoms allows to overcome the thickness self-limitation active on Ni when using borazine alone. The nucleation density and the shape of the 2D-BN domains are clearly related to the Ni substrate preparation and to the growth parameters. Based on spatially-resolved photoemission spectroscopy and on the detection of the π plasmon peak, we discuss the origin of the N1s and B1s components and their relationship with an electronic coupling at the interface. After optimization of the growth parameters, a full 2D-BN coverage is obtained, although the material thickness is not evenly distributed. The 2D-BN presents a granular structure on (111) oriented Ni grains, showing a rather poor cristallographic quality. On the contrary, high quality 2D-BN is found on (101) and (001) Ni grains, where triangular islands are observed whose lateral size is limited to ∼20µm.

Position-controlled quantum emitters with reproducible emission wavelength in hexagonal boron nitride.

Single photon emitters (SPEs) in low-dimensional layered materials have recently gained a large interest owing to the auspicious perspectives of integration and extreme miniaturization offered by this class of materials. However, accurate control of both the spatial location and the emission wavelength of the quantum emitters is essentially lacking to date, thus hindering further technological steps towards scalable quantum photonic devices. Here, we evidence SPEs in high purity synthetic hexagonal boron nitride (hBN) that can be activated by an electron beam at chosen locations. SPE ensembles are generated with a spatial accuracy better than the cubed emission wavelength, thus opening the way to integration in optical microstructures. Stable and bright single photon emission is subsequently observed in the visible range up to room temperature upon non-resonant laser excitation. Moreover, the low-temperature emission wavelength is reproducible, with an ensemble distribution of width 3 meV, a statistical dispersion that is more than one order of magnitude lower than the narrowest wavelength spreads obtained in epitaxial hBN samples. Our findings constitute an essential step towards the realization of top-down integrated devices based on identical quantum emitters in 2D materials.

Bright Luminescence from Indirect and Strongly Bound Excitons in h-BN.

A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (h-BN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ∼50% is found for h-BN at 10 K comparable to that of direct band-gap semiconductors. This bright luminescence at 215 nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk h-BN. Ab initio calculations of the exciton dispersion confirm the indirect nature of the lowest-energy exciton whose binding energy is found equal to 300±50 meV, in agreement with the thermal stability observed in luminescence. The direct exciton is found at a higher energy but very close to the indirect one, which solves the long debated Stokes shift in bulk h-BN.

Advanced synthesis of highly crystallized hexagonal boron nitride by coupling polymer-derived ceramics and spark plasma sintering processes-influence of the crystallization promoter and sintering temperature.

Hexagonal boron nitride nanosheets (BNNSs) are promising 2D materials due to their exceptional chemical and thermal stabilities together with their electrical insulation properties. A combined synthesis method involving the polymer-derived ceramics (PDCs) route and the spark plasma sintering (SPS) process is proposed, leading to well-crystallized and pure layered h-BN crystals, prone to be exfoliated into large BNNSs. Here we focus more specifically on the influence of two key parameters of the process to be optimized: the Li3N concentration (0-10 wt%) and the SPS temperature (1200 °C-1950 °C). The presence of Li3N, added as crystal promoter in the pre-ceramic powder, significantly improves the crystallinity level of the product, as evidenced by XRD, SEM and Raman spectrometry. SPS temperature strongly modifies the size of the resulting h-BN flakes. The influence of SPS temperature on both purity and crystallinity is studied using cathodoluminescence. h-BN flakes larger than 200 µm2 (average flake area) are obtained. Few-layered BNNSs are successfully isolated, through exfoliation process.

Impact of Reabsorption on the Emission Spectra and Recombination Dynamics of Hybrid Perovskite Single Crystals.

Understanding the surface properties of organic-inorganic lead-based perovskites is of high importance to improve the device's performance. Here, we have investigated the differences between surface and bulk optical properties of CH3NH3PbBr3 single crystals. Depth-resolved cathodoluminescence was used to probe the near-surface region on a depth of a few microns. In addition, we have studied the transmitted luminescence through thicknesses between 50 and 600 µm. In both experiments, the expected spectral shift due to the reabsorption effect has been precisely calculated. We demonstrate that reabsorption explains the important variations reported for the emission energy of single crystals. Single crystals are partially transparent to their own luminescence, and radiative transport is the dominant mechanism for propagation of the excitation in thick crystals. The transmitted luminescence dynamics are characterized by a long rise time and a lengthening of their decay due to photon recycling and light trapping.

Dimensionality effects on the luminescence properties of hBN.

Cathodoluminescence (CL) experiments at low temperature have been undertaken on various bulk and exfoliated hexagonal boron nitride (hBN) samples. Different bulk crystals grown from different synthesis methods have been studied. All of them present the same so-called S series in the 5.6-6 eV range, proving its intrinsic character. Luminescence spectra of flakes containing 100 down to 6 layers have been recorded. Strong modifications in the same UV range are observed and discussed within the general framework of 2D exciton properties in lamellar crystals.

P-doped diamond grown on (110)-textured microcrystalline diamond: growth, characterization and devices.

The formation and properties of (110)-textured P-doped microcrystalline CVD diamond were studied. Based on several microscopy techniques, with a special emphasis on electron backscattered diffraction, a detailed determination of the grain orientations with respect to the exact [110] axis is given. The different orientations present in the film, in combination with low phosphine concentrations in the gas phase, lead to a variation in P incorporation that can vary over three orders of magnitude, as determined with cathodoluminescence mapping. The role of the surface morphology in the observation of these large incorporation differences is explained. Hall measurements confirm that the films are n-type conductive with a thermal activation energy of 0.56 eV. Based on B-doped substrates, pn junctions were created, showing a rectification ratio of nearly 10(4) at ± 25 V.