J.-Y. Veuillen

Single photon emitters in exfoliated WSe2 structures.

Crystal structure imperfections in solids often act as efficient carrier trapping centres, which, when suitably isolated, act as sources of single photon emission. The best known examples of such attractive imperfections are well-width or composition fluctuations in semiconductor heterostructures (resulting in the formation of quantum dots) and coloured centres in wide-bandgap materials such as diamond. In the recently investigated thin films of layered compounds, the crystal imperfections may logically be expected to appear at the edges of commonly investigated few-layer flakes of these materials exfoliated on alien substrates. Here, we report comprehensive optical micro-spectroscopy studies of thin layers of tungsten diselenide (WSe2), a representative semiconducting dichalcogenide with a bandgap in the visible spectral range. At the edges of WSe2 flakes (transferred onto Si/SiO2 substrates) we discover centres that, at low temperatures, give rise to sharp emission lines (100 μeV linewidth). These narrow emission lines reveal the effect of photon antibunching, the unambiguous attribute of single photon emitters. The optical response of these emitters is inherently linked to the two-dimensional properties of the WSe2 monolayer, as they both give rise to luminescence in the same energy range, have nearly identical excitation spectra and have very similar, characteristically large Zeeman effects. With advances in the structural control of edge imperfections, thin films of WSe2 may provide added functionalities that are relevant for the domain of quantum optoelectronics.

Heteroepitaxy of metallic and semiconducting silicides on silicon

Abstract Recent developments in the epitaxial growth of metallic and semiconducting silicides on silicon are reviewed. The structural, electronic and electrical properties of these silicide-silicon interfaces are examined with the aid of results obtained with a large variety of in-situ and ex-situ surface techniques. The paper will focus on two topics: (i) recent progress on the epitaxial growth of thin metallic CoSi 2 films on Si(111), (ii) epitaxial growth of thin semiconducting β-FeSi 2 films on Si(111), (100) and (100) vicinal faces.

Role of pseudospin in quasiparticle interferences in epitaxial graphene probed by high-resolution scanning tunneling microscopy

Pseudospin, an additional degree of freedom emerging in graphene as a direct consequence of its honeycomb atomic structure, is responsible for many of the exceptional electronic properties found in this material. This paper is devoted to providing a clear understanding of how graphenes pseudospin impacts the quasiparticle interferences of monolayer (ML) and bilayer (BL) graphene measured by low-temperature scanning tunneling microscopy and spectroscopy. We have used this technique to map, with very high energy and space resolution, the spatial modulations of the local density of states of ML and BL graphene epitaxially grown on SiC(0001), in presence of native disorder. We perform a Fourier transform analysis of such modulations including wave vectors up to unit vectors of the reciprocal lattice. Our data demonstrate that the quasiparticle interferences associated to some particular scattering processes are suppressed in ML graphene, but not in BL graphene. Most importantly, interferences with 2(qF) wave vector associated to intravalley backscattering are not measured in ML graphene, even on the images with highest resolution where the graphene honeycomb pattern is clearly resolved. In order to clarify the role of the pseudospin on the quasiparticle interferences, we use a simple model which nicely captures the main features observed in our data. The model unambiguously shows that graphenes pseudospin is responsible for such suppression of quasiparticle interference features in ML graphene, in particular for those with 2qF wave vector. It also confirms scanning tunneling microscopy as a unique technique to probe the pseudospin in graphene samples in real space with nanometer precision. Finally, we show that such observations are robust with energy and obtain with great accuracy the dispersion of the p bands for both ML and BL graphene in the vicinity of the Fermi level, extracting their main tight-binding parameters.

Oxidation of thin erbium and erbium silicide overlayers in contact with silicon oxide films thermally grown on silicon

Pure Er and co-evaporated Er and Si layers were deposited near room temperature in UHV on SiO2 films grown on Si(100) wafers and were subsequently annealed at increasing temperature up to 1153 K. The samples were characterized in situ by X-ray photoelectron spectroscopy following deposition and each annealing step. The co-evaporated samples were also post-examined by Rutherford backscattering spectroscopy after the final annealing. The results show that both the Er and the ErSix adlayers react readily with the SiO2 upon increasing temperature to give Er2O3, silicon suboxides and elemental silicon. The erbium oxide remains stable up to 1073 K and then transforms back to erbium silicide with a simultaneous loss of oxygen from the surface via the volatile SiO. This behavior is rationalized in terms of a number of solid phase reactions taking place in the overlayer.

Scanning tunneling microscopy study of the Si(111)-(√3 × √3)-Pb mosaic phase

Abstract The atomic and electronic structure of the Pb-induced (√3 × √3)R30 mosaic phase on Si(111) substrates has been studied by means of scanning tunneling microscopy (STM). A systematic voltage dependent STM imaging analysis combined with scanning tunneling spectroscopy measurements have enabled us to interpret the different height contrast between Si and Pb adatoms observed on STM images. This contrast difference may be attributed to a large charge-transfer between the different adatom dangling-bond orbitals.

Interface structure of graphene on SiC: an ab initio and STM approach

High temperature treatment of SiC surfaces is a well-established technique for producing graphene directly on top of an insulating substrate. In this domain an important question is the influence of the substrate on the atomic and electronic structure of the graphene layers. This requires a detailed investigation of the interactions at the graphene-SiC interface. Surface science techniques and ab initio calculations are well suited for that purpose. In this paper, we present a brief review of the recent investigations performed in this domain by scanning tunnelling microscopy (STM) and ab initio simulations. It is largely based on the work performed in our group, but it also provides a survey of the literature in these fields. Both the so-called Si and C face of the hexagonal 6H(4H)SiC{0 0 0 1} substrates will be considered, as they show markedly different types of behaviour.

The influence of growth techniques on the structure of epitaxial ErSi1.7 on Si(111)

Abstract The very low lattice mismatch between the ErSi1.7(0001) and Si(111) allows a convenient epitaxial growth of the former on silicon substrates. However, the morphology of the films formed is found to depend greatly on the epitaxy techniques used. We have thus undertaken a study of the different possible techniques (single metal deposition, alternate evaporation and co-evaporation) so as to determine the procedure which gives silicide thin films of good quality. Best results are obtained by co-evaporation: the films are free of pinholes and present a flat surface. The formation of the silicide films was followed in-situ by LEED and XPS. The ErSi1.7 silicide is characterized by a sharp ( 3 × 3 )R30° LEED pattern which always appears after annealing at temperatures ⩾ 650°C whatever deposition technique is used. The Si KLL spectra, on the other hand, are found very appropriate to detect the presence of holes. The texture of the films was studied ex-situ by SEM. Glancing incidence X-ray diffraction and SIMS have also been used to get insights into the crystalline parameters and the in-depth composition of the silicide layers. The lattice of the epitaxial film is laterally extended to adapt to the Si(111) lattice and compressed axially. No evidence of ordered Si vacancies in the silicide volume is detected. The Er : Si ratio is the same throughout the film. The formation of ErSi1.7 is discussed in relation to the nucleation controlled reaction and the atomic structure of the compound.

Atomic and electronic structure of monolayer graphene on 6H-SiC(0001)(3 3) : a scanning tunneling microscopy study.

We present an investigation of the atomic and electronic structure of graphene monolayer islands on the 6H-SiC(0001)(3 3) (SiC(3 3)) surface reconstruction using scanning tunneling microscopy (STM) and spectroscopy (STS). The orientation of the graphene lattice changes from one island to the other. In the STM images, this rotational disorder gives rise to various superlattices with periods in the nm range. We show that those superlattices are moir e patterns (MPs) and we correlate their apparent height with the stacking at the graphene/SiC(3 3) interface. The contrast of the MP in STM images corresponds to a small topographic modulation (by typically 0.2 A) of the graphene layer. From STS measurements we nd that the substrate surface presents a 1 :5 eV wide bandgap encompassing the Fermi level. This substrate surface bandgap subsists below the graphene plane. The tunneling spectra are spatially homogeneous on the islands within the substrate surface gap, which shows that the MPs do not impact the low energy electronic structure of graphene. We conclude that the SiC(3 3) reconstruction eciently passivates the substrate surface and that the properties of the graphene layer which grows on top of it should be similar to those of the ideal material.

Growth and morphology of epitaxial ErSi1.7 films on Si(111)7×7 studied by scanning tunneling microscopy

Abstract The formation of epitaxial erbium silicide films on Si(111)7 × 7 has been followed in situ by means of atomic resolution scanning tunneling microscopy (STM) images. The erbium silicide presents two different crystalline phases. For low Er coverage (below 1 ML) a p(1 × 1) phase is formed whereas for higher Er coverages one observes a (√3 × √3)R30° phase. STM images recorded on films grown by depositing less than one Er monolayer have shown that the formation of the p(1 × 1) phase implies the disruption of the 7 × 7 reconstruction and thus adatoms and rest-atoms are incorporated into the film. STM images directly exclude the presence of vacancies at the surface layer for both p(1 × 1) and (√3 × √3)R30° phases. The (√3 × √3)R30° superstructure could be due to a relaxation of the topmost layer. The morphology of the films is characterized by large, flat and ordered terraces; however a closer inspection at a microscopic level shows many defects such as holes and stacking faults which are discussed in the text.

Field emission interferometry with the scanning tunneling microscope

A scanning tunneling microscope, operated in the near field emission regime, is used to obtain the phases of very low energy electrons reflected from a sample surface. A simple theoretical model shows that the spectrum of the electron standing waves, formed in the vacuum gap between the tip probe and the sample, is directly related to the complex amplitudes of the reflected electron waves. The surface sensitivity of the interferometric spectra is demonstrated in the analysis of different reconstructions of the Pb/Si(111) system.